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 Bureau of Mines Information Circular/1985 
 
 Continuous Radiation Working-Level 
 Detectors 
 
 By R. F. Drouilard and R. F. Holub 
 
 -^sr^ 
 
 UNITED STATES DEPARTMENT OF THE INTERIOR 
 
 75: 
 
 w 
 
 'If/NES 75TH AV»^ 
 
Information Circular 9029 
 
 Continuous Radiation Working-Level 
 Detectors 
 
 By R. F. Droullard and R. F. Holub 
 
 UNITED STATES DEPARTMENT OF THE INTERIOR 
 
 Donald Paul Model, Secretary 
 
 BUREAU OF MINES 
 Robert C. Morton, Director 
 

 Library of Congress Cataloging in Publication Data: 
 
 DrouUard, R. F. (Robert F.) 
 
 Continuous radiation working-level detectors. 
 
 (Information circular ; 9029) 
 
 Bibliography: p. 17-18. 
 
 Supr. of Docs, no.: 1 28.27:9029. 
 
 1. Nuclear counters. 2. Radon— Measurement. 3. Uranium mines 
 and mining— Safety measures. 4. Mine gases— Measurement. I. Holub, 
 Robert F. II. Title. III. Series: Information circular (United States. 
 Bureau of Mines) ; 9029. 
 
 TN295.U4 [TN490.U7] 622s [622.8] 85-600009 
 
1 
 
 2 
 
 2 
 
 2 
 
 4 
 
 7 
 
 8 
 
 9 
 
 9 
 
 9 
 
 10 
 
 10 
 
 11 
 
 12 
 
 13 
 
 15 
 
 15 
 
 17 
 
 19 
 
 CONTENTS 
 
 Page 
 
 Abstract 
 
 Introduction 
 
 Three CWL detector designs and measurement methods 
 
 Backside of filter counting 
 
 Filter surface counting 
 
 Ambient air counting 
 
 ^ - Ai r sys terns 
 
 _- Factors affecting accuracy 
 
 _ Volumetric airflow variations 
 
 Radon daughter mixture variations 
 
 Plateout losses 
 
 A Other sources of error 
 
 ^ Calibration 
 
 ~ Attachments for model B-lOO CWL detector 
 
 '^ CWL detector applications 
 
 O Commercial CWL instruments 
 
 Conclusions 
 
 References 
 
 Appendix. — Method for measuring efficiency of a beta detector 
 
 ILLUSTRATIONS 
 
 1. Unassembled model B-lOO CWL detector 3 
 
 2. Model B-lOO CWL detector 3 
 
 3. Block diagram of CWL monitoring system 4 
 
 4. Unassembled model FC CWL detector 5 
 
 5 . Model FC CWL detector 5 
 
 '^ 6. Model FC CWL detector with alpha-detecting diode 6 
 
 »-^ 7. Model FC CWL detector with beta shield 7 
 
 *J 8. Model Beta-8 experimental CWL detector 8 
 
 — 9. Block diagram of typical air system for use with a CWL detector 8 
 
 — 10. Inherent uncertainty plot for a gross alpha CWL detector 9 
 
 —r- 11 . Inherent uncertainty plot for a gross beta CWL detector counting from back 
 
 "^ side of filter 10 
 
 ^ 12. Inherent uncertainty plot for a gross beta CWL detector counting from col- 
 lection side of filter 10 
 
 13. Two model B-lOO CWL detectors equipped with input tubes 12 
 
 14. Effect of input screen on attached-unattached radon daughter activity 12 
 
 15. Effect of input screen on attached-unattached radon daughter activity in 
 
 the Twilight Mine 12 
 
 16. Two model B-lOO CWL detectors equipped with Zeleny tubes 13 
 
 17. Model B-lOO CWL detector with Zeleny tube containing a charged wire 13 
 
 18. Charged particle collection using a negatively charged wire in a Zeleny 
 
 tube on a model B-lOO CWL detector 14 
 
 19. Effect of a ventilation system using a heat exchanger on the working-level 
 
 exposure in a dwelling 14 
 
 20. A commercial CWL monitoring system in operation in a mine 15 
 
 21. A commercial CWL monitoring system undergoing tests 15 
 
 22. Comparison of working-level values measured by alpha and beta CWL 
 
 detectors 16 
 
 23. Commercial version of model B-lOO CWL detector 17 
 
11 
 
 TABLE 
 
 1. Effect of filter-to-detector distance on count rate, 
 
 Page 
 6 
 
 
 UNIT OF MEASURE ABBREVIATIONS 
 
 USED 
 
 IN 
 
 THIS 
 
 REPORT 
 
 cm^ 
 
 square centimeter 
 
 irnii 
 
 
 
 millimeter 
 
 cm-^ 
 
 cubic centimeter 
 
 9 
 
 
 
 square millimeter 
 
 cpm 
 
 count per minute 
 
 p,m 
 
 
 
 micrometer 
 
 dis/min 
 
 disentegration per minute 
 
 |J,S 
 
 
 
 microsecond 
 
 h 
 
 hour 
 
 pCi 
 
 /L 
 
 
 picocurie per liter 
 
 L 
 
 liter 
 
 pCi 
 
 /(L/WL) 
 
 picocurie per liter 
 
 
 
 
 
 
 per working level 
 
 L/min 
 
 liter per minute 
 
 pet 
 
 
 
 percent 
 
 MeV 
 
 million electron volts 
 
 V 
 
 
 
 volt 
 
 min 
 
 minute 
 
 WL 
 
 
 
 working level 
 
 mg 
 
 milligram 
 
 yr 
 
 
 
 year 
 
 tag/cm^ 
 
 milligram per square 
 centimeter 
 
 
 
 
 
CONTINUOUS RADIATION WORKING-LEVEL DETECTORS 
 
 By R. F. Droullard ^ and R. F. Holub^ 
 
 ABSTRACT 
 
 The Bureau of Mines has used gross alpha and gross beta detectors to 
 continuously measure radiation working levels for a number of years. 
 During this time, improvements have been made in the design and per- 
 formance of continuous working-level (CWL) detectors. This report dis- 
 cusses the improved designs and some of the operating principles and 
 applications of CWL detectors in the measurement of radon daughter 
 products in mines and dwellings. 
 
