Measurement of Radon and Radon Daughters in Air (Ncrp Report, No 97)

NCRP REPORT No. 97

MEASUREMENT OF RADON AND RADON DAUGHTERS IN AIR Recommendations of the NATIONAL COUNCIL O N RADIA1-ION PROTECTION AND MEASUREMENTS

Issued November 15. 1988 National Council on Radiation Protection and Measurements 7910 WOODMONT AVENUE I BE'THESDA,M D 20814

LEGAL NOTICE This report was prepared by the National Council on Radiation Protection and Measurements (NCRP).The Council strives to provide accurate, complete and useful information in its reports. However, neither the NCRP, the members of NCRP, other persons contributing to or assisting in the preparation of this report, nor any person acting on the behalf of any of these parties (a)makes any warranty or representation, express or implied, with respect to the accuracy, completeness or usefulness of the information contained in this report, or that the use of any information, method or process disclosed in this report may not infringe on privately owned rights; or (b)assumes any liability with respect to the use of, or for damages resulting from the use of any information, method or process disclosed in this report, under the Civil Rights Act of 1% Section 701 et seq. as amended 42 U.S.C. Section 2000e et seq. (Title VZZ) or any other statutory or common law theory governing liability

Library of Congress Cataloging-in-Publication Data National Council on Radiation Protection and Measurements. Ionizing radiation exposure of the population of the United States. (NCRP report ;no. 97) Bibliography: p. Includes index. ISBN 0-913392-97-9 1. Atmospheric radon - Environmental aspects - Measurement. 2. Atmospheric radon - Isotopes - Environmental aspects - Measurement. 3. Atmospheric radioactivity - Measurement. I. Title. 11. Series. TD885.5. R33N38 1988 88-15155CIP 363.7-392 - dc19

Copyright O National Council on Radiation Protection and Measurements 1988 All rights reserved. This publication is protected by copyright. No part of this publication may be reproduced in any form or by any means, including photocopying, or utilized by any information storage and retrieval system without written permission from the copyright owner, except for brief quotation in critical articles or reviews.

Preface Exposure of the population of the United States to radon and radon daughters has been recognized by NCRP as a potential public health problem (NCRP, 1984% 198413, 1988a). The scientific program of the 1988 Annual Meetmg of the NCRP was devoted exclusively to issues associated with radon and radon daughters. The need to perform meaningful measurements of radon and radon daughters has been recognized for some time. This report, drafted by NCRP Scientiik Committee 61, describes the instrumentation available for measurement of radon or radon daughters. It evaluates the instrumentation in light of the purpose of the measure ment and it prwides recommendations for sampling strategies. The International System of Units (Sl)is used in this Report, followed by conventional units in parentheses, in accordance with the p d m s set forth in NCRP Report No. 82, SI Units in Radiation Protection and Measurements. Serving on Scientific Committee 61 during the preparation of this report were: Naomi H. Harley, C h u i m n Department of Environmental Medicine New York University Medical Center New York, New York

Isabel M. Fisenne

Richard G. McGregor

Environmental Measurements Laboratory New York, New York

Bureau of Radiation and Medical Devices Ottawa, Ontario

John H. Harley

Anthony V. Nen,

Hoboken. New Jersey

~awrenGJ3erkeleyNational Laboratories Berkeley, California

Marvin H. W i e n i n g New Mexico Institute of Mining and 'Ikhnology Socorn, New Mexico NCRP Secretariat. William M. Beckner The Council wishes to express its appreciation to the members of the Committee for the effort they have made to produce this Report. Bethesda. Maryland September 22, 1988

Warren K. Sinclair President, NCRP

Contents ...

Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1.11 1. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1. 2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7. 3. Physical Properties of the Radon Isotopes. . . . . . . . . . . . . . . . . . . .9 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9. 3.2 Primordial Radionuclide Series . . . . . . . . . . . . . . . . . . . . . . . . .9 3.2.1 Discovery of the Radon Isotopes . . . . . . . . . . . . . . . . . 13 . 3.2.2 Radiometric Properties of the Radon Isotopes . . . . . . . .13 3.3 Physical Properties of Radon Gas . . . . . . . . . . . . . . . . . . . . . 14 . 4 Properties and Behavior of the Short-Lived Radon Daughter Products . . . . . . . . . . . . . . . . . . . . . . . . . . . .16 . 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16 4.2 Radioactive Equilibrium . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 . 4.3 Formation of Unattached Radon Daughters . . . . . . . . . . . . . .19 4.4 Behavior of Unattached Radon Daughters . . . . . . . . . . . . . . .20 4.5 Formation of Attached Radon Daughters . . . . . . . . . . . . . . . .21 4.6 Behavior of Attached Radon Daughters . . . . . . . . . . . . . . . . .21 . 4.7 Aerosol Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22 4.8 Measured Values of the Equilibrium Factor (F) . . . . . . . . . . . .23 4.9 Modeling Daughter Product Behavior. . . . . . . . . . . . . . . . . . .25 . 4.10 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25 5. Distribution of Uranium, Thorium, Radium and Radon intheEnvironment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .26 . 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .26 . 5.2 Uranium and Thorium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .-26 5.3 Radium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .28 . 5.4 Radon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .29 . . 5.4.1 Radon in the Atmosphere . . . . . . . . . . . . . . . . . . . . . . 30 5.4.2 RadoninWater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31 . 5.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .32 . 6 Radon Emanation and 'Ransport . . . . . . . . . . . . . . . . . . . . . . . . .33 . 6.1 Emanation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .33 . 6.2 Earth-to-Air nansport Processes . . . . . . . . . . . . . . . . . . . . . 35 . 6.3 llansport in the Atmosphere . . . . . . . . . . . . . . . . . . . . . . . . .36 . 6.4 Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 .

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vi I CONTENTS

7. Measurement of Radon Isotopes in Air ................... -40 7.1 Intduction ......................................40 7.2 Sampling .........................................40 7.3 Calibration....................................... -41 7.4 Instruments for Measlrrement of ?ZORnin At ............42 in Air ............43 7.5 Instruments for Measurement of z2gRn 7.5.1 Ionization Chambers ...........................43 7.5.2 Scintillation Cells ..............................46 7.5.2.1 Development of Scintillation Cells .......... 47 7.5.2.2 Scintillation Cells for Continuous Measurements ..............50 7.5.3 Passive Monitors ..............................51 7.5.3.1 Solid State Nuclear %a& Detection and Photographic Detection ............. 52 7.5.3.2 Passive Diffusion and Electrostatic Collection ........................... -53 7.5.3.3 Passive Monitor for Direct Measurement .....57 . 7.5.3.4 Passive Adsorption Measurement.......... -58 7.5.4 Solid State System for Continuous Measumment 60 7.5.5 Air Filtration for '"Rn Estimation ................61 7.5.5.1 Single Filter Sampling ...................61 7.5.5.2 ' h o Filter Method .......................61 7.6 Summary......................................... 64 8 Measurement of Individual Radon Daughters ...............65 8.1 Introduction ...................................... 65 8.2 Sampling ......................................... 65 8.3 Gross Alpha Counting...............................66 8.3.1 Tsivoglou and Modified Tsivoglou Methods.........66 8.3.2 Cliff Method ..................................68 8.4 Alpha Spectrometry ................................ 69 8.5 Gamma Spectrometry ............................... 71 8.6 Thoron Daughter Measurement .......................71 8.7 Summary .........................................72 9 Meaeurement of Unattached Radon Daughters 74 9.1 Introduction ......................................74 9.2 Properties of Unattached Radon Daughters..............77 9.3 Samplers for Unattached Radon Daughters..............79 9.3.1 Diffusion lbbes ...............................79 9.3.2 Electrostatic Collectors .........................82 9.3.3 Screen Samplers...............................83 9.3.4 Other Samplers ...............................86

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CONTENTS 1 vii

9.4 Relationship of Unattached Radon Daughters and Condensation Nuclei ...........................86 9.5 Calculational Txhniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . .89 9.6 Quality Assurance for Unattached Radon Daughter Measurements ...........................90 9.7 Calculation of the Unattached Fraction . . . . . . . . . . . . . . . . .90 . 9.8 Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .91 10 Measurement of the Potential Alpha Energy Concentration or Equilibrium Equivalent Daughter Concentration . . . . . . . .92 10.1 Introduction.....................................92 10.2 Kusnetz Method and Modifications ..................93 10.3 Rapid Measurements and Alpha Spectrometry .........98 10.4 Integrating Monitors.............................101 10.5 Personal Dosimeters . . . . . . . . . . . . . . . . . . . . . . . . . . . . .102 10.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .104 11 Radon Flux Measurements ............................105 11.1 Introduction ....................................105 11.2 Laboratory Methods ............................. 106 . 11.3 Field Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 11.3.1 Accumulation Method . . . . . . . . . . . . . . . . . . . . . .106 11.3.2 Flow Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . .108 11.3.3 Adsorption Method ........................108 11.3.4 Vertical Profile Method . . . . . . . . . . . . . . . . . . . . .109 11.3.5 Soil Concentration Gradient Method ...........109 11.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .110 . 12 Calibration, Standardization and Quality Assurance . . . . . . . .111 . 12.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 12.2 Calibration and Laboratory Standards ...............112 12.3 Standardization and Working Standards . . . . . . . . . . . . .112 . 12.4 Commercial Working Standards . . . . . . . . . . . . . . . . . . . 115 12.5 Quality Assurance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .116 12.6 Calibration Chambers . . . . . . :.....................117 13. Strategy for Measurement of Radon and Radon Daughters... 119 13.1 Introduction ....................................119 13.2 Short-Brrn versus Long-'brm Measurements..........120 13.3 Reporting of Data ............................... 122 13.4 Radon versus Radon Daughter Measurements.........123 13.5 Sampling Location. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .124 13.6 Scientific Studies ................................125 13.7 Radon Surveys for Distributional Studies ............125 13.8 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .126

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viii I CONTENTS

Appendix A: Critical Level (LC)and Lower Limit of Detection (LLD) . . . . . . . . . . . . . . . . . . . . . . . . . .127 Appendix B: Units and Conversion Factors for Concentrations . of Radon and Radon Daughters . . . . . . . . . . . . . . . . . . . . . . . . . 129 Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .131 . References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .136 TheNCRP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .157 . NCRP Publications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .164 . Index. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .173

1. summary AU of the gaseous radon members of the three primordial series, are radioactive headed by 23SU(lIgRn),'"Th P20Rn) and Z"U (222Rn) alpha-particle ernitters.~Thehalf-lives of 2'9Rn(actinon, t,,, 3.96 s) and ZZORn (thoron, t,,, 55.6 s) are short, and they have a low abundance relative to 222Rn(radon, t,,, = 3.82 d). Because of these properties and dosimetric considerations (the alpha dose delivered t o the tracheobronchial tree from 220Rnis 113 that from 2'2Rn per unit that is generally of exposure) it is the measurement of radon (222Rn) primary interest. Special situations, either occupational or environmental, could pose unusual circumstances where measurement of actinon ("ORn) or thoron ("Rn) would be the primary concern but so far these situations have been rare. All of the radon isotopes are noble gases and are inert with no large sinks in the environment. Radon is relatively soluble in water and, for this reason, water transport has been a significant mechanism for bringing radon into some underground mines and into some homes where ground water (well water) is used instead of surface water. Like all noble gases. radon does not readily form chemical compounds. However, several compounds have been produced and used for experimental purposes. The short-lived daughter products of radon, ""Po. *"Pb, ""i, and 2"Po, are of most s i ~ i c a n c efor human exposure since it is these products' which are inhaled and deliver the bronchial radiation dose that is implicated in bronchogenic carcinoma. The four daughters do not exist in any environment. whether occupational or environmental, in aggregates sufficient to be distinguished as the chemical elements. It is their radiometric properties that are important. The two polonium isotopes, 2LToand 2 1 4 P ~ , the alpha emitters, and these are the daughters of dosimetric interest. The overall effective half-life for the sequence of the four daughters is about 30 minutes. The majority of 2'8Pois formed as positively charged ions which rapidly change charge and size. The diffusion coefficient of the daughter is of fundamental importance since it controls the movement of the species in the environment. Most of the daughter species, including finPo,become

2 I 1. SUMMARY

attached to the ambient aerosol, and it is the much lower diffusion coefficient of this form which detennines movement of these daughters in the atmosphere. Diminution of the attached daughters in a given air space is a function of the deposition velocity which in turn depends upon the existing air flow, i.e., convection currents, turbulence and filtration, if it exists. Radioactive decay is also a major removal mechanism. The short-lived daughters are essentially never found in equilibrium with the gaseous parent radon. The concentrations of "'Rn and its four short-lived daughters are usually expressed in a sequence of ratios such as 110.910.610.4. This denotes that, relative to a unit concentration of 222Rn, Z ' 8 Pis~ 0.9, ZIAPb is 0.6, and both "'Bi and "'Po are 0.4. The daughter equilibrium is also given as a fraction of the potential alpha energy or working level (see Section 4.1) for the radon in equilibrium with its daughters; this quantity is called the equilibrium factor (F).For example, an equilibrium factor of 0.4, which is typical of indoor environments, means that the ratio of the working level to the concentration of radon (pCi 1-I),multiplied by 100, is 0.4. The term equilibrium equivalent concentration (EEC)is frequently used in the European literature. The EEC is the radon concentration in equilibrium with its short-lived daughters, that would have the same potential alpha-particle energy per unit volume as the existing mixture. The equilibrium factors for various environmental and occupational situations are described in detail in later sections of this Report. The concentration of radon in the atmosphere is somewhat dependent upon the uranium and radium concentrations in soil. The actual abundance of uranium in soil varies widely over geographic areas. Igneous rocks generally exhibit higher Z,'hU than sedimentary rocks with the exception of some shales and phosphate rocks. The concentration of radium and uranium in soil is generally different from that of the earth's crust as a whole. This is because soils reflect the amount of leaching, porosity, augmentation, and precipitation from groundwater etc. A typical value for 238Uand 226Rain soil is about 40 Bq k g 1 (1pCi gl). This gives rise to & average measured value of 17 rnBq m-2s-' (0.45 pCi m-' s-') for the exhalation or emanation rate or flux of radon from soil. The actual value will depend upon soil moisture content, porosity, temperature and barometric pressure changes at the time of the measurement. Once in the atmosphere, turbulent (eddy) diffusion originating from thermally driven convective processes or winds is effective in radon transport. Diurnal and seasonal radon courses are consistent with atmospheric stability caused by nighttime inversions and with

1. SUMMARY 1 3

turbulent diffusion models. Changes in radon concentration are site specific with mountain sites showing classical nocturnal drainage winds and temperature stability. Marine environments can show land-sea breeze effects where on-shore winds carry little radon. Radon, thoron and their daughter products play an important role in steady-state atmospheric ionization, with both chains typically producing about 5,000 ion pairs per liter per second. For radon to be released into the air spaces in soil efficiently requires that the radon atom be formed within 20 to 70 nm of the mineral surface for most common minerals. The presence of water in capillaries or in a film around soil particles increases the release of radon into soil air. With water, the probability that the recoiling radon atom will stop in the pore is greatly increased and measurements verify this concept in ordinary soils and mill tailings. Movement from the soil to the atmosphere is then controlled by diffusion and pressure gradients in soil caused by various processes such as barometric pressure changes. The measurements of radon and thoron in air are separated into three categories, instantaneous, continuous or timeaveraged methods. Ionization chambers and scintillation cells are utilized primarily for instantaneous or grab samples, but may be modified to accommodate continuous measurements. Passive methods to detect radon are popular since largescale environmental surveys require that a detector be located in a suitable location for extended periods of time in order to assess annual exposure. The passive methods actually rely on the short-lived daughter products to provide the signal. There are basically three methods of passive radon detection, solid-state nuclear track detectors (etched tracks in plastic films), charcoal adsorption, and thermoluminescent dosimeters. The etched track detector relies upon a latent damage hole or pit forming when an alpha particle penetrates the film, and this is revealed upon etching with a caustic solution and viewing under light microscopy. The f i is preferably exposed in a fixed volume cell with a passive diffusion barrier to permit radon entry but to block daughter entry. Bare track detectors suffer from the vagaries of variable plateout of the daughters on the detector surface and other factors. Inside of a fixed volume the short-lived daughters deposit on the film and most of the pits or holes counted are related to them rather than the alpha particles from radon itself. These detectors can be deployed for very long times, and one year is common. Charcoal canisters of a variety of sizes, with or without water vapor removal media and diffusion barriers can be deployed for up to about a week. Radon gas adsorbs on the charcoal but it is the gamma-emitting

4 1 1. SUMMARY

daughters, ""b and ""i, which build up to radioactive equilibrium with the radon, and are counted. Counting is usually performed with a sodium iodide gamma scintillation counting system or spectrometer. Thermolurninescent dosimeters (TLDs)can also be used to measure radon. These are placed in a fixed volume and either passive diffusion or a small flow device brings radon into the detector. The charged daughters are collected at a negatively charged electrode to enhance the signal, and the TLD material is placed under or on the electrode so that alpha and beta particles emitted by the daughters interact with the TLDs. Radon daughters are measured with relative simplicity using a filtered air sample and three subsequent alpha counts. This method originally devised by Tsivoglou and modified by Thomas, Scott, and others provides the actual air concentration of each daughter species with a total counting time of about 30 minutes. This more detailed information is better for scientific purposes than a single count method which is used to determine the working level. However, it is more labor-intensive and in large surveys or in occupational situations the time involved in sampling and counting may be a consideration. The alpha spectrometer can replace the alpha counter in the detection process. This reduces the counting error of the ""Po measurement by reducing background. The measurement of unattached daughters has in general been unsatisfactory. Measurements are made with diffusion tubes which allow measurement of the radionuclides in a filtered air sample after the air stream has passed through a tube. This value, compared with the reference air concentration and a correction for ""Po formed from radon within the tube, provides the necessary information to calculate the unattached fraction. The rapid change in the ""Po diffusion coefficient in some atmospheres invalidates some of the assumptions made in the calculation of results with diffusion tube samplers. Screen samplers are popular because they are not as difficult to use in field measurements as diffusion tubes. The unattached species of radon daughters, with a relatively large diffusion coefficient, can be collected efficiently with 60 mesh or finer screen sizes. Interception of the attached species on the screens and the selection of the appropriate diffusion coefficient is also a problem in sampling with screen samplers. l b o or three screens in series with simultaneous counting of the screens is necessary to determine the screen efficiency and therefore the appropriate diffusion coefficient. This is an area of radon daughter research that needs additional effort.

1. SUMMARY / 5

The measurement of working level was appealing historically because it r e q a a fiveminute filtered air sample followed by a single alpha count. The original method was devised by Kusnetz and later modified by many investigators. The most widely used modification is that developed by Rolle. I t requires a fixed sampling time and the counting interval can be selected to provide the optimum (minimum) error in estimation of the working level. Various instruments have been developed which measure the integrated activity from radon daughters and relates this to working level. The most widely used version as of this writing is the radon progeny integrated sampling unit (RPISU) which was developed by Schiager. Radon flux measurements are of some scientific value to identify the radon source term. Accurate radon flux measurements are difficult in that the measurement must not perturb the existing diffusion or pressuredriven flows. Various types of accumulator cans have been devised which are basically containers with one open face. This face is sealed onto the surface being measured or embedded in the soil. Many types of accumulators have been used including drums, boxes and cylinders. The flux rate is determined from the initial slope of the radon growth curve through sequential sampling of "2Rnin the accumulator, the accumulator volume and the area of surface covered by the accumulator. Flow methods are also utilized to more closely simulate natural conditions. The exit air from the accumulator is collected in a trap and air flow adjusted to approximate that which occurs naturally. Charcoal canisters are also used and these are sealed to the surface to be measured. The charcoal has the advantage that back diffusion due to the build-up of radon in the accumulator does not occur. However, alI methods necessarily perturb the -measured system somewhat. Comparison of flux measurements is advantageous in remediation of some homes with high radon levels. Flux measurement has also been used to determine whether significant differences exist in the flux from basement walls and floor. Standardization, calibration and quality control are important aspects of any radon or radon daughter measurement program. The only primary standards are the vials of '=Ra solution prepared by the National Bureau of Standards (NBS)[Name changed to the National Institute of Standards and ~ o l o g (NIST) y as of August 23, 1988 by provisions of the Omnibus 'Ikade and Competitiveness Act]. Radon calibrations are thus based on laboratory standards that are referred to as being traceable to NBS. None of the available counters is an absolute counter and so instrument standardization is required.

6 1 1. SUMMARY

Standardization of a radon counter is accomplished by transferring a known amount of gas to the counting system. There are two approaches to generating the known amount of gas. The first is to allow radon to come into equilibrium with a standard radium source in a container of known volume, and the second is based on the production rate of radon from a known amount of radium. Both of these depend on the complete release of radon from the source which is simple for radium in solution but more difficult for radium as a solid source. There are presently no standards for the short-lived radon daughters. Measurements of the daughters must be made with devices which have meticulously calibrated flow rates, frequent efficiency measurements of the counting device and replicate samples in order to ensure stability. Quality assurance should consist of some form of standard samples, along with duplicates and blank samples. Results on standards are a measure of the accuracy of the system and results on duplicates are a measure of the precision. One practical approach to standardization is to establish a room or chamber and attempt to maintain a fixed radon concentration. I t is easier to maintain a constant radon atmosphere than a constant radon daughter atmosphere in a mom-sized chamber, particularly when sampling. The purpose of the survey or measurement usually dictates a particular measurement strategy. Generally there are four primary purposes for measurement, evaluation of human exposure, identification of high radon areas, scientific studies to determine fundamental mechanisms, and diagnostic measurements made when carrying out remedial action. If human exposure evaluation is the purpose, long-term measurements or multiple-spot measurements should be made. In contrast, short-term measurements are more appropriate for screening for geographic areas of high exposure. Scientific studies are usually more intensive with continuous monitoring and measurements of radon and several related variables. Any of the methods described in this report can provide adequate high-quality information depending upon the care taken to calibrate and standardize the measurement equipment. Proper decisions regarding remedial action, environmental or occupational exposure and risks cannot be made without valid basic information on radon concentrations and consideration of all exposure conditions.

2. Introduction With the realization that indoor radon concentrations can reach levels that q u i r e remedial action, interest in the measurement of radon and radon daughters has escalated dramatically. There are many methods which have been used to measure radon and radon daughters in both occupational and environmental situations. It is the purpose of this report to describe measurement techniques and to indicate which of the methods are appropriate or pertinent to particular situations. This report describes the properties of radon and particularly the properties of radon daughters which are important in developing proper methods of measurement and in understanding the dosimetric significance of exposures. The techniques for measurement of radon are described as well as methods for measurement of radon daughters. Although it is the radon daughters which deliver the alpha dose to target cells in bronchial epithelium (implicated in bronchogenic carcinoma),it is the parent radon which supports the daughters, and its measurement is often the measurement of choice. Radon daughters are simpler to measure when instantaneous or grab sampling is desired but radon measurement is easier for continuous or integrated sampling. The relationship between radon and radon daughters is described in detail in Section 4 for occupational exposures and for indoor and outdoor environmental exposures. At present there are no exposure chambers with stable, standardized concentrations for radon daughters but standard chambers are available for radon. For this reason measurement of radon is becoming the method of choice, especially in environmental situations where accurate estimates of long-term average exposure are desired for risk estimation. The strategies for measurement in the most typical situations, environmental exposure surveys, screening for areas of high radon and scientific studies to determine fundamental factors of radon behavior are detailed in the last section of the report. Throughout the report, calibration, standardization, and quality control are emphasized so that reported results are meaningful. Without this emphasis and without the reporting of quality control

8 1 2. INTRODUCTION

data, little reliance can be placed on assessment of any of the implications of exposure to indoor radon or on the decisions made regarding remedial action. The report is not intended to be a cookbook, rather it is intended to provide the background information and description of the available instruments and techniques for measurement of radon and radon daughters so that the appropriate choices can be made when obtaining data. The advantages and disadvantages of the measurement methods are described. Each of the methods described is of value and can provide useful data depending upon the specific purpose considered. Therefore, it is not possible to grade methods for quality.

3. Physical Properties of the Radon Isotopes 3.1 Introduction All known isotopes of radon, a noble gas, am radioactive. Three isotopes, 2'8Rn,2mRnand 222Rn,occur in nature as members of the primordial actinium, thorium, and uranium series, respectively. Their importance in the exposure of man ta terrestrial radiation stems. principally, from their transport from surfaces into the atmosphere where their decay products may be inhaled. This section summarizes the available information on the physical properties of radon. Little is known of the chemical properties of radon because of the short half-livesof all of its isotopes. 3.2 Primordial RadionuclideSeries A thorough review of the actinium, thorium, and uranium radionuclide series appears in NCRP Report Nos. 45, 77, and 94 (NCRP, 1975, 1984a, 1987) and some of the material is repeated here for convenience. AU isotopes of the elements of natural origin with atomic numbers greater. than 83 (bismuth) are radioactive. They are members of chains of successive decays and constitute the primordial radioactive series. The three series, uranium, thorium and actinium, are headed, respectively, by 238U,232Th,and Za6U(Figures 3.1, 3.2, and 3.3). In an undisturbed state, that is one not subjected to any physical or chemical separation, a radioactive decay series reaches radioactive equilibrium. In this state, the rate of radioactive emissions of each member of the series is essentially equal to the rate of emission of the nuclide heading the series, commonly called the parent nuclide. In the environment, the members of these series are subject to physical and chemical changes so that radioactive equilibrium among the members is not generally expected. The parent and longer-lived

10 1 3.PHYSICAL PROPERTIESOF THE RADON ISOXlPES

Ra-226

4.8 MeV

Rn-222 5.5 MeV

5.0 6 1.2 MeV

Fig.3.1 Principal decay scheme of the u m n i w series

A

3.2 PRIMORDIAL RADIONUCLIDE SERIES I 11

Th-232 1 . 4 ~10'Oy 4.0 MeV

Th-228 (RdTh) 1.91 y ~5.3,5.4 MeV A~-228(MsTh11)' 6.13 h H0.4 - 2.2 MeV Ra-228(MsThI)' Ra-224 (ThX) 5.8 y 3.64 d 10.1 MeV 5.7 MeV

I

Rn-220 (ln) 6.3 MeV

'pJ I PO-216 (ThA) U.IJJ

6.8 MeV

I

~7

I Bi-212 fThC)I

/-60.;1-',~ 2.2 MeV P 6.1 MeVa Pb-212 (ThB)

Alpha Decay ,

,

1 .Pb-208 (ThD) Stable

0.3, 0.6 MeV TI-208 iThC"I

Fig. 3.2 Principal decay scheme of the thorium series

,

12 1 3. PHYSICAL PROPERTIESOF THE RADON ISOTOPES

U-235 (AcU) 7.1 x 108y 4.4 MeV

#

'

Pa-231 3.2 xI04y 5.0 MeV

Th-231 (UY) 25.5 h 0.09 - 0.30 MeV

Th-227 (Rd Ac) 18..2 d ~ 5 . -86.0 MeV

Ac-227 21.6 y 0.05 MeV

'

Ra-223 (AcX) 11.4d 5.5 - 5.7 MeV

I

7.4 MeV Bi-211 (AcC) 2.1 5 min A 6.3, 6.6 MeV

Pb-211 (AcB) 36.1 min 1.4, 0.5 MeV

Beta Decay Alpha Decay

'

Pb-207 Stable TI-207 (AcC") 4.79 min 1.44 MeV

'

Fig. 3.3 Principal &cay scheme of the actinium series

3.2 PRIMORDIAL RADIONUCLIDE SERIES 1 13

radionuclides of these series exist in soil and rock at concentrations of a few parts per million. The shorter-lived nuclides are present in extremely small concentrations and their behavior tends to deviate from the chemical mass action law. Disequilibrium among series members occurs through physical processes such as soil erosion, radioactive recoil and absorption which influence the mobility of the nuclides. In this respect the radon isotopes are the most mobile due to their unreactive, inert gas structure.

3.2.1 Discovery of the Radon Isotopes Owens and Rutherford observed erratic electrometer readings when measuring thorium salts. In 1899 the cause of the erratic readings was found to be the diffusion of a radioactive substance from the thorium salt through the ionization chamber (Rutherford, 1900a, b). Later that year actinium was separated from pitchblende and was found to give off an active emanation. The following year Dorn (1901) found that radium salts emitted a gas similar to that from thorium and actinium. In 1902 Rutherford and Soddy succeeded in condensing radon. Five years later Soddy was able to prove that radon was a member of the inert gas family (Ramsay,1907).

3.2.2 Radiometric Properties of the Radon Isotopes The radioactive decay properties of the three naturally occurring radon isotopes are summarized in Table 3.1. a. 2'9Rn(Actinon),a member of the actinium series that starts with the long-lived radionuclide 'YU,is the least abundant radon isotope. The concentration of "'LT by weight in rocks and soils is generally Ra(or L"Ra in the thorium series) the "'Rn (or ""Rn) atoms are free to diffuse through the interstices between mineral or soil particles where they become a minor constituent of the soil gas. The concentrations of ""Rn in soil capillaries, 30 to 100 kBq m:',(0.8 to 3 pCi m :') several meters below the earth's surface exceeds that of ordinary outdoor air, 8 Bq m.:', (200 pCi m :') by a factor of the order of a thousand or more. Hence, a "current" or flux of radon atoms proceeds from soil to air at the earth-air interface. Factors which influence the flow of radon atoms from soil to air are (1) the concentration of the parent radium in the soil and rocks, (2) the emanation power or fraction of radon released from the material, (3) the porosity of the soil or rock material, (4) atmospheric pressure differentials across the interface, (5) the degree of water saturation, and (6) minor influences (Schery et al., 1984). The measured flux or exhalation rate of "'Rn from soil to air varies widely from place-to-place and is dependent upon soil characteristics and barometric pressure differentials a t the time of measurement. A listing of radon flux at 21 sites in the U.S. over a wide distribution of soil types yielded 20 mBq m-2s-I (0.55 pCi m-29)with a range of 2 to 50 mBq m-2s-' (0.05 to 1.4 pCi m-2s-I) (Colle, et al., 1981). A summary of 29 published values h m the US, Europe, the USSR, Japan, and the Philippines, also for a wide variety of soils and measurement methods, gave a mean of 16 mBq m-2s-' (0.42 pCi m-Zs-I)(Wilkening,et aL,1972). A figure of 17 mBq m-2s-'(0.45 pCi m-2s-I)can be considered representative for continental soils. The total transfer of 222Rn from the earth to the atmosphere is about 2 TBq s-I (55 Ci s-') based upon the flux from the area listed above. The ocean surface gives up only about 0.1 mBq m-Zs-' (0.003 pCi m-2 S-') of 222Rn to the atmosphere; hence, the oceans of the world contribute only about 2% of the total input of 22ZRnto the earth's atmosphere. Contributions from lava covered areas, the oceans, and the ice caps of the Antarctic and Greenland are negligibly small.

