United States Office of EPA 520/5-83-020
Environmental Protection Radiation Programs September 1983
Agency Washington, DC 20460
Radiation
c/EPA
Evaluation of
Track-Etch Detectors
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Disclaimer
Track-EtchVx is a registered trade name for nuclear track detectors
made by Terradex Corporation, Walnut Creek, California. Evaluation of
this product by EPA does not constitute its endorsement by EPA.
Throughout the paper any reference to "nuclear track detectors" are those
manufactured by Terradex.
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Evaluation of Track-Etch Detectors
E. 0. Savage
Eastern Environmental Radiation Facility
P. 0. Box 3009
Montgomery, Alabama 36193
May 26, 1983
EPA
U.S. Environmental Protection Agency
Office of Radiation Programs
Waterside Mall East
401 M Street, S.W.
Washington, D.C. 20460
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Abstract ii
List of Tables iii
List of Figures iii
I. Introduction
II. Study Objectives 3
III. Methodology 5
IV. Results 15
V. Conclusions 23
References 25
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Abstract
A study was conducted to evaluate the precision and accuracy of
passive radon detectors manufactured by the Terradex
Corporation. Four configurations of these detectors were exposed to known
radon concentrations in the Eastern Environmental Radiation Facility radon
chamber under varying exposure regimines and sent to Terradex for
processing and readout.
Data reported by Terradex and our own exposure concentrations were
used to calculate calibration numbers for each configuration which were
then compared to calibration numbers used by Terradex.
Results of our study indicate that groups of detectors exposed
together and processed together demonstrate similarity in response.
Groups of detectors exposed and processed at different times, however, did
not always agree with each other nor with published Terradex calibration
numbers.
ii
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List of Tables
Table 1. Number of Detectors Exposed 14
Table 2. Actual Delivered Exposures 14
Table 3. Summary of Calibration Factors 16
Table 4. Average Calibration Factors Compared to
Terradex Reported 17
List of Figures
n
Figure 1. Four Configurations of Terradex Track-Etch Detectors 6
Figure 2. The EERF Radon Calibration Facility 8
Figure 3. Detectors Deployed in the Exposure Chamber 11
Figure 4. Response of Filter Cup Detector, Batch 8 19
Figure 5. Response of Filter Cup Detector, Batch 10 20
Figure 6. Response of Open Cup Detector, Batch 8 21
Figure 7. Response of Open Cup Detector, Batch 10 22
o
Registered tradename.
iii
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Introduction
For the past several years the U.S. Environmental Protection Agency's
Office of Radiation Programs (ORP) has been involved in several studies
designed to assess ambient and indoor concentrations of radon and radon
daughter products. The data gathered in these studies are being used in
turn to help evaluate the health impact of population exposure to radon
and its decay products and to determine the efficiency of remedial and
preventative techniques.
Radon daughter measurements are relatively difficult due to several
contributing factors: radon itself is a fairly unreactive noble gas that
decays into particles having short half-lives and a great affinity for
respirable airborne dust (condensation nuclei). Much scientific attention
has been directed of late toward characterizing the relationship between
radon and its decay products and developing techniques for sampling and
measuring these quantities accurately under dynamic conditions. Several
measurement techniques are now commonly used to measure both radon and
daughter product concentrations with a fair degree of precision and
accuracy. Radon is commonly sampled in evacuated scintillation cells and
subsequently counted on photomultiplier tubes according to generally
accepted procedures (Lu57, Ge76). The minimum detectable levels generally
achievable by this method are about 0.1 pCi/liter. Other investigators
draw air samples into glass flasks and metal containers and determine
radon concentrations by a pulse ionization technique (Ha72). This
procedure usually results in a lower detection limit of 0.05 pCi/1.
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Variations on these measurement techniques have been incorporated by
several equipment manufacturers into commercial devices with integrating
flow-through characteristies and automatic sampling features. Radon
daughter concentrations and subsequent WL determinations can be
accomplished by drawing air samples through a filter and quickly counting
the filter alpha activity. By using the techniques and assumptions of
several investigators (Ku56, Th72, and Ha69), and applying alpha
spectroscopy or gross alpha counting, sensitivities of .0005 WL. are
possible. Long-term integrated samples in the field are accomplished with
Radon Progeny Integrated Sampling Units (RPISU's) which employ
thermoluminescent detectors to measure alpha activity trapped on filter
membranes(Sc74). Automatic sampling and measurement devices for Working
Level determination have been developed by Argonne National Laboratory
(ANL78) and commercially by several companies. These are commonly known
as Instant Working Level Monitors (IWLM).
In most cases, the procedures and equipment necessary to accomplish
all of the aforementioned measurements are cumbersome, expensive (or
both), generate heat and/or noise and require electrical power in the
field. In addition, the techniques do not lend themselves well to large
scale screening measurements or to extremely long (months or more)
measurement time periods.
It is for these logistical and economic considerations that
inexpensive and passive radon detectors are so attractive. The ORP has
experience with two passive systems at the present time. The first is
known as the Passive Environmental Radon Monitor (PERM) which is based on
a design by George (Ge77). This device uses a battery to impose a voltage
potential across a metal funnel and a center electrode bearing a
thermoluminescent dosimeter chip (TLO). The alpha-emitting radon
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daughter products exist as free positive ions which are attracted to and
subsequently expose the TLD. The second type of passive detector which we
have used is Terradex's Track-EtcfV-'Detector. This detector consists of
a material in which tracks are formed on bombardment by a alpha particle.
When suitably processed and "developed" the tracks become visible, are
counted microscopically, and are related to radon concentrations via an
empirically derived calibration factor having units of tracks/square
millimeter per pCi/liter-day.
Study Objectives
The objective of this study was to assess the accuracy and precision of
the detectors as environmental radon monitors under several controlled
exposure conditions. Field testing nuclear track detectors is an
objective of a much larger environmental study currently in progress at
Butte, Montana. Existing Terradex Track-Etch^ data were statistically
analyzed and the results formed the basis for designing a study to address
the following issues:
1. Reliability; the extent to which different detectors from the
same production lot yield similar results when exposed together.
2. Lot-to-lot variability; differences in detector response between
2 different production lots.
3. Linearity of response; consistency of detector response over a
range of total exposures.
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Our study evaluates detector response characteristics by exposure to
known concentrations of radon and comparison of resultant calibration
factors to published values. Terradex calibration factors were derived
from exposure data sets that exhibited considerable variability. The
statistical analysis of the results of this study consist of estimations
rather than hypothesis testing.
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Methodology
Detectors
The detectors used were standard nuclear track detectors purchased
from Terradex in four configurations: the filter cup, membrane cup, open
cup and bare badge (see Fig. 1). The detectors are supplied by the
manufacturer in the open cup and bare card configuration. The customer
then assembles the required paper filter or membrane filter configuration
with provided snap ring and filter material s.
The detectors are provided in differing configurations to satisfy
varying sampling requirements or environmental conditions. The open cup
is a plastic cup with the 0.8 x 2.5 x 0.2 cm plastic nuclear track
detector fastened to the inside bottom with two strips of tape. Because
it is open to the atmosphere, the open configuration detector responds to
both radon and plated out radon daughter alpha activity. The manufacturer
suggests that plate-out characteristics of the sampled environment may be
derived from this type of detector.
The membrane cup is fitted with a clear, semi-permeable plastic
membrane that retards the diffusion rate of gases through it. By virtue
of differences