 'Supervisory geophysicist. 
 ^Research physicist. 
 Denver Research Center, Bureau of Mines, Denver, CO. 
 
INTRODUCTION 
 
 The inhalation of radon daughter prod- 
 ucts may result in lung cancer. These 
 products can be an especially significant 
 problem for underground miners in uranium 
 and other types of mines where the daugh- 
 ter products of 222Rn ^^^ always present 
 in the mine atmosphere. Exposure to the 
 ionizing radiation of 222^^ is measured 
 in working levels (9^).^ One working lev- 
 el denotes any combination of the short- 
 lived radon daughter products in 1 L 
 of air that will result in the ultimate 
 emission of 1.3 x 10^ MeV of alpha 
 energy. 
 
 The most common method of measuring 
 working-level exposures is to collect 
 a grab sample on a filter and measure 
 the alpha and/or beta activity. While 
 this approach is satisfactory for some 
 applications, it did not meet the needs 
 of much of the radiation hazard research 
 work performed by the Bureau, and so 
 the Bureau initiated efforts to develop 
 a CWL detector (5). A CWL detector 
 
 differs from other automated samplers in 
 that air sampling is continuous, whereas 
 other methods use an electromechanical 
 grab sampler that samples for predeter- 
 mined periods. Radon daughter measure- 
 ments using this latter method of sam- 
 pling provide information that represents 
 conditions at the time the sample is 
 collected. Most of the CWL detectors 
 discussed in this report have a response 
 time that lags behind actual activity 
 levels by about 1 h. 
 
 Earlier work by the Bureau on CWL de- 
 tectors was reported in 1977 (4^). Since 
 that time, improvements have been made in 
 CWL detector designs and applications (2, 
 6). CWL detectors developed by the Bu- 
 reau and industry have been evaluated us- 
 ing the Bureau's radiation-hazard test 
 facilities (3) , and in operating mines 
 and dwellings. This report provides up- 
 dated information on the use of CWL de- 
 tectors for measuring radon daughter ac- 
 tivity over extended time periods. 
 
 THREE CWL DETECTOR DESIGNS AND MEASUREMENT METHODS 
 
 All but one of the CWL detectors dis- 
 cussed in this report measure the gross 
 radon daughter activity collected on a 
 filter by means of a vacuum pump. The 
 type of radioactivity measured depends on 
 the type of detector used. In any case, 
 the detector output, in the form of 
 pulses, is counted, and, through calibra- 
 tion, each count is assigned an equiva- 
 lent amount of alpha energy, which is 
 expressed in working levels. Three CWL 
 detector designs developed by the Bureau 
 are discussed in this section. 
 
 BACKSIDE OF FILTER COUNTING 
 
 Figure 1 shows an unassembled model 
 B-lOO CWL detector. Figure 2 shows an 
 assembled detector mounted on a metal box 
 that contains a high-volpage power supply 
 for the Geiger-Mueller (GM) tube and an 
 electronics system for signal processing. 
 
 ^Underlined numbers in parentheses re- 
 fer to items in the list of references 
 preceding the appendix. 
 
 In the model B-lOO CWL detector, the 
 beta activity of the radon daughters col- 
 lected on the filter is measured by the 
 GM tube from the back side of the filter 
 collection surface. The primary reason 
 for this method of measuring the activ- 
 ity is to provide an unobstructed path- 
 way to the surface of the filter. This 
 reduces problems with radon daughter 
 plateout (removal of radon daughters from 
 the air by their attachment to surfaces) 
 and makes attachment of accessories less 
 difficult. (Attachments for the B-lOO 
 detector are discussed in a later 
 section. ) 
 
 The B-lOO detector uses a type-7311 GM 
 tube with a diameter of 5.36 cm and a 
 thickness of 1.53 cm. The tube has a 
 mica window with an areal density of 
 about 2 mg/cm^. An aluminum foil is 
 placed over the GM-tube window to prevent 
 detection of alpha particles. The foil 
 also helps to protect the interior of the 
 GM-tube housing from moisture. The de- 
 tector is operated with an anode resist- 
 ance specified by the manufacturer and a 
 
o 
 
 mi-^ 
 
 mmmtimsmmmf'w: 
 
 FIGURE 1. - Unassembled model B-100 CWL detector. From left to right: back cover plate with signal and 
 air connectors, housing for Geiger-Mue 1 ler (CM) tube with CM tube clamp and O-ring seals, pancake-type GM 
 tube, filter backup screen, glass-fiber fi Iter, rubber fi Iter gasket, and fi Iter c lamping plate w ith reto ining screws. 
 
 FIGURE 2. - Model B-lOO CWL detector mounted on metal enclosure for associated electronics, with beta 
 shield. 
 
bias voltage determined by measuring the 
 tube's operating plateau. 
 
 The signal-processing electronics con- 
 sist of an amplifier, discriminator, 
 dead-time control, and line driver. The 
 system dead time is around 50 |is. A 
 block diagram of a CWL monitoring system 
 used by the Bureau is shown in figure 3, 
 
 ll5Vac 
 
 In some applications of the B-lOO CWL 
 detector, beta particles from the decay 
 of airborne 214gj (RaC) have sufficient 
 energy to reach the GM tube through the 
 filter and its backup screen. These un- 
 wanted particles can be eliminated by 
 using a beta shield as shown in figure 2. 
 The shield is spaced about 2 cm from the 
 detector housing to avoid losses of radon 
 daughters through plateout on the shield. 
 
 FILTER SURFACE COUNTING 
 
 115 Vac 
 
 Aloha or 
 
 beta 
 detector 
 
 Signal- 
 processing 
 electronics 
 
 Controlled 
 air 
 flow 
 
 Detector 
 
 bias 
 
 supply 
 
 
 Counter 
 timer 
 
 
 
 
 1 
 
 
 
 Delay 
 timer 
 
 -* 
 
 TTY 
 scanner 
 
 — 
 
 Chronometer 
 
 
 
 ^' 
 
 
 
 
 Printer 
 
 and tape 
 
 punch 
 
 
 FIGURE 3. - Block diagram of CWL monitoring system. 
 