30 1 5.DISTRIBUTION OF U, Th, Ra,AND Rn

Flux figures for ""Rn (thoron)are about 1.5 Bq m.' s ' (40 pCi m-'s-') (Crozier.1969; Israel et d.1968). The longer half-life of ?'Rn relative to "'ORn favors its survival as a trace component of the atmbphere; Other sources of atmospheric radon have been listed by NCRP (1984b).They include contributions from evaporation and transpiration of ground water, phosphate residues, uranium mill tailings, burning of coal and natural gas, and human exhalation: All except the potential contribution from ground water are much less than one percent of that coming from the land surface of the earth. 5.4.1 Radon in the Atmosphere

When radon (222Rnor 220Rn)passes from soil to air it is mixed throughout the lower atmosphere by eddy diffusion and the prevailing winds. Average concentrations of 222Rna t 1 m above ground in the continental US vary from 4 to 15 Bq m-3(0.1 - 0.4 pCi I-') with a' mean of about 9 Bq me3(0.25 pCi I-') (Gesell, 1983). Normal values. may be exceeded by a factor of 10 or more when radon, like atmospheric pollutants originating a t or near the surface. is trapped by temperature inversions. The results of measurements by Fisenne (1985) a t Chester, New Jersey over a seven-year period show considerable scatter, as expected, but yield an arithmetic mean of 8 Bq m"(0.22 pCi 1 I). Complete sets of meteorological data are availat~lein this study including temperatures and dewpoints a t one meter above ground, wind speed and direction, barometric pressure, and precipitation. "No significant correlation was evident for any of these parameters" according to the author. Other typical ranges of "'Rn concentrations are: air over the oceans, 0.02-0.2 Bq m-:'(0.0005-0.005 pCi I-');indoor air, 11-300 Bq m.:' (0.38.0 pCi I-'); caves, 0.37-11 kBq m-.'(10-300 pCi 1');soil air 18-180 kBq m-"500-50000 pCi I-'); unventilated uranium mines, 37-3700 kBq m..:' (1000-100,000 pCi 1.'). Once in the atmosphere, turbulent diffusion originating from thermally driven convective processes or winds is effective in dispersing the radon. Diurnal and seasonal courses of atmospheric radon have been shown to be consistent with a diffusive transport model of this type. The diurnal maxima occur at night and in early morning hours when the atmosphere tends to be stable. Minima occur in the afternoons when vertical mixing due to turbulent diffusion is a maximum. Seasonal patterns show maximum and minimum values that are affected also by surface heating and by wind and moisture patterns. The details

5.4 RADON 1 31

of how radon, thoron, and their decay products participate in atmospheric mixing processes are treated by Reiter (1978). Mountain sites show fluctuations in radon concentration that are characteristic of nocturnal drainage winds and temperature stability patterns (Clements and Wilkening, 1981). Marine environments exhibit low values of radon concentrations and show striking land-sea breeze effects. The radon concentration in winds from offshore on the east coast of the island of Hawaii during the day average about 0.07 Bq m.-:' (0.002 pCi 1.') compared with 1.5 Bq m ,' (0.04 pCi 1-I) in the land breeze during the night (Wilkening, 1976). Radon, thoron, and their daughter products play an important role in atmospheric electricity. Near the earth's surface, almost half of the ionization of the air is due to ""Rn, "'Rn. and their daughters. The alpha emitters from these chains typically produce about 5,000 ion pairs per second per liter (Ikebe,1970). Ionization from radon, thoron, and their decay products is important for its effect on atmospheric electrical characteristics, including positive and negative ion densities and electrical conductivity in both outdoor and indoor environments. Ions resulting from the decay of radon or daughter atoms have a high probability of being positiveIy charged a t the instant of formation but are rapidly neutralized. These ions have a mobility comparable to ordinary small ions in the atmosphere and are efficiently deposited by diffusion deposition in the bronchial tree during the normal breathing process. Additional information on the unattached fraction is given in Section 4. The radon daughter positive ions make up less than one percent of the total positive ions in the atmosphere yet have been found useful as tracers in certain studies of thunderstorms and electrical properties of the atmosphere in general (Wilkening, 1977). Jonassen (1984) has shown how radon daughter ions can be removed from the indoor environment by means of electric fields. 5.4.2 Radon in Water

Radon is readily soluble in water. Since ground and surface waters are in close contact with soil and rocks containing small quantities of radium, i t is not surprising to find radon in public water supplies. UNSCEAR (1982) gives typical concentrations of "'Rn in surface water of less than 40 Bq m-3(1pCi 1 ') and in ground water from 4 to 40 kBq m-' (100 to 1,000 pCi 1-I). Water from deep wells and mineral springs may carry dissolved Y2"Ra to the oceans where radioactive equilibrium between "%a and "'Rn is

32 1 5.DISTRIBUTION OF U, Th,Ra,AND Rn

approached. Both nuclides may be present in sea water to the extent of about 4 Bq m-"0.1 pCi 1.'). The soluble "'Rn atoms cannot escape readily from the water surface with the result that marine air masses contain only about one percent as much fi'Rn as air over the continents. Radon concentrations in drinking water range up to many thousand Bq m-" Measurements of radon in water have tended to be concentrated in areas of high levels. Broad surveys aimed at developing an average exposure for sizeable populations are lacking; however, a compilation of results from measurements of water supplies in the US showed 74% to be less than 74 kBq m-.'(2,000 pCi 1.') and five percent greater than 370 kBq m-:' (10,000 pCi 1 I ) (Gesell and Prichard, 1977). On the other hand McGregor and Gourgon (1980) found a mean of 6,300 kBq m-Yl70,OOOpCi I-') in 11 drilled wells in Nova Scotia. Wells in Sweden and Finland likewise have exhibited averages of 1,800 and 630 kBq m-R(50,000 and 17,000 pCi 1.').These and other studies have been reviewed by Nero (1983),Hess, et d , (1985) and Nazaroff et d , (1987).I t has been shown that radon released from faucets and showers during domestic water use adds a significant amount to the total indoor radon exposure (Gesell and Prichard, 1977). An approximate relationship has been derived that 400 kBq m :' (10.000 pCi 1.')of radon in water will add about 40 Bq m-:I (1 pCi 1 I) to the average indoor air radon concentration. 5.5 Summary

Uranium-238 and 2R2Th am present in all rock and soil in the earth's crust. These am the series parents of "'Rn and 2x'Rn.A typical value for 226Ra in crustal rock and soil is about 40 Bq kg-' (1 pCi g-I) and this supports 22'Rnconcentrations in soil gas of approximately 30-100 kBq m-3.The concentration of '=Rn in soil air exceeds that in outdoor air by several orders of magnitude and so a flux of '"Rn atoms travels from soil to air a t the soil interface due to diffusion and other processes. The measured flux is 17 mBq m-' s-' (0.45 pCi m-' a').Radon-222 in the outdoor atmosphere averages about 8 Bq m-:'(200 pCi m-:')and follows diurnal and seasonal patterns consistent with nighttime stability of the atmosphere due to temperature inversion and thermally driven convective forces and wind. Radon is soluble in water and although public surface water supplies are usually low ( < 40 Bq m-:')some p u n d water from wells is elevated. Radon-222 in water can contribute to indoor 222Rn concentrations during domestic water use, approximately 400 kBq m-3(10,000pCi 1-I) of water can contribute 40 Bq m-:'(1 pCi 1 I ) to the average indoor air concentration.

6. Radon Emanation and Pansport 6.1 Emanation Radon emanation refers to the processes by which radon atoms escape from a given material. Emanation power or "coefficient of emanation" is defined as the ratio of the number of radon atoms that escape from a quantity of material to the total number of atoms formed by radioactive decay of radium in the material in unit time. Emanation power varies from only about 0.02 for recent lavas to 0.7 for welldeveloped, fine grained soil (Wilkening,1974); hence, it is important to understand factors influencing emanation from source materials. When a 226Raatom decays, most of the energy available for the reaction (4.87 MeV) is carried off by the 4.78 MeV alpha particle. The remainder, about 0.1 MeV, is carried by the recoiling "'Rn atom. If the atom is within a mineral grain, the recoiling radon atom decaying 22RRa has a range of 20 to 70 nm for minerals of common density. (Quet et aL, 1975). A radon atom that is directed toward the boundary has a chance to escape into the pore. If the pore is filled with air, the range of the recoiling atom is equal to the fraction of its remaining kinetic energy or multiplied by its recoil range in air, which is about 63 pm for 222Rn, 83 pm for '"Rn (Flugge and Zimens, 1939). If a recoiling atom has enough energy to traverse the pore, it penetrates the adjacent solid surface and forms a pocket a t a depth comparable with the recoil range in the solid material reduced by the fraction of energy already expended. In natural materials two main factors affect the ability of the newly formed radon atoms to become a part of the soil gas by the recoil method; either the grain sizes are much larger than the recoil range in the grains so that new atoms cannot escape from the grains in which they originate, or the pores are much smaller than the recoil range in air with the result that the recoil paths do not terminate in the pores.

34 1 6. RADON EMANATION AND TRANSPORT

Consequently, the direct-recoil contribution t o emanation power is less than one percent in dry, compacted, granular materials. If the pores or soil capillaries are fitled with water, however, the range of the recoil atom is only about 0.1 pm, and the probability that it will stop in the pore is greatly increased. Thus the presence of water in the capillaries or in a film surrounding a soil particle increases the directrecoil fraction of emanation power. Strong and Levins (1982) made a study of the effect of moisture on emanation power. They found that when radon flux density was measured from columns containing dry, moist, and water-saturated uranium mill tailings, the highest flux came fmm the column filled with moist tailings having a water content of 5.7% by weight. Flux from the moist column exceeded that from the dry tailings by a factor of 3.5 and that from the water-saturated column by a factor of 54. The "'Rn apportions between the gas and liquid phase rapidly in accordance with Henry's Law, thus becoming available for transport. An extensive study of the effects of moisture on radon emanation from uranium ores has been made also by Thamer e t aL (1981). They found that emanation coefficients were consistently lower when the ores were dry than when they were moistened to 5 to 2070 of saturation. A model using measured pore-size distributions suggested that the radium mineralization may be confined to annular layers about 20 pm thick around pores. Pure thermal diffusion of radon atoms formed within relatively large sand-sized grains into pore spaces is not a significant contributing factor to emanating power since diffusion coefficients are of the order of only 10-'mm's-' or less for radon isotopes in ionic crystals a t room temperatures. Such small diffusion coefficients limit movement to only a few lattice constants during the mean life of '"Rn and even less for ""Rn. Studies of the effects of moisture, adsorption and grain size on emanating power have been reviewed by Tmnner (1980). He concludes that "most minerals and nearly all rocks and soils emanate radon isotopes to a far greater degree than can be accounted for by recoil or by diffusion from major crystals that are structurally intact and in which the radium isotope precursor is uniformly distributed." Thls is explained by the fact that uranium and thorium atoms, which are not compatible with the crystal structure of the major rock-forming minerals, normally reside in smaller accessory minerals adsorbed on clay minerals or are occluded in finegrained or amorphous cements and other coatings. The succession of radioactive recoils from uranium and thorium, together with leaching processes, make the radon isotopes

6.2 EARTH-TOAIR TRANSPORT PROCESSES 1 35

even more available to networks of capillaries and intergranular boundaries. %ner (1980) concludes that "radon-222, radon-220 or radon-219 atoms come from radium isotopes distributed in secondary crusts or films or in the shallow surface layers (approximately only as deep as the recoil range) of intact crystals of the host minerals. The principal mechanism by which the radon isotopes enter the pores, capillaries, or microfractures is radioactive moil in liquid-containing spaces or diffusion from solid material appreciably smaller than the diffusion length of the most short-lived isotope observed."

6.2 Earth-teAir 'Ilansport Processes The large differences between the concentration of radon in air in soil capillaries and in outdoor air is well known. An average net flow of about 17 mBq m-' s-I (0.45 pCi m-' s I ) from soil to air results. Radon concentrations increase with depth underground while above the earth's surface they decrease with height. Processes affecting transport in the soil will be addressed in this Section whereas the of 2Y2Rn dispersal of radon in the atmosphere will be treated in Section 6.3. Pansport in the soil is made up of two main components: (1) molecularlatomic diffusion through pores and capillaries and (2) pressure-induced flow through soil capillaries. Both processes are related directly to the size and configuration of the space occupied by the soil gas. These spaces may vary from molecular interstices to large underground caverns. The openings may be isolated, interconnected, or dead end. The pore volume may be small or a substantial fraction of the gross soil volume. All of these characteristics are important to the radon transport problem. In diffusive transport, a constituent of the soil gas flows in a direction opposite to that of the increasing concentration gradient (Fick's Law). Theory and experiment confirm that the concentration increases with depth reaching within three percent of the concentration at infinite depth at five meters when typical parameters for dry soils are used. Variation with depth was shown by Schery et d (1984) and Lindmark and Rosen (1985). In addition to diffusive flow there is frequently a pressureinduced flow, governed by Darcy's Law, which governs flow of a viscous fluid in a porous medium (Clements and Wilkening, 1974). Solutions of the resulting equations for diffusive and fluid flow with appropriate boundary conditions and assumptions have been used extensively (Colleet d ,1981; UNSCEAR, 1982). The results

.

36 1 6. RADON EMANATION AND TRANSPORT

show that the "'Rn flux density at the surface depends upon the following factors: (the numerical values in parentheses are typical values for a dry, sandy loam soil) a. The production rate of "'Rn atoms per unit volume of the pore space. This is the "effective" "',Ra concentration 13 Bq kg-' (0.36 pCi g-I) times the density of the soil (1.5 g cm-")divided by the porosity b. The free air soil porosity-ratio of free air void to bulk volume (0.35)

c. The effective diffusion coefficient for radon in the soil gas (0.01 cm' s-') d. The permeability of the soil (10 cm') e. The decay constant, A, for "'Rn (2.1 x 10 '; s I ) f. The dynamic viscosity of air in the soil at 20" C (1.8 x 10 'g cm's') g. The fluid volume flowing per unit time and per unit area-from Darcy's Law (10-kms I) h. The vertical pressure gradient in the soil (gcm ' s-I). The "'Rn concentration at a depth greater than about three meters for the characteristics shown would be about 55 kBq m " (1,500 pCi 1.'). Among the parameters listed, the effective "',Ra concentration, the porosity and permeability of the soil, and sources capable of creating pressure gradients up to only a few percent above or below ambient atmospheric pressure are the critical factors controlling exhalation rates from the soil.

6.3 k s p o r t in the Atmosphere Pioneer work on the change of radon with height in the atmosphere was done by Jacobi and Andre (1963).A detailed account of atmospher ic mixing processes and their effects on radon and its daughters has been provided by Reiter (1978). Radon transport in the atmosphere is controlled by turbulent (eddy)diffusion and vertical components of the wind. The effective diffusion coefficients of radon in the atmosphere due to turbulence are from lo5 to 10"imes as great as in the soil because of the marked difference in diffusion coefficients noted in 'Ihble 6.1. Hence, most of the radon in outdoor air originates from the first few meters below the surface, even though radon can be found throughout the troposphere (10 to 20 krn above sea level). Essentially no 222Rn has been detected in the stratosphere. I t is not surprising that radon

6.4 SUMMARY 1 37 T A B L E 6.1-Some effective diffusion coefficients (AIPHandbwk of Physics, 1972) Atoms or ions

Effective diffusion coefficient (m2s-1)

Solids Water Soil Air Air parcels in the atmosphere

concentrations in air just above the soil surface are negligible with respect to concentrations in the soil only a short distance below. The turbulent diffusion coefficient in the atmosphere is not constant with height; hence, departures from simple diffusion theory are expected in practice. Moore, et a l , (1973)compiled numerous measure ments showing the decrease of radon above the earth's surface (see Figure 6.1). The vertical distribution of "'Rn, ""Rn and their decay products together with their diurnal oscillations have been treated by Staley (1966) taking into account turbulent diffusion and radioactive decay. 6.4 Summary

Radon behaves as a gas and moves with diffusional and pressure gradients and flows. Radon is formed by decay of ""Ra which is distributed generally nonuniformly in the mineral grains composing rock and soil. Unless the "'Rn atom is formed near the mineral surface, the diffusion coefficient in solids is too small to permit the atom to enter the soil air or pore space, and it will decay in situ. If the "SRn atom is liberated into the air-filled pores it has the potential for transport into the atmosphere. Pore spaces containing a capillary layer of water facilitate "'Rn transfer to soil air upon decay because the recoiling "=Rn atom is stopped in the water layer where the gas is rapidly apportioned between gas and water phases according to Henry's Law. The emanating power or coefficient of emanation is the ratio of the number of "'Rn atoms that escape per unit mass to the total number of atoms formed. Emanating power varies from about 0.02 to 0.7 depending upon the mineral structure and the water content. The average flux from the ground surface based upon measurements is 17 mBq m-2s-' (0.45 pCi m-Ys-I).At a depth in soil of a few meters, a steady-state "'Rn

38 1 6. RADON EMANATION AND TRANSPORT

14

I

I

I

I

I

I

I

12

-

-

10 -

-

-

-

-

E 8-

Y

6

-

w

3 .C

2

6-

4-

2-

0 1

I

I

2

5 222

10

I

I

20

50

I

100 200

500

3

Rn Concentration, dpm /m STP

Fig. 6 1 Pmfiles of mdon concentmtion (disintegmabns per minute per cubic m e w in the lower atmosphere over continental arms, as measured by different investigators (From Moore. e t aL, 1973, Journal of Geophysical Research, copyright by the American Geophysical Union) (1)Bradley and Pearson (1970) (2) Kirichenko (1962)

(3)Wexler et a l , (1956)

( 4 ) Moore et aL, (1973) (5) Wilkening (1970) (6) Nazarov et aL, (1970)

6.4 SUMMARY 1 39

concentrationexists, reflecting the absence of net transport loss. Above ground =*Rnis transported by turbulent (eddy)diffusion and vertical and horizontal components of the wind. Radon can be found throughout the troposphere while essentially no radon has been detected in the stratosphe~.

7. Measurement of Radon Isotopes in Air 7.1 Introduction Three radon isotopes, 219,220.222 Rn, exist in the environment as members of the primordial radioactive series. With rare exceptions, only 222Rn (3.82 d) has been diredly measured in air. Since 235U is only 0.73% by weight of natural uranium, its daughter products, including 219Rn (4 s), are not found in the environment in significant concentrations. Although the concentration of 220Rn (55 s) near the surface of the earth is comparable to that of 22%n,it's half-life precludes direct measure ment with relatively portable instruments. For these reasons, measure ment of n9Rn has been omitted and only one method for 220Rn measurement is discussed in this Section.

7.2 Sampling The collection of a valid sample is equal in importance to an accurate analysis in carrying out a program for evaluating the exposure of man - to radionuclides in the environment. This is a particular problem in the case of the radon isotopes whose concentrations change markedly with time and location. Both indoor and outdoor radon concentrations are subject to diurnal and seasonal variations (Lockhart, 1962; Fontan et d , 1966; Malakhov and Chernysheva, 1967; Fisenne, 1985; Harley and %rilli,1986). The sampling protocol and method selection for the determination of radon isotopes will be heavily dependent upon the scale of the program. Survey programs, such as the 12,000 dwellings under study in Sweden (Hildingson, 1982), require simple, inexpensive and unobtrusive devices, while indepth investigations of the behavior of the radon isotopes in a single house require more intensive and complex instrumentation.

7.3 CALIBRATION 1 41

Field collection of air for "2Rn analysis is usually an instantaneous or "grab" sample in a relatively small vessel holding one liter or less. The sampling period may be extended up to 60 hours by such techniques as metered filling of Mylar bags (Sill, 1969) or W a r bags which yield timeaveraged results. A few samplers for collection of grab samples are shown in Figure 7.1.

7.3 Calibration The fundamental quantity in the measurement of a radioactive material is the number of atoms decaying per unit time (ICRU, 1980). It is seldom possible to make an absolute measurement of the activity but rather it is necessary to obtain a calibration factor for the detection system based on a standard of the same substance. Improper calibration of a debction system is the most prevalent cause of erroneous activity measurements. Proper calibration of a radon detection system depends upon the determination of the background and counting effi-

Fig. 7.1 Field collectors for mRn in air as used by various labomtories (Samplers collected during U.S. Department of Energy Environmental Measurements Laboratory intercomparison exercise)

42 1 7. MEASUREMENT OF RADON IN AIR

ciency of each detector in use. All radioactivity measurements made for regulatory purposes in the United States must be traceable to the U.S. Department of Commerce, National Bureau of Standards through use of their standard reference materials (SRM)for the calibration of radiation detection systems. For the case of 222Rnmeasmments, a range of activities of 22BRa SRM's are available as solutions for calibrating detection systems by the radon emanation method (see Section 12). As an alternative, an elevated radon atmosphere may be produced in a chamber, samples drawn and measured in systems previously calibrated by radon emanation from an NBS 226Ra SRM, Other radon detectors may then be filled from or exposed in the chamber and standardized based on this "secondary" standard. Several United States facilities maintain radon calibration chambers based on this secondary standard concept. The U.S. Department of Energy at the Environmental Measurements Laboratory in New York City, the U.S. Bureau of Mines in Denver, Colorado and the U.S. Environmental Protection Agency in Montgomery, Alabama maintain chambers a s of this writing.

7.4 Instruments for the Measurement of mRn in Air Measurements of 55s 2mRnmust be performed on site and essentially instantaneously. Israel and Israel (1965) developed a system for use in research on meteorological parameters. The unit consisted of two 324liter ionization chambers, a 2,300 1 decay chiunber and two vibrating reed electrometers. Filtered air at a flow rate of 11.7 1 s-I was drawn into one ionization chamber where the combined 2mRnand z22Rnactivities were recorded. The air passed through the decay chamber with a 160 s transit time, sufficiently long for > 90% of the 220Rnto decay, leaving only 222Rnto be measured in the second ionization chamber. The 2aORnconcentration was obtained from the difference between the ionization chamber readings. Israel and Israel, 1965 performed 2z0Rnmeasurements a t each of three locations for a period of three months. At heights of 3.5 and 1 m above ground level. the average air concentrations were 1 Ad 1.8 Bq 220Rn m-3(30and 50 pCi m-3)mpectively. The observation periods were sufficiently long to demonstrate the diurnal variation but not the seasonal pattern of 2z0Rnconcentrations in air a t the sites. Additional measurements of atmospheric "Rn have been performed by Crozier and Biles (1966) and Schery and Zarcone (1985) in the

7.5 INSTRUMENTSFOR THE MEASUREMENTOF PZRnIN AIR 1 43

United States, Ikebe and Shimo (1972)in Japan; D d e t and Fontan '(1973)in France; Bakulin et al (1970)and Filistovich et al (1979)in the Soviet Union and Israelsson (1980) in Sweden using other types of hstnunents.

7.5 Instruments for the Measurement of "Rn in Air The methods of collection and measurement of 222Rn in air may be separated into three categories: instantaneous, continuous (real time) and time-averagingmethods (Budnitz,1974; George, 1982). These cate gories may be further subdivided into active and passive methods, dependent upon direct measurement of "2Rn only, measurement of 2"Rn and its short-lived daughter products or measurement of the short-lived 222Rn products for the estimation of the gas concentration. 'Igble 7.1 lists instruments and methods in use for the collection andlor measurement of 222Rn. Each will be described in more detail in the following sections. As an indication of the relative sensitivity of each instrument, a detection limit has been calculated when possible, The particular procedure for this, in this chapter, was developed by Pasternack and Harley (1971). These authors define the lower limit of detection (LLD) of a measurement system as the smallest amount of sample activity that will yield a net count for which there is confidence at a predetermined level that activity is present. It is necessary that the measurement system characteristics, detection efficiency and background count rate, and the typical measurement period be known to estimate the LLD (see Appendix A). These characteristics are not always given for each system but an undefined "sensitivity" value is quoted. When possible, the LLD at the 95% confidence level is calculated for the measurement system. "Sensitivity" limits m given only when insufficient information is available to calculate the LLD. The LLD (or sensitivity) for selected instruments is also given in 'Igble 7.1. 7.5.1 Ionization Chambers

As noted earlier, the radon isotopes were discovered during the course of measurements with ionization chambers. The interest in

ionization chamber measurements of "'Rn is primarily historical since less costly instruments are now in general use. The exception is in their

TABLE 7.1-Instnunents and methods for the estimation of 22ZRnwncentmtions Detection System

Ionization chambers 2 1pulse

Sampler l)pe

Flask

Sam ler

~olumehfeight

Sampler Filling

Method

Evacuation or Flow through

Measurement Lower Limit of Period Detection (95%) lo00 min

Sensitivity

Reference Harley, (1972)

Scintillation cell Evacuation Evacuation Flow through

60min 60min 60min

Lucas. (1957) Kristanetand Kraner al.,Kobal, (1964)(1973)

Continuous scintillation cell Pittendngh. (1954) Thomas and Countess, (1979)

Flow through Flow through Electrostatic collection ZnS tofilm ZnS to PMT

ZnSto PMT SS to MCA SS to MCA PERM (LIF) EGARD (LIF) 222Rn only ZnS to PMT Charcoal adsorption

Filtration Single filter kfilter m o filter kfilter Nuclear track detector

Diffusion Diffusion Diffusion Diffusion Diffusion Diffusion Diffusion

170 h 40 min 40 min 40 min. 10min 7d

Diffusion

60 min

Costa-Ribeimet al., (1969) Wrenn et d.(1975)

Negro and Watnick, (1978) Negro and Watnick. (1978) Watnick et al., (1986) George and Breslin, (1977) Maiello and Harley, (1987)

72h 50 h (?)

Pensko, (1983) George, (1984) b h e n and Nason, (1986)

3 month

Harley (1953) Jacobi (1963) Scheryet al., (1980) Harley, (1978) Alter and Fleischer, (1981)

10 min-3 h filtration 140 1decay chamber continuous filtration 600 I decay chamber continuous filtration 100 1decay chamber continuous filtration

7.5 INSTRUMENTS FOR THE MEASUREMENT OF 222Rn IN AIR 1 45

use currently as highly accurate measurement systems in laboratory intercomparisons. Ionization chambers are usually brass or steel cylinders with a central collecting mode (Urry, 1936; Harley et d ,1951; Wilkinson, 1950). "Sealed" and flow-through chambers with sensitive volumes of one to several hundred liters have been constructed for "Tin measurements (Waters and Howard, 1969; Israelsson et d , 1973; Hogberg and Gustavsson, 1973). The chambers are usually operated at or slightly above atmospheric pressure. With both sealed and flow-through chambers, the air to be measured is filtered prior to entry into the sensitive volume to remove atmospheric aerosols including the radon daughter products. In sealed chambers, the short-lived daughter products formed in the chamber are allowed to attain radioactive equilibrium with the gas, while, in a flow-through chamber, only a fraction of the equilibrium daughter concentration is attained. In the dynamic or flow-through ionization chambers, the current produced within the sensitive volume is measured with an electrometer and strip chart recorder. Commercially available electrometers are accurate to plus or minus one percent a t about 1 x 10-l3amperes. Depending on the background and the chamber volume, detection limits of 4 Bq m-"0.1 pCi 1.') have been reported. Present day measurements are performed with "sealed" chambers, usually after the removal of water vapor. By removing electronegative impurities, principally oxygen, as well as water vapor from the gas to be measured, it is possible to perform fast-pulse counting (Davis,1947). Agednitrogen, nitrogen-hydrogen, argon, or helium is usually used as the transfer gas. The detection limit of sealed ionization chambers for 222Rnmeasurements is basically controlled by the background of the system. One laboratory employing 2 1 stainless steel chambers with background count rates of 6 counts per hour (cph)and an efficiency in equilibrium with its factor of 6,200 cph Bq-I (230 cph pCi-I)of 222Rn short-lived daughter products reports an LLD for an overnight count of 0.7 mBq (0.02 pCi) for 222Rn(Harley, 1972). These pulse ionization chambers are shown in Figure 7.2. Flow-through ionization chambers are essentially field instruments but because of the size required to accurately assess environmental "%n levels, their application has been limited to research programs. The sealed ionization chambers are laboratory instruments. Samples are collected in the field and returned for laboratory assay. Radon-222 from large volumes of air may be concentrated cryogenically or on activated charcoal and then desorbed into the chamber. Smaller

46 1 7. MEASUREMENT OF RADON IN AIR

Fig. 7.2 Sealed 2-liter ionizakbn chambers for alpha pulse counting of radon (Courtesy of U.S. Department of Energy Environmental Measurements Laboratory)

volume samples may be transferred directly into the chamber for measurement. For all practical purposes, the measurement of "2Rn in air with ionization chambers has been superseded by scintillation cell measurements.

7.52 Scintillation Cells Single alpha particles emitted by naturally-occurring radionuclides were observed as light flashes from a ZnS screen in Sir Walter h o k e s ' spinthariscope. Curran and Baker (1948) substituted a phototube for the human eye to observe alpha radiation with ZnS. The photomultiplier tube and silver-activated ZnS in a light-tight housing are the essential components of many alpha detection systems in current use. ZnS(Ag)is an inorganic white opaque powder of small crystal size with . light in the range of 450 nrn and has a a density of 4.1 g ~ m -I ~t emits photon yield of 2, relative to anthracene. The light decay time of ZnS (Ag)is 40 times greater than that of anthracene. Photomultiplier tubes

7.5 INSTRUMENTS FOR T H E MEASUREMENT OF 222Rn IN AIR 1 47

of spectral class S 1 2 with a peak response at 410 nm are generally used in systems employing a ZnS(Ag) scintillator. ZnS(Ag) is essentially

insensitive to any form of natural background radiation other than alpha particles. Although the pulses detected in photomultiplier tubes from the Zns(Ag)-alphainteractions are not suitable for spectral analysis, the low background count rates and high detection efficiencies attainable from such systems make them the instruments of choice for many applications. Closed vessels with interior surfaces coated with ZnS(Ag),commonly known as scintillation cells, are the most widely used collection and in air. Although their detection systems for the determination of 222Rn commonest application is as an instantaneous samplerdetector, the necessary equations have been developed so that scintillation cells may be used as continuous, flow-through detectors in suitable situations (Busigin et al., 1979).

7.5.2.1 Development of ScintiUution CeUs A system for the determination of '"Rn in geochemical samples was described by Damon and Hyde (1952).The interior walls of a 40 cm3 F'yrex glass tube were coated with 20 mg ~ mZnS(Ag) - ~ phosphor, while the exterior surfaces, except for the curved base and sidearm filling tube, were silvered to reflect scintillations. The sidearm tube was used for evacuating and filling the sensitive volume. This tube was painted black for protection against light and presumably closed with tubing and a clamp. The tube was coupled to a curved photomultiplier tube in a light-tight shield. The background count rate of the system was 300 counts per hour (cph) attributed primarily to the phosphor. Although the detection efficiency of the system was not given, the authors stated that the sensitivity of the system approached that of ionization chambers and had the advantages of being insensitive to the composition of the gas filling the chamber or to the presence of ions or condensable vapors. Van Dilla and 'lhysum (1955)simplified the earlier scintillation cell design for their determination of 222Rn in exhaled breath from animals injected with radium. All interior surfaces of a 120 cm3 cylindrical glass cell were coated with a thin layer of ZnS(Ag)powder. A single bellows valve for evacuating and filling the cell was fixed at one end plate. The outer surfaces, except for the face plate, were painted white to aid in light reflection from the phosphor. The cell was placed on a methyl

48 1 7. MEASUREMENT OF RADON IN AIR

methacrylate light-pipe on a planar photomultiplier tube inside a lighttight housing. The background count rate of the system was 12 to 18 counts per hour (cph).The detection efficiency with air or nitrogen as the filling gas was 8,000 cph Bq-' (300 cph pCi-I) of '12Rn in equilibrium with its short-lived daughter products. With helium as the filling gas, the detection efficiency increased by seven percent. The detection efficiency with nitrogen as the filling gas was unaffected by pressure in the range of 0.5 to 1 atmosphere. Methyl methacrylate was substituted for the glass cell resulting in a reduction of a factor of two in the background count rate. However, an appreciable radon loss was observed h m these cells over a period of days. The authors attributed the deviation from the theoretical radon decay curve to diffusion of radon into the cell surfaces. The LLD at the 95% confidence level for this instrument was 2 to 3 Bq m-9 (0.05 to 0.07 pCi 1.') for a 60 minute measurement interval. Lucas (1957)developed low background scintillation cells exhibiting reproducible pulse height spectra and detection efficiencies. The 96 cmS cell consisted of a metal shell, a quartz window and a micro stopcock for filling and evacuation. The interior surfaces of the metal shell were spray coated with a suspension of ZnS(Ag) in ethylene dichloride to a minimum thickness of 20 mg ~ mof-phosphor. ~ The inner surface of the quartz window was coated with a transparent layer of conducting tin oxide to eliminate an induced negative charge from the photocathode when the cell was placed on the face of a planar photomultiplier tube. Without the conductive coating, the 222Rn daughter products tended to deposit on the charged window resulting in variations in detection efficiency of up to 15%. The scintillation cell and photomultiplier tube were covered with a light-tight cap. The background count rate of the system averaged five counts per hour (cph).The detection efficiency of the cell with helium as the filling gas was 8,600 cph Bq-' (320 cph pCi-') of '12Rn and daughter products. The LLD a t the 95% confidence level for a 60 minute measurement period was 1 mBq (0.03 pCi). Since the detection efficiency of the cell was reduced by only 10% when filled with air, ~ u c a suggested s that environmental levels of radon could be measured directly, that is, without sample preconcentration. The simplicity and low cost of the system, with the scintillation cell serving as both the collector and detector, encouraged the adaptation of the cell for field studies. Lucas cells are widely used because of their reliability and general availability Kraner et aL,(1964)constructed a 450 cm3right cylindrical cell from methyl methacrylate. The interior surfaces were a t t a c h e to a metal shield for electrical grounding. Presumably a valve was fixed in the wall

7.5 INSTRUMENTSFOR THE MEASUREMENT OF Z22Rn IN AIR 1 49

of the cylinder body for evacuation and gas filling. The cell was placed between two 12.7 cm diameter photomultiplier tubes in order to maximize the detection efficiency. The background count rate of the system was 50 cph with a detection efficiency of 7,600 cph Bq-I (280 cph pCi-'1 of radon and daughter products. The LLD at the 95% confidence level for a 60 minute measurement period was about 4 rnBq (0.1 pCi). Difficulties encountered in collecting air samples in mines with commercially produced "Lucas" cells led Kristan and Kobal (1973) to design a scintillation cell with two stopcocks for filling on site by use of a hand pump. The interior surfaces of the 140 cms Pyrex glass shell were coated with ZnS(Ag).The exterior surfaces, except for the window were coated with an opaque paint. The authors considered that the cell window need not be treated with a conductive coating because the photocathode of their photomultiplier tube was electrically grounded. The background count rate of the system was 12 to 30 counts per hour (cph) with detection efficiencies of 5,400 to 7,300 cph Bq-' (200 to 270 cph pCi-I) of radon and daughter products. The LLD at the 95% confidence level for a 60 minute measurement time was about 4 mBq (0.1 pCi). The flow-through (double stopcock) scintillation cell has virtually replaced the single stopcock design for field collection and measure ment of radon. The most prevalent cell design is a right cylinder of methyl methacrylate with two valves on the top face. These cells are simple to use, inexpensive to construct and rugged under field conditions. The details of construction and phosphor coating procedure for this type of cell have been published by George (1976). Basically all interior surfaces are coated with a thin layer of ZnS(Ag)powder but no reflective or opaque coating is applied to the exterior surfaces. George (1976) reported that no appreciable radon leakage was observed for periods up to 72 hours, perhaps due to increased wall thickness or better sealing techniques than previously observed by Van Dilla and 'bysum (1955).Methyl methacrylate flow-through cells with volumes of up to 1.5 1 are used for field studies by a number of groups. The photocathode of the multiplier tube should be electrically-groundedto eliminate induced charge effects, although small effects, about five percent, have been observed even with grounding (Lucas,1977). A largevolume (3 l), high sensitivity scintillation cell was developed by Cohen, (Cohen et aL, 1983). Background is reported to be 30 cph with a calibration factor of 16 cph per Bq m-3(600 cph per pCi 1-I). For a 4-hour count, 0.4 Bq m-3(0.01 pCi 1.') 22ZRn is the lower limit of detection.

50 1 7. MEASUREMENT OF RADON IN AIR

I t should be noted that single stopcock scintillation cells are assumed to be leak tight yet most of these samplers tested for leakage e aL, 1986). This may have integrity were found to be faulty ( F i s e ~et an adverse effect on field collection of samples unless the cells are evacuated at the field site. A few types of scintillation cells are shown in Figure 7.1. 7.5.2.2 Scintillation Cells for Continuous Measurements

The need for more convenient and rapid measurement of the 222Rn concentrations in occupational environments, particularly uranium mines, led to the development of instruments for on-site, continuous measurements (Pittendrigh, 1954; Harris et al., 1957). In their simplest form, these continuous 222Rnmonitors consisted of a ZnS(Ag)coated cell which was optically coupled to a photomultiplier tube. Air was drawn through a filter to remove radon daughter products and particulates, passing into and then out of the sensitive volume of the cell. Despite the relatively small volumes of the cells (about 150 cm3)and flow rates of about 3 1min-', radon daughter products deposited on the cell walls, resulting in increased backgmund count rates and attendant calibration difficulties. Contaminated cells were cleaned and replaced but proper adjustments in the calibration factors of the instruments were difficult. In addition, the possibility of 220Rninterference was ignored. These early instruments were considered adequate for continuous radon measurements at concentrations above 1 kBq m-3(25 pCi 1.'). Aiken et d,(1977) in describing the calibration of a flow-through radon monitor noted a delay or lag time for the detection of short-lived daughter products in the cell. Thomas and Countess (1979)described a generalized set of equations to calculate the average radon concentration obtained from continuous monitors for time periods of 30 minutes or less. The radon concentration during a particular interval was calculated from the net counts obtained in that interval and the contribution from previous sampling intervals (sometimes referred to as the "lookb a c k method). The equations accounted for the relative contribution of deposited daughter products in the present interval and h m p r e vious measurement intervals. The constants in the equations were dependent upon the cell volume, the flow rate through the scintillation cell and the measurement interval. The empirically defined constants were found to be independent of relative humidity in the range of 0 to 85 percent. In their 1.46 1 flow-through scintillation cell, the background count rate was found to be influenced by the presence of 220Rn.

7.5 INSTRUMENTS FOR THE MEASUREMENT OF mRn IN AIR 1 51

The addition of a decay chamber prior to the filter removed the Z20Rn. The system was deemed adequate for measurements of radon concentrations of about 40 Bq m-3(1 pCi 1-I). The method suggested by Thomas and Countess (1979) for the calibration of the continuous cell monitor was lengthy though accurate. The method required placing an uncontaminated cell in the system and drawing through a known concentration of radon for the chosen measurement period to obtain the counts, Y.. Immediately following the initial exposure to the known radon concentration, radon-free air was drawn through the cell and the counts obtained in successive equal time periods, Y ,, Y . . . . These results were used to obtain the constants in the equations. Busigin et d ,(1979)described a simplified method for the calibration of flow-through scintillation cells. Again, the method depends on flowing a known concentration of radon through an uncontaminated cell. With a known cell volume and flow rate, the counts were recorded at preselected time intervals. Although commercial flow-through scintillation cell systems are available, the instrument packages are designed primarily for use in measuring radon in static cells. Generally, no calibration equation is supplied with the flow-through cell configuration An additional difficulty arises in changing the configuration from static to flow-through. The manufacturers set the counting plateau voltage of the photomultiplier tube for optimal measurements with the static scintillation cell. In calibrating flow-through cells in preset commercial instrument packages it was found that adjustment of the photomultiplier tube voltage was required to ope& the instrument on the counting plateau. Once the operating voltage of the photomultiplier tube was adjusted for the flow-through cell configuration. the instrument background count rate decreased by as much as onethird. The flow-through scintillation cell system has been used for radon measurements in mines and indoor air. I t is also widely used to monitor the concentration in radon exposure chambers used for research and instrument standardization. I t must be emphasized that these systems yieId average radon concentrations during the measurement periods and substantial e m r s result under rapidly changing conditions.

. . +

+,

7.5.3 Passive Monitors Passive devices for the determination of '=Rn concentrations are essentially dependent upon the diffusion of the gas to the detector.

52 1 7. MEASUREMENT OF RADON IN AIR

Passive monitors are of two types: real time and time-integrating. Real time monitors are designed to measure varying radon concentrations, usually on an hourly basis, while the timeintegrating monitors yield a single value representing the average concentration during the e x p sure period. With one exception, passive monitors for the detection of 222Rnare based on the measurement of the gas and its short-lived daughter products or the daughter products alone. An intercomparison exercise a t the USDOE Environmental Measurements Laboratory in 1984 tested four types of passive integrating monitors. Radon concentrations of 2,700 and 160 Bq m-3(70 and 4pCi 1-I)were maintained for 7 and 60 days respectively. The measured concentrations were reported to be within a factor of three of the "true" value (Georgeet d ,1985).

7.5.3.1 Solid State Nuclear h

k Detection and Pbtogmphic

Detection

Geiger (1967) introduced a radon film badge employing a cellulose nitrate solid state nuclear track detector (SSNTD).A sheet of cellulose nitrate film was set inside the badge housing and covered with a thin light-tight cover. The 222Rn diffused into a central recess through small holes in the badge housing. After exposure, the film was etched in an alkali solution to enlarge the alpha tracks, and the number of alpha particles from the 222Rnand its shorblived daughter products were determined by optically counting the tracks in the film. A set of standardization exposures showed that the number of tracks was directly proportional to the "'Rn exposure. The sensitivity of the method was limited by the number of fields examined under a micre scope. A single 7 mm2 area of film yielded 21 tracks after a 40 h exposure to 40 kBq m-S(1,000 pCi I-') of "'Rn. This technique has been further developed by a number of investigators (Bedrosian,1969; Fleischer et d,1972; Domanski et d ,1975; Frank and Benton, 1973; Ward et d ,1977; Likes et u.!, 1979; Alter and Fleisher, 1981; Urban and Piesch, 1981). Cellulose nitrate is still used as the film material but other plastics which develop damage trails from energy deposited by alpha particles are also popular such as polycarbonate and ally1 diglycol carbonate (CR-39). Both SSNTD and photographic film monitoring of '"Rn based on pure diffusion of the gas to the detector are fairly insensitive techniques. At environmental radon concentrations, the exposure period required to obtain a single timeaveraged measurement is three months or longer. Despite the low sensitivity, SSNTD detectors have been used in large scale radon surveys because of their simplicity. As with all

7.5 INSTRUMENTS FORTHE MEASUREMENT OFZaRnIN AIR 1 53

systems, careful standardization over the range to be measured is required to obtain reliable results. The usual standardization method is atmosphere of known concento expose a group of detectors to a 222Rn tration for varying time periods. This is especially important to eliminate errors due to the differences among batches of the SSNTD film. Bare nuclear track detectors have been shown to have poor precision and to have standardization factors that change significantly with degree of daughter equilibrium, particularly when standardized to measure radon daughter working level. For this reason, track detectors are utilized to measure 22%nconcentration rather than daughters and are rarely placed for measurement without a containing vessel. When the track detector is placed in a closed container with 222Rnentry by passive diffusion, precision and the stability of the calibration factor improves markedly. Commercial versions of the track detector generally have standardization factors of approximately 0.001 tracks mm-' day1 per Bq m-S.For an exposure time of three months, background tracks are reported to be from 0.3 to 0.8 tracks mm2.The value of the standardization factor changes somewhat with the size and materials of the container. Electrically conducting plastic containers are reported to give better precision than non-conducting pIastics. Based upon a well defined background, and the efficiency values given above, and an area of track detector for measurement of about 15mm2,the LLD for a three month exposure would be from 2 to 8 Bq m-3(0.05to 0.2 pCi 1-9. Nuclear track detectors can be deployed to determine timeaveraged concentrations for long time intervals. Up to one year exposures are common. This is not practical with other types of integrating monitors. 7.5.3.2 Passive Diffusion and Electrostatic CoUection

Costa-Ribeiro et d , (1969) developed a system for measuring inkgrated 222Rnexposures based on the diffusion of the gas through a polyurethane foam, electrostatic collection of the short-lived daughter products and registration of the alpha decays on photographic film. The 222Rn diffused into a 28 cm3 volume detection chamber where the 218Po atoms formed from the radon decay were collected on an aluminized mylar film maintained at -60 volts potential. The aluminized mylar film was the outer layer of the detector package which included a ZnS(Ag) disc and a photographic film. The alpha particles from the decay of the anPo and 21Toproduced scintillations in the phosphor which were recorded on the photographic film. After exposure, the film was removed, developed, and read by conventional densitometry. The

54 1 7. MEASUREMENT OF RADON IN AIR

diffusion coefficient of radon through the polyurethane foam was determined to be 0.032 cm2s-'. The detection sensitivity of the instrument was reported to be 1.5 kBq m-3(40 pCi 1.') of "'Rn over a 170 h exposure period, limiting its use to uranium mines. Based on this work. Wrenn et al. (1975)increased the decay chamber volume to about 0.9 1 and substituted a photomultiplier tube for the photographic film. A small fraction of the "'Rn alpha decays were detected along with those from "8Po and "To. The system was instrumented for repeated readouts on a 40-minute time basis. Under steadystate conditions, equilibrium between the radon concentration in the atmosphere and within the decay chamber was achieved in 15 min. A time lag between a change in the ambient radon concentration and the instrument response was related to the diffusion coefficient for radon through the polyurethane foam and the build-up time of the daughter products on the Mylar film. The background count rate of an uncontaminated system was 30 cph with a calibration factor of 1.1 cph per Bq m-"40 cph per pCi 1.') of "'Rn, yielding a lower limit of detection at the 95% confidence level of 26 Bq m-3 (0.7 pCi 1-')for a 40-minute measurement period. Negro and Watnick (1978)built a 3 1 decay chamber volume version of the previously described instrument and noted some difficulties. It was found that the photomultiplier tube response was erratic when a negative potential was applied to the aluminized Mylar film. ?b eliminate the problem, the Mylar film was grounded and a positive potential applied to the hemispherical metal sieve used to support the polyure thane diffusion barrier. The instrument was also found to be adversely affected by humidity changes. A linear humidity dependency caused a 6% decrease in detection sensitivity for a 10% increase in relative humidity over the test range of 10 to 85% relative humidity at a constant temperature of 27°C. The instrument also failed to attain its original sensitivity after exposure to high humidity conditions. The original sensitivity was restored by placing the unit under vacuum to remove water vapor. ?b eliminate the humidity effect, a layer of activated silica gel desiccant was placed over the foam to dry the air before it diffused into the decay volume. Although this eliminated the humidity effect on the sensitivity of the unit, two drawbacks were noted. First, the lag time for diffusion of radon into the decay volume was increased from 30 minutes to 3 hours. Second, the desiccant had to be replaced about every two weeks. The sensitivity of the instrument was independent of temperature in the range of 21 to 38°C. With a background count rate of 12 counts per hour (cph) and a humiditydependent calibration factor ranging from 2,700 to 4,300 cph Bq-' (100

7.5 INSTRUMENTS FOR THE MEASUREMENT OF mRn IN AIR 1 55

to 160 cph pCi-') of "'ZRn, the lower limit of detection at the 95% confidence level was about 4 to 7 Bq m-3(0.1 to 0.2 pCi I-') of 2"Rn for a 40-minute measurement period. Negro and Watnick (1978) also constructed an instrument with a specially-manufactured 300 mm2 active area silicon surface barrier detector. The detector was mounted in the center of one end of a 10 1 aluminum cylinder, while the opposite end was covered with a metal mesh screen and polyurethane foam. The cylinder walls were operated at + 1.5 kV potential with respect to the detector. The alpha particles were recorded with a multichannel analyzer. A from the 218Poand 214P0 humidity dependency of the sensitivity was also observed with this instrument. With dry air (10% relative humidity), the collection efficiency of the daughter products on the solid state detector was 58% with a ratio of the observed counts in the 214Po region to the counts in the 218Poregion of 1.2:l. In a high humidity atmosphere (about 80% relative humidity), the collection efficiency of the daughter p d u c t s decreased to 20% and the ratio of the observed counts on the "To to 2L8Po regions was 2:l. The authors postulated that the decrease in the "8Po collection on the solid state detector was due to charged ion loss by recombination in the high humidity atmosphere. With the extremely low background count rates for solid state detectors of 0.06 to 0.12 cph, in the region of interest, and an average calibration factor of 1.9 cph per Bq m-S(70 cph per pCi 1.') of radon, the lower limit of detection at the 9590 confidence level for a 40-minute measurement period was about 0.7 Bq m-3(0.02 pCi 1-I). The real time instruments based on the diffusion of 222Rninto a sensitive volume and the measurement of the short-lived daughter products require a correction in the count rate in the present m e a s m ment period for the counts contributed fmm the daughter products deposited on the detector during the previous measurement period. This correction for a "memory" effect is of particular importance in measurement of environments with changing 2nRn concentrations. Because the calibration factors for this type of instrument are based on steady-state radon concentrations, changes from high to low radon concentrations yield overestimates. Conversely, changes from lower to higher radon concentrations are underestimated. Busigin et al. (1979) developed the mathematical formulae necessary to correct for the concenmemory effect of any radon monitor which estimates the 222Rn tration through its short-lived daughter products. This can be done with usually less than a 20%error. A timeaveraging 2"Rn monitor operating on the principle employed by Costa-Ribeiro et al. (1969) was developed by George and Breslin

56 1 7. MEASUREMENTOF RADON IN AIR

(1977). In the Passive Environmental Radon Monitor (PERM),radon diffuses through a 2 kg silica gel bed and a filter paper into an inverted 1.5 1 volume metal funneL The neck of the funnel is sealed with a rubber stopper through which a brass rod is inserted to serve as a cathode. A molded plastic pedestal attached to the end of the rod holds an alphasensitive lithium fluoride (LiF) thermolurninescent chip. A 900 V negative collecting potential is applied to the rod with dry cell batteries. The and 2 1 4 ions P ~ formed from the decay of 222Rn m collected charged 218Po on the LiF chip. After a suitable exposure period, the chip is removed and read on a thermoluminescent dosimeter (TLD) analyzer. The gamma-ray exposure during the period is measured with a second chip shielded from the alpha radiation. The thermoluminescence, after COP rection for gamma-ray background, is directly proportional to the time integrated radon exposure rate. Although the diffusion time into the sensitive volume is about 5 h, over long monitoring periods the average concentration in the funnel will equal the average ambient concentration wen in varying radon atmospheres. The PERM is widely used in remedial action programs with a usual exposure period of one month. for a The lower limit of detection is about 1 Bq m-"0.03 pCi 1.') of 222Rn oneweek sampling period. Maiello and Harley (1987) designed a passive environmental radon monitor similar to the PERM but considerably smaller, lighter, less expensive and requiring no electrical power source. This environmental gamma-ray and radon detector (EGARD), relies on TLDs as the inte grating detection system. A thin aluminum box of one 1 volume houses three 2.54 cm (1 inch) metal ring and disc assemblies attached to the center of three solid walls. Each assembly houses two TLD chips (3 x 3 x 0.38 mrn ribbon type) placed in shallow wells. One assembly is ~ )detects environmental covered with aluminized Mylar (0.9 mg ~ m -and gamma-ray and cosmic-ray exposure. The second assembly is covered with a 25 pm thick lkflon FEP (fluorinated ethylenepropylene)electret charged to 1,200 volts. The electret collects positively charged radon daughters formed within the collection volume while the TLDs inte grate the exposure rate from the radon daughters plus environmental gamma and cosmic rays. The third assembly contains TLD chips irradiated with a known gamma-ray exposure to determine "fading" during field deployment (none was found). The threesided box cover has two 2.54 cm (1inch) diameter access ports on each of two sides. The access ports are backed with a polyurethane foam which acts as a particulate and radon daughter filter so that only radon diffuses into the collector volume.

7.5 INSTRUMENTS FOR THE MEASUREMENT OF *Rn IN AIR 1 57

Laboratory measurements show that 35% of the 218Po and slightly more of the 214Pbformed within the sensitive volume are collected by the electret, yielding an effective collection efficiency of 70% for the charged daughters in the electret voltage range of -900 to -1,500volts. The radon exposure to the TLD is apportioned between the "'Po and "'Po alpha particles (75%)and the 2HPband 2'4Bibeta particles (25%). The collection efficiency is unaffected by relative humidity in the range of 10 to 85%. With normal indoor environmental conditions. the elec tret retains 50% of its initial charge for 150 days. Field testing of the EGARD detection system was carried out in 37 unoccupied houses in Uravan, Colorado. The results for average radon and gamma-ray exposures of 120 Bq m-3(3.2 pCi 1.')and 17 uR h-I (4 x C kg'h-')w k consistent with relatively high soil concentrations of uranium and high altitude. The LLD for radon is 3.3 kBq d-' m-3(90pCi d-' I-') and 1.3 mR (3.4 x 10" C kg-') for the combined gamma-ray and cosmic-ray exposure. 7.5.3.3 Passive Monitor forDirect Measurement

Chittapom et d,(1981) developed a passive, continuous monitor capable of detecting 222Rn at environmental levels without interference from or dependence on the measurement of the short-lived daughter products. The instrument consists of a photomdtiplier tube, a ZnS(Ag) phosphor lining a metal cylinder, a negatively charged electret, a lighttight housing with a thin diffusion barrier and an electronic package. The electret is a 25 pm thick sheet of FEP Tbflon foil. alumhimi on one side and capable of maintaining a negative electrostatic potential for more than one year. The detector is a 13 cm diameter by 14 cm high metal cylinder (1.861 volume) lined with ZnS(Ag) phosphor on a Mylar substrate (Fisenne and Keller 1981). The electret acts as a collection electrode for *laPo removal as these atoms are formed from =12Rndecay within the sensitive volume. The electret establishes a collecting field within the sensitive volume of 100 V cm-'. The electret is located at a height selected so that neither the nOPocollected from the radon decay nor the 214F%which is formed on the electret will interact with the scintillation phosphor. When two precautions are taken, the authors report that about 10% of the short-lived alphaemitting radon decay products not collected by the electret contribute to the count rate. First, a large mesh wire screen is placed on the photomultiplier tube face in electrical contact with the metal detection chamber housing. This establishes a primarily axial

58 1 7.MEASUREMENT OF RADON IN AIR

electrical field within the chamber and maintains the photocathode at ground potential, preventing radon daughter product deposition on the glass photomultiplier tube face. Second, the Mylar-backed ZnS(Ag) phosphor must be in electrical contact with the metal cylinder to eliminate induced charge and perturbation of the electret-induced field. The diffusion half-time into the sensitive volume is 23 minutes, so that '*Rn is eliminated by decay. Because the instrument is not dependent on the measmment of daughter products, the detection sensitivity is independent of humidity in the range studied experimentally, 14 to 80% relative humidity. With improvements made in the detection system, the background count rate is 8 cph with a calibration factor of 4,300 cph Bq-' (160 cph pCi-I) 22Ttn.The lower limit of detection at the 95% coinfidence level for a 60-minute measwement period is about 4 rnBq (0.1 pCi) which corresponds to an air concentration of 2.2. Bq m-3 (0.06 pCi 1-I). 7.5.3.4

Passive Adsorption Measurement

Rutherford (1900~) was the first to show that charcoal can be used to adsorb either 222Rn,2ZoRn, or z19Rn.He also proposed quantitative measurement of "2Rn in air and emanation rate from soil using passive adsorption. Short-term, integrated estimates of 222Rnconcentrations may be obtained from samples collected on activated charcoal by passive diffusion. The theoretical considerations of the adsorption of 222Rnonto a charcoal bed have been discussed by Cohen and Cohen (1983). The method is convenient and simple but requires rigorous control on mrrection and calibration factors. Pensko (1983)enumerated the factors which influence the accuracy of the measurement as exposure time, weight of charcoal, radioactive decay of the adsorbed 222Rn, accuracy of the gamma spectrometry measurement and water adsorption from the atmosphere. The correction for adsorbed water, which is obtained by weighing the canister before and after exposure-to the atmosphe~, may be as high as 50%. Pensko (1983)found that the 2z2Rn concentration was overestimated for dry (4% dative humidity) a t m e spheres and underestimated at high humidity (80%)conditions. The 222Rnconcentration, after correction for humidity, is obtained from gamma-ray measurements of the 352 keV P4Pb)or 609 keV P B i ) gamma-ray energies of the radon daughter products (ora measurement spanning this energy range.) Pensko (1983)estimated that, for a 7.8 cm diameter by 8.2 cm high canister containing 150 grams of charcoal. a

7.5INSTRUMENTS FOR THE MEASUREMENT OF 2mRnIN AIR 1 59

sensitivity of about 1 Bq rnJ (0.03 pCi 1.') of 222Rn could be achieved with an enor of 50%. George (1984)described a simple and practical activated charcoal sampler package for monitoring the indoor environment, principally in homes. The original sampler package, a World War I1 gas mask canister developed by the U.S.Army Chemical Corps, is no longer available and a replacement has been designed, calibrated and field tested (George, 1984).The charcoal canister is a cylindrical metal can, 5 cm high, with a cross-sectional area of 80 cm; containing 80 grams of activated charcoal. The charcoal occupies 4.5 crn of the depth of the canister and is held in place by a metal screen and a metal retainer ring. Adsorbed moisture is removed before exposure by heating the canister in a 120" C oven for several hours. In the field, the top of the can is removed and radon is allowed to adsorb onto the charcoal bed for a period of three days. After the collection period, the lid is replaced, taped closed and returned to the laboratory for NaI(Tl) gamma spec trometry measurement. After correction for humidity effects (basedon weight gain of the sampler),George(1984)calculated that for a 72-hour exposure period the LLDs for 2"Rn, when measured 3 and 72 hours post-exposure, were about 7 and 11 Bq m-3(0.2 and 0.3 pCi I-'), respectively George (1984) also noted that with extreme radon concentration changes under experimental conditions the timeaveraged radon concentration could be underestimated by 40% or overestimated by 250%, because 222Rndesorbs from as well as adsorbs onto charcoal. Such varying conditions are not encountered or expected in normal indoor environmental conditions. Prichard and Marien (1985)suggested that the charcoal adsorption devices could be improved by the addition of a diffusion barrier between the charcoal and the ambient atmosphere. With prototype samplers consisting of a 20 ml glass vial containing 1 g of activated charcoal and sealed except for a diffusion tube, 24hour sample collection periods were possible. For maximum sensitivity the authors suggested 20 g of charcoal and measurement by liquid scintillation counting (Prichardand Marien, 1983). Based on theoretical considerations of adsorption of radon on charcoal, the measurement method and the desirability of reducing water adsorption due to the relative humidity at the sampling site, Cohen and Nason (1986) devised a low cost sampler for monitoring radon in homes. The sampler consists of a 7.62 cm diameter by 2.54 cm high (3 inch by 1 inch)metal can containing a 1.5 cm thick(25 g) bed of coconut shell charcoal retained by a wire mesh screen The diffusion banier is a

*

60 1 7. MEASUREMENT OF RADON IN AIR

double layer of fine mesh nylon screen glued over a 1.90 cm diameter inch) hole punched in the can lid. A dessicant bag (3 grams of finegrained absorbent) is taped to the nylon screening. The charcoal is heated overnight at 120" C and weighed prior to assembling the sampler. The combination diffusion barrier and moisture reduction lid is placed on the body of the can and the assembly sealed together with tape. The hole in the lid top is covered with aluminum foil and sealed in place by tape. In the field, the foil-tape is removed, the sampler exposed for up to 168 h, resealed with the foil-tape cover and returned to the laboratory for measurement by gamma-ray spectrometry. After measurement, the sampler is weighed, the lid removed and the sample heated in an oven overnight prior to redeployment in the field. Basically, the addition of the diffusion barrier reduced the quantity of charcoal required for a given exposure period The authors state that for a sampler exposed to 40 Bq m (1pCi I-') (exposure period unspecified but assumed to be 50 h) the measurement yields a result with a standard deviation of 17%. As an integrated sampler the charcoal passive device does offer advantages, up to seven days sampling period and high cost effective ness. Its disadvantages include a short integration period and the difficulty of obtaining a true average over an interval with widely concentration. varying 222Rn

7.5.4 Solid State System for Continuous Measurement The newest instrument for the continuous monitoring of 222Rn concentrations incorporates electrostatic collection and solid state alpha spectrometry (Watnicket aL,1986). The air stream is dried and filtered prior to entry into a 2 1 volume aluminum chamber containing a Tbflonhoused silicon solid state detector with a 300 rnrn2 active area. An electrostaticcollection field of 3,500 volts is applied between the chamber walls and the detector. The flow rate through the active volume is maintained at 1 1 per minute. The authors state that about 95% of the radon daughter products in the filtered, dried air are positively charged and deposit on the face of the detector. Under these conditions, equilibrium between 218Poand radon is achieved in about 10 minutes. The overall detection and measurement efficiency for 218Po and 'I4I)O are given as 45% and 65% respectively. The background count rate of the system is less than 0.1 cph. The rapid response of the detection system to changes in radon concentrations may yield extremely refined environmental data but is of particular assistance in monitoring of radon

7.5 INSTRUMENTS FOR THE MEASUREMENT OF 222Rn IN AIR 1 61

levels in exposure chambers. The stated Poisson error at the 40 Bq m-3 (1pCi I-') level is + 23%. 7.5.5