 Figure 4 shows an unassembled model FC 
 CWL detector. Figure 5 shows an assem- 
 bled model FC detector mounted on a metal 
 box that contains the signal-processing 
 electronics (same as those used with the 
 B-lOO detector). Collection air passes 
 through the filter, which faces the de- 
 tector, and then to the back side of the 
 detector housing by means of a U-shaped 
 piece of plastic tubing. 
 
 The model FC CWL detector can also be 
 used with an alpha detector by replacing 
 the GM tube with a SOO-mm^ surface bar- 
 rier diode. An appropriate preampli- 
 fier is used along with the necessary 
 detector bias supply. Figure 6 shows 
 this arrangement. 
 
 The beta detector used in the model FC 
 CWL detector is a type-7242 end-window GM 
 tube. It has a diameter of 2.54 cm and a 
 thickness of 1.33 cm. It operates at an 
 anode voltage of 750 V and has a mica 
 window with an areal density of 2 mg/cm^. 
 An aluminum foil is cemented over the GM- 
 tube opening in the housing to prevent 
 alpha measurements and to protect the GM 
 tube from moisture. 
 
 The model FC detector uses a 25-mm-diam 
 filter, which tends to clog sooner than 
 the 47-mm-diam filters used in the model 
 B-lOO unit. The filter-to-detector dis- 
 tance can be changed in the model FC, 
 The most frequently used distance is 
 about 1 cm. At this distance, unwanted 
 airborne beta activity can penetrate to 
 the detector. This can be prevented by 
 using a shield as shown in figure 7. 
 
 Measuring beta activity on the col- 
 lection side of the filter results in 
 
FIGURE 4. - Unassembled model FC CWL detector. From left to right: back cover plate with 
 signal and air connectors, end-window GM tube, plastic mounting ring for GM \u\)e, housing for 
 CM tube, spacers, filterring, 0-ringfilter gasket, glass-fiber filter, and filter holderwith backup 
 screen and tubing connector. 
 
 5 
 
 / 
 
 FIGURE 5. - Model FC CWL detector mounted 
 on electronics box. 
 
 greater sensitivity using the model FC , 
 as compared with the model B-100. At a 
 given airflow rate, the model FC is about 
 2,7 times more sensitive to radon daugh- 
 ters. This results, in part, from a 
 greater efficiency in measuring the soft- 
 er 214ptj (RaB) beta particles. Typical 
 efficiencies are 11.7 pet for RaB and 
 13.2 pet for RaC. For the model B-100, 
 typical efficiences are 3.5 pet for RaB 
 and 10.6 pet for RaC. A method for mea- 
 suring these efficiencies is given in the 
 appendix. 
 
 Filters used for collecting radon 
 daughters should have a high efficiency 
 for collecting particles smaller than 
 1 pun; this applies to both membrane- and 
 glass-fiber-type filters. The membrane 
 filter is a surface collector, while the 
 glass-fiber filter is a depth collector. 
 This characteristic of the glass-fiber 
 filter makes it prone to self-absorption 
 for alpha and beta particles. Beta- 
 absorption values for five 47-mm-diam 
 glass-fiber filters, using a 210b , beta 
 source located on the opposite side of 
 the filter from a GM detector, were as 
 follows, in percent: 17.5, 20.0, 17.6, 
 17.7, and 17.5. 
 
FIGURE 6. - Model FC CWL detector with alpha-detecting diode. The metal enclosure contains a 
 preamplifier. 
 
 It is important to keep the filter-to- 
 detector distance constant to maintain 
 calibration in all CWL measuring systems. 
 Table 1 shows the effect on count rate of 
 changing the source-to-detector distance, 
 using a 210gj beta source and a beta de- 
 tector such as the type used in the model 
 B-100. At the filter-to-detector dis- 
 tance used in the B-100, a 1-mm change in 
 distance results in a change in the cali- 
 bration factor of several percent. The 
 filter must be kept in place against the 
 filter backup screen. It will usually 
 stay in place while air is applied to the 
 CWL detector. The calibration of alpha 
 
 TABLE 1. - Effect of filter-to-detector 
 distance on count rate 
 
 (Using 210b i beta source, glass-fiber 
 filter, and aluminum backup screen) 
 
 Distance, mm 
 9.39 
 
 10.89 
 
 12.89 
 
 15.89 
 
 18.89 
 
 21.89 
 
 26.89 
 
 Count , cpm 
 
 2,913 
 2,736 
 2,573 
 2,314 
 2,100 
 1,861 
 1,553 
 
 Std dev, cpm 
 
 ±27.3 
 ±26.5 
 ±25.7 
 ±24.4 
 ±23.3 
 ±21.9 
 ±20.1 
 
FIGURE 7. - Model FC CWL detector with beta shield. 
 
 and beta detectors is affected by changes 
 in the air-path density between the de- 
 tector and the source. 
 
 AMBIENT AIR COUNTING 
 
 Several years ago, the Bureau began 
 evaluating a method of measuring working- 
 level exposures by measuring the beta 
 activity of the air surrounding a beta- 
 detecting system. Figure 8 shows one of 
 the prototype detectors developed by the 
 Bureau and tested in the Bureau's exper- 
 imental uranium mine. This detector, 
 called the model Beta-8, consists of two 
 
 counting sections. The lower section has 
 four GM tubes operating in parallel with 
 no shielding. These tubes can be seen at 
 the lower end of the detector in figure 
 8. The upper section also has four GM 
 tubes of the same type as the lower sec- 
 tion, but these tubes are shielded 
 against the ambient beta activity, re- 
 sulting in gamma ray activity measure- 
 ments only for this section. By sub- 
 tracting the count from the upper section 
 from the count from lower section, a 
 fairly good approximation of the ambient 
 beta activity can be made without the use 
 of a filter and air system. 
 
Air exhaust 
 
 FIGURE 8. - Model Beta-8 experimental CWL 
 detector. 
 
 Airflow 
 meter 
 
 Air pump 
 
 Airflow 
 controller 
 
 Differential 
 pressure 
 
 Airflow 
 sensor 
 
 Electronic 
 manometer 
 
 To CWL detector To data- 
 
 and air input aquisition system 
 
 FIGURE 9. - Block diagram of typical air sys- 
 tem for use with a CWL detector. 
 
 Mine tests of the Beta-8 detector 
 showed that it has a rapid response time 
 to changes in working level when the con- 
 densation nuclei level exceeds about 
 25,000 particles per cubic centimeter. 
 