Air Filtration for 222Rn Estimation

Historically, the estimation of 222Rn concentration from filtered air samples was the only alternative to measurement of the gas in ionization chambers. Filtered air samples could be collected and measured in the field. The early methods were designed to fill the need for monitoring occupational environments, principally uranium mines. The later introduction of the twefiIter method for "2Rn concentration estimation led to the adaptation of this technique to continuous measurement of environmental radon levels. 7.5.5.1 Single Filter Sampling

Harley (1953) described a method for interpreting measured daughter product activities on a single air filter for estimation of both the radon and daughter product concentrations. Air was drawn at a known flow rate through a filter paper for a fixed time period, usually 10 to 180 minutes. The sample was allowed to decay for one hour prior to alpha activity measurements so that the daughter products had reached the linear portion of the 31 minute exponential decay curve. The integrated radon daughter activity on the filter was extrapolated back to the end of sampling by graphical methods, and the extrapolated value was further corrected by a sampling-time dependent factor. The corrected alpha activity could then be substituted into an equation to yield the radon concentration. Harley noted several difficulties with the method. The major problem was the need to assume radioactive equilibrium between radon and its short-lived daughter products. Underestimates of the radon concentration by up to a factor of 2 could result under non-equilibrium conditions. Other difficulties related to the retention of submicron particles on the filter and the alpha absorption in the filter material and dust loading on the filter. Howwer, the method was deemed sufficiently accurate for use in uranium mine atmospheres. 7.5.5.2

?loo Filter Method

Fontan (1964) described a double or "two filter" system for the continuous measurement of atmospheric concentrations of radon and

62 1 7. MEASUREMENT OF RADON IN AIR

thoron. The term "twefilter" method has become generic and is used to describe the estimation of radon and thoron concentrations for shortterm collections in mines (Blanc et a!,1967; Breslin et d , 1969) or continuous outdoor monitoring (George,1978; Harley, 1978). Basically the equipment consists of a primary high efficiency filter which retains atmospheric aerosols, particularly the radon and thoron daughter decay products, a tube or chamber, an exit filter for the collection of daughter products formed within the chamber, and a pump to draw air t h u g h the system. The activity present on the exit filter is independent of the degree of equilibrium between the parent gases and the daughter products in the inlet air stream. The activity of the second filter when corrected for growth and decay of the daughter products and for loss of daughters to the walls yields an estimate of the gaseous parent concentration. The activity of the outlet filter is generally measured by ZnS(Ag)scintillation. In short-term collections the exit filter is removed from the decay chamber or tube for measurement. For continuous measurements, two types of systems are common. In the first the exit filter is actually a movable iilter paper strip upon which the daughter products are collected for a specified time period (Fontan, 1964). At the end of the collection period the filter paper tape is automatically moved to an alpha scintillation detection system for measurement while the next sample of daughter products is collected on a b h piece of the filter paper tape. The second type of c~ntinuousmeasurement depends upon the simultaneous collection and measmment of the daughter products on a stationary exit filter. The face of the exit filter is exposed to an alpha scintillation detection system. When only the radon concentration in air is of interest, it is common practice to place a decay chamber of sufficient volume in front of the system to eliminate thoron before it can reach the inlet filter, This reduces the mathematical computations required to obtain the radon concentration. Jacobi (1963)replaced the outlet filter with an electrostatic collector. The walls of the 140-liter volume polyvinylchloride decay chamber were covered with a conducting film to which a positive potential was applied. In place of the exit filter, a negatively charged aluminked polyester foil facing a ZnS(Ag) phosphor and photomultiplier tube formed the collector-detector assembly. The sensitivity of this device was found to be independent of the air flow rate as long as the residence t h e in the decay chamber was s m d compared to the half-time of Rn or Tn and the air velocity was small compared to the velocity of the ions in the electric field. The system was reported to be capable of

7.5 INSTRUMENTS FOR THE MEASUREMENT OF mRn IN AIR 1 63

measurement of thoron concentrations as low as 0.4 Bq m-3(10 pCi m-3) in the presence of radon concentration as high as 4 Bq m-3(100 pCi m-3) and the simultaneous determination of radon and thoron daughter products on the primary filter. Taylor and Lucas (1966)found that the marked dependence of collection efficiency on relative humidity made the electrostatic collection method unsuitable for use as a radon monitor. A twofilter system was also constructed based on the original design of Fontan with a movable second filter. The collection efficiency of the freshly-formed daughter products on the outlet filter was found to be highly dependent on the flow rate of air but independent of the relative humidity. The authors attempted to describe the loss of the radon daughter products in the decay chamber by use of the Gormley-Kennedy equation for diffusion losses in circular tubes (Gorrnley and Kennedy, 1949). The equation predicted that 80 to 92% of the radon daughter products should pene trate the decay chamber at flow rates of 60 to 600 1 min-I. Experiments showed that 80% of the daughter products were collected at a flow rate of 50 1 min-I but only 30% were collected at flow rates of greater than 500 1 min-I. Thomas and LeClare (1970)noted the lack of an equation for calculating the radon concentration from the activity collected on the second filter. ?b properly describe the collection, the equation had to include a correction factor for the loss of the radon daughter atoms in the decay chamber by "plate out" or diffusion. The Gormley-Kennedy equation was not applicable because of the formation of the daughter atoms within the decay chamber volume. They developed an equation for a decay tube of simple cylindrical geometry.(52cm long, 3.6 cm inside diameter giving a volume of 0.53 1) in which the time of flight of the gas through the tube (air flow rate of one 1 min-') essentially precluded the collection of radon daughter products other than 218Po.At relative humidities in the range of 20 to 90% the diffusion coefficient for 218Po was found to be 0.085 cm2 s-I and was 0.053 crn2s-I for relative humidities < 20%. These authors felt that the flow rate must be monitored and controlled because the diffusive losses were a function of flow rate. Newstein et d ,(1971)showed that flow rate changes of the order of 20 to 30% are unimportant, resulting in only a 2 to 3% error in the calculated radon concentration. Schery et d , (1980) improved the twcfilter monitor using stateoftheart electronics and reported a sensitivity of better than 0.4 Bq m-3 (0.01pCi 1-I)for 222Rn with no interference from 220Rn.

64 1 7. MEASUREMENT OF RADON IN AIR

Although small two-filter systems have been used to monitor the indoor environment on a short-term basis, the size and complexity of continuous systems make them unsuitable for long-term indoor measurements.

Accurate and precise instruments and methods exist for the measurement of radon concentrations in occupational and environmental situations. Sealed ionization chambers measure radon gas introduced into them with high accuracy. These iwe not readily available for monitoring and rnused in laboratory situations for calibration. Scintillation cells are used either as instantaneous grab samplers or as flowthrough chambers t o continuously measure the particular environment. Other techniques for continuous monitoring are available, such as the two-filter method and the electrostatic collection of 2'8Poand counting with solid state surface barrier detectors. Passive monitors such as solid state nuclear track detectors (SSNTD), activated charcoal samplers or thennoluminescent dosimeters (TLD) are used for timeintegrated average radon concentration measurements. The SSNTD detectors may be employed for long time intervals, one year for example, while charcoal may be left at location for a maximum of one week and TLDs for intermediate periods of up to about 6 months. The selection of an appropriate method for radon estimation depends on the scope of the monitoring program and the resources available.

8. Measurement of Individual Radon Daughters 8.1 Introduction For research purposes and for surveys it is often desirable to measure concentrations of individual radon daughters. In an equilibrium atmosphere of radon and radon daughters, all short-lived daughters have activities equal to that of the parent. In other cases. relative activities of the individual daughters can be used to determine the degree to which equilibrium is achieved and to calculate the potential alpha energy concentration (PAEC). 2'4Bi,and 214Po, only Of the short-lived radon daughters, 2'8Po,2L4Pb, Z ' 8 ~ and 214Poare alpha emitters. Several measurement techniques make use of three independent gross alpha counts to determine individual concentrations of the short-lived daughters. Alpha spectrometric methods employing surface barrier or diffused junction detectors with multichannel analysis can also be used for individual daughter measurements. Recent advances in instrumenbtion with the addition of computer control has allowed the development of continuous monitoring instruments for unattended operation with automatic sampling, analysis, and recording of concentration levels (Bigu and Raz, 1984). Standard gross alpha count methods and alpha spectrometric methods can be implemented for both radon and thomn daughters. The individual daughter concentrations in an atmosphere are important for detailed bronchial dosimetry and for scientific aerosol studies.

8.2 Sampling

All methods require that a known volume of air be drawn through a filter in a known time, and that the radon daughter activity on the filter be measured during or after sampling. Holmgren et d , (1977)reviewed relative filter efficiencies for sampling radon daughters in air. They

66 1 8. MEASUREMENT OF INDIVIDUAL RADON DAUGHTERS

concluded that high-efficiency membrane filters with pore sizes less than 1 pm have collection efficiencies of 100 percent or slightIy less. Membrane fdters show little dependence of efficiency upon either flow rate or filter loading whereas cellulose fiber filters exhibit an unacceptable variability even under uniform sampling conditions.

8.3 Gross Alpha Counting 8.3.1 Tsivoglou and Modified Tsivoglou Methods The first technique for the determination of individual radon daughter concentrations was described by Tsivoglou et al. (1953).The classical Tsivoglou method requires measurement of the gross alpha count rates at 5, 15, and 30 minutes following a 5, 10 or 30 minute sampling period a t flow rates of 5 to 10 1 per minute. I t is necessary that the response of the alpha counter be independent of the alpha energy in the range 6.00 to 7.69 MeV. The concentrations of maPo,'14Pb, and ""Bi are then determined by solving three simultaneous equations. Breslin et aL (1969)showed that the count-rate method was not very accurate, particularly for determination of 21sPoand that with 100-liter sample volumes a t concentrations of 0.5-5 WL, replication e m r s of 15-25% and 25-35% were found for 2J4Pb:21SPo and 2"Bi:218Poratios, respectively. The modified Tsivoglou method developed by Thomas (1970, 1972) ,improves the above technique by using a scaler to record total counts rather than count rates and by optimizing the count intervals. Thomas recommended a 5-minute sampling period at a flow rate of 10 1 per minute, followed by alpha counts from 2 to 5, 6 to 20, and 21 to 30 minutes after the end of sampling. The detection limit of the method is of the order of 40 Bq m-"1 pCi 1-l)for each nuclide. The concentrations of n'8Po,214Pband 2 1 4 Pare ~ calculated by the followingequations (Thomas, 1972):

214Bi (pCi 1.I) = [-0.0225Cl + 0.0332~~-0.0377C~-0.058F&,]NE(8.3) where C, = total counts in interval2-5 minutes C, = total counts in interval 6-20 minutes c9= total counts in interval21-30 minutes R,= background count mte, cpm

8.3 GROSS ALPHA COUNTING 1 67

V = volumetric sampling rate, 1 min-' E = alpha coun ting-efficienq counts per disintegration If the errors in V and E are known to be within 1-3% then the standard deviations for the individual nuclide concentrations in pCi 1.' are calculated by the following equations:

where T, = background count time. Concentrations and the error terms may be expressed in Bq m-3by multiplying by 37. If counter background is well established and the average of many values used for Rbin Eq 8.1 to 8.3, then the term RJT, in Eq 8.4 to 8.6 should be replaced with the square of the standard error of the mean background count rate. Scott (1981)devised a modified Tsivoglou method which he calls the MRK method. The count times are optimized for the best detection limit for 218Po.The first two count intervals are fixed and the third interval can vary from 30 minutes to one hour post sampling time. In this way, more samples may be analyzed in a given time period. The second or third count interval can be used directly to calculate working level. The second is identical to the Rolle method and the third count, if measured at 40 minutes after sampling (typical for the Kusnetz method), is identical to the Kusnetz method. The modified Tsivoglou method has been used to measure the potential alpha energy concentration, producing results that are comparable to those obtained by the modified Kusnetz method (Budnitz, 1974). The fractional contributions of ,'To, ,I4Pband "Bi to the potential alpha energy concentration under conditions of equilibrium are 0.105,0.516 and 0.379 respectively. It follows that if the concentrations in air of 2'To, ,I4Pb, and ,14Bi are Q,, Q,, and Q, respectively then the working level is given by WL

=

WL

=

(0.105 Q, + 0.516 Q, + 0.379 Q,)/3700,where Q,, Q,, Q, are in Bq m3 10-3(l.05Q, + 5.16Q2 + 3.79Q3)whereQ,, Q,, Q, areinpCikl

Busigin and Phillips (1980) Busigin et aL, (1982)and Pogorski et m! (1982)optimized the Tsivoglou method to take into account the effects of concentration and flow rate uncertainties. The selected counting intervals of 2 to 5, 7 to 15, and 25 to 30 minutes provided greater insensitivity to flow rate and concentration fluctuations. These

68 / 8. MEASUREMENT OF INDIVIDUAL RADON DAUGHTERS

intervals resulted in an improvement in the precision of the "'PO measurement from &3.5% for the modified Tsivoglou method to lt 1.3% in the Busigin method. A Monte Carlo simulation m ~ d e l developed to quantify the overall precision of the three-count total alpha sampling and counting technique found the relative standard deviation of the measurement was independent of flow rate variations of less than 10% and variations in liter to liter airborne radon daughter concentrations of less than 50% (Pogorski and Phillips, 1985). Precision estimates based on counting statistics alone are reliable for potential alpha energy concentrations exceeding 0.02 WL. A generalized method for calculating the coefficients of the equations for both 222Rn and 220Rn daughters for any sampling and measurement interval was devised by Fu-Chia and Chia-Yong (1978).Nazaroff (1984)optimized the counting error by selecting somewhat different count intervals than the Thomas method. Beginning the count at 1 minute post sampling time reduces the statistical uncertainty associated with 2'rPo. Raabe and Wrenn (1969) utilized a least squares method to count over the entire decay interval from end of sampling to 30 minutes post sampling. This method is reported to provide the concentrations of individual daughter products of both 220Rn and 222Rn.

8.3.2 CliffMethod With all sampling methods where gross alpha counting takes place only after sampling, the uncertainty in the determination of 2 ' 8 P ~ concent'ration is largely due to the short half-life of ahPo.In the Thomas method, 62% of thenRPodeposited on the filter paper will have decayed before the first count. James and Strong (1973)designed an instrument that permitted gross alpha counting as sampling was in progress. A second count was recorded for a similar period after sampling. From the ratio for the two counts and from prepared tables, the 218Po concentration can be determined from the first count and the potential alpha energy concentration from the second count. Cliff (1978a, b) extended the concept of James and Strong to include a third alpha count. He showed that for a sampling time of 5 minutes and a total measurement period of 35 minutes, the sensitivity of the "#Po determination may be improved by a factor of 3 to 5, depending upon the degree of disequilibrium, with counting intervals of 0 to 5 , 6 to 25 and 26 to 35 minutes. By extending the sampling time to 10 minutes and the overall measurement time to 57 minutes, the limit of detection of ""0 is an order of magnitude lower than the modified Tsivoglou method.

8.4 ALPHA SPECTROMETRY 1 69

8.4 Alpha Spectrometry

Alpha spectrometry is a useful approach to the measurement of individual radon daughter concentrations. The sample is collected at a known flow rate for a specified period of time on a membrane filter with a nominal pore size less than 1 pm. The sample is then counted at least twice using an alpha spectrometer to obtain separate determinations of the '18Po and "To activities on the filter. Semiconductor detector systems, such as surface barrier or diffused junction detectors, can be used for individual daughter measurements. The system is r e q u i d to resolve the 6.00 MeV and 7.69 MeV alpha energies of 2'8Poand 21To respectively. For practical purposes it can be assumed that the concentrations of 214Bi and 214Po (in terms of activity per unit volume)in any given atmosphere are always equal. A method developed by Martz e t aL, (1969) employs alpha spectrometry using a semiconductor detector and multichannel analyzer to measure the count rates of "To and "'Po present on a membrane filter sample at two periods, 5 and 30 minutes after sampling. Only two simultaneous equations are needed (compared to the three required in the Tsivoglou method) to determine the relative daughter concentrations. In experimental comparisons with the Tsivoglou method, Martz e t al. (1969) found that the spectrometry method gave more reliable estimates of the individual radon daughter concentrations, particularly for the 218Pocomponent (8% standard deviation compared to 29% for the Tsivoglou method). Duggan and Howell (1968) developed a method employing alpha spectrometry that required a measurement of the activity on the filter while sampling was in progress. They collected airborne daughter products by drawing air at about 80 1 per minute through a membrane filter, the collectingface of which was viewed by a silicon surface bamer detector. The first count was recorded during the 30-minute sampling period and a second 30-minute count was recorded starting 30 minutes after cessation of sampling. The method is sufficiently sensitive for measurement of daughter concentrations usually found in the atmosphere. Gournnerova and Minev (1972) presented a method involving a and 2 1 4 at P~ two-channel analyzer to measure the alpha peaks of 218Po oneminute intervals for 20 minutes after a 5-minute collection on a membrane filter. The concentrations of the three daughters were obtained by plotting the decay curves of Z18Po and Z"Poseparately and extrapolating to the time of the end of collection and substituting the activities obtained into three simultaneous equations. Jonassen and

70 1 8. MEASUREMENTOF INDIVIDUAL RADON DAUGHTERS

Hayes (1974a)chose to integrate the alpha activity due to 218Poand 2'4Pofor the counting intervals of 120 to 320 sec. and 8 to 20 minute following a 10-minute sampling period. The method has a sensitivity of about 20 Bq m-3(0.5 pCi 1.') with a relative standard deviation of 50%. Jonassen and Hayes (197413)pointed out that when measuring the '"Po and 2"Po activity on filter samples by alpha spectrometry, it is sometimes necessary to take into consideration the effect of energy degradation of the measured spectrum if the sampling time is longer than 10 minutes. The energy degradation correction is most important for the 2'8Pomeasurements due to possible overlap by the *14Popeak. Kritidis et al. (1977)chose a sampling time of 7 minutes and counting intervals of 1 to 10 and 11 to 20 minutes after sampling. Comparison of the Kritidis method with the Martz et uL, (1969) method, considering counting statistics only, shows sensitivities for a 50% relative standard Sensitivities deviation of 3 vs. 30 Bq m-3(0.08 vs. 0.8 pCi 1.') for Z18Po. for 2'4Pband 214Biby the Kritidis alpha spectrometric method are reported as about 20 and 4 Bq m-a(0.5and 0.12 pCi I-'), respectively. Nazaroff et al. (1981)optimized the twecount-interval technique for choice of counting intervals to minimize the standard deviation of the measurement associated with the random nature of radioactive decay. In general, the best results are obtained by minimizing the delay between sampling and the first counting interval. For a manually operated instrument, they selected a one-minute interval as a reasonable minimum. Greater overall precision is achieved with longer measurement time. With modest flow rates and detector efficiencies, a measurement time of 40 minutes allows the individual daughters to be measured with a 20% relative standard deviation a t about 40 Bq m.S(1 pCi 1.'). Nazaroff (1983) constructed an automated instrument, the radon daughter carousel (RDC),which collects radon daughters on one of seven filters. The filters are subsequently rotated to position them under an alpha spectrometer detector and are counted during two intervals. For fixed total measurement times, ranging from 30 to 60 minutes, concentrations as low as about 20 Bq m3(0.5 pCi 1') can be measured with less than 20% uncertainty. determination increases sharply as the The uncertainty in the 218Po count start time is delayed, thus 'hmblay et a! (1979) developed a two-interval, alpha spectrometric method to count while sampling. The accuracy of the 214Pband 214Bi concentrations is improved if there is a separation between the 214Pocount intervals 1 and 2 although a tradeoff in increasing separation must be made with resulting decreases in total counts. Tkemblay et al. (1979) chose optimum sampling times of 15 or 20 minutes and counting intervals of 0 to 20 for

8.6 THORON DAUGHTER MEASUREMENT 1 71

*'@Poand 0 to 20 and 25 to 40 for 214P0 (or 0 to 15,0 to 15 and 20 to 35) where zero minutes corresponds to the start of sampling. Alpha spectrometry was also used by Andrews et d (1984) to measure radon daughters deposited on an aluminum foil tape. The daughters were first charged in a corona discharge and then collected by electrostatic deposition onto the foil. This system also employed automated control of the sampling for continuous monitoring. Other techniques using alpha spectrometry and two counting intervals are reported by Kerr (1975) and Hill (1975). Holub (1980) evaluated the statistical counting error in four different "two-count" methodologies and found little difference among them.

8.5 Gamma Spectrometry An alternative method of determination of the relative concentrations of "@Po,2 1 4 Pand ~ "'Bi in the atmosphere by gamma spectrometry has been described by Irfan and Fagan (1979). The method is based on the regression analysis of integrated counts of the 1.76-MeV gamma-ray observed in several time intervals of a few minutes' duration. Interferences from other naturally occurring gamma-ray emitters make general application difficult. The much higher counter background for gamma-ray detectors compared with alpha counters renders the method inferior to alpha counting techniques.

8.6 Thoron Daughter Measurement It is generally considered when measuring radon daughters that any contribution from the daughters of thoron (ZmRn)is negligible. The half-lifeof thoron (55 seconds) reduces its mobility in the environment compared to radon (half-life of 3.82 days). However, one thoron has a relatively long half-life (10.6 h) compared to the daughter, 21ZPb, short half-lives of the radon daughters. As a result, one air change every 10 minutes limits the thoron daughter PAEC to about one percent of the maximum, whereas the same ventilation rate limits the PAEC from radon daughters to about 18 percent of the maximum. European studies (Stranden, 1980; Steinhausler,-1975)have indicated that the indoor air concentration of the thoron daughter, 212Pb, can be significant.

72 I 8. MEASUREMENT OF INDIVIDUAL RADON DAUGHTERS

The measurement of thoron daughters is complicated by the presence of radon daughters. Gross alpha counting techniques, alpha spectrometry and energy-sensitive etched track methods can all be used to discriminate between radon daughter and thoron daughter concentrations and PAEC's. The modified Tsivoglou method employing gross alpha counting was extended by Harley and Pasternack (1973) to include "'Pb and 212Bi by adding two counting intervals and two more equations. Measurements were performed on air samples collected on membrane filters for 60 minutes at 10 1mini at intervals of 2-5, 6-20, 21-30, 200-300 and 1440-2440 minutes after sampling. Extending this concept to counteract flow rate and concentration deviations, Khan et al. (1982) optimized the counting intervals at 2-4, 5-20, 30-60, 140-200 and 270-320 minutes after a 10-minute sampling period, thus reducing the total counting-period to less than 5 112 hours. A comparison of the 10- and 60-minute sampling periods (Pogorski et d,1982) shows an improvement in variance for the 10-minute sampling period for 218Po, and no change in the variance for "Tb, "'Bi and 2nPb. Greater improvement in precision of the estimated concentrations can be obtained by the method of counting during and after sampling and utilizing least squares data reduction (Zhang and Luo, 1983). Alpha particle spectrometry can be applied to the measurement of both radon and thoron daughter concentrations on an air filter sample. I t is impossible to resolve the activity on the filter due to the 6.05- and 6.09-MeV alpha particles from '12Bi and the 6.00-MeV alpha particle from ']To but the activity can be partitioned between 21BPo and "'Bi using the 8.78-MeV alpha from 212Po.Kerr et al. (1978) described a method of calculating the concentration of 'I2Pb and '"Bi from two counts of the 8.78-MeV alpha particle activity of "'Po at intervals of 2-12 and 15-30 minutes aker sampling. The method can also be applied to evaluating the airborne concentrations of daughters of 222Rn, "ORn and 2'9Rn (Perdue e t al., 1980). A three-channel alpha spectrometric technique (Cote and Tbwnsend, 1981) counted "8Po and 212Biin the first channel, 214Po in the second and 212Po in the third channel. The measurement procedure was then optimized to minimize uncertainty in the estimated concentration.

8.7 Summary Individual radon daughter nuclides may be measured in a filtered air sample by alpha spectrometry or a series of timed total alpha counts. The selection of times is controlled by the relative half-lives of the

8.7SUMMARY 1 73

individual daughter products. The timing may be optimized to reduce the error for an individual daughter product or to spread the m r more uniformly The individual daughter concentrations in an atmosphere are important for the detailed bronchial dosimetry and for scientific aerosol studies.

9. Measurement of Unattached Radon Daughters 9.1 Introduction At the moment of 222Rndecay, the asPo (RaA) daughter appears primarily as a singly charged positive ion. Although the daughter is formed with two excess electrons, removal of electrons during recoil of the daughter nucleus renders over 90% of the ions positively charged (Dua et d , 1978, 1983; Dua and Kotrappa, 1981). The daughter ion undergoes rapid chemical changes and most of the resulting molecules become attached to aerosol present in the atmosphere. Polonium-218 (RaC),therefore exist and its successive daughters, 2"Pb (RaB)and 214Bi as both the unattached and attached species in any environment. The unattached species are called, by various authors, the free ion, unattached or uncombined fraction, or radioactive ions (Billard et d, 1971). The unattached species can exist in positively charged, negatively charged or neutral forms, therefore the term free ion fraction is not strictly correct. In this section, the term unattached fraction is used and is defined as the fraction of a radon daughter species that is not attached to an ambient aerosol particle. The measurement of unattached fraction and its interpretation are complicated because 218Po (RaA) changes chemical, charge and size characteristics from the moment of birth. These characteristics are discussed in Section 4. The accurate measurement of each of the unattached species of the three radon daughters is difficult but is important. The small sizes of unattached daughters lead to efficient deposition in the bronchial tree with consequently higher dose from the unattached 21sPo than the attached 218Po, as an example, even though the air concentration of the unattached is perhaps 1/10 of the attached. Thus the measurement of the unattached fraction improves the estimate of the lung deposition chara&ristics of radon daughters. In humans, bronchial deposition cannot be measured directly but must be modeled. Deposition of very small particles is essentially complete when they reach any surface, so their deposition is a function of the diffusion coefficient which regulates

9.1 INTRODUCTION 1 75

the fraction of particles deposited. Therefore, a knowledge of the diffusion coefficient as well as the air concentration of the unattached species is of primary interest. The measured particle size can be used to calculate the diffusion coefficient in some cases, while in others, the measured diffusion coefficient may be used to infer particle size. The particle size distribution for the ambient aerosol varies with time and location, depending on atmospheric characteristics. There are few detailed studies but Figures 9.1 and 9.2 show the data for New York City and an arid region in New Mexico. There are now at least three definitions for the unattached species commonly found in the literature. The unattached fraction can be expressed as a fraction of the activity of the specific nuclides, as a fraction of the radon activity, or as the fraction of the total potential alpha energy that is in the unattached form. Care must be exercised to determine which definition has been used in the literature since the values are very different numerically for a given air concentration of the unattached nuclides.

OUTDOORS NEAR STREET

CN = 300,0000.05 pCiA Rn 500,000/m3

PARTICLE DIAMETER, nm F i g . 9.1 Bimodal sue distribution of 2zRn daughter products measured in New York City (From Sinclair, 1986, reprinted with permission from the publisher of Aerosol

Science and 'kchnology, copyright 1986 by Elsevier Science Publishing Co., Inc.)

76 1 9. MEASUREMENTOF UNATTACHED RADON DAUGHTERS

AMD 80nm

PARTICLE DIAMETER, nm Fig. 9.2 Bimodal size distribution of mRn daughter products measured outdoors in Socorn, NM (Fmm Sinclair, 1986, reprinted with permission from the publisher of Aerosol Science and Ichnology, copyright 1986 by Elsevier Science Publishing Co.. Inc.)

This Section describes the properties of the unattached daughters, existing measurement techniques, the relationship of the unattached concentration to atmospheric aerosol content and the problems which arise in the measurements. Some attention is given to quality control since many of the reported data are of dubious value. The measurement of radon-222 daughters is discussed in particular but some of the measurements performed have incIuded thoron (2"Rn)daughters, and these data are discussed briefly. A good review of the measurement techniques for nanometer aerosols may be found in Sinclair (1986).A theoretical treatment of the growth mechanism is reported by Raes et al. (1987).

9.2 PROPERTIES O F UNAlTACHED RADON DAUGHTERS 1 77

9.2 Properties of Unattached Radon Daughters Within short times after formation (probably seconds), the polonium ion combines with oxygen in the atmosphere to become polonium dioxide (Goldstein and Hopke, 1985). The charge on the polonium ion is initially positive but PoO, can be readily neutralized. The size of the ion at formation is that of a single atom, however, condensation occurs rapidly along with other possible chemical reactions (Busigin et d , 198 1) and particle growth of an order of magnitude occurs readily The short-lived daughter products of radon are Z18Po, 214Pb, 214Bi, and 2'4P0,respectively. Thoron daughters follow a similar decay pattern with asPo as the first product, but the half-lives are markedly different. (See the decay schemes in Section 3, Figures 3.1 and 3.2.) The half-life of "'"Po is 3.05 minutes. A value of 3.11 minutes (Van Hise e t d , 1982) has been reported but the 3.05-minute value is used in this report and the half-life of "6Pois 0.15 seconds. Because of the very short half-life of 21To,its daughter, " T b (ThB),is formed with essentially the same charge characteristics as its '16Po parent. Over 90% of "To (and 99% of the "6Po) is formed with a single positive charge due to recoil during alpha emission (Dua and Kotrappa, 1981). I t is theoretically possible to have a small percentage of ions that are negatively charged, since two excess electrons are present from the parent's electronic structure. Measurement of the diffusion coefficient of "'"Po was first performed by Wellisch (1914) and found to be 0.045 cm2 s-l. Chamberlain and Dyson (1956) reevaluated the diffusion coefficient and found it to be 0.054 crn2s-', in good agreement. Several investigators then attempted to study the kinetics following formation and a range, with values from 0.0025 to 0.08 cm2 s - I , has been observed for widely differing experimental conditions (Raabe, 1968; Porstendorfer and Mercer, 1979; Kotrappa e t d , 1975; Raghumath and Kotrappa, 1979; Frey et d , 1981; Knutson et d,1983; Holub, 1984; Porstendorfer, 1984; Goldstein and Hopke, 1985). The marked change in diffusion coefficient over very short times is now well-documented (Busigin et at!, 1981; Frey et d , 1981; Goldstein and Hopke, 1985). This is the fundamental problem in an accurate measurement of unattached 218Pobecause the collection efficiency of the measurement device and the diffusion coefficient are directly related. Measurements of the diffusion coefficient of radon and similar gases indicate a value on the order of 0.1 crnZs-l. The diffusion coefficient of freshly formed 218Po or 2 1 6 P in~a normal environmental atmosphere has ordinarily been measured as about 0.05 cm2 s-', considerably less than that for a gaseous species. Goldstein and Hopke (1985) believe that the

78 1 9. MEASUREMENT OF UNATTACHED RADON DAUGHTERS

decreased mobility may be due to increased interaction of the ionic charge with induced dipoles in the neighboring gas molecules. The presence of electronegative gases which retain electrons strongly may also affect the mobility of the charged species but detailed mechanisms are still not available. Following formation of the positively charged asPo ion, normal atmospheric components of oxygen, water vapor, and certain trace gases allow neutral polonium compounds to be formed rapidly. The progression of events has been described by Goldstein and Hopke (1985)as follows. Polonium forms polonium dioxide within seconds of formation through interaction with atmospheric oxygen. They measured the ionization potential of PoO, to be within the range of 10.35 to 10.53 eV, substantially greater than polonium metal (8.43 eV). Charged polonium dioxide is able to acquire an electron via two mechanisms, either directly from gases with ionization potentials less than i t s own or through a scavenging mechanism whereby electronegative trace gases such as nitrogen dioxide scavenge electrons in the path of the alpha particle and transfer the electrons to the charged polonium dioxide. Polonium dioxide as a neutral species has a diffusion coefficient of about 0.08 cm2s-'. Neutralization occurs with a mean life of 15-20 seconds. The polonium dioxide atom then grows by an unspecified mechanism (cluster formation) (see Busigin e t d,1981 for a review plus additional data on charge state and Raes et d,1987) and the diffusion coefficient is diminished by wer an order of magnitude (Porstendorfer and Mercer, 1979; Kotrappa and Mayya, 1976; Knutson e t d,1983). The smallest diffusion coefficient was inferred by Knutson et ah, (1983)from plateout experiments as 0.0025 cm2 s-' (corresponding to particles of about 0.004 pm diameter) for normal aged room air. Particle size measmments made in New York and New Jersey homes with a high flow rate diffusion battery s h o d a bimodal distsibution of particle sizes, the smallersized mode having a diameter ranging b m 0.01 to 0.04 pm. I t was realized later that this small-sized fraction was probably the unattached daughters (George and Breslin, 1980). A positive charge is -regained during growth or attachment to ambient aerosol and investigators report a collection ability of over 50% using charged plates, wires and electrets (Soilleu, 1970; Khan and Phillips, 1984; Wilkening et al., 1966; Wilkening, 1973; Jonassen, 1984; Maiello and Harley, 1987). Jonassen (1984) performed experiments with electrostatically charged disks and demonstrated that, in filtered air, over 20% of polonium-218 ions and 50% of all daughters in the unattached form are collected.

9.3 SAMPLERS FOR UNATIACHED RADON DAUGHTERS 1 79

This area of research deserves considerable attention as the overall kinetics are not well defined. This important work is necessary to assess the validity of past measurement techniques and ultimately for evaluating human lung dose. 9.3 Samplers for Unattached Radon Daughters

The unattached daughters may be measured by utilizing their large diffusion coefficient and their resultant rapid interaction with surfaces. The charge of the daughters might be utilized to measure electrical mobility and therefore size, but rapid neutralization of some of the daughter ions would requitti recharging of daughter particles.

9.3.1 Diffusion Thbes The first measurements of the unattached fraction were made by Chamberlain and Dyson (1956). They found that in a typical mine environment about 10% of the *I8Po(expressed as the equilibrium or radon equivalent amount of activity) was in an uncombined form. They measured the diffusion coefficient as 0.054 cm2s-', in good agreement with the value that had been determined by Wellisch in 1914. Chamberlain and Dyson (1956)used diffusion tubes 150 and 600 rnrn long by 18 rnrn diameter and the diffusion equations developed by Gonnley and Kennedy (1949) for deposition of particles from fluids flowing in cylindrical tubes to determine the unattached fraction by loss to the walls. This is still a valid approach but not an easy field measurement to cany out accurately. The Gonnley and Kennedy diffusion equations pennit calculation of the material deposited by diffusion an the walls of cylindrical tubes and are widely used. The equations are: C/C, = 0.819 exp(-14.63 4

+ ...

+ 0.0976 exp(-89.22 n) + 0.0325 exp(-228n)

where C = mean concentration of aerosol in the stream C, = initial concentration of aerosol in the stream A = diffusionpameter = DW4Ur2 D = diffusion coefficient (cm2a3 L = length of tube (cm) U = mean axial velocity of fluid (cm s-I) r = tube radius (cm)

(9.1)

80 1 9. MEASUREMENTOF UNATTACHED RADON DAUGHTERS

If A is less than 0.0078, the solution requires more terms in the series. Gormley and Kennedy gave an asymptotic solution to avoid this difficulty.

Ingham (1975)d c u l a t e d the diffusion equations and presented the following expression which is somewhat simpler to use, since it does not q u i r e the two solutions necessary in the Gormley-Kennedy equations. The Ingham formula is

C/c, = 0.819 exp(-14.63 A) + 0.0976 q ( - 8 9 . 2 2 A) + 0.0325 exp(-228 A) + a exp(-b Au3) where a and b are constants a = 0.0509

(9.3)

b = 125.9

Chamberlain and Dyson (1956) utilized diffusion tubes to measure the uncombined radon daughter fraction in underground uranium mines and in a thorium factory. The reported values of unattached "8Po of about 10% for mines are in good agreement with more recent measurements. One of the first studies to utilize the diffusion tube in field studies was that of Craft et d (1966).Diffusion tubes 600 rnrn long and 37 mm diameter were operated in pairs, one with a reference filter on the inlet and another filter for correction purposes on the outlet (see Figure 9.3). The second diffusion tube had a single Glter at the outlet and this measured the sample activity (S).Calibration was performed using uranium ore in a steel drum as a radon source. At a flow rate of 1 liter per minute, the fractional penetration (F)of the unattached fraction of radon daughters in the measurement tube was found to be 0.114, companding to a diffusion coefficient of 0.045 cmZs-'. The reference diffusion tube utilized the filter at the tube inlet to determine the total air radioactivity concentration (R) and the outlet filter to determine a comtion (C) for the buildup of activity from radon within the tube. These three values of measured radioactivity were used in the expression to determine the unattached activity (X).

X = (R-S+ C)I(l-F) Unattached % = 100 XIR

(9.4)

9.3 SAMPLERS FOR UNATI'ACHED RADON DAUGHTERS 1 81

Fig. 9.3 Diffusion-tube samp&r for memurement of unattached WRn daughter products in mines (From Craft et aL, 1969, reprinted with permission by the American Industrial Hygiene Association Journal. vol. 27, 154)

82 1 9. MEASUREMENT OF UNATTACHED RADON DAUGHPIlERS

Measurements were made with the diffusion sampler in mines in southern Utah and Colorado, assuming constant F, and values for unattached activity ranging from 0 to 73% were reported. With the present-day considerations of the rapidly changing particle size and diffusion coefficient of the unattached daughters, it can be seen that calculational difficulties arise because of the potential for variable penetration fraction (F)with a changing diffusion coefficient. However, these data were from the first largescale study to attempt an estimate of occupational exposure to the unattached fraction.

9.32 Electmstatic Collectors Evans and Goodman (1940) used electrostatic precipitation to measure thoron daughters in 1940. Chapuis et al (1970)measured the unattached fraction using an electrostatic collector consisting of a brass cylinder with an axial wire maintained at a high potential. The ion fraction in a uranium mine atmosphere (withboth + and - charges) was less than 5%. The neutral species was determined using a diffusion battery (multiplediffusion tubes in parallel). Wilkening (1952)charged the environmental aerosol and measured the electrical mobility (particle velocity per unit field strength-cm s-' per volt cm-')of the carriers with an electrostatic precipitator. He found that most particles were in the 0.001 to 0.04 pm size range. Wilkening (1987)measured the positively charged maPoand 2"Pb ions in outdoor air and found their activity to be 6.5% and 0.65% of the '"Rn activity and the mean lifetime to be 12 seconds. Soilleux (1970) performed extensive studies of the charge on environmental radon daughter aerosols using an electrostatic collector. Outdoor air was sampled at a high flow rate and the particle size of radon daughters could be described' in all cases as a lognormal cumulative frequency distribution. Less than 5% of the daughters were on particles less than 0.02 pm diameter as determined by their electrical mobility. This is probably the fraction of unattached daughters, neglecting the neutral species contribution. Since the distribution was displayed lognormally, no dear distinction between unattached and attached species could be made. The positively charged fraction of all daughters ranged from 50 to 95%. The negatively charged fraction was about 25% but was highly variable. The highest percentage of positively charged daughters (95%)was measured during a snowstorm when presumably the atmosphere was lowest in aerosol concentration.

9.3 SAMPLERS FOR U N A W H E D RADON DAUGHTERS 1 83

Because electrostaticcollectors retain only ions, they must be used in conjunction with another type of sampler such as a diffusionbattery to determine the total unattached fraction of radon daughters.

9.3.3 Screen Samplers For many years, the screen sampler was more of a novelty than a quantitative sampling device for small atmospheric particles. Sinclair (1979)described various samplers relying on diffusion for analyzing the particle size of aerosols. These samplers are commonly called diffusion batteries and generally have an array of screens or diffusion tubes. The tubes can take the form of straight cylindrical tubes, rectangular channels, collimated hole structures, multielement screens or porous carbon filters. Sinclair (1986)was instrumental in the development of diffusion batteries and empirical penetration curves to analyze the particle size and in their application to environmental aerosol size measurements. Singlescreen diffusion batteries are now widely used because of their portability and simplicity in sampling. Thomas and Hinchliffe (1972) were the first to describe useful equations for the efficiency of screens for collection of unattached radon daughters. The expression they derived for screen efficiency was based conceptually upon the Gonnley and Kennedy equations. It is:

E = 1-10.82 exp(-0.223H) + 0.18 exp(-16.7 H)! where E = Scmen collection efficiency H = (100iWdDN) M = Screen mesh (openings cm.9 d = Wire diameter (cm) D = Diffusion coefficient (emzs-') V = Linear air velocity (cm s"). This expression has been widely used and compared with values obtained with diffusion tubes. Stranden and Berteig (1982b) used a screen sampler with two screens to check the applicability of the equation. The ratio of the activity on the backup screen (N,) to that on the prescreen (Nl)is

NJNl = E(1-E)B or, E = 1-NJN,

84 1 9. MEASUREMENT OF UNATTACHED RADON DAUGHTERS

This device was tested for various values of the parameter H, and the measured efficiency was in good agreement with that calculated using equation (9.5).This sampling device with only one screen was used in an iron mine to determine the unattached fractions. The fractions of the 21nPo (RaA),2'4Pb(RaB),and 2'4Po(RaC) airborne activity that existed unattached were found to be 0.123, 0.057, and 0.032, respectively. Similar values have been reported in other mines (Busigin et d,1983). surprisingly, is rather low. Scott (1984) The ratio of free 21sPo1214Pb1214Po. has remarked that the ratio of unattached 2'8P012"Pbshould be greater than 511, and possibIy is not because of collection of some attached daughters on the screens. Theoretical calculations (NCRI?, 1984b3 indicate that for normal condensation nuclei (CN) concentrations of about lo4CNlcm3the unattached 2'8PoP"Pbratio is 1011. Changes in the diffusion coefficient with time following formation and with atmospheric constituents (trace gases and condensation nuclei) may cause difficulty in the accurate assessment of the unattached fraction if only a single screen is used. The idea of the dual screen technique is useful and, although more difficult to employ in a field study than a single screen, it can be used to check the validity of adopting a single value for the screen efficiency. An alternative approach is to sample with a single screen a t different flow rates or with single screens of different mesh size. If the correct effective diffusion coefficient is used, the measured air concentrations will be the same for all sampling regimes. One additional consideration when using screen samplers is the potential for retention of a small fraction of the attached species of radon daughters. Van der Vooren et al. (1982) have shown that the efficiency of many samplers for unattached daughters has not been corrected for interception of attached daughters. The much greater concentration of attached species in most atmospheres means that the contribution of a small fraction of the attached species can be overwhelming. George (1972) indicated that there was no measurable interception for screens coarser than 60 mesh. McLaughlin and Jonassen (1983)devised a method for testing screen samplers. If the particle concentration is increased to lo6cm", using an appropriate source of particles, the radon daughter equilibrium approaches 100% and the unattached fraction approaches 0%. In such an atmosphere, screen samplers can be checked for the retention of the attached species. Cheng and Yeh (1980) and Cheng et al. (1980) reported on the development of expressions for the retention of particles on screen samplers. These expressions are:

9.3 SAMPLERS FOR U N A W H E D RADON DAUGHTERS / 85

For a particle diameter less than 0.2 pm.

E = 1 - exp (-A n Pe-") (9.8) where E = efficiency of retention of n screens. n = number of screens in the diffusion battery Pe = Peclet number = wU/D w = wire diameter (cm) D = diffusion coefficient (cm2 s-I) U = axial fluid velocity (cm s-I.) A = 4Bcuh/~(I-or)w B = 2 7 for fan model B = 29(-0.5lnor + cr -0.25clla-0. 75)j0 for pamllel staggered cylinder model h = screen thickness (cm) or = solid volume fractiono f filter = 1 - porosity A second equation was derived (Cheng et al, 1980) which corrects for particle interception by the screen wire. It is, in terms of screen penetration, P, for n screens. log P = -n (1.96Pe-w8+ 3.37 R 2 + 1.94 Pe-li2RUB) (9.9) R = interception parameter = particle diameterlwire diameter. Cheng and Yeh (1980)prefer the fan model p more accurately describe stacks of screens with rows randomly oriented. Since single screen efficiency is considered here, the p d e l staggered cylinder model is the best approximation. In the work of Cheng and Yeh (1980)the value of A for the staggered cylinder model was 10.56 and the screen a = 0.345. The screens reported by Thomas and Hinchliffe (1972) can be i n f e d from their published description to have an or of 0.34 and a value of A = 8.18 for use in equation 9.8. Thble 9.1 uses these measured values of A = 8.18 and a = 0.34 in the 3 equations for screen efficiency for a few of the air flow velocities reported by Thomas and Hinchliffe (1972). I t is clear that quite different values are obtained depending upon the equation used. Holub and Knutson (1987)indicate that the fan model value for B of 2.7 in Equation 9.8 is preferred even for single screens. This would reduce the screen efficiencies calculated in 'lhble 9.1 by about a factor of 2 for most entries. As discussed in Section 9.3.1, some diffusion batteries also utilize screens as the collection device. From 1 to 60 screens are common for the size analysis of nanometer aerosols.

86 1 9. MEASUREMENT OF UNATI'ACHED RADON DAUGHTERS T ABLE 9.1-Screen efficiency calculated with three different equations andfZow velocities. Note: Calculation is for a screen such as used in Thomas and Hinchliffe, with specificstwns of 32 mesh (openings cm-'), volume fmction of screen c m A = 8 1 8 in euuation 9.8

0.34, wire duuneter= 0.013 Screen Efficiency Linear flow (cm s-1)

< Particle >

20

50

0.98 1.00 0.49

0.68 0.96 0.84

0.44 0.83 0.62

0.0033 0.0033 0.0033

0.32 0.67 0.46

0.21 0.36 0.22

0.15 0.21 0.12

0.0000068 0.0000068 0.0000068

0.006 0.018 0.010

0.001 0.007 0.004

0.001 0.004 0.002

Modela

Diameter (pml

TH CY CKK

0.0009 0.0009 0.0009

0.06 0.06 0.06

TH CY CKK

0.004 0.004 0.004

TH CY CKK

0.1 0.1 0.1

a

=

D lcmz s-1)

TH:Thomas and Hinchliffe (1972) CY Cheng and Yeh (1980) CKK: Cheng et d ,(1980)

5

Equation 9.5 " "

9.8 9.9

9.3.4 Other Samplers

Mercer and Stowe (1969)constructed a novel sampler which has been used to measure unattached daughters. Mercer called it an impactor stage since it was designed as the first stage in a cascade impactor. Figure 9.4 is a schematic diagram of this device. A larger version of this sampler was constructed by the U.S. Department of Energy Environmental Measurements Laboratory and used by George and Hinchliffe (1972)for measurements in 16 uranium mines to determine the uncombined fraction. This sampler was checked for penetration of 0.01 and 0.05 pm particles and 100% penetration was reported. Depending upon the age of the mine air measured, the measurements taken with this sampler may somewhat underestimate the unattached hction. 9.4 Relationship of Unattached Radon Daughters and Condensation Nuclei

Many investigators have demonstrated a relationship between the condensation nuclei (CN) concentration and the unattached fraction

9.4

RADON DAUGHTERS AND CONDENSATION NUCLEI 1 87 Diffusion Sampler for Uncombined Radon Daughters Air Inlet

Lower Disc

.,

Spacer Ring

Air bullet

Fig. 9.4 Impactorstage sampler designed by Mercer and Stowe (1969)(From George and Hinchliffe (1972), reproduced from the journal Health Physics by permission of the Health PhysicsSociety)

(Duggan, and Howell 1969; Raabe, 1969; Mohnen, 1967). Raabe (1969) describes the unattached fraction as a function of time following formation of the daughter species, condensation nuclei concentration and particle size. Experimental measurements made by George et al. (1975) have been compared (NCRP, 1984b)with the theoretical predictions of Mohnen (19671 and Raabe (1969). F i p 9.5 is reproduced here from the NCRP reference to show the nature of the agreement between measured unattached fraction and calculated values. The calculations are in reasonable agreement with measurements so that, if no direct measurement can be performed in a particular case. an estimate may be obtained by measurement of condensation nuclei. For steady state atmospheres (over 100 minutes old with no fresh influxof radon)expressions relating concentration of condensation nuclei to unattached fraction were derived by Raabe (1969) and values for atmospheres containing from 103 to 106 CN were tabulated in NCRP Report No. 78 (NCRP, 198413).Scott (1984)has tabulated the unattached 218Fb, 214Pb,212Pb and 212Po fractions for an extensive variety of attach-

88 1 9. MEASUREMENT OF UNATTACHED RADON DAUGHTERS

Particle Concentration (ern-') Fig.9.5 Vhiztion of unattached fmetion NCRP Report No. 78. NCRP, 1984b)

of

with aemsol concentmhbn (From

2 1 8 ~

ment and ventilation rates in typical environmental and mining atme spheres. A few typical calculationsare summarized in 'Ihble 9.2. In general, the unattached fraction of anPomay be approximated using the expression: x (9.10) X + kz + Ap +, = lb218 &cay constant = 0.22 m i d A, = plateout removal mte 0.2to 0.5 min" k = Z5 x 1Q6(em8min-I)Porstendorfer(1994) z = condensation nuclei concentration(CN em*) Unattmhedfraction =

9.4

RADON DAUGHTERS AND OENSA!l'ION NUCLEI 1

89

TABLE 9.2-Colculaled unattached and attached mdon drucghters for vcuiorcs attachment mtes wul removal mtes in environmentaland mine atmospheres fora mdon concentmtion Attachment

Rateth-1)

b

ofIOa0Bum-3(AfterScott, 1984) Bq m-3 blnOVd Rate (h-') Unattached Attached 218W 2leFob 21aPb ~14Pbb 211Pbb

%Equil.

Concentration of unattached species in Bq m-3 Concentration of attached species in Bq m-3 This corresponds to mining conditions with mechanical ventilation

Notes: 1. Attachment rates based on attachment coefficient of 4.5 x

(Porstendorfer and

Mercer 1980).Attachment rate (h-1)= (k)(CN cm-3). 2. Condensation nuclei (CN)concentration range =

103 to 4

x

3. Indoor surface to volume ratio = 2 m-1. 4. Removal rate for both unattached and attached radon daughters is due to ventilation. deposition and fdtration. I).pical environmental rates are 15 h-1 for u n a W e d and 1 h-1 for attached daughters.

The samplers used for collectjing the unattached fraction require the same radioactivity measurements as those for direct measurement of atmospheric concentration of each radon daughter. Several methods am? available such as those proposed by Thomas (1970,1972).Scott (1981), and Raabe (1969). These usually require a 5- to 10-minute sampling period followed by 3 alpha caunts at various time intervals within about 30 minutes post sampling. Three simultaneous equations are

90 1 9. MEASUREMENT OF UNATTACHED RADON DAUGHTERS

then solved to determine individual daughter species. These methods are described in detail in Section 8.

9.6 Quality Assurance for Unattached Radon Daughter Measurements Field measurements are generally the most difficult to insure against loss of quality. Duplicate measurements are desired, whenever possible as one check for precision. Rigorous calibration of the apparatus prior to field use should be performed to estimate potential problems such as changes in efficiency between test and field atmospheres and retention of attached species in the unattached daughter sampler. See Section 12 for a fuller discussion of standardization, calibration, and quality assurance techniques. Back-up measurements with a condensation nuclei counter are perhaps not as rigorous a test; however, they are relatively simple and lend support, at least, for the unattached fraction of 2'8Po.

9.7 Calculation of the Unattached Fraction There are currently three ways for reporting unattached fraction. It is not always clear which method is being used by various investigators and quite different numerical results are obtained for the same unattached daughter concentration in air. These methods are: 1. Unattached fraction referenced to radon concentration f = Unattached 218Po, 214Pb or Z 1 4(Bq P ~ m-3or pCi 1-Iper Bq m-3 or pCi 1-Iof ZnRn). (9.11) This calculation was utilized primarily by ICRP (1959) until the release of ICRP Publication 32 (ICRP,1981). 2. Unattached fraction referenced to the daughter species. f = Unattached Concentration of 218Po, 214Pb or Z 1 4 (Bq P ~ m-3or pCi 1-I per Bq m-3or pCi 1-Iconcentration) for the daughter species. (9.12) This is frequently used by experimenters when reporting data. 3. Unattached daughters as a fraction of total potential energy.

9.8 SUMMARY 1 91

Where RaA*, RaB*, RaC*

unattached 218Po, 214Pb, 214p10in Bq m-Sor pCi 1-I

= concentration of

RaA, RaB, RaC = total concentration of daughter species Bq m-$or pCi -I1 This form of reporting unattached fraction was introduced by Jacobi (1973) and used in ICRP Publication 32 (ICRP, 1981). I t was an attempt to produce a generalization comparable to the working level. The numerical value of the fraction of potential energy existing as unattached species, $,,is approximately a factor of 3 to 4 smaller than the fractions in the two preceding definitions. There is a current trend to equate the older definitions with f,, which is not correct.

9.8 Summary

The unattached fraction of atmospheric radon daughter activity has been described in this section. The difficulties in measurement arise because the polonium ion changes size, chemical form and charge immediately after formation. It is the diffusion coefficient, as well as air concentrations of each unattached species, that is the quantity of interest because of its application to bronchial deposition and human dose. Few if any of the measurements reported so far are f m of calculational or measurement difficulties. Refined measurements in this area are sorely needed.

10. Measurement of the Potential Alpha Energy Concentration or Equilibrium Equivalent Concentration 10.1 Introduction

This Section, treats methods for measuring the potential alpha energy concentration (PAEC), often measured in units of Working Level. Another quantity of interest is the equilibrium equivalent concentration (EEC)' measured in units of activity per unit volume. For radiation protection purposes, PAEC is usually the quantity of interest. The direct measurement of PAEC can be accomplished with only a single alpha count of a filtered air sample. I t is important to note that some of the methods for individual daughters discussed in Section 7, can be used to calculate PAEC. ?ko types of PAEC measurements are used in mines and other situations of interest: a. Measurements of PAEC a t a given time or place, b. Measurements of integrated exposure either for a person or for an area; these are usually expressed as working level hours (WLH)or working level months (WLM). We shall emphasize (a) first, leaving a discussion of dosimeters to the end. The traditional unit of PAEC, the WL, is defined as "any combination of short-lived radon daughters in one liter of air that will result in the ultimate emission of 1.3 x lo6MeV (2 x 10-Voulem-3)of potential alpha energy" (Rocket a l , 1970,1971). Of the daughters, 2"Pb and ?14Bi are betalgamma emitters. But, even though only the alphas from 218Po I

EEC is the concentration of radon in equilibrium with its short-lived daughters which has the same potential alpha energy as the existing daughter concentration mixture. EEC (pCi 1-I) = 100 (Working Level)or EEC (Bq m-3) = 3,700 (WorkingLevel).

10.2 KUSNETZ METHOD AND MODIFICATIONS I 93

and 214Po decay need be measured, the energy sum considered in the PAEC contains contributions from the 2MPoalphas that arise from decay of the beta-emitting daughters in the sample air. Figure 10.1 (Evans, 1969) shows the growth of PAEC with time in initially pure radon. The use of an air filter is a feature common to many of the methods. The filter collects radon daughters, and is subsequently counted for alpha activity If only alphas are counted, then what matters is the activity of 21SPo and 2'To, the alpha-emitting daughters. Figure 10.2 (Holaday et d.1957) shows the build-up and decay of alpha activity from individually isolated radon daughter isotopes, each with an initial decay rate of 10 disintegrations per minute (0.17 Bq or 4.5 pCi).

10.2 Kusnetz Method and Modifications The most common method for measuring PAEC or "working level" is the Kusnetz method Originally developed in 1956 (Kusnetz, 1956). it has been the mainstay of radon daughter monitoring in uranium mines since then, and was recommended by ANSI as the "standard method" (ANSI, 1973). The basic method and then certain later improvements are described below. and The Kusnetz method employs an air sampler (pump and an alpha counter (usually an io ation chamber or zinc-sulfide scintillation detector). The basis of niz the method is that, by roughly an hour after sampling, counts arise only from *14Poformed on the filter by decay of the previous daughters, and the rate corresponds reasonably well to the potentid alpha energy concentration at the time of sarnpling. Air is sampled at 2 to 20 liters per minute for five minutes. After a delay of from 40 to 90 minutes (most commonly 40), the count rate is measured, using a rate meter. The disintegrations per minute (dpm)per liter of air sampled are determined by correcting for the efficiency of filter collection and alpha detection, and dividing by liters of air sampled. The alpha disintegration rate per liter of air sampled is then divided by a scale factor (150 if the delay time is 40 minutes) to obtain working level. As pointed out in Harley and Pasternack (1969), this scale factor or Kusnetz factor is relatively constant and independent of daughter equilibrium, assuming a delay of about 40 minutes is always used. Scale factors for delay times of 50,60,70, 80, and 90 minutes are 130.1 10,90,75 and 60 respectively The main feature which commends the Kusnetz method is its relative insensitivity to the concentration ratios of the three daughters *18Po, 214Pb and 214Po. For example, if there is a 40-minute delay before count-

94 1 10. MEASUREMENT OF PAEC OR EEC

0

30

60

90

Time in Minutes Fig. 10.1 Contributions o f R a A (*/dPo),RaB PI4Pb) and RaC fl4Bil to thepotential alpha energy concentration as a function of time from initially pure ***Rn (From Evans, 1969, reproduced from the journal Health Physics by permission of the Health Physics Society)

10.2 KUSNETZ METHOD AND MODIFICA!I'IONS 1 95

DECAY TIME IN MINUTES Fig. 10.2 Build-up and decay of alpha activity of R d ff"Pol and RaC' f f 4 h ) from indiuidclaUy isolatedRd, RaB fl4Pb) and RaC Pf'Bi), each hwingan initialdecay mte of 0.17 Bg (IOdisintegmtionsperminute) (From Holaday et aL, 1957)

96 1 10. MEASUREMENT OF PAEC OR EEC

ing and if the 21Wx214Pb:"4Po mncentrations are in the ratios of 100:100:100, 100:90:80, 100:45:35, or 100:15:6, the intrinsic w r in determining WL h m the data is only + 7%, + 8%, and -t- 2 and -77'0, respectively. Rock et al, (1970; 1971), Groer (1972), and Borak et al (1982) have shown that in very "young" air, such as air in which only 2'RPohas had much chance to grow from the parent 2nRn,the Kusnetz method underestimates the true WL by as much as 25%. Even so, since these ermrs are smaller than typical uncertainties in sampling, the Kusnetz method can be said to be intrinsically accurate enough for most purposes. The precision of the Kusnetz method using an alpha counter with a rate meter and a 100 liter filtered air sample was measured in a uranium mine by Breslin et al. (1969)and is shown in Fig. 10.3. The precision is about & 15% at 0.3 WL but degenerates rapidly a t lower levels. This increased e m r is dominated by statistical fluctuations in the rate measurement (Breslin,1972) and, hence, is unavoidable except by using a scaler, as discussed below or by increasing the volume of air sampled. A full discussion of the various sources of error in the Kusnetz method by Loysen (1969) indicates that, with appropriate care, e m r s from

Oo

1

3 Working Level

2

4

5

Fig. 10.3 Precision of measurement forthe potential alpha energy concentration (working leveU by the Kusnetr method (FromBreslin et al,1969)

10.2KUSNETZ METHOD AND MODIFICATIONS 1 97

sampling can be kept smaller than the counting (statistical) fluctuation. The modified Kusnetz method substantially improves sensitivity by using a scaler to count for a fixed period, rather than performing a rate measurement. In a laboratory environment, Breslin (1972)countid for four minutes (from + 38 to + 42 minutes) using an alpha scintillation1 scaler instrument. Reproducibility was found to be + 4%. +1490, and 35% at mean levels of 0.41, 0.0029, and 0.00046 WL,respectively; sensitive enough for almost any application. Although the sensitivities just cited corresponded to a 5-minute sample at 10 1 min-', most measurements are performed using lightweight pumps that sample at 2 1 min-' (Rock et &, 1970; 1971), somewhat degrading sensitivity. Membrane filters are widely used because of their 99 + 90retention of submicron particles and because most particles are deposited at the filter surface, minimizing selfabsorption of alpha particles (ANSI. 1973). Glass fiber filters,equally efficient as collectors, can suffer from more penetration and hence more self-absorption. Moisture on the filter face can cause absorption pmblems for any of the filters as well as pressure problems in pumping (Rock et d ,1970; 1971). Air pump flow meters may cause problems because their calibration is densitydependent and therefore sensitive to pressure differences across filter media and to the different altitudes at which measurements are made. The details of this problem are discussed in the Bureau of Mines Handbook (Rock et aL,1970,1971), which also discusses calibration procedures. An advantage of the Kusnetz method is that the alphadetection system, which must respond only to 7.69 MeV alphas, need not have an energy independent response. Perhaps the biggest drawback of the Kusnetz method is the minimum 45-minute delay from start to finish. This inherent difficulty has stimulated the development of other techniques for measurement of WL. A method developed by Rolle (1969) and subsequently described further (Rolle, 1972),makes possible much more refined measurements, using equipment identical to that of the Kusnetz method. Rolle describes how the choice of counting time affects systematic error, and indicates that intrinsic e m r s can be kept below about -+ 12% with counting for 10 minutes after about a 5-minute wait. Roue also discusses in detail the way volumetric and radiometric errors limit ultimate uncertainties to the t 20% range. The expression for a combination of sampling, measuring and decay times suggested by Roue is:

98 1 10. MEASUREMENT OF PAEC OR EEC

WL

=

(samplecount - background count) (212.7) (volumesampled)(measurementperiod)(detectionefficiency)

where: Volume sampled is in liters, Measurement period is 5 minutes for a 10-minute sample or 10 minutes for a 5-minute sample, The waiting or decay time from the end of sampling to the beginning of the count interval is 4.35 minutes, Detection efficiency is the count rateldisintegration rate of a standardized source. A single count regime using long count intervals was also described by Nazaroff (1980) to better estimate the WL at low environmental concentrations. The Kusnetz method was extended to a two-count regime following sampling so that both the 222Rnand 220RnWL could be measured (Strong and Duggan, 1973; Ogden, 1974; 1977; Cote and Bwnsend, 1981). A rapid, simple field method for both radon isotopes using three count intervals has been reported ( n t h ,1984).