 At lower concentrations, plateout of ra- 
 don daughters on the detector causes 
 response and accuracy problems. At pres- 
 ent, no practical method exists for pre- 
 venting the radon daughter plateout. 
 
 AIR SYSTEMS 
 
 All methods for measuring radon daugh- 
 ters collected on a filter paper require 
 a known air volume or volumetric flow 
 rate. In most of its work with CWL de- 
 tectors, the Bureau uses instrumentation 
 to measure flow rate continuously. In 
 addition, a mechanical flow controller is 
 used to set and hold the flow rate at a 
 fixed value over a wide range of differ- 
 ential pressures across the filter. Fig- 
 ure 9 shows a block diagram of a typical 
 air system. 
 
 Continuous airflow rate measurements 
 are made either by mass flow sensors and 
 converted to volumetric flow rate or by 
 direct measurement with volumetric flow 
 sensors using a laminar flow rate. Flow 
 
 rates are also checked with a dry test 
 meter connected in an appropriate part of 
 an air system. 
 
 Most of the air systems used in the Bu- 
 reau's studies of CWL detectors include a 
 pressure gauge to measure the differen- 
 tial pressure across the collection fil- 
 ter. The gauge serves two purposes: 
 
 (1) It indicates the presence of a leak 
 around a newly installed filter, and 
 
 (2) it indicates when the filter should 
 be changed because of clogging. 
 
 The sensitivity of a CWL detector is 
 directly proportional to the airflow 
 rate. In uranium mine applications, a 
 flow rate of 1 to 2 L/min provides good 
 sensitivity. For sites where the radon 
 
daughter activity is low, such as in 
 dwellings, sampling flow rates can be 
 
 increased to improve the statistical ac- 
 curacy of the measured count. 
 
 FACTORS AFFECTING ACCUEIACY 
 
 There are a number of factors that can 
 affect the accuracy of a working-level 
 measurement made by CWL detectors. Many 
 of these factors apply to any measurement 
 of radioactivity and have been discussed 
 in the literature. Several of the more 
 important factors that affect the accu- 
 racy CWL detectors of the type covered in 
 this report are discussed below. 
 
 VOLUMETRIC AIRFLOW VARIATIONS 
 
 Volumetric airflow variations are an 
 obvious problem, but one that is not eas- 
 ily resolved. Various approaches to flow 
 control have been used, including differ- 
 ential flow controllers, electromechan- 
 ical systems that control air-pump 
 speeds, and the use of either mass or 
 volumetric airflow rate sensors, which 
 usually require monitoring by the data- 
 acquisition system. Airflow rate varia- 
 tions are a common source of error for 
 CWL systems. 
 
 and 214pq) is represented by a line that 
 originates at the midpoint of the hypote- 
 nuse and bisects the triangle. The line 
 showing no inherent uncertainty passes 
 through the mixture of RaA, RaB, and RaC 
 upon which the calibration is based. 
 
 Figure 10 shows the inherent uncertain- 
 ty triangle for a CWL detector measuring 
 alpha particles. In this case, the error 
 is quite low and can be ignored for most 
 measurements. The error direction is to- 
 ward overestimation of the working level 
 as the radon daughter mixture becomes 
 dominated by RaA. This provides some er- 
 ror compensation for plateout losses as- 
 sociated with radon daughter mixtures 
 high in RaA and environments with rela- 
 tively low concentrations of condensation 
 nuclei. 
 
 The inherent uncertainty triangle for a 
 model B-lOO CWL detector measuring beta 
 activity is shown in figure 11. Differ- 
 ences between the radon daughter mix- 
 ture and the calibration mixture have a 
 
 RADON DAUGHTER MIXTURE VARIATIONS 
 
 When airborne radon daughter mixtures 
 differ from the mixture used for cali- 
 brating a gross-count CWL detector, the 
 working-level measurement will have an 
 error that is called inherent uncertain- 
 ty. This error originates from assigning 
 an average energy to each count measured 
 when making the calibration. The degree 
 of inherent uncertainty depends on how 
 much the average energy per count dif- 
 fers from the calibration mixture and 
 whether alpha or beta particles are being 
 measured. 
 
 The degree of inherent uncertainty can 
 be shown by a triangular graph ( 16-17 , 
 19) . Briefly, the graph represents all 
 possible mixtures of RaA, (218 Po)» RaB, 
 and RaC equivalent to 1 WL. One leg of 
 the triangle represents RaA activity in 
 picocuries per liter per working level, 
 and the other leg represents the RaB ac- 
 tivity in the same units. RaC-C (214qj 
 
 RaB, pCi/(L/WL) 
 50 100 150 
 
 FIGURE 10. - Inherent uncertainty plot for a gross 
 alpha CWL detector, showing percent inherent uncer- 
 tainty. fPlus sign indicates equilibrium mixture.) 
 
10 
 
 RaB, pCi/(L/WL) 
 50 100 150 
 
 RaB, pCi/(L/WL) 
 50 100 
 
 50 
 
 FIGURE 11. - Inherent uncertainty plot for a gross 
 beta CWL detector counting from back side of filter, 
 showing percent inherent uncertainty. (Plus sign in- 
 dicates equilibrium mixture.) 
 
 FIGURE 12. - Inherent uncertainty plot for a gross 
 beta CWL detector counting from collection side of 
 filter, showing percent inherent uncertainty. (Plus 
 sign indicates equilibrium mixture.) 
 
 greater effect on this method of mea- 
 suring working-level exposures. This is 
 also true for the model FC CWL detector, 
 whose inherent uncertainty triangle is 
 shown in figure 12. In both these 
 graphs, the measured efficiencies for 
 beta activity from RaB and RaC were used 
 to construct the error lines. For gross 
 beta measurements, the inherent uncer- 
 tainty error increases as the radon 
 daughter mixture becomes dominated by 
 RaA. In most applications, this type of 
 error seldom exceeds 8 pet. 
 
 PLATEOUT LOSSES 
 
 CWL detector errors caused by plateout 
 losses are not easy to assess. Errors 
 from plateout become noticeable when the 
 condensation nuclei concentration drops 
 below 10,000 particles per cubic centime- 
 ter and may be quite significant below 
 5,000 particles per cubic centimeter. 
 Plateout losses are a greater problem 
 with detector systems designed to count 
 the collection side of the filter; they 
 do not pose a significant problem for 
 beta detectors that measure from the back 
 side of the filter. 
 
 OTHER SOURCES OF ERROR 
 
 Other sources of error previously men- 
 tioned include self-absorption by the 
 filter and changes in the distance and 
 air density between the collecting sur- 
 face of the filter and the detector. 
 
 The use of depth filters, such as those 
 made of glass fiber, results in some loss 
 due to self-absorption. However, most of 
 this loss is compensated for when the 
 detector is calibrated. Surface filters, 
 such as membrane filters, present little 
 or no self -absorption problems with most 
 aerosols when measurements are from the 
 collection side of the filter. However, 
 surface filters tend to clog much sooner 
 than depth filters, making them less 
 suitable for use in environments with 
 relatively high smoke levels. Self- 
 absorption problems encountered when 
 counting from the back side of a filter 
 are similar for both surface and depth 
 filters. 
 
 Tests on the Bureau's model B-lOO CWL 
 detector show that there is a significant 
 change in the count rate when the dis- 
 tance between the source and the beta 
 detector is changed. (This effect is 
 
11 
 
 discussed in the previous section, "Fil- 
 ter Surface Counting.") 
 
 Dust and diesel smoke can affect the 
 detector's response by increasing absorp- 
 tion of alpha and beta activity and by 
 changing source-to-detector distances. 
 This problem can be prevented by timely 
 filter replacements. 
 
 In uranium mines, where CWL detectors 
 are operated during periods when the 
 
 ventilation system is off, significant 
 amounts of 210g j activity will build up 
 on the filter, causing an error in the 
 measurements. This problem can be pre- 
 vented by weekly filter changes. 
 
 Thoron daughters can also cause errors, 
 but this has not been a problem in domes- 
 tic mines. 
 
 CALIBRATION 
 
 CWL detectors are usually calibrated by 
 operating them in an atmosphere where the 
 radon daughter activity can be accurately 
 determined. The best place to do this is 
 in a properly designed radon test cham- 
 ber, where the factors that affect cali- 
 bration can be controlled reasonably 
 well. Calibration can also be done on- 
 site, such as in a mine, but a mine envi- 
 ronment has constantly changing condi- 
 tions that make calibration based on grab 
 sampling difficult. This problem can be 
 overcome by using a calibrated CWL detec- 
 tor to calibrate the unknown unit. 
 
 Prior to calibration, a system using a 
 GM tube must be adjusted for operation at 
 the midpoint of the detector plateau, and 
 the system dead time must be known. For 
 systems that use an alpha detector, a 
 lower level discriminator must be set to 
 a point where beta activity, as well as 
 the detector noise, is not measured. 
 
 The first step in calibrating a system 
 that uses a GM detector is to measure the 
 background radiation (primarily gamma ac- 
 tivity) at the operating site of the CWL 
 detector. The Bureau uses a separate 
 background detector operated near the CWL 
 detector for this purpose. The gamma ray 
 response between the two detectors is de- 
 termined and used to correct the count 
 from the CWL detector on a continuous ba- 
 sis. Alpha detectors should have in- 
 significant background levels in most 
 applications. 
 
 Before applying air to the CWL detector 
 using a GM tube, the detector should be 
 standardized with a gamma ray source, 
 such as 137(^5. The source should be 
 placed at a fixed distance from the de- 
 tector and the detector's response re- 
 corded for future reference. For systems 
 
 using alpha detectors, a 230-ph source on 
 a disk that replaces the filter paper can 
 be used for standardization. 
 
 Following the standardization proce- 
 dure, air at a known flow rate is applied 
 to the CWL detector. After a period of 
 4 h or more in a controlled radon daugh- 
 ter atmosphere , the count rate is mea- 
 sured and corrected for dead-time losses 
 and background. Modified Tsivoglou (21) 
 or alpha spectroscopic methods are used 
 to measure the levels of radon daughter 
 activity. This information is converted 
 to working-level values , and a count con- 
 version factor is determined by the 
 equation 
 
 Kp = (WL/Na)/F, 
 
 (1) 
 
 where 
 
 and 
 
 Kp = count-to-working-level con- 
 version factor, 
 
 Na = average counts per minute 
 corrected for dead time and 
 background, cpm, 
 
 WL = average working level for 
 calibration period, 
 
 F = calibration flow rate, 
 L/min. 
 
 Correction for the system dead time can 
 be made by using the equation 
 
 N = 
 
 1-nt 
 
 (2) 
 
 where N = true count rate, cpm, 
 
 n = observed count rate, cpm. 
 
12 
 
 and 
 
 t = dead time, tain. 
 
 The dead time for the counting system can 
 be measured using an oscilloscope. 
 
 Another method for determining the 
 count-to-working-level conversion factor 
 for beta detectors is given in the 
 appendix. 
 
 ATTACHMENTS FOR MODEL B-lOO CWL DETECTOR 
 
 The unobstructed filter-collection in- 
 put of the model B-lOO CWL detector fa- 
 cilitates attaching devices to modify the 
 detector's response. One such device is 
 shown in figure 13. Input tubes have 
 been mounted on the filter clamp rings of 
 two model B-lOO units. One of the tubes 
 has a 60-mesh wire screen (13) at its en- 
 trance. Figure 14 shows the effects of 
 this screen relative to the amount of 
 condensation nuclei in a test chamber at- 
 mosphere. The measurements were made 
 with the chamber mixing fans off and with 
 condensation nuclei generated by a nebu- 
 lizer using tap water. The ratio of the 
 working-level values measured by the two 
 B-lOO detectors (designated B-102 and 
 B-101) reflects the amounts of attached 
 and unattached radon daughter particles 
 in the test chamber atmosphere. 
 
 Figure 15 represents similar measure- 
 ments made in the Bureau's Twilight 
 
 FIGURE 13. - Two model B-lOO CWL detectors 
 equipped with input tubes. Detector on left has a 
 60-mesh screen at the tube input. 
 
 experimental uranium mine over a period 
 of several days. The results show the 
 effect of diesel smoke on the ratio of 
 attached-to-unattached radon daughters. 
 