10.3 Rapid Measurements and Alpha Spectrometry The methods just described all require a considerable time delay between the start of sampling and the end of counting: the Rolle method takes about 20 minutes, the Kusnetz upwards of 45 minutes. This has motivated the development of several instruments with more complex measurement schemes that take less time. These instruments typically use a surface barrier detector for alpha detection, often distinguishing between alphas from ""Po and "To (see discussion of alpha spectrometry later in this Section). The more complex devices have an additional detector for measuring the beta emitting daughters. An early instrument of the latter type (Groer et al, 1973a) has been referred to as an"Instant Working Level Meter" (IWLM),but actually measures individual daughter concentrations, from which the PAEC is calculated in WL. Such instruments are described here, although background on the principles involved is presented more fully in the discussion of individual daughter measurements. Schiager began development, at Colorado State University, of a working level meter employing a surface barrier detector to measure ""Po and "To separately during a single very short counting period (Schiager, 1970). A oneliter sample is collected on a filter paper tape using a manual pump (the tape is advanced for each measurement).The

10.3 RAPID MEASUREMENTS AND ALPHA SPECTROMETRY 1 99

potential alpha energy concentration (in WL)is calculated (electronically) from the 2'8Pocount plus about ten times the 2'To count total. Figure 10.4 shows how such a p d u r e yields results within 10-1590 of the actual PAEC over most of the range from equilibrium to complete disequilibrium. However, the largest uncertainty, for low daughter concentrations, arises from poor counting statistics. For the completed prototype, the standard deviation of measurements taken with a 17second counting period (and a total measurement period of about 2 minutes) was 2990 at 0.6 WL and 7190 at 0.25 WL. Schiager, (1977) estimates that increasing the air sample size to 3 liters, the counting period to 100 s, and the detector size would improve the sensitivity by about a factor of 20, making it a useful survey instrument for measw ments in mines and mills; the full measurement would take 3-4 minutes. Borak and Holub (1984)report that for some counting protocols, the alpha spectrometric method does not yield better precision than the conventional Kusnetz or Rolle methods for determining Working Level. James and Strong (1973)use a surface barrier detector to count gross alpha activity on a filter during the collection period and during an additional counting period, of equal length, that immediately follows the collection period. 'Ib a rough approximation, the first count gives 600 USEFUL RANGE

)

RaA + 10 RaC'

1 &

-

400 -

-2 .-

RaA + 8 RaC'

3

3

8 200 -

...

3

0

w

t

w a

m

0

3

0W

0

6

-

5 .-L 9 .-

U)

a

Z

200 400 600 800 EQUlLlBRlLlM INDEX - pCi RaA per WL - liter

w

El

1000

Fig. 10.4 Theoretical alpha spectrometer response (counts per minute WL-'I as a function of degree of *ZZRn d w g h t e r pruduct equilibrium (pCi RaA P'nPo]l-lWL-I)(From Schiager, 1970)

100 1 10. MEASUREMENT OF PAEC OR EEC

the 218Poconcentration, and the second - for a range of equilibrium conditions - gives the daughter concentration in WL. For count periods of 5 minutes, a total measurement time of 11 minutes, and a 10 1 min-' sampling rate, the statistical uncertainty is given as 1% a t 1 WL, which compares favorably with more complex methods. Cliff et a! (1979)have reoptimized this approach, ending the counting collection period at 2.5 minutes, and beginning the second count period (also 2.5 minutes) 2 minutes afterwards. Results of tests with a 50 1 min-' sampling rate indicate statistical uncertainties of about 20% at 0.005 WL. Counting during sampling has the'advantage over the Kusnetz method in that information on concentration of the short-lived Z18Po is not lost before counting begins. Instruments that measure total alphas for three counting periods or and 21Toalphas for two periods are treated in the that measure ZIBPo discussion of individual daughter measurements (Section 8). Such measurements can be used to obtain the potential alpha energy concentration with no assumptions about the state of equilibrium between 2'8Po, and 2"Pb, and ""i. (Because of its very short half-life, 21T'ois always assumed to be in equilibrium with its precursor, 214Bi.) The instrument class called the "Instant Working Level Meter" based on the work of Groer et al. (1973a)uses three count channels, two alpha (from a surface barrier detector) and one beta (from a plastic scintillator). Groer et aL, (1973b)improved this device by using a 12 1 min-' sampling rate and a very thin (0.08 mm.) beta detector. After a one-minute background sampling period, the filter paper tape is advanced to position the collection area between the detectors; a t w e minute counting period then begins. Radon daughter concentrations and potential alpha energy concentration (in WL) are calculated electronically and are available on demand. The useful range of the instrument is given as 0.01 - 100 WL, with intercomparison tests indicated 10-20% discrepancies in the range 0.3 -1 WL (Groer et al., 1973b).This instrument has been developed further for the EPA to an "environmental working level monitor" with 3-minute background, collection, and counting periods, and a sampling rate of 30-40 1 min-' (Keefe et d , 1978). A commercial version is now available. Shreve et aL (1977)have developed a different type of "instant working level meter", with (total) alpha and beta detectors. For a range of realistic equilibrium conditions, the alpha and beta count totals (for sampling, delay, and counting times of about two minutes each) give a good indication of the PAEC, both theoretically and according to comparison tests.

10.4 INTEGRATING MONITORS 1 101

The solid state silicon surface barrier detector has been built into several microcomputerbased working level monitors. One such unit measures WL for both radon and thoron daughters as a function of time using a constant-flow pump with alpha counting data stored in computer memory. The unit is small enough to be used either as an area 1985). monitor or a personal dosimeter in mines (Rozeet d,

10.4 Integrating Monitors Integrating working level monitors have been developed, using thermoluminescent dosimeter chips to accumulate the signal from the alpha decay of material collected on air filter. A "Radon Progeny Integrating Sampling Unit" (RPISU) based on this principle and sampling 2-3 1 min-' was developed by Schiager (1971,1974),and Franz et d ,(1977),for the State of Colorado and (ultimately) the EPA. Modified versions of the initial instrument have been employed for indoor studies in Florida's phosphate regions (Guimond et d.1979),and in Cole rado's uranium milling regions (Schiager,1971, 1974). An instrument that is similar in principle has been developed by the DOE Environmental Measurements Laboratory (Guggenheim et d,1979), and is capable of measuring average concentrations as low as 0.0005 WL for week-long sampling periods. Small portable or fixed devices that inte grate radon daughter levels are treated in the next section. Latner (1981)developed a portable integrating working level monitor utilizing a silicon surfacebarrier detector, constant-flow pump and direct readout of the integrated alpha count. The lower limit of detection for a one hour count is 0.0004 working level. Abu-Jarad and Fremlin (1981)designed an integrating working level monitor utilizing a high flow rate (65 1 min-') and nuclear track detection with LR115 plastic of the alpha particles emitted from radon daughters collected on a glass fiber filter. Working level may also be estimated by measurement of the daughter beta activity. A beta channel is included in the instant working-level monitors described above, but simple beta counting has also been used in a continuous working-level monitor. Droullard and Holub, (1977) developed a continuous monitor for mines which measures the back of the air filter with a pancaketype Geiger-Muller tube. The error in estimating the working level by beta counting only is reported to be +8% for high concentrations. This unit has been automated and is now utilized as an area monitor. A microcomputerbased data acquisition system allows real time analy-

102 1 10. MEASUREMENT OF PAEC OR EEC

sis of WL and other atmosphere characteristics such as pressure and relative humidity (Briggset d ,1985).

10.5 Personal Dosimeters

The usefulness of dosimeters for workers is obvious: a portable instrument which could accurately integrate WL exposure over time would help to provide for the radiological monitoring of occupationally exposed individuals (e.g., miners) by providing direct recording of individual exposures. Present dosimetry is done largely by measuring daughter potential alpha energy concentrations in mine areas and correlating this information with the amount of time spent in each of the various working locations by a worker. This method has served a valuable purpose over a long period and has certain advantages over dosimeters, among which is that measurements are made by trained personnel with less inconvenience for the miner. Such daughter measurements will always be required to supplement any personnel dosimetry system that achieves wide acceptance. Desirable features for a personal dosimeter for occupational use are: that it be sensitive down to an integrated exposure of about 1 WL-h; that it be capable of weekly or bi-weekly readout; that it properly sample the air being breathed; and that it be light, rugged and fail-safe. A number of efforts in a several-year period around 1970 were devoted to development of personal dosimeters employing film, solidstate nuclear track detectors (SSNTD) or TLD alpha detectors for recording radon or radon daughter exposures in active or passive systems. Breslin (1972)summarized results of several evaluations of these devices performed at the EML by White (1969, 1970,1971). Eberline's alpha track-count film dosimeter (Geiger, 1967) and the New York University ZnSIfilm system (Costa-Ribeiroet d ,1969),similar in principle to the Polaroid monitoring technique of Bedrosian (1969),were designed for radon dosimetry. Laboratory standardization tests (White,1969) of one dosimeter gave responses varying by up to a factor of 10 in repeated runs; responses of the other varied by up to a factor of 3 and increased by a factor of only 3 when the concentration increased by a factor of 10. Of the six daughter dosimeters, five gave substantially less than satisfactory performance in the EML tests, and one proved somewhat better (White, 1969, 1970, 1971). The daughter dosimeters performed with varying degrees of success in the laboratory, but the harsh condi-

10.5 PERSONAL DOSIMETERS 1 103

tions in mines (mechanicalabuse, high humidity, mine dust and mud, corrosion, pump failms) uniformly caused difficulty. For the units from Oak Ridge National Laboratory (Auxier et d ,1971),Massachusetts Institute of Bchnology (Evans and Kolenkow, 1968), Colorado State University (McCurdyet aL, 1969),and General Electric (Lovett, 1969; Rock et d , 1969),reproducibility and mine comparison results were usually poor. The sixth dosimeter, the "MOD" unit from EML, profited from previous efforts, particularly the TLD units (Evans and Kolenkow, 1968; McCurdy et d ,1969),but was still not fully satisfactory (White, 1971),largely because of pump failures and TLD response to external gamma and beta radiation. Subsequent attention to these problems has produced a unit that appears to be dependable in mine conditions and with a range extending as low as 0.2 WLh (Breslin et d ,1977). The same sampling head (including a filter and two TLDs, one a control shielded form the filter) and pump have been incorporated in the area-integrating daughter monitor described above (Guggenheim et d ,1979). The 1 kg weight of the pump, which is worn on the belt, is an impediment to acceptance by miners. A concurrent effort by the Bureau of Mines has developed a personal dosimeter employing a surface barrier semiconductor detector in the sampling head (Durkin, 1979; Drodard, 1982). Alpha-induced pulses are counted in the unit's circuitry for subsequent readout. The unit shows good agreement in recent comparison tests for exposures of the order of 1 WLh. Gammage et d (1976)have suggested use of ceramic Be0 dosime ters that are read for both thermoluminescence and thermally stimulated electron emission (TSEE). Because of differing thermoluminescence and electron-emission response to alpha and gamma radiation, a single detector could be used to measure alpha emitters; however, the Be0 detectors are delicate. The use of cellulose nitrate plastic for integrated nuclear track radon daughter monitoring has been investigated further by Frank et d (1977) at the University of San Francisco. Results indicate that an active dosimeter, in which airborne daughters are collected on a filter facing a plastic detector, gives useful information-under optimum conditions-for an exposure extending below 1 WLh. On the other hand, a passive dosimeter and a passive area monitor, each using two detectors (one protected from radon daughters by a filter), were found to have low sensitivity and poor reproducibility, respectively. The personal alpha dosimeter developed by The Commissariat a 1'Energie Atomique (CEA) in France utilizes cellulose nitrate LR115 nuclear track detectors and has been through several generations of

104 1 10. MEASUREMENT OF PAEC OR EEC

devices. It is a self-contained unit weighing 400 grams, with a plastic casing housing an air mover (0.11 min-I),hygrophobic filter, collimator, absorber, nuclear track detector and a gamma-ray TL detector (Yoshida, 1985). Three collimator plus absorber channels separate the LR 115 and the collection filter to allow alpha particles from 2'8Poand 212 ~ , "T i O and 2'2Poto be identified uniquely. Thus, the WL from both '*'Rn and "ORn daughters can be m e a d . All underground miners in France have utilized this personal dosimeter since 1982 (NEAIOECD, 1985).

10.6 Summary The WL may be calculated from measurements of the individual short-lived 222Rn or '?ORn daughter product concentrations as described in Section 8. However, Kusnetz showed in 1956 that a single alpha count of the "To of a 5-minute filtered air sample at 40 minutes post sampling will yield a value within 10% of the "true" WL over a very wide range of daughter equilibria Variations of the Kusnetz method, such as the Rolle method yield only marginally better error but do d o w samples to be counted at times closer to end of sampling, which is of practical value. Although developed for measurements in mines, a low background (a few count per hour) alpha counter provides measurements which are useful in rapid estimation of indoor levels. Other methods, involving two alpha counts of short-term filtered air samples d o w "'Rn and "ORn WLs to be determined. Integrating monitors utilizing filtered air samples with measurement by nuclear track detectors, thermoluminiscent dosimeters or solid state detectors permit the average WL over time periods of a month to be measured.

11. Radon Flux Measurements 11.1 Introduction Radon flux,or exhalation, is the transfer of radon across the interface between a solid phase and the atmosphere. The solid may be soil, building materials, or other substances while the atmosphere may be that of a closed laboratory vessel, a building, or the outdoor air. This process has been described in Section 6. Exhalation is the source of the radon that is of interest in this Report and a measurement of the flux (in Bq m-2s-' or aCi ~ ms-') - provides ~ information on source strength. In soil, exhalation is a twestage process with the radon being emanated from the solid grains into the pore space between the grains or into the water surrounding the grains. The radon is then transported into the atmosphere by concentration or pressure driven forces. Under laboratory conditions, these p m s e s can be modelled successfully, e.g.. Jonassen (1983); Samuelsson and Petterson, (1984), but, in the field, the effect of interference by the measuring system itself or of other factors such as soil moisture or porosity has limited the utility of flux measurements. Such problems were noted as early as 1972 by Wilkening et aL, (1972) and have not been completely clarified. Field flux measurements are still useful, however, for comparing different soil m a s or different building m a s (mppan, 1985). The detailed descriptions of the various methods am given in the original literature, so this Section will memly describe the general approaches to flux measurement and note some of the differences in procedures. The average exhalation rates from soil have been estimated by WIlkening et al. (1972)as 16 mBq m-2s-I (42 aCi cm-9-') and by Birot (1971) as 18 mBq m-2s-I (50 aCi m-Ps-I). Building materials are quite variable but Folkerts et al. (1984)and Poffijn et d , (1984)indicate that the exhalation rates are mostly lower than for soil.

11.2 Laboratory Methods

Methods for laboratory measurements on soils and building materials are generally based on placing a sample of the material in a closed chamber and sampling the air of the chamber. M i c a l proce dures have been described by Folkerts et al. (1984), Ingersoll et aL, (1983),and Stranden (1979).Samuelsson (1987)has published a critique of the method.

11.3 Field Methods

Exhalation measuring systems were classified by the National Bureau of Standards (Colle et d ,1981) into six groups, of which the following five are of interest here. a. Accumulation Method. b. Flow Method. c. Adsorption Method. d. Vertical Profile Method. e. Soil Concentration Gradient Method. A typical accumulator is shown in Figure 11.1, while the accumulator, flow, adsorption and vertical profile methods are shown schematically in Figure 11.2 11.3.1 Accumulation Method

This was the earliest procedure and consists, basically, of s e w the open end of a vessel to the surface being measured. The radon concentration inside the vessel is then measured, either at some selected time or serially over a period of several hours. The vessels have ranged from flat cylinders to 55-gallon drums and the sampling arrangements have been equally variable (WiUreningand Hand, 1960; Kraner et d ,1964). Ackers (1984)has described a system which includes an alpha scintillation counter to provide measurements every half hour. Wilkening et aL, (1972)listed certain requirements for the accumulation method, namely that the accumulation time be short compared with the 3.82 d half-life of 222Rn, that the concentration in the vessel be lower (perhaps 10% or less) than that in the soil gas and that the measuring device does not significantly affect the radon exhalation. This latter criterion is difficult to meet.

11.3 FIELD METHODS 1 107

HAND EQUALIZING

Fig. 11.1 Accumulator formeasurement ofradon f l u (FromWilkeninget aL,1972)

VERTICAL PROFILE

ACCUMULATOR

FLOW CHARCOAL ABSORPTION

E.LN A At

N

'A

I

E = IZni A zi /A

= ANIA

Fig. 11.2 Methods used in measuring radon /Iwc E is the f2u, A is the area c o v e d by the ckvice or system, t is the accumu2atbn time, N is the qwmtity of radon and A is the decay constant for radon.

108 1 11. RADON FLUX MEASUREMENTS

11.3.2 Flow Method In the original flow method, the air in the accumulation vessel was continually circulated (at a low flow rate) through a charcoal trap which collected the radon. This system has the advantage that the concentration merit is not changed by build-up of radon in the accumulator. The general system has been described by Pearson and Jones, (1965) and its application by Pearson, (1967). The authors indicated that the air flow should resemble natural conditions and developed a long, narrow accumulator with air inlet and outlet a t the ends. Israel et aL (1968) substituted direct measurement of the radon concentration in the flow stream with an ion chamber for the trap plus counter, while Styro et aL (1970)used a similar system for "ORn measurement. Schery and Gaeddert (1982) have also developed a flowthrough accumulator which employs continuous monitoring of radon in the outlet airstream. Another variation of the flow method has been developed by Watnick et d,(1984). They used a hemispherical accumulator placed on the ground from which air was drawn, at a rate of 0.2 1 min-', through filters to remove moisture and radon daughters before being admitted to a counting chamber. Ions formed from decay of the 2"Rn were collected electrostatically on a semiconductor detector where "8Po and 2"Po activities were counted. While this system can operate continuously with good reliability, it does apply a negative pressure to the area being sampled, possibly disturbing the system.

11.3.3 Adsorption Method Rutherford (1900~) mentioned that the adsorption of radon on charcoal was a possible method for measuring the exhalation of radon from soil. The technique was first applied by Megumi and Marnuro (1972).In their procedure, the charcoal was essentially laid on the ground within a covered frame, using a gauze base to hold the charcoal and allow its recovery. After exposure, the charcoal was transferred to a counting container and the gamma radiation from the 214Biradon daughter measured. Estimates of 2mRn(thoron)exhalation could also be made by thoron daughter. gamma counting the 208T1 A simpler technique was devised by Countess, (1976, 1977% 1977b) who proposed using a charcoal gas mask canister (U.S.Army M11) for the accumulator. Exposure was generally for one day and the entire canister was measured in a gamma spectrometer. Countess used a NaI crystal spectrometer to measure gamma radiation from both the "'Pb and "'"Biradon daughters.

11.3 FIELD METHODS 1 109

The canister procedure is simple, and the sampler can be readily sealed to vertical surfaces with modeling clay or flexible caulking material. On the other hand, it requires calibration against more conventional accumulators and the degree to which it perturbs the system being measured is not known. In addition, the small sampling area of the canister means that its sensitivity is lower than the preceding methods. Countess (1977a)reported a lower limit of detection of 2.5 Bq (70 pCi) which amounts to a flux of about 4 rnBq m-2s-' (0.1 pCi m-2s-') for a oneday exposure.

11.3.4 Vertical Profile Method This procedure is only applicable to large areas and rests on a number of assumed conditions that may not always be met. The basis is that the total amount of radon in a vertical air column of fixed area represents the radon that has been exhaled by the same area of soil. This procedure has been used in the Soviet Union by Malakhov et al. (1966)and Kirichenko (1970)and in the U.S. by Wilkening et al. (1972). The procedure requires aircraft or balloons for sampling and is thus not generally applicable.

11.3.5 Soil Concentration Gmdient Method The transport of radon from soil to the atmosphere is driven by concentration and pressure gradients. If measurements can be made under stable pressure conditions and when rain and wind do not interfere, it is possible to establish the soil concentration gradient and to estimate the corresponding flux. As Clements and Wilkening (1974) showed, this requires an independent measurement of the diffusion constant of radon in soil as well as determinations of the soil radon concentration, as the soil-air interface is approached. The method has been applied by Senko (1968) and by Mochizuki .and Sekikawa (1980) but is aver-elaborate for most measurement requirements. Schery et al. (1984)used concentrations of "OPb in the surface layers of soil as an indication of the timeaveraged gradient of radon concentration (210Pb is the long-lived member of the 222Rn series).Their results for flux rate agree with other measurements made with closed and flowtype accumulators. Another method that uses solid state nuclear track detector (SSNTD) techniques in determining soil radon gradients has been described by Fleischer (1980).Passive SSNTD monitors were placed in

cups a t different depths and radon concentrations were inferred from track densities. The problem involved in applying Fickian diffusion only to radon transport must be resolved in this case. The use of alphasensitive materials and of SSNTD detectors is being investigated in other laboratories. The diffusion barrier charcoal adsorption collectors described by Cohen and Nason (1986) have potential application in soil gradient studies also.

11.4 Summary A variety of techniques are available for measuring radon flux in the laboratory and in the field. The problem remains that the field techniques disturb the system being measured. An ideal measurement would not disturb the surface being measured nor would it interfere with meteorological effects such as those due to rain or wind. Much needs to be done before the real spatial and temporal variations in radon flux can be understood in detail

12. Calibration, Standardization and Quality Assurance 12.1 Introduction The production of valid data for measurements of environmental concentration of radon and its short-lived daughters requires considerable care. The usual difficulty in such efforts is compounded by the gaseous nature of the parent and the rapid decay of the daughters. This Section will address two facets of the problem - calibration or standardization and quality assurance. Calibration, sometimes referred to as standardization, specifies the determination of the factor relating the instrument response to the absolute amount of radioactivity present. Calibration will be used in this section to describe the determination of instrument response with a laboratory standard derived from a "%a solution certified by the National Bureau of Standards (NBS).Standardization will refer to the determination of instrument response with worlung standards. There is no system for absolute counting-of either radon or its daughters, so calibration or standardization must be in terms of a mgniz,edstandard. Standardization or calibration is usually carried out at levels of radioactivity that are sufficiently high that counting statistics and counter background do not introduce measurable errors. A satisfactory standardization or calibration of the instrument involved is no proof that valid measurements can be made a t environmental levels. This requires a continuing quality assurance (QA) program, and such a program will be outlined here. The measurement of environmental concentrations of radon and its short-lived daughters for health protection purposes seldom requires accuracy or precision better than 20 percent. Many measurements, particularly in early monitoring of mines, have had only genego requirements to see if some action was required and accuracy and precision were of little interest, provided the results fell on the right side of the dividing line. These latter data are always disappointing, since a little more effort would have produced data that are useful for estimating the distribution of population exposures and other quantities. The second use of measurements of environmental concentrations is in scientific studies of ventilation, radon transport and daughter product transport, and removal mechanisms. These studies may require five

112 1 12.CALIBRATION,STANDARDIZATION AND QA

percent accuracy but seldom better than that. Thus the accuracy of a NBS radium solution is not required, but it would not be cost effective to de-;clop parallel standards.

12.2 Calibration and Laboratory Standards The term calibration is used in two senses. The most common is the determination of a factor for converting the response of an instrument into a quantity of radioactivity. The second sense is the determination that a measured quantity of radioactivity is consistent with national standards laboratories which, in turn, should be consistent with inter national standards. In the particular case being considered in this report, the only certified standards are the "'%a standards issued by the National Bureau of Standards. These are solutions of "6Ra in weak acid with added carrier and are available in several concentrations. Strictly speaking, they are only standards for measuring gamma activity in the same configuration, since any vial that is opened for other use is no longer an NBS standard. In practice, it is considered that a vial can be opened and transferred to another vessel while maintaining its quality, even though it loses the NBS imprimatur. Careful handling plus checking of the empty vial and glassware for residual radium activity can produce valid laboratory standards. The radon required for most calibrations or standardizations is readily released from aqueous solutions by bubbling nitmgen or other gas through the solution. Gas bubblers, such as the one shown in Figure 12.1, allow transfer with a relatively few bubbler volumes of gas and the radon can be collected in a suitable scintillation cell or ionization chamber. Completeness of transfer is readily checked by continued bubbling to fill a second cell or chamber and correcting any radioactivity found for radon that builds up after completion of the initial transfer. The overall accuracy of calibration, including preparing aliquots from the NBS vial, gas transfer, and alpha counting can be held to five percent or better. As of 1988, there are no primary standards for the short-lived daughter products of radon.

12.3 Standardization and Working Standards The vials of 226Ra solution prepared by the National Bureau of Standards are not in the form or concentration required for standardizing the various systems used for counting radon and its short-lived daugh-

12.3 STANDARDIZATION AND WORKING W

S 1 113

-

Radon Bubblers Py= 1. Main chamber about 15 cm. long, base to neck, with capacity about 50 ml. 2. Top of chamber to have 19/38 standard taper joint. 3. Stopcocks 2 rnm bore with tapered Teflon plug - Side arms 7 mrn 0 . D

4. All side tubing to be 7 mm (not as shown).

5. Glass rod structural support between main chamber and side tube, 1.5 cm in length. 0.4 cm diameter.

6. Medium porosity glass frit in bottom of chamber. 7. Glass tubing from stopcocks 2.5

cm long.

Fig. 12.1 Gas bubbler for transfer ofmRn from a B R u solution to a counting system (From Harley, 1972)

114 1 12.CALIBRATION,STANDARDIZATION AND QA

ters. As mentioned, once the vials are opened, they are no longer certified standards and the necessary dilutions and transfers remove them further from the certified class. Standardization is thus based on working standards that are usually referred to as being traceable to the National Bureau of Standards. This latter term is poorly defined but should mean that the NBS standard was the starting point in a meticulous chain of production for the workmg standards. Standardization of a radon counter involves transfer of a known amount of the gas to the counting system. There are two approaches to generating the known amount of gas, the first is based on allowing the radon to come into equilibrium with a standard radium source in a container of known volume, the second is based on the known production rate of radon from a standard radium source. Both of these depend on the complete release of radon fmm the source, which is a simple process for radium in solution but more difficult for radium as a solid source. The known volume technique can be approached with a solid radium source having only a few micrograms of carrier in the container and taking care that the source is physically stable. A more certain approach is to have a radium solution in a bubbler and to recirculate the air in the container through the bubbler with a small pump. Both of these require care to avoid construction materials that absorb or adsorb radon. Air samples that are a small fraction of the container volume can be withdrawn and considered as working standards. The flow technique passes a known, stable flow rate of air through the bubbler or over a solid source, and the concentration of radon in the exit gas is readily calculated. Since the accuracy of the standard depends directly on the accuracy of the flow rate, the system again yields a working standard. Minor additions to the system include humidification of the entering gas and, often, drying of the exit gas. The method is limited to standardization of counters that allow flow through the system. Variations include the standardization of large-volume ion chambers where the equilibrium amount of radon in a bubbler can be completely transferred to the chamber and systems where the equilibrium amount of radon or the radon from a known flow rate and time is collected in a charcoal trap and then transferred to the counter with a small volume of carrier gas. Only measurements of 22%ncan be considered as based on a trace able standard and all low-level measurements of the daughter products are based on other standards. Higher activity levels of the daughters that can be gamma counted could conceivably be based on an NBS radium standard in equilibrium, but the need for this seldom arises.

12.4 COMMERCIAL WORKING STANDARDS 1 115

The "'8Poto 214Po chain has a n effective half-life of about 30 minutes, so they cannot be standards themselves. Additionally, this time constraint means that they must be measured directly without any chemical processing. Most radon daughter measurements involve alpha counting, whether with electronic counters, photographic film, solid state nuclear track detectors, or thennoluminescent dosimeters. There are a number of long-lived alpha emitting radionuclides that might be used for working and 21"o. The standards, but none with energies close to those of 218Po other problem arises in getting the sample and standard into the same configuration for counting. The solid standards, usually transuranics, are best when electroplated onto a metal backing. The radon daughter samples are usually collected on filters with varying d e p s of penetration and consequent energy or particle loss. Impactors or electrostatic collectors can lay down the sample on a metal backing so they can approach the proper configuration. Dust loading can, of course, reduce counting efficiency for any sample. In the absence of anything better, common practice is to accept these errors or to make a rough correction for loss of counting efficiency in the sample.

12.4 Commercial Working Standards Pylon Electronic Development Co., Ltd. markets a series of radon and radon daughter standardization systems based on a proprietary dry powder radium source that is claimed to release 100% of the radon. They are calibrated against NBS standards to within four percent. These are the only commercially available radon standardization devices. The various forms are: 1. A source contained in a volume that appears to be about fiveliters and which allows transfer of a 111000 fraction to a Lucas cell or other counter. 2. A sealed Lucas cell with a self-contained source. 3. A flow-through system designed to feed a chamber with radon. 4. A passive source for placement in a chamber.

The stock items all have very high radon release levels and seem intended to standardize at the level of several hundred picocuries. This allows short counting periods but does not provide adequate standardization for measurements at environmental concentrations. Pylon also offers a radon daughter calibrator which consists of a small chamber with a radium source and a holder for 25 mm or 47 mrn

116 1 12. CALIBRATION. STANDARDIZATION AM) QA

filters. The daughters deposited on the filter are described by the company as a standard for counting.

12.5 Quality Assurance A quality assurance program can not only furnish reassurance to the laboratory but can supply data that can be used externally to show that the overall performance in measurements is satisfactory. This is becoming more critical with the general increase of litigation in the United States on health questions, and the possible effects from radon and its daughters are going to be a major subject of contention. This Section will first describe what is desired in an ideal QA program and then attempt to show how the principles can be applied to measurement of radon and its daughters-a far from ideal case. There are three components in our ideal system - standards, duplicates, and blanks. Standards for QA are different than calibration standards since they should be in the same form as the actual samples being analyzed. The results on standard samples are a measure of the accuracy of the system. Duplicates, as always, are a measure of the precision of the system. Depending on the amount of radioactivity in the samples, blanks may be a critical feature of the program, may be required once in a while, or may be dispensed with altogether. The difficult requirement for quality assurance is that the QA samples should be submitted to the analyst blind. If they are not, they will obviously receive best effort analysis and wiU not test the day-today performance of the laboratory. The arrangements for such a program require a tactful approach. Experience has shown, however, that blind samples are accepted willingly by a competent analyst and that good results h m the program are a considerablesource of pride. A second general requirement is that the results from the QA p r e gram are used as a basis for action. The analyst should see the results, and any necessary changes in procedure or care in analysis should be discussed as soon as possible. The need for action can become apparent earlier if a quality control chart of the QA data is maintained. It is obvious that action requires that suitable guidelines for precision and accuracy be set up in advance. This is not an easy process, since it involves setting goals for the overall measurement program. A third requirement is that the QA results be published, preferably along with the data sets to which they apply. In this way, whoever intends to use the data is constantly aware of their value. A corollary is that data associated with bad QA results should not be published. This is a painful step, but less so than knowing that dubious data are being published.