 Figure 16 shows two Zeleny tubes 
 mounted on the filter clamp rings of two 
 model B-lOO detectors. The tubes are 
 identical except for a charged wire along 
 the axis of one of them (fig. 17). These 
 
 FIGURE 14. - Effect of input screen on attached- 
 unattached radon daughter activity, with changes ir 
 concentration of condensation nuclei. 
 
 No diesel smoke 
 
 Diesel smoke 
 
 55 56 
 
 57 
 
 58 
 
 59 60 61 62 
 DAY OF YEAR 
 
 53 
 
 64 
 
 65 66 
 
 FIGURE 15. - Effect of input screen on attached- 
 unattached radon daughter activity in the Twilight 
 Mine. 
 
13 
 
 FIGURE 16. - Two model B-lOO CWL detectors 
 equipped with Zeleny tubes. Detector on right con- 
 tains a charged wire. 
 
 tubes are patterned after work by Busigin 
 (J^) . Figure 18 shows the results of 
 operating with negative 1,000 V on 
 the wire of detector B-102 and changing 
 
 FIGURE 17. - Model B-lOO CWL detector with 
 Zeleny tube containing a charged wire. 
 
 the amount of condensation nuclei in a 
 radon test chamber under the same condi- 
 tions as indicated above (for the screen 
 measurements) . 
 
 CWL DETECTOR APPLICATIONS 
 
 Commercial CWL monitors have been used 
 primarily to monitor dwellings and radio- 
 active waste disposal sites. Their use 
 in mining has been very limited (14-15, 
 18). 
 
 The Bureau has used CWL monitors in its 
 radiation hazards research program for 
 over 8 yr (,3_, J_l_) . CWL monitors have 
 been especially useful in calibrating ra- 
 don daughter dosimeters and in studies of 
 radon daughter levels in various mine 
 atmospheres. Franklin (J^) describes an 
 alarm system designed to warn of working- 
 level exposures that exceed a preset val- 
 ue. Such a system is useful in providing 
 mine operators with timely information on 
 radiation levels at work sites and in 
 main airways. 
 
 The Bureau's model B-lOO CWL detector 
 has been used to investigate the effi- 
 ciency of a ventilating system equipped 
 with a heat exchanger in removing radon 
 daughters from the basement of a dwell- 
 ing. Figure 19 shows the working-level 
 measurements for a period of 2 weeks. 
 During this time, air exchanges between 
 the basement and the outside were nor- 
 mal except during the period marked on 
 the graph when the heat-exchanger system 
 was operating at 0.5 air exchanges per 
 hour. The graph shows that the system 
 was effective in lowering the working- 
 level exposure to acceptable values for a 
 portion of the house (where the bedrooms 
 were) . 
 
14 
 
 151 
 
 m 
 
 
 > 
 
 
 LU 
 
 _l 
 
 1.0 
 
 CD 
 
 
 2 
 
 .9 
 
 :^ 
 
 
 DC 
 
 o 
 
 .8 
 
 ^ 
 
 
 
 7 
 
 o 
 
 
 ^^ 
 
 
 m 
 
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 \ 
 
 
 LU 
 
 .5 
 
 > 
 
 
 LU 
 
 
 _l 
 
 .4 
 
 o 
 
 
 z 
 
 ?^ 
 
 ^ 
 
 
 oc 
 
 
 o 
 
 ? 
 
 ^ 
 
 
 (N 
 
 1 
 
 o 
 
 
 m 
 
 
 
 DAY 
 152 
 
 \ B-102 
 
 V B-101 
 
 ^A,- 
 
 153 
 
 -CN introduced 
 
 I I 1 I I I I I J I I r I I I I I I J I I I I I I I 
 
 10^ 
 
 10 
 
 - 10 
 
 6 
 
 12 
 
 18 
 
 12 
 
 IS 
 
 10' 
 
 24 
 
 DC 
 LU 
 
 h- 
 LU 
 
 HI 
 O 
 
 o 
 
 m 
 
 Z) 
 
 o 
 
 DC 
 LU 
 Q. 
 
 O 
 
 HIO ^ 
 
 UJ 
 
 —I 
 
 o 
 
 3 
 
 24 6 
 
 TIME.h 
 
 FIGURE 18. - Charged particle collection using a negatively charged wire in a Zeleny tube 
 a model B-lOO CWL detector. A positive charge on the wire also results in the collection of 
 don daughters. 
 
 < 
 CO 
 
 z 
 
 LU 
 Q 
 
 Z 
 
 o 
 o 
 
 on 
 ra- 
 
 
 0.05 
 
 _l 
 
 .04 
 
 LU 
 
 
 > 
 
 
 LU 
 
 _l 
 
 .03 
 
 (D 
 
 
 Z 
 
 .02 
 
 DC 
 
 
 O 
 
 .01 
 
 
 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 
 
 DAY 
 
 FIGURE 19. - Effect of a ventilation system using a heat exchanger on the working-level ex- 
 posure in a dwelling. 
 
15 
 
 COMMERCIAL CWL INSTRUMENTS 
 
 Figures 20 and 21 show two C\^ monitor- 
 ing systems that are available from com- 
 mercial sources (]_~^) ' Both of these 
 systems measure alpha activity and use a 
 membrane filter. Figure 22 shows a com- 
 parison of measurements made by a commer- 
 cial (alpha) system and by the Bureau's 
 model B-lOO (beta) detector during the 
 monitoring of a basement in a dwelling. 
 The commercial system was operated with 
 60-min readout intervals, whereas the Bu- 
 reau's system measured for 60 min and 
 
 then delayed for 40 min. There is no 
 significant difference between the re- 
 sults obtained using the two systems. 
 
 Figure 23 shows a commercial version of 
 the Bureau's model B-lOO CWL detector 
 with the access doors open. The unit was 
 built under a Bureau contract (20) . Un- 
 like the two commercial systems mentioned 
 above, the system shown in figure 23 does 
 not include a data-acquisition portion, 
 which must be supplied by the user. 
 
 CONCLUSIONS 
 
 CWL detectors have been widely used in 
 the Bureau's radiation-hazard research 
 program for the past several years. 
 Their application has been very useful 
 in long-term calibration of personal do- 
 simeters, working-level alarm systems 
 for mines, and the study of long-term 
 
 relationships between radon and radon 
 daughters in mine atmospheres. Future 
 applications are expected to include the 
 use of these detectors in characterizing 
 radon daughters and their relationship to 
 aerosols found in mine environments. 
 