12.6 CALIBRATION CHAMBERS 1 117

A fourth requirement is that the QA program be a continuing one. There is some value to the occasional QA measurement, but it is weak support for a program involving a steady flow of sample data. A good starting point is to include about 10% of the analytical effort as QA samples. This may be adjusted with experience but effort less than 5% is almost certain to be inadequate. The final requirement is that the data to be published be reviewed by a competent scientist. This helps to remove impossible results and leads to consideration of results that seem extreme but may be correct. Such a review is simplest for data sets that should be uniform with time or location, but may not be helpful for a small number of unusual analyses. Such a group of samples, in fact, may not be amenable to QA at all unless a disproportionate effort is expended. Quality assurance standards for radon at environmental concentrations are generally not available, so reliance is placed on duplicate measurements. This introduces the variability of the sampling system into the measurement but this is not necessarily a disadvantage. Also, the variability for radon gas should not be large. If the results on high level standards and duplicates are satisfactory, occasional intercomparisons with competent laboratories should give sufficient reassurance. The operation and results of one such intercomparison program has been described (Fisenneet al.,1986). For short-lived radon daughters, the situation is less satisfactory, since sampling variability can be large. In addition, intercomparisons can only be run by simultaneous sampling and rapid measurement, which i n t d u c e s logistic problems.

12.6 Calibration Chambers The most practical approach to standardization, in many cases, has been to set up a room or a smaller chamber and to attempt to maintain a fixed radon concentration. This can be done regardless of minor leakage and losses by absorption since the atmosphere is only a secondary standard on the basis of measurements. Different groups have produced chambers that differed with respect to size, construction materials and complexity of controls but there is no consensus on the best approach. I t is easier to maintain a constant atmosphere in a room-sized chamber, particularly when sampling, but other factors are a matter of preference Figure 12.2 shows the U.S. Department of Energy "2Rn calibration room which has been maintained for many years.

118 1 12.CALIBRATION,STANDARDIZATION AND QA

When using a chamber as a standard source of daughter products, the problems mutiply. Radon daughters are readily lost to surfaces, with the rate depending on ambient particle size and concentration and on air movement within the chamber. In spite of efforts to control such factors, artificial radon daughter atmospheres are only of value for direct, simultaneous comparisons of measuring systems. The Department of Energy chamber has been described by Fisenne et al., (1983).The Environmental Protection Agency has set up chambers at its Eastern Environmental Facility in Montgomery, Alabama, which are being used chiefly for research and for proficiency testing of radon measurement vendors. The usual radon chambers have been designed for other purposes than standardization and quality assurance. They normally have only a limited range of radon concentrations available, and these are too high for serious QA work. They are invaluable, however, for radon intercomparisons a t high concentrations. Radon daughter intercomparisons are limited by the volume of the chamber and simultaneous sampling by the participants in more realistic environments is to be preferred.

Fig. 12.2 Hadon calibration chamber (19 m3) maintained by the U S . Department of Energy Environmental Measurements Laboratory (courtesy U S . Department of Energy Enuironmental Measurements Laboratory)

13. Strategy for Measurement of Radon and Radon Daughters 13.1 Introduction The purpose of the survey or measurement usually determines a particular strategy. The primary purposes for measurement of radon and radon daughter concentrations in various situations so far are: 1. Evaluation of human population exposure in terms of lung cancer

risk for either environmental or occupational settings. 2. Identification of geographic areas with high indoor radon levels and subsequent high individual risk. 3. Scientific studies to determine fundamental properties and mechanisms of radon or radon daughter behavior. 4. Diagnostic measurements when carrying out remedial action. This Section will indicate the methods that are applicable to these specific purposes. In underground mines, frequent grab sampling, with weighting by occupancy time is used extensively for estimating annual exposure. Makepeace (1985)and Borak (1986) have proposed a randomization procedure for use in mines which allows better estimation of annual exposure for miners from grab samples. Occupational exposure is considered in other NCRP documents and the required measurements are carried out by the company safety group and by federal agencies such as the Mine Safety and Health Administration (MSHA). For this reason, although the procedures detailed in ths report am general and may be used in any situation, this section stresses environmental exposures, particularly indoors, since these deliver the major human population exposure. The risk h m human exposure can only be evaluated in terms of the total lifetime exposure. This means that long-term data are necessary and, at least, an annual average exposure in a home is required to estimate lifetime exposure. At times, only spot or grab sampling is

120 1 13. STRATEGY FOR MEASUREMENTOF Rn AND Rn

DAUGHTERS

possible, and this poses a difficult problem for estimating exposure. I t is emphasized that an attempt should be made to obtain the best data which can be used in estimating annual averages. Identification of areas with a high radon risk can be carried out with short-term measurements of either radon or the daughter products. These measurements are usually performed under conditions designed to maximize the result, for example, sampling after the house has been closed for some time and sampling in the basement rather than the living quarters. Scientific studies may make use of any of the techniques described, but tend to lean on instantaneous or continuous recording devices. Additionally, such studies frequently are designed to seek out sources or pathways and aim at sampling hnited areas within a house. Measurements made while conducting remedial action are generally short-term or grab sampling to identify a persistent source. Once remediation is completed, long-term measurements are necessary to demonstrate that the annual average exposure has been reduced to desired levels. In some instances, where elevated and unevenly distrib u k d 226Ra in soil is the source, flux measurements made at many spots on walls and floors can help to identify anomalously high entry points for radon.

13.2 Short-Term Versus Long-Term Measurements The choice of method for exposure assessment is usually dictated by the instrumentation and effort available. The simplest methods are short-term or grab sampling methods and require repeat measurements for the estimation of annual average exposure. The simplest on-site method is the single alpha count of a filtered air sample by the Kusnetz or Rolle method. This provides a rapid measure ment of the radon daughter working level. I t is more prudent to alpha count this same sample three times rather than only once since this allows evaluation not only of the WL but also the concentrations of the specific daughters nuclides. The Thomas method or the Scott MRK, (Scott 1981) method provides these data and the Scott method utilizes the second or third count interval to directly compute working level. Alpha counting a filtered air sample provides immediate results and is useful in screening. Low background, high-efficiency alpha counters are available so that even outdoor radon daughter concentrations may be measured this way. Samples should be recounted after from 3 hours

13.2 SHORT-TERM VERSUS LONG-TERM MEASUREMENTS 1 121

to one day to ensue that thoron daughter activity is not an interference. If it is possible to utilize an alpha spectrometer for the measure ment, the detection limit is usually improved because of the ability to determine 218Po and 214Po separately, and to distinguish these immediately from any thomn daughters. If a WL meter is available, this prwides a spot check of the WL present at one point in time but repeated measurements are necessary to provide an estimate of annual exposure. Spot radon measurements are possible by collecting air samples. The collecting vessel is usually a counting flask lined with zinc sulfide alpha phosphor. This can be placed directly on a photomultiplier tube for measurement. The containers, often called Lucas flasks, vary in volume from 0.1 liter ta many liters. Counting can commence as soon as radioactive equilibrium between radon and the daughters is established (about three hours). These flasks are equipped with either one or two entrance ports for filling. The singleport flask is less desirable since it must generally be evacuated and taken to the site and opened for filling. This relies on the integrity of the vacuum seal. The two-port flask allows air to be drawn through the flask by a small pump at the site, and filling is complete and unambiguous. Counting of either type is usually performed upon return to a central site or laboratory. The radon grab sample provides immediate results and is useful for smening purposes but has the same drawbacks as any instantaneous sample for estimating annual exposure and repeat measurements are necessary. Longer-term radon measurements are possible with charcoal Samplers (two to seven days) and this technique is easy to implement. Samplers can be deployed by individual home owners and samplers can be sent out and returned by mail to the central site for gamma counting of the radon daughters. These are in equilibrium with the adsorbed radon on the charcoal within a few hours. Repeat measurements are necessary for determining an annual average radon exposure. Harley and %fi (1988) report that, based on two different homes studied for 18 months, two oneweek measurements, taken one in summer and one in winter, and averaged, will yield within k 50% of the actual measured annual average 222Rn concentration. Other longer-term radon monitoring measurements, with deployment for periods of up to a few months, can be performed with TLD based monitors and for up to a fuJl year with solid state nuclear track detectors. A strategy for rapid measurement of the radon lwd in a home is sometimes necessary. This usually is required for the buying or selling

122 1 13. STRATEGY FOR MEASUREMENT

of homes but other situations such as assessment of a workplace or public building can arise. In this case a valid measurement of radon concentration over a short time interval with charcoal, working level monitor or even grab radon samples taken in a living or working area may suffice to provide some guidance. This measurement should not be confused with measurements for actual human exposure which must be conducted on a longer term basis. A short term (say two to four day) sample taken in the basement of an average home in the winter might yield a value of perhaps 200 Bq m-S(4 pCi I-'). A decision that exposure of the occupants is in the range necessary for remedial action cannot be made from this measurement. First floor and second floor radon concentrations generally average 2 to 5 times lower than basement concentrations dependling upon season and other variables related to the specific dwelling such as type of heating. Thus, 200 Bq m-% the basement most likely indicates the typical value of 40 to 100 Bq m-% the upstairs living space. The single rapid measurement provides little useful information concerning compliance with any established guidelines for exposure of individuals within a dwelling (and thus the need for remediation). A short term measurement can provide information that a home has unusually high radon concentrations. Radon concentrations in excess of 2000 Bq m-"regardless of season or location within the dwelling will assure that some radon reduction technique (NCRP, 1988b)is necessary in the dwelling to conform with guidelines (NCRP, 1984a). Valid exposure measurements can only be obtained with long term follow-up integrating measurements or several measurements of a week's duration taken during a minimum of two seasons per year. The measurements for exposure must be conducted in the living space within the home.

13.3 Reporting of Data The reporting of data should reflect the sampling intervals involved. The best estimate of exposure on an annual basis is with long term monitoring and the use of time weighting to include exposure out of doors and in buildings other than the home. Time weighted average exposures are calculated as follows:

ANNUAL EXPOSURE (WLM) = (11170)[ (WL),(Hours),

+ . . . + (WL),(Hours),]

(13.1)

13.4 RADON VERSUS RADON DAUGHTER MEASUREMENTS / 123

Where (WL),. . . (Hours),. . .

.

average exposure in working levels in locations one to n. = Time spent annually (in hours) at locations one to n. =

Reporting data in concentration or activity units such as Bq m-= 22PRn(gCi 1-I 22'Rn) or WL may indicate a spot or grab sample. If calculated values of WLM y-'are made from a single measurement, for example, this should be specifically stated in reporting the data. Reporting data in WLM y-' should indicate that multiple measure ments or long-term measurements have been made to assess annual exposure.

13.4 Radon Versus Radon Daughter Measurements Radon daughters deliver the significant dose t o cells in the bronchial epithelium which is the site of lung cancer. The short-lived daughters (effective t,,, = about 30 minutes) originate only fmm the parent gas radon, so an argument can be made for the measurement of either radon or daughters. One problem in the measurement of radon daughters is that there is no radon daughter standard available. The individual investigator is left to provide the necessary calibration and quality control associated with the flow rate for the filtered air sampling and the alpha counting efficiency. Also, there is no technique, as of this writing, to allow participation in an authorized intercomparison program. One potential method for generating a radon daughter standard atmosphere was suggested by McLaughlin and Jonassen (1983),once a valid radon measurement technique has been established. This is to raise the condensation nuclei concentration to a high level ( >lo5CN cm-7 and to allow the radon daughter concentration to approach equilibrium with the radon. Both radon and radon daughters are then measured to insure the validity of the daughter measurement. Radon, on the other hand, has a long enough half-life so that a standard atmosphere can be developed in a closed chamber. This allows intercomparison programs to be camed out. The DOE, for example, has sponsoml two intercomparison programs per year since 1981 at the Environmental Measurements laboratory in New York. This allows an independent check on the quality of the data, so radon measurement appears to be the measurement of choice. The EPA initiated a quality control program in 1986 a t their Eastern Environmental Radiation Facility in Montgomery, Alabama This program is designed particu-

124 / 13. STRATEGY FOR MEASUREMENT OF Rn AND Rn DAUGHTERS

lady for commercial firms performing radon measurements. Some states have instituted validation programs in order to qualify vendors performing radon measurements within the state. The exposure in working level months per year (WLM y-') may be obtained by using average values for the daughter equilibrium. Extensive measurements in Canada, Federal Republic of Germany and Swe den (McGregor et d., 1980; Wicke, 1984; Keller and Folkerts 1984; Swedjemark, 1983) indicate that the equilibrium factor, F, for radon daughters in homes averages about 0.4. The problem of calibration of radon daughter products is not an insignificant one. There has been some discussion that occupational exposures should be monitored in terms of radon concentration with frequent checks on the degree of radon daughter equilibrium. The EPA has set its guidelines for exposure in the home in terms of both radon and radon daughter concentration (EPA, 1986); assuming 50% daughter equilibrium (0.02 WL equals 4 pCi 1.').

136 Sampling Location Sampling for exposure assessment should be performed where the pertinent human exposure takes place. Detailed assessment is best done by using the occupational approach of a timeweighted average for each location in the dwelling and for exposures outdoors and in other buildings. In most cases, this detailed a study is not practical but measurements in main living quarters and bedrooms are advisable with general average values used for other contributions from outdoors and other buildings. Overemphasis has been placed on measurement of basement levels of radon and radon daughters. I t is known that soil is the source of most indoor radon in single family dwellings and considerable effort has been expended on measurements in basements on the assumption that this is the maximum potential exposure. This is usually true, and while a basement measurement is a rapid approach to identifying high areas,it does not allow accurate assessment of actual exposure. The EPA (Ronca-Battista et d., 1987) has published a protocol for measurement of "2Rn and 222Rndaughters in homes. This protocol, however, is directed toward locating homes with high potential for exposure. Samples are collected in the lowest living level under "closed house" conditions. Such measurements, of course, are not useful for determining actual population exposure.

13.7 RADON SURVEYS FOR DISTRIBUTIONAL STUDIES 1 125

13.6 Scientific Studies Radon or radon daughter measurement for scientific purposes poses mow stringent ~equirementsthan monitoring p d u t e s since basic or fundamental factors are sought and measurement m r must be minimized.Such studies usually require considerable effort in data collection and intensive measmments a~ taken. Hourly data may be required, for example, to study radon excursions associated with factors which influence radon entry into homes such as temperature and pressut.e changes, and &all. Continuously recording monitors are most useful for scientific studies if detailed meanmmentsare necessary, Radon flux measurements are sometimes made for scientific purposes. Radon entry into basements changes markedly with the integrity of the basement soil barrier. I t is difficult to predict indoor radon levels, for example, with soil or interior flux measurements without extensive coverage with flux monitors. However, flux measurements can be very informative for specific purposes such as rate of diffusional flow through intact walls or exhalation from d i f f e ~ ntypes t of soils.

13.7 Radon Surveys for Distributional Studies Survey data are u t M to determine the population distribution of radon and daughter exposure. The exposure distributions for single family dwellings that have been observed to date appear to yield lognormal distributional data (McGregor e t al., 1980; Scott, 1983c; New et al., 1986; Cohen, 1986). This is most probably due to the potential for many source term modification factors to be present when buildings are closely coupled to the ground. I t would not be surprising if the data for apartments or homes built with a separation between the soil and the living space, such as trailer homes, were normally distributed. Any sampling or measurement technique for a population which has more variability due to lack of time averaging, such as with single measurements or which has significant measurement error will give an inherently larger standard deviation or geometric standard deviation in the data. Scott (1983~)showed this in the lognormal distribution derived from single radon measurements versus that for the means of a t least ten measurements made over a oneyear period in Canada. The geometric standard deviation increased by a factor of 1.3 when the variability in the homes due to the single measurement was present in this distribution. Harley (1986)showed that a measurement error (coefficient of variation) of greater than about 50% increased the geometric

126 / 13. S T R A m G Y FOR MEASUREMENT

standard deviation so that it could significantly distort the health risk predictions based upon the distribution of exposures.

13.8 Summary A rigid standard measurement protocol is not required to effectively measure radon or radon daughter concentrations and human exposures in homes. What is necessary is accurate measurement of either radon or the daughters and, if risk estimation is a goal of a study, then a valid annual average exposure for the population is required. This means that long-term integrated measurements must be made or that multiple spot or grab samples must be taken to average out seasonal and daily variations in radon concentration. Radon measurements are preferred to daughter measurements because there are facilities capable of maintaining well-defined radon concentrations whereas, as of the writing of this report, no standard radon daughter atmospheres have been established. Radon measure ments are suitable for human risk assessment since the radonlradon daughter ratio in environmental situations is reasonably predictable. Radon measurements are used for assessment of occupational exposure in a number of countries. While they might be technically preferable, regulatory controls in the U.S.are based on radon daughter (WL)measurements.

APPENDIX A

Critical Level (LC) and Lower Limit of Detection (LLD) (See NCRE 1985,Pasternack and Harley 1971)

' h o criteria are of importance when measurements are made. One criterion for reporting data is that radioactivity is present above background. The second is a characteristic of a measurement device which describes the smallest sample count rate which must be present in order to yield a net count sufficiently large so as to imply the presence of -radioactivity. That is, the net count rate is greater than the critical level (L,). The data criterion L, is important for evaluating counting data to determine whether it is distinguishable as being above background. The critical level is equal to

k, = the value of the upper percentile of the standardized normal variate corresponding to the preselected risk (cx) for concluding falsely that activity is present. Alpha is usually taken to be 0.05 with k, = 1.645. So = the estimated standard e m r for the net sample count rate wh& the sample activity is actually zero. Usually a mean background rate from a number of background counts is used whenever counting data are converted to radioactivity.

If a mean background and standard deviation have not been determined %In may be replaced with RdT,.

T,,= sample and background count times. R, = background count rate. S, = standard deviation of multiple background measurements. n = number of background measurements in the mean value.

128 1 APPENDIX A

Mean background is desirable because it allows smaller values of sample radioactivity to be detected. Values of net sample count rate above LCindicate the presence of sample radioactivity with a preselected risk (a). The second criterion is more descriptive of the counting system and is termed lower limit of detection (LLD). LLD = (k, + k,) So = 3.29 So k,

=

The value of the standardized normal variate for a predetermined degree of confidence (1-P) for detecting the presence of activity. Beta (0)is usually selected as 0.05 and k, = kp = 1.645

The LLD indicates the net count rate necessary in a sample in order to detect its presence. The LLD and LCmay be converted to activity through the counter efficiency factor. EXAMPLJ3 As an example of a calculation of LCand LLD, consider a large scintillation cell (1.3 1 volume) used for grab 222Rnsamples. The background has been measured for 1,500 minutes, 10 different times and the mean and standard deviation of the mean rate is 14 -t 0.61J10 counts per hour (CPH).The calibration factor (which includes daughter counts) is 3.2 CPH Bq-I 222Rnm3 (120 CPH pCi-I 1). For a two hour sample count, the LCand LLD are,

L, LLD

= =

(1.645)(2.6) = 4.3 CPH 3.293, = 8.7CPH

Therefore, for a two hour count, only a sample with a net count rate of a t least 8.7 CPH will yield net count rate, 95 times out of 100, above 4.3 CPH (the level below which a count is not distinguishable from background). In eporting data, for this example, any measured net counting rates less than 4.3 CPH should not be reported as being different from background.

APPENDIX B

Units and Conversion Factors for Concentrations of Radon and Radon Daughters

SI Units '"Rn

WL = 2.8 x (A) + 1.4 x lo-' (B) + 1.0 x (C) EEC in Bq m-3= 0.105(A)+ 0.516(B) + 0.379(C) where A, B, C = concentrations of 2'8Po,214Pband 214Bi in Bq m-3 EEC in Bq m-3= (3,700)(WL)

220Rn WL = 3.3 x (B) + 3.1 x lo-' (C) EEC in Bq m-a= 0.91(B)+ 0.09(C) where B, C = concentrations of *12Pband 112Bi in Bq m-$.The contribution of 216Poto WL and EEC is negligible EEC in Bq m-" 275 (WL) Either J m-3= 5.6 x (EECin Bq m-9 J h m-3= (J r n 9(hoursexposed) Historical Units

a2Rn WL = 1.05 x 10-3(A)+ 5.16 x

(B)+ 3.79 x

(C)

EEC in pCi I-' = 0.105(A) + 0.516(B)+ 0.379(C) where A, B, C = concentrations of 21BPo, 214Pband 211Bi in pCi I-' EEC in pCi 1 - I = (100)(WL)

130 1 APPENDIX B

220Rn WL = 0.122(B)+ 0.0116(C) EEC in pCi I-' = 0.91(B)+ 0.09(C) where B, C = concentrations of "'2Pband "ZBiiripCi I-'. The contribution of 2'6Poto WL, and EEC is negligible EEC in pCi I-' = (7.5)(WL)

and '"Rn, WLM = (WL)(hours exposedll70) For 222Rn Equivalents 1 WL 1WL 1 WL 1WLM 1 WLM

= 100 pCi I-' (EEC) =

2 x 10"(J m-3)

= 3,700 Bq m-3(EEC) = 3.5 x (Jh m-3) = 6 x 10"q h m-3(EEC)

EEC is the equilibriunl equivalent concentration

GLOSSARY activity median diameter (AMD): The median diameter of the size distribution of the aerosol with associated radioactivity. adsorption: See charcoal adsorption method. aerodynamic diameter: The diameter of a unit density particle that has the same settling velocity as the particle described. aerosol: A suspension of solid or liquid particles in a gas. ambient aerosol: The aerosol existing in the environment of interest. attached daughters: The short-lived daughter products of radon that ~IV attached to the ambient aerosol. calibration: A measurement to determine the response of an instrument to a known amount or concentration of radioactivity. calibration chamber: A chamber that can be maintained at a fixed, known concentration of radon and having sufficient volume that the desired samples can be taken without disturbing the concentration in the chamber. certified standard: In this report, a vial of '"Ra solution certified for amount of radioactivity by the Natural Bureau of Standards that has a known rate of production of Radonzz2.

charcoal adsorption method: A method of estimating radon concentration where activated charcoal is exposed to the atmosphere being sampled for a fixed time. Radon concentration is estimated by measuring the gammaemitting short-lived radon daughter products. condensation nuclei (CN): Any small particle or ion capable of serving as a site for the condensation of vapor.

132 1 GLOSSARY

continuous monitor: In this report, an instrument for estimating the concentration of radon or its short-lived daughter products that gives an instantaneous direct reading or records the concentration at intervals. diffusion: Brownian movement with a net transport of particles or gas molecules through a gas under a concentration gradient. diffusion battery: An instrument designed to separate particles by size according to their diffusion coefficients, based on measurement of particle losses during laminar flow through the battery. diffusion coefficient: The constant of proportionality that relates the flux of aerosol particles or gas molecules through a gas and the concentration gradient. diffusion tube: An instrument designed to estimate either the particle size or the fraction of an aerosol with a given particle size in an air sample. The sample is passed through a tube and the particle concentration is measured before and after passage. A knowledge of the amount deposited in the tube for different flow rates allows estimation of particle size. If the material deposited is known or assumed to be of a particular size, the fraction of the total aerosol with this size is estimated. dosimeter (radonlradon daughter): A term used to describe any device that can be used to estimate the concentration of radon or its daughter products. emanation: In this report, the release of radon from a solid into the sumunding gas or liquid phase. exhalation: In this report, the transport of radon from the surface of soil or other material into the atmosphere. equilibrium (secular radioactive):A steady-state relationship between a parent radionuclide and its daughter product(s) such that their decay rates are equal. equilibrium equivalent concentration (EEC):The radon concentration, in equilibrium with its short-lived daughters, that has the same potential alpha energy per unit volume as exists in a sample mixture.

GLOSSARY 1 133

equilibrium fraction (F):The simplified factor describing the degree of radioactive equilibrium between radon and its short-lived daughters.

filtered air sample: A sample of the ambient aerosol, including'the short-lived daughter products of radon, obtained by drawing a known volume of air through a filter. flux: In this report, flux is the rate of transfer of radon from a solid matrix to the atmosphere.

instant working level meter: An instrument designed to give rapid, if not instantaneous, direct readingsof the working level in an atmosphere integrating monitor: An instrument designed to estimate the cumulative exposure to radon or radon daughter productsin an atmosphere. inversion:. A stable atmospheric condition near the earth's surface by which the temperatureincreases with altitude. ionization chamber (alpha): An instrument designed to estimate the quantity of radon, within its fixed volume, either by alpha pulse counting or by total ionization measurement. Kusnetz method: The original method of estimating the working level by making a single measurement of alpha activity on a filtered air sample. solution prepared from an laboratory standard: In this report, a 226Ra NBS certified standard and used to deliver a known amount or known rate of production of 222Rn.

Lucas cell: A specific form of scintillation cell developed by Lucas (1957). nuclear track detector: See solid state nuclear track detector. passive detector An instrument designed to estimate the concentration of radon or radon daughter products without any moving parts such as a sampling pump. potential alpha energy concentration (PAEC): The concentration of radon daughter products, in air, in terms of the alpha energy that will be released during complete decay through 214Po.

134 1 GLOSSARY

radon daughter products: In this report, refers to the short-lived daughter products of radon. radon decay products: Radon daughter products. radon progeny: Radon daughter products. respirable fraction: The fraction of airborne material that can be inhaled and possibly deposited in the lung. Rolle method: A modification of the Kusnetz method for estimating working level by making a single measurement of alpha activity on a filtered air sample. scintillation cell: A vessel lined with alpha-sensitive phosphor designed to estimate the quantity of radon contained within its volume by counting the scintillations produced with a photomultiplier tube. scintillation counter: An instrument, consisting of an alpha, beta or gamma-sensitive phosphor designed to estimate the quantity of radioactivity in a sample by counting the scintillations with a photomultiplier tube. soil gas: The gas filling the free air space within a volume of soil. solid state counter: In this report, an instrument designed to measure radioactivity, where the detector is a solid which produces an electrical pulse for each interaction. The pulse size is proportional to the energy deposited in the detector and the incident energy spectrum may be measured. solid state nuclear track detector (SSNTD):An integrating detector where heavy particles interact with a plastic film to produce incipient tracks. These can be enlarged chemically and counted visually or with specialized instruments. standardization: Often used interchangeably with calibration. In this report, usually, a measurement made with a stable or easily reproduc ible source of radioactivity (working standard) to test the operating status of an instrument.

GLOSSARY / 135

thermoluminescent dosimeter (TLD): An integrating detector where radiation energy is absorbed (trapped) and can be read out later by thermal excitation of the detector. Thomas method: A modification of the Tsivoglou method for estimating the concentrations of individual short-lived daughter products of "'Rn, involving a change in the counting protocol. TRACK-ETCH: The trade name for a specific brand of solid state nuclear track detector. Tsivoglou method: The original method of estimating the concentrations of the individual short-lived daughter products of 2"Rn by making three sequential measurements of alpha activity on a filtered air sample. twefilter method: A method of estimating radon concentration based on removing the radon daughters from an air sample with an input filter, allowing decay in a chamber and measuring the freshly formed radon decay products on a second filter. unattached daughters: The short-lived daughter products of radon that are not attached to the ambient aerosol. unattached fraction: The fraction of any short-lived daughter product of radon that is not attached to the ambient aerosol. working level (WL): Any combination of short-lived radon daughter products in one liter of air that wiU result in the emission of 1.3 x 10" MeV of potential alpha energy. working level month (WLM): The cumulative exposure equivalent to exposure a t one working level for a working month of 170 hours. working standard: In this report, a stable or easily reproducible source of radioactivity used to test the operating status of an instrument.

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156 1 REFERENCES WICKE, A. (1984)."Exposure of the population to radon daughters-problems associated with the assessment of the annual dose," Radiat Prot. Dosimetry 7,337. WILKENING, M.H. (1952)."Natural radioactivity as a tracer in the sorting of aerosols according to mobility," Rev. Sci. Instrum. 23,13. W ILKENING, M.H. A N D HAND, J.E. (1960)."Radon flux at the earth-air interface,'' J. Geophys. Res 65,3367. WILKENING, M.H., KAWANO, M., A N D LANE, C.(1966). "Radon daughter ions and their relation to some electrical properties of the atmosphere," 'Ibllus 18, 679. W ILKENING, M.H. (1970). "Radon-222 concentrations in the convective patterns of a mountain environment:' J. Geophys. Res. 75,1733. WILKENING, M., CLEMENTS, W.E., AND STANLEY, D. (1972). "Radon-222 flux measurements in widely separated regions," page 717 in The Natural Radiation Environment II, ADAMS,J.A.S., LOWDER, W.M. AND GESELL, TE,Eds. USAEC Report CONF-720805-P2,(National Whnical Information Service, Springfield, Virginia). W ILKENING , M. (1973)."Daughter ions of 222Rnin the atmosphere:' Proceedings American Geophysical Union Annual Meeting, (American Geophysical Union, Washington). W ILKENING, M. (1974)."Rn-222 from the island of Hawaii: Deep soils are more important than lava fields or volcanos:' Science183,413. W ILKENING, M. (1976). "Offshoreonshore flow patterns on the island of Hawaii,'' Reprint Volume on Coastal Meteorology (AmericanMeteorological Society, Boston). W ILKENING, M. (1977)."Influence of the electric fields of thunderstorms on radon-222 daughter ion concentrations," page 54 in Electrical Processes in Atmospheres. (Dietrich Steinkopff Verlag, Darmstadt, Federal Republic of Germany). W ILKENING , M. (1987)."Effect of radon on some electrical properties of indoor air," page 253 in Radon and Its Decay Products, American Chemical Society Symposium Series 331, HOPKE, PK.,Ed. (American Chemical Society, Washmgton). WILKINSON, D.H. (1950).Ionization Chambers and Counters, Cambridge Monographs on Physics (CambridgeUniversity PI.ess, London). W RENN , M.E.,SPITZ, H. A N D COIIEN, N. (1975). "Design of a continuous digital-output environmental radon monitor," IEEE %ns. Nucl. Sci., NS-22, 645. YOSHIDA. K. (1985). "Aerodynamic parameter determination in inlet orifice of CEA alpha dosimeter," page 123 in Occupational Radiation Safety in Mining, STOCKER, H., Ed. (Canadian Nuclear Association, lbronto). ZARCONE, M.J.. SCHERY. S.D.. WILKENING. M.H., AND MCNMEE. E. (1986). "A comparison of measurements of thoron, radon and their daughters in a test house with model predictions," Atmospheric Environ. 20,1273. ZHANG, C. A N D LUO, D. (1983). "Measurement of mixed radon and radon daughter concentrations in air," Nucl. Instrum. and Methods., 215,481.