 FIGURE 20. - A commercial CWL monitoring 
 system in operation in a mine. 
 
 FIGURE 21. - A commercial CWL monitoring 
 system undergoing tests in a Bureau laboratory. 
 
16 
 
 
 
 18 19 20 21 
 
 22 23 24 25 
 DAY OF YEAR 
 
 26 27 28 29 
 
 FIGURE 22. - Comparison of working-level values measured by alpha and beta CWL detec- 
 tors in a dwelling. 
 
17 
 
 FIGURE 23. ■ Commercial version of model B-lOO CWL detector (mounted at bottom of box at 
 left)- Metal box on right contains an oir system for use with the CWL detector. 
 
 REFERENCES 
 
 1. Busigin, C. J., A. Busigin, and 
 C. R. Phillips. Measurement of Charged 
 and Unattached Fractions of Radon and 
 Thoron Daughters in Two Canadian Ura- 
 nium Mines. Health Phys., v. 44, 1983, 
 pp. 165-167. 
 
 2. Droullard, R. F. Instrumentation 
 for Measuring Uranium Miner Exposure to 
 Radon Daughters. Paper in Radiation Haz- 
 ards in Mining: Control, Measurement, 
 and Medical Aspects, ed. by M. Gomez 
 (Proc. Int. Conf., Golden, CO, Oct. 4-9, 
 1981). Soc. Min. Eng. AIME, Littleton, 
 CO, 1981, pp. 332-338. 
 
 3. E>roullard, R. F., T. H. Davis, 
 E. E. Smith, and R. F. Holub. Radiation 
 
 Hazard Test Facilities at the Denver Re- 
 search Center. BuMines IC 8965, 1984, 
 22 pp. 
 
 4. Droullard, R. F., and R. F. Holub. 
 Continuous Working Level Measurements 
 Using Alpha or Beta Detectors. BuMines 
 RI 8237, 1977, 14 pp. 
 
 5. . A Continuous Working Level 
 Monitor^ HASL No. 325, ERDA Radon Work- 
 shop, New York, Feb. 1977, pp. 43-47. 
 
 6. Droullard, R. F., and R. F. Holub. 
 Method of Continuously Determining Radia- 
 tion Working Level Exposures. U.S. Pat. 
 4,185,199, Jan. 22, 1980. 
 
18 
 
 7. Eberline Instrument Corp. (Santa 
 Fe, NM). WLM-l/WLK-1 Radon Working Level 
 Monitoring System. 1982, 20 pp. 
 
 8. EDA Instruments, Inc. (Toronto, 
 Canada) . Operating Manual for WLM-300. 
 1981, 14 pp. 
 
 9. Evans, R. D. Engineer's Guide 
 to the Elementary Behavior of Radon 
 Daughters. Health Phys . , v. 17, 1969, 
 pp. 229-252. 
 
 10. Franklin, J. C, P. E. Barr, K. D. 
 Weverstad, and C. T. Sheeran. Alarm 
 System for Radiation Working Level, Fan 
 Operation, and Air Door Position. Bu- 
 Mines IC 8903, 1982, 17 pp. 
 
 11. Franklin, J. C, and R. F. Droul- 
 lard. Instrumentation Developed by the 
 Bureau of Mines for Continuously Monitor- 
 ing Radon and Radon Daughters. ISA 
 Trans., v. 22, No. 4, 1983, pp. 25-32. 
 
 12. Fu-Chia, Y. , and T. Chia-Yang. A 
 General Formula for the Measurement of 
 Concentrations of Radon and Thoron Daugh- 
 ters in Air. Health Phys., v. 34, 1978, 
 pp. 501-503. 
 
 13. George, A. C. Measurement of the 
 Uncombined Fraction of Radon Daughters 
 With Wire Screens. Health Phys., v. 23, 
 1972, pp. 390-392. 
 
 14. Haider, B., and W. Jacobi. Long- 
 term Measurements of Radon Daughter Ac- 
 tivity in Mines. Paper in Proceedings 
 of the Third International Congress of 
 the International Radiation Protection 
 Association (Washington, DC, Sept. 9-14, 
 
 1973). U.S. AEC CONF-730907-P2, 1973, 
 pp. 920-925. 
 
 15. Holmgren, R. M. Working Levels of 
 Radon Daughters in Air Determined From 
 Measurements of RaA and RaC, Health 
 Phys., V. 27, 1974, pp. 141-145. 
 
 16. Holub, R. F. Evaluation and Mod- 
 ification of Working Level Measurement 
 Methods. Health Phys., v. 39, 1980, 
 pp. 425-447. 
 
 17. Holub, R. F., and R. F. Droullard. 
 Evaluation of Various Radon Daughter Mea- 
 surement Methods. Paper in Workshop on 
 Methods of Measuring Radiation In and 
 Around Uranium Mills, ed. by E. D. Har- 
 vard (Albuquerque, NM, May 23-26, 1977). 
 Atomic Industrial Forum, Inc., Washing- 
 ton, DC, 1977, pp. 197-219. 
 
 18. Kawaji, M. , H, Lin Pai , and C. R. 
 Phillips, Use of Gross Filter Activities 
 in a Continuous Working Level Monitor. 
 Health Phys., v. 40, 1981, pp. 543-548. 
 
 19. Schiager, K. J., T. B. Borak, and 
 J. A. Johnson (Alara, Inc.). Radiation 
 Monitoring for Uranium Miners: Evalua- 
 tion and Optimization (contract J0295026, 
 Alara, Inc.). BuMines OFR 149-82, 1981, 
 132 pp.; NTIS PB 83-102681. 
 
 20. Strombotne, T. R. , and A. L. 
 Beggs. Continuous Working Level Detector 
 System (contract H0212005, TSA Systems, 
 Inc.). BuMines OFR 2-83, 1982, 21 pp. 
 
 21. Thomas, J. W. Measurement of Ra- 
 don Daughters in Air. Health Phys., v. 
 23, 1972, pp. 783-789. 
 
19 
 
 APPENDIX. —METHOD FOR MEASURING EFFICIENCY OF A BETA DETECTOR 
 
 The efficiency of a beta detector for 
 measuring 214p and 214g. beta activity- 
 can be determined by the method described 
 in this section. 
 