The NCRP The National Council on Radiation Protection and Measurements is a nonprofit corporationchartered by Congress in 1964 to: 1. Collect, analyze, develop, and disseminate in the public interest information and recommendations about (a) protection against radiation and (b) radiation measurements, quantities, and units, particularly those concerned-with radiation protection; 2. Provide a means by which organizations concerned with the scientific and related aspects of radiation protection and of radiation quantities, units, and measurements may cooperate for effective utilization of their combined resources, and to stimulate the work of such organizations; 3. Develop basic concepts about radiation quantities, units, and measurements, about the application of these concepts, and about radiation protection; 4. Cooperate with the ~nternationalCommission on Radiological Protection, the International Commission on Radiation Units and Measurements, and other national and international organizations, governmental and private, concerned with radiation quantities, units, and measurements and with radiation protection. The Council is the successor to the unincorporated association of scientists known as the National Cor;lmittee on Radiation Protection and Measurements and was formed to carry on the work begun by the Committee. The Council is made up of the members and the participants who serve on the over sixty scientific committees of the Council. The scientific committees, composed of experts having detailed knowledge and competence in the particular area of the committee's interest, draft proposed recommendations. These are then submitted to the full membership of the Council for careful review and approval before being published. The following comprise the current officers and membership of the Council: Officers President Vice President Secretary and T-easurer Assistant Secretary Assistant lkasurer

W ARREN K.S I N C I . A I R S.JAMESAT)EI.STEIN. W. ROGERNEY CARLD. HOBEI.MAN J AMES E BERG

158 1

THE NCRP Members

SEYMOUR ABRAHAMSON S. JAMES ADELSTEIN PETER R ALMOND EDWARD L. ALPEN JOHN A. AUXIER WILLIAM J. BAIR MICHAEL k BENDER BRUCE B. BOECKER JOHN D. BOICE. JR ROBERT L. BRENT ANTONE BROOKS THOMAS F.BUDINGER MELVIN W.CARTER RANDALLS. CASWELL JAMES E. CLEAVER FRED T.CROSS STANLEY B. CURTIS GERALD D. DODD PATR~CIA W.DURBIN JOE A ELDER THOMASS. ELY JACOB I. FABRIKANT R J. MICHAELFRY ETHEL S GILBERT ROBERT A. GOEPP JOEL E. GRAY ARTHUR W. GW E RIC J. HALL NAOMI H. HARLEY WILLIAM R. HENDEE DONALD G. JACOBS A EVERETIT JAMES.J R BERNDK AHN KENNETH R. KASE CHARLES E. LAND GEORGTR. L ~ o m m RAY D.LLOYD

ARTHUR C. LUCAS

J R I ~ S. N TAYLOR,Hommry

President

THE NCRP 1 159

Currently, the following subgroups are actively engaged in formdating mommendations: Basic Radiation Protection Criteria SC 1-1Probability of Causation for Genetic and Development Effects SC 1-2 Risk Estimates for Radiation Protection Medical X-Ray, Electron Beam and Gamma-Ray Protection for Energies Up to 50 MeV - Equipment Performanceand Use X-Ray Protection in Dental Offices Standards and Measurement of Radioactivity for Radiological UseMedical and Biological Applications Biological Aspects of Radiation Protection Criteria SC 40-1Atomic Bomb Survivor Dosimetry SC 40-1A Biological Aspects of Dosimetry of Abmic Bomb Survivors Radiation Associated with Medical Examinations Radiation Received by Radiation Employees Operational Radiation Safety SC 46-2 Uranium Mining end Milling-Radiation Safety F'rograms SC 46-3 ALARA for Occupationally Exposed Individuals in Clinical Radiology SC 46-4 Calibration of Survey Instrumentation SC 46-5 Maintaining Radiation Protection Records SC 46-6 Radiation Protection for Medical and Allied Health Personnel SC 46-7 Emergency Planning SC 46-8Radiation Protection Design Guidelinesfor Particle Accelerator F a d t i e s SC 46-9 ALARA a t Nuclear Plants Methods for Detenninationof Dose Equivalent and Related Quantities Conceptual Basis of Calculations of Dose Distributions Internal Emitter Standards SC 57.2 Respiratory P a c t Model SC 57.5 Gastrointestinal %ct Models SC 67-6Bone Problem SC 57-8Leukemia Risk SC 57-9Lung Cancer Risk SC 57-10 Liver Cancer Risk SC 57-12 Strontium SC 57-14 Placental l'k-ansfer SC 57-15 Uranium Human Population Exposure Experience Radiation Exposure Control in a Nuclear Emergency SC 63-1 Public Knowledge About Radiation SC 63-2Criteria for Radiation Instruments for the Public SC 63-3Emergency Exposure Critsria for Specializsd Categories ' of Individuals Environmental Radioactivity and Waste Management SC 64.6 Screening Models SC 64-7 Contaminated Soid as a Source of Radiation Exposure SC 64-8Ocean Disposal of Radioactive Waste

,

160 1 THE NCRP

SC 66: SC 66: SC 67: SC 68: SC 69: SC 70:

SC 64-9 Effects of Radiation on Aquatic Organisms SC 64-10 Xenon SC 64-1 1 Low Level Waste Quality Assurance and Accuracy in Radiation F'rutection Measurements Biological Effects and Exposure Criteria for Ultrasound Biological Effects of Magnetic Fields Microorocessorsin Dosimetrv ~ffica'cyof Radiographic Prokdures Quality Assurance and Measurement in Diagnostic Radiology Radiation Exposure and Potentidy ~ e l a t e d ~ n j u r y Radiation Received in the Decontamination of Nuclear Facilities Guidance on Radiation Received in Space Activities Effects of Radiation on the EmbryoFetus Guidance on Occupational and Public Exposure Resulting from Diagnostic Nuclear Medicine Procedures Practical Guidance on the Evaluation of Human Exposures to Radiofrequency Radiation Extremely Low-Frequency Electric and Magnetic Fields Radiation Biology of the Skin (Beta Ray Dosimetry) SC 80-1 Hot Particles on the Skin Assessment of Exposures from Therapy Control of Indoor Radon

Study Group on Comparative Risk Ad Hoc Group on Video Display ~ m i n a l s 'IBsk Force on Occupational Exposure Levels

In recognition of its responsibility to facilitate and stimulate cooperation among organizations concerned with the scientific and related aspects of radiation protection and measurement, the Council has created a category of NCRP Collaborating Organizations. Organizations or groups of organizations that are national or international in scope and are concerned with scientific problems involving radiation quantities, units, measurements, and effects, or radiation protection may be admitted to collaborating status by the Council. The present Collaborating Organizations with which the NCRP maintains liaison are as follows: American Academy of Dermatology

American Association of Physicists in Medicine American College of Medical Physics American College of Nuclear Physicians American College of Radiology American Dental Association American Industrial Hygiene Association American Institute of Ultrasound in Medicine American Insurance Association

THE NCRP 1 161 American Medical Association American Nuclear Society American Occupational Medical Association American Podiatric Medical Association American Public Health Association American Radium Society American Roentgen Ray Society American Society of Radiologic 'lbchno1ogist.s American Society for Therapeutic Radiology and Oncology Association of University Radiologists Atomic Industrial Forum BioelectromagneticsSociety College of American Pathologists Conference of Radiation Control Program Directors Federal Communications Commission Federal Emergency Management Agency Genetics Society of America Health Physics Society Institute of Nuclear Power Operations National Electrical Manufacturers Association National Institute of Standards and ~ h n o l o g y Radiation Research Society Radiological Society of North America Society of Nuclear Medicine United States Air Force United States Army United States Department of Energy United States Department of Housing and Urban Development United States Department of Labor United States Environmental Protection Agency United States Navy United States Nuclear Regulatory Commission United States Public Health Service

The NCRP has found its relationships with these organizations to be extremely valuable to continued p m m s s in its program. Another aspect of the cooperative efforts of the NCRP relates to the special liaison relationships established with various governmental organizations that have an interest in radiation protection and measurements. This liaison relationship provides: (1)an opportunity for participating organizations to designate an individual to provide Liaison between the organization and the NCRP; (2) that the individual designated will receive copies of draft NCRP reports (at the time that these are submitted to the members of the Council) with an invitation to comment, but not vote; and (3) that new NCRP efforts might be discussed with liaison individuals as appropriate, so that they might have an opportunity to make suggestions on new

162 1 THE NCRP

studies and related matters. The following organizations participate in the special liaison program: Australian Radiation Laboratory Commission of the European Communities Cornmisariat a I'Energie Atomique (France) Defense Nuclear Agency Federal Emergency Management Agency Japan Radiation Council National Institute of Standards and 'kchnology National Radiological Protection Board (United Kingdom) National Research Council (Canada) Office of Science and 'kchnology Policy Office of Bchnology Assessment United States Air Force United States Army United States Coast Guard United States Department of Energy United States Department of Health and Human Services United States Department of Labor United States Department of 'Itansportation United States Environmental Protection Agency United States Navy . . United States Nuclear Regulatory Commission

The NCRP values highly the participation of these organizations in the liaison program. The Council's activities are made possible by the voluntary contribution of time and effort by its members and participants and the generous support of the following organizations: Alfred I? SIoan Foundation Alliance of American Insurers American Academy of Dental Radiology American Academy of Dermatology American Association of Physicists in Medicine American College of Nuclear Physicians American College of Radiology American College of Radiology Foundation American Dental Association American Hospital Radiology Administrators American Industrial Hygiene Association American Insurance Association American Medical Association American Nuclear Society American Occupational Medical Association ' American Osteopathic College of Radiology American Pediatric Medical Association American Public Health Association American Radium Society

THENCRP 1 163 American Roentgen Ray Society American Society of Radiologic ~ h n o l o g i s t s American Society for Therapeutic Radiology and Oncology American Veterinary Medical Association American Veterinary Radiology Society Association of University Radiologists Battelle Memorial Institute Center for Devices and Radiological Health College of American Pathologists Commonwealth of Pennsylvania Defense Nuclear Agency Edison Electric Institute Edward Mallinckrodt, Jr, Foundation Electric Power Research Institute Federal Emergency Management Agency Florida Institute of P h o s ~ h a t eResearch Genetics Society of America Health Physics Society Institute of Nuclear Power Operations James Picker Foundation Lounsbery Foundation National Aeronautics and Space Administration National Association of Photographic Manufacturers National Cancer Institute National Electrical Manufacturers Association National Institute of Standards and 'Ibchnology Nuclear Management and Resources Council Radiation Research Society Radiological Society of North America Society of Nuclear Medicine United States Department of Energy United States Department of Labor l Agency United States ~ n i i r o n m e n t aProtection United States Navy United States Nuclear Regulatory Commission

?b all of these organizations the Council expresses its profound appreciation for their support. Initial funds for publication of NCRP reports were provided by a grant from the James Picker Foundation and for this the Council wishes to express its deep appreciation. The NCRP seeks to promulgate information and recommendations based on leading scientific judgment on matters of radiation protection and measurement and to foster cooperation among organizations concerned with these matters. These efforts are intended to serve the public interest and the Council welcomes comments and suggestions on its reports or activities from those interested in in its work.

NCRP Publications NCRP publications are distributed by the NCRP Publications' office. Information on prices and how to order may be obtained by directing an inquiry to: NCRP Publications 7910 Woodmont Ave., Suite 800 Bethesda, MD 20814 The currently available publications are listed below. Proceedings of the Annual Meeting No. 1

Title Perceptions of Risk, Proceedings of the Fifteenth Annual Meeting, Held on March 14-15, 1979 (Including Thylor Lecture No. 3) (1980) Quantitative Risk in Standards Setting, Proceedings of the Sixteenth Annual Meeting, Held on April 2-3, 1980 (Including 'kylor Lecture No. 4) (1981) Critical Issues in Setting Radiation Dose Limits, Proceedings of the Seventeenth Annual Meeting, Held on April 89, 1981 (Including 'kylar Lecture No. 5) (1982) Radiation Protection and New Medical Diagnostic Procedures., Proceedings of the Eighteenth Annual Meeting, Held on April 6-7, 1982 (Including Bylor Lecture No. 6) (1983) Environmental Radioactivity, Proceedings of the Nineteenth Annual Meeting, Held on April 6-7, 1983 (Including Thylor Lecture No. 7) (1984) Some Issues Important in Developing Basic Radiation Protection Recommendations, Proceedings of the lbentieth Annual Meeting, Held on April 4-5, 1984 (Including 'kylor Lecture No. 8) (1985) Radioactive Waste, Proceedings of the Wenty-First Annual Meeting, Held on April 3-4, 1985 (Includmg 'kylor Lecture No. 9) (1986) Nonionizing Electromagnetic Radiation and Ultrasound Proceedings of the Wenty-Second Annual Meeting, Held on April 2-3, 1986 (Including 'kylor Lecture No. 10) (1988) 164

NCRP PUBLICATIONS 1 165 9

New Dosimetry at Hiroshima and Nagasaki and Its Implications for Risk Estimates, Proceedings of the ItrventyThird Annual Meeting. Held on April 8-9, 1987 (Including 'hylor Lecture No. 11) (1988) Symposium Fhadhgs

The Control of Exposure of the Public to Ionizing Radiation in the Event ofAccident or Attack, Proceedings of a Symposium held April 27-29.1981) (1982)

Lauriston S. 'ZgylorLectures No. 1

Title and Author The Squares of the Natuml Numbers in Radiation Protec tion by Herbert M. Parker (1977) Why be Quantitative About Radiation Risk Estimates? by Sir Edward Pochin (1978) Radiation Protection - Concepts and ?)uLde Offs by Hymer L. Friedell (1979)[Availablealso in Perceptions of Risk, see above] From "Quantity of Radiation" and "Dose" to "Exposure" and "Absorbed Dose " - A n Historical Review by Harold 0.Wyckoff (1980)[Available also in Quantitative Risks in Standards Setting, see above] How Well Can We Assess Genetic Risk? Not Very by James F.Crow (1981)[Available also in Critical Issues in Setting Radiation Dose Limits, see above] Ethics, Z h d m f f s and Me&d Radiation by Eugene L. Saenger (1982)[Available also in Radiation Protection and New Medical Diagnostic Approaches, see above] The Human Environment - Past, Present and Future by Merril Eisenbud (1983)[Available also in Environmental Radioactivit> see above] Limitation and Assessment in Radiation h t e c t i o n by Harald H. Rossi (1984) [Available also in Some Issues Important in Developing Basic Radiation h t e c t i o n R e c ommendations, see above] %th (and Beauty) in Radiation Measurement by John H. Harley (1985) [Available also in Radioactive Waste, see above] Nonionizing Radiation Bioeffects: Cellular Properties and Interactions by Herman I? Schwan (1986)

166 1 NCRP PUBLICATIONS

How to be Quantitative about Radiation Risk Estimates by Seymour Jablon (1987) How Safe is Safe Enough? by Bo Lindell(1988)

NCRP Commentaries Cornmerltar3' Title No. 1 Krypton85 in the Atmosphere - With Specific Reference to the Public Health Significance of the Proposed ConmUed Release a t Three Mile Island (1980) Preliminary Evaluation of Criteria for the Disposal of Dansuranic Contaminated Waste (1982) Screening nchniques for Determining Compliance with Environmental Standards (1986) Guidelines for the Release of Waste Water fmm Nuclear Facilities with Special Reference to the Public Health Significance of the Proposed Release of nested Waste Waters at Three Mile island (1987)

NCRP Reports No. 8

Title Control and Removal of Radioactive Contamination in Laboratories (1951) Radioactive Waste Disposal in the Ocean (1954) Maximum Permissible Body Burdens and Maximum Per missible Concentration of Radionuclides in Air and in Water for Occupational Exposure (19591[Includes Addendum 1 issued in August 19631 Measurement of Neutron Flux and Spectra for Physical and Biological Applications (1960) Measurement of Absorbed Dose of Neutrons and Mixtures of Neutrons and Gamma Rays (1961) Stopping Powers for Use with Cavity Chambers (1961) Safe Handling of Radioactive Materials (1964) Radiation Protection in Educational Institutions (1966) Medical X-Ray and Gamma-Ray Protection for Energies Up to 10 MeV - Equipment Design and Use (1968) Dental X-Ray Protection (1970) Radiation Protection in Veterinary Medicine (1970) Precautions in the Management of Patients Who Have Received Therapeutic Amounts of Radionuclides (1970)

NCRP PUBLICATIONS 1 167 Protection Against Neutron Radiation (1971) Protection Against Radiation from Brachythempy Sources (1972)

Specifications of Gamma-Ray Bmhythempy Sources (1974) Radiological Factors Affecting Decision-Making in a Nuclear Attack (1974) Krypton85 in the Atmosphere - Accumulation, Biological Significance, and Control Dchnobgy (1975) Alpha-Emitting Particles in Lungs (1975) Ditium Measurement Dchniques (1976) Radiation Protection for Medical and Allied Health Personnel (1976) Structural Shielding Design and Evaluation for Medical Use of X Rays and Gamma Rays of Energies Up to 10 MeV (1976)

Environmental RatEiation Measurements (1976) Radiation Protection Design Guidelines for 0.1-100 MeV Particle Accelerator Facilities (1977) Cesium-137from the Environment to Man. Metabolism and Dose (1977) Review of NCRP Radiation Dose Limit for Embryo and Fetus in Occupationally Exposed Women (1977) Medical Radiation Exposure of Pregnant and Potentially Pregnant Women (1977) Protection of the Thyroid Gland in the Event of Releases of Radioiodine (1977) Instrumentation and Monitohg Methods for Radiation Protection (1978) A Handbook of Radioactivity Measurements Procedures, 2nd ed. (1985) Operational Radiation Safety Program (1978) Physical, Chemical, and Biological Properties of Radiocerium Relevant to Radiation Protection Guidelines (1978) Radiation Safety ?)rzining Criteria for Industrial Radwgmphy (1978) Ditium in the Environment (1979) m t i u m and Other Radionuclide Labeled Organic Compounds Incorporated in Genetic Material (1979) Influence of Dose and Its Distribution in Time on DoseResponse Relationships for Low-LET Radiations (19801 Management of Persons Accidentally Contaminated with Radionuclides (1980) Mammography (1980)

168 1 NCRP PUBLICATIONS

Radiofrequency Electromagnetic Fields - Properties, Quantities and Units, Biophysical Interntion, and Measure ments (1981) Radiation Protection in Pediatric Radiology (1981) Dosimetry of X-Ray and Gamma-Ray Beams fir Radiation Therapy in the Energy Range 10 keVto SOMeV(1981) Nuclear MedicineFactors Influencing the Choice and Use of Radwnuclides in Diagnosis and Therapy (1982) Operational Radiation Safety - lfaining (1983) Radiation Protection and Measurement for Low Voltage Neutron Generators (1983) Protection in Nuclear Medicine and Ultrasound Diagmstic Procedures in Children (1983) Biological Effects of Ultmsound Mechanisms and Clinical Implications (1983) Iodinel29: Evaluation of Releases from Nuclear Pbwer Generation (1983) Radiological Assessment: Predicting the Ifansport, Bioac cumulation, and Uptake by Man of Radionuclides Released to the Environment (1984) Exposures from the Uranium Series with Emphasis on Radon and its Daughters (1984) Evaluation of Occupational and Environmental Exposures to Radon and Radon Daughters in the United States (1984) Neutron Contamination from Medical Electron Accelerators (1984) Induction of Thyroid Cancer by Ionizing Radiation (1985) Carbon-14 in the Environment (1985) SI Units in Radiation Protection and Measurements (1985) The Experimental Basis for Absorbed-Dose Calculations in Medical Uses of Radionuclides (1985) Geneml Concepts for the Dosimetry of Internally Deposited Radionuclides (1985) Mammography - A User's Guide (1986) Biological Effects and Exposure Criteria for Radiofrequency Electromagnetic Fields (1986) Use of Bioassay h c e d u r e s for Assessment of Intemul Radionuclide Deposition (1987) Radiation Alarms and Access Control Systems (1987) Genetic Effects of Internally Deposited Radionuclides (1987)

NCRP PUBLICATIONS 1 169

Neptunium Radiation Protection Guidelines (1987) Recommendations on Limits for Exposure to Ionizing Radiation (1987) Public Radiation Exposure from Nuclear Power Generation in the United States (1987) Ionizing Radiation Exposure of the Population of the United States (1987) Exposure to the Population in the United States and Canada from Natural Background Radiation (1987) Radiation Exposure of the U.S. Population from Consumer Products and MisceUaneous Sources (1987) Comparative Carcinogenicity of Ionizing Radiation and Chemicals (1988) Measurement of Radon and Radon Daughters in Air (1988) Binders for NCRP Reports are available. ?kro sizes make it possible to collect into small binders the "old series" of reports (NCRP Reports Nos. 8-30) and into large binders the more recent publications (NCRP Reports Nos. 32-97). Each binder will accommodate from five to seven reports. The binders cany the identification "NCRP Reports" and come with label holders which permit the user to attach labels showing the reports contained in each binder. The following bound sets of NCRP Reports are also available. Volume I. NCRP Reports Nos. 8,16, 22 Volume 11. NCRP Reports Nos. 23,25,27,30 Volume 111. NCRP Reports Nos. 32,33,35,36,37 Volume IV. NCRP Reports Nos. 38,40,41 Volume V. NCRP Reports Nos. 42,44,46 Volume VI. NCRP Reports Nos. 47,48,49,50,51 Volume VII. NCRP Reports Nos. 52,53,54,55,57 Volume VIII. NCRP Reports No. 58 Volume IX. NCRP Reports Nos. 59,60,61,62,63 Volume X. NCRP Reports Nos. 64,65,66,67 Volume XI. NCRP Reports Nos. 68,69,70,71,72 Volume XII. NCRP Reports Nos. 73,74,75,76 Volume XIII. NCRP Reports Nos. 77,78,79,80 Volume XIV. NCRP Reports Nos. 81,82,83,84,85 Volume XV. NCRP Reports Nos. 86,87.88,89 Volume XVI. NCRP Reports Nos. 90,91,92,93 (Titles of the individual reports contained in each volume are given above.)

170 I NCRP PUBLICATIONS

The following NCRP Reports are now superseded andlor out of print: No.

Title X-Ray Protection (1931)[Superseded by NCRP Report No. 31 Radium Protection (1934).[Superseded by NCRP Report No. 41 X-Ray Protection(1936).[Superseded by NCRP Report No. 61

Radium Protection (1938). [Superseded by NCRP Report No. 131 Safe Handling of Radioactive Luminous Compounds(1941). [Out of Print] Medical X-Ray Protection Up to n o Million Volts (1949). [Superseded by NCRP Report No. 181 Safe Handling of Radioactive Isotopes (1949).[Superseded by NCRP Report No. 301 Recommendations for Waste Disposal of Phosphorus-32 and Iodine-131 forMedical Users (1951).[Out of Print] Radiological Monitoring Methods and Instruments (1952). [Superseded by NCRP Report No. 571 Maximum Permissible Amounts of Radioisotopes in the Human Body and Maximum Pennissible Concentmtions in A i r a d Water (1953).[Superseded by NCRP Report No. 22) Recommendations for the Disposal of Carbon-14 Wastes (1953).[Superseded by NCRP Report No. 811 Protection Against Radiations from Radium, Cobalt-60 and Cesium-137(1954).[Superseded by NCRP Report No. 241 Pmtection Against Betatron-SynchrotronRadiations Up to 100 Million Electron Volts (1954).[Superseded by NCRP Report No. 511 Safe Handling of Cadavers Containing Radioactive Isotopes (1953~1. [Superseded by NCRP Report No. 211 Pennissible Dose from External Sources of Ionizing Radiation (1954)including Maximum Pennissible Exposure to Man, Addendum to National Bureau of Standards Handbook 59(1958).[Superseded by NCRP Report No. 391 X-Ray. Protection(1955).[Superseded by NCRP Report No. 261 Regulation of Radiation Exposure by Legislative Means (1955).[Out of Print]

NCRP PUBLICATIONS 1 171

Pmtection Against Neutron Radiution Up to 30 Million Electmn Volts (1957). [Superseded by NCRP Report No. 381 Safe Handling of Bodies Containing Radioactive Isotopes (1958).[Superseded by NCRP Report No. 371 Protection Against Radiations fmm Sealed Gamma Sources (1960).[Superseded by NCRP Report Nos. 33.34, and 401 Medical X-Ray Pmtection Up to Three MiUion Volts (1961). [Superseded by NCRP Report Nos. 33,34.35, and 361 A Manual of Raalioactivity Procedures (1961). [Superseded by NCRP Report No. 581 Exposure to Radiutian in an Emergency (1962).[Superseded by NCRP Report No. 421 Shielding for High Energy Electron Accelemtor Installations (1964).[Superseded by NCRP Report NO. 511 Medical X-Ray and Gamma-Ray Pmtection for Energies Up to 10 MeV - Structuml Shielding Design and Evaluation (1970).[Superseded by NCRP Report No. 491 Basic Radiation Protection Criteria (1971). [Superseded by NCRP Report No. 911 Review of the Current State of Radiution Protection Philosophy (1975).[Superseded by NCRP Report No. 911 Natural Background Radiation in the United States (1975). [Superseded by NCRP Report No. 94) Radiation Exposure from Consumer Products and MisceUaneous Sources (1977). [Superseded by NCRP Report No. 951 A Handbook of Radioactivity Measurements Procedures [Superseded by NCRPReport No. 58,2ndedl

Other Documents The following documents of the NCRP were published outside of the NCRP Reports and Commentaries series:

"Blood Counts. Statement of the National Committee on Radiation Protection," Radiology 63.428 (1954)

"Statements on ~ a x & u mPermissible Dose from 'lblevision Receivers and Maximum Permissible Dose to the Skin of the Whole Body," Am. J. Roentgenol.. Radium Ther. and NucL Med. 84,152 (1960)and Radiology 75,122 (1960)

172 1 NCRP PUBLICATIONS Dose Effect Modifying Factors In Radiation htection, Report of Subcommittee M-4 (Relative Biological Effectiveness) of the National Council on Radiation Protection and Measurements. Report BNL 50073 (T-471)(1967) Brookhaven National Laboratory (Nationallkhnical Information Service, Springfield. Virginia). X-Ray Protection Standards for Home 7bleuision Receivers, Interim Statement of the National Council on Radicrtion Pmtection and Measurements (National Council on Radiation Protection and Measurements, Washington, 1968) Specification of Units of Natuml Uranium and Natuml Thorium (National Council on Radiation Protection and Measurements, WashingLon. 1973) NCRP Statement on Dose Limit for Neutrons (National Council on Radiation Protection and Measurements, Washington, 1980) Control of Air Emissions of RadionucZkks (National Council on Radiation Protection and Measurements, Bethesda, M q l a n d , 1984)

Copies of the statements published in journals may be consulted in libraries. A limited number of copies of the remaining documents listed above am available for distribution by NCRP Publications.

INDEX Actinium series 12,13.27 adinon (219Rn)13 decay scheme 12 'hble of properties 27 Actinon (219Rn)13 Aerosol characteristics 22,23 particle size 22 lhble 23 Altitude variation of radon 36.38 Figure 38 Attached daughter products 21 Calibration 41.111.112.117 standards 112 chambers 117 Condensation nuclei 86 Conversion factors 129 Deposition velocity 20.22 attached daughters 22 unattached daughters 20 Diffusion Coefficient 20,37,77 gases 37,77 21"Po 77 lhble 37 unattached daughters 20 Diurnal variations of radon 30 Equilibrium. radioactive 9.18 Equilibrium, equivalent daughter concentration (EEC)17.92 d e f ~ t i o n17 measurement 92 Equilibrium factor (F)18,23,24 definition 18 measured values 23 lhble 24 Exhalation of radon 105 Flux-see Radon flux Instant WL meter 98,100 Lower limit of detection (LLD)44,128 definition and equations 128 Table for PzRn methods 44

PAEC measurement 92,93,97,98, 101,102 alpha spectrometry 98 integrating monitors 101 Kusnetz method 93 modified Kusnetz method 97 personal dosimeters 102 Roll6 method 97 RPISU 101 Plateout rate 20,22 attached daughters 22 unattached daughters 20 Potential Alpha Energy Concentration (PAEC)17.92 equations 17 measurement 92 Quality assurance 111,116

Radium distribution 26 Radon 9, 13-15,26,29,30,32,33,36 atmospheric transport 36 distribution 26 emanation 29.33 in the atmosphere 30 in water 32 physical properties 9,14 Table 15 radiometric properties 13 lhble 14 see a h radon measurement see also radon methods Radon daughter products 16,18,21 attached 21 behavior 21 equilibrium 18 'Pable of properties 16 see also radon daughter measurement see also radon daughter methods Radon daughter measurement 65,71,74 individualdaughters 65 sampling 65 t h m n (220R.n)7 1 unattached 74

INDEX 1 174 Radon daughter methods 65-69,71.74 alpha spectrometry 69 Cliff 68 gamma spedrometry 71 modified Tsivoglou 66 Scott (MRK)67 Thomas 66 Tsimglou 66 Radon flux 29,35,105 average value. soil 35 measurement 105 see radon flux methods Radon flux methods 106109 accumulation 106 adsorption 108 field 106 Figure 107 flow 108 laboratoly 106 soil concentration gradient 109 vertical profile 109 Radon measurement 40-43,105,111. 114,119

calibration 41,111

flux 105 quality assurance 116 220Rn in air 42 2nRn in air 43 sampling 40 standardization 114 strategy 119 see also radon methods Radon methods 42,43,50,52,56,58, 60.61 air filtration 61 charcoal adsorption 58 continuous monitors 50,60 ionization chambers 43 nuclear track 52 scintillation cells 46 TLD 56 Reporting of data 122

Sampling 40,65,77,119,124 daughter products 65 EPA p10tocol124 location 124 radon 40 strategy 119 unattached daughters 77 ~eaaonalvariation of radon 30

Standardization 111,112,114,115 commercial standards 115 laboratory standards 112 working standards 114 Thorium series 11,13,26,27 decay scheme 11 distribution 26 'Igble of properties 27 thoron (mRn)13 Thoron (mRn)13.42.72 daughter measurement 72 measurement 42 radiometric properties 13 Thoron daughter products 71 measurements 71 Unattached fraction 7 4 7 6 definitions 75 of PAEC 75 Unattached daughter products 19,20,74, 77,79,87,88,90

and condensation nuclei 87 behavior 20 d a d a t i o n s 88.90 formation 19 properties 77 sampling 79 see unattached daughter methods Unattached daughter methods 74.79.82, 83.86,90

diffusion tubes 79 electrostatic collectors 82 impactor sampler 86 quality assurance 90 screen samplers 83 Units 129 Uranium series 10,26,27 decay scheme 10 distribution 26 'Igble of properties 27 Working Level (WL) 17,92 definition 17 measurement 92 Working Level Month (WLM)17 definition 17