 The following equations are used: 
 
 and 
 
 and 
 where 
 
 I, = Eg Nb, + Ec Nc, 
 
 I2 = Eb Nb2 + Ec Nc^, 
 
 (A-1) 
 (A-2) 
 
 I] = count measured in first 
 counting period. 
 
 I2 = count measured in second 
 counting period, 
 
 Eg and Eq 
 
 Nb,. Nc 
 
 Nb2> a"d Nc2 
 
 and Nb,, Nc,, 
 
 unknown efficiencies for 
 RaB and RaC , 
 
 nuiuber of atoms that de- 
 cay on the filter paper 
 during the two measure- 
 ment periods. 
 
 The number of atoms (Ng , etc.) must be 
 determined independently by measuring the 
 radon daughter activity, using either the 
 Tsivoglou (^) or alpha spectroscopic 
 method. 
 
 Solving the linear simultaneous equa- 
 tions A-1 and A-2 for Eg and Eq provides 
 the desired information. Repeated exper- 
 iments show a reproducibility within ±10 
 pet of the individual efficiencies, re- 
 flecting the counting statistics in both 
 I) and I2 and in the determination 
 of Ng, , etc. 
 
 The accuracy of the beta efficiencies. 
 Eg and Eq, can be checked by comparing 
 their weighted sum to the steady-state 
 equilibrium efficiency of a continuously 
 operating beta detector using the follow- 
 ing equation: 
 
 fc = 
 
 Ng + Nc 
 
 Both of these values are determined at 
 the equilibrium at which the steady-state 
 efficiency was determined. 
 
 A typical procedure for measuring the 
 beta efficiencies is outlined below, 
 
 1. Measure RaA, RaB, and RaC using the 
 modified Tsivoglou method. 
 
 2. Measure the background of the beta 
 detector with no activity on the collec- 
 tion filter. 
 
 3. Collect a 15-min sample at a known 
 flow rate. 
 
 4. Count the beta activity on the fil- 
 ter for 10 rain, starting -2 min after the 
 end of collection (2 min to 12 min). 
 Make dead-time corrections and normalize 
 to 1.0 L/min. Subtract background. The 
 resulting difference is I^. 
 
 5. Count the beta activity on the fil- 
 ter for 10 min, starting 17 min after the 
 end of collection (17 min to 27 min). 
 Make dead-time corrections and normalize 
 to 1.0 L/min. Subtract background. This 
 difference is I2. 
 
 6. After completing the collection us- 
 ing the beta detector, measure the RaA, 
 RaB, and RaC activity again. 
 
 7. Determine the mean RaA, RaB, and 
 RaC values. 
 
 8. Calculate the Ng and Nq values for 
 the 2-12 and 17-27 time intervals (steps 
 4 and 5), using equations from reference 
 12. These values become Ng and Nc„ for 
 the 2-12 interval and Ng and Nc for the 
 17-27 interval. 
 
 9. Solving equations A-1 and A-2, find 
 the determinant 
 
 DET = Ng, X Nc, - N2 X Nc,, 
 
 ^B, TOTAL - fe >< Eg + fc X E( 
 
 (A-3) 
 
 then Eg = 
 
 I, X Nr_ - lo X Nf 
 
 DET 
 
 (A-5) 
 
 where 
 
 fR = 
 
 Nh + Nf 
 
 and 
 
 Ec = 
 
 - I 
 
 DET 
 
 (A-6) 
 
20 
 
 An example of this method for determin- 
 ing beta efficiencies for the model FC 
 (beta) CWL detector is given below. 
 
 Step 1 
 
 RaA = 743.05 pCi/L 
 
 RaB = 303.17 pCi/L 
 
 RaC = 167.32 pCi/L 
 
 Exposure = 2.942 WL 
 
 Step 2 
 
 Background = 71.8 cpm 
 
 Steps 3-6 
 
 Flow rate = 1.893 L/min 
 
 Dead time = 47 |is 
 
 Gross count (2-12) = 38,720 
 
 Gross count (17-27) = 32,894 
 
 Net count (2-12) = 38,120 
 
 Net count (17-27) = 32,261 
 
 Count normalized to 1 L/mln (2-12) 
 
 = 20,137 
 
 Count normalized to 1 L/min (17-27) 
 
 = 17,042 
 
 Steps 7-8 
 
 Nb,(2-12) = 90,139 dis/min at 1 L/min 
 
 Nc,(2-12) = 72,558 dis/min at 1 L/min 
 
 Nb2( 17-27) = 62,700 dis/min at 1 L/min 
 
 Nc2( 17-27) = 73,477 dis/min at 1 L/min 
 
 Step 9 
 
 DET = (90,139)(73,477) - (62,700)(72,558) 
 
 „ _ ( 20,137)(73,477) - (17,042) (72,558 ) 
 ^B - DET 
 
 = 0.1172 (11.72 pet) 
 
 ( 17,042)(90,139) - (20,137)(62,700 ) 
 C DET 
 
 = 0.1319 (13.19 pet) 
 
 Using the above values of RaA, RaB, and 
 RaC (from step 1) and normalizing them to 
 1 WL and 1 L/min, the equilibrium filter 
 activity would be as follows: 
 
 Nr = 11,399 dis/min, 
 
 eq 
 
 Nr = 14,987 dis/min, 
 
 eq 
 
 and No + Np = 26,386 dis/min, 
 
 °eq ^ eq 
 
 Using equation A-3, the total beta 
 count can be calculated as follows: 
 
 Eb, total = (0.432)(0.1172) 
 
 + (0.568)(0.1319) 
 
 = 0.1255 (12.55 pet). 
 
 In this example, 12.55 pet of 26,326 dis/ 
 min gives a count rate of 3,313 cpm, and 
 the reciprocal of this number gives a 
 eount-to-working-level conversion factor 
 of 0.000302. The conversion factor de- 
 termined by the usual calibration method 
 described in the main text was 0.000307, 
 which is in good agreement with the val- 
 ue determined using the detector beta 
 efficiencies. 
 
 irU.S. GPO: 1985-505-019/20,066 
 
 INT.-BU.OF MINES, PGH., PA. 28014 
 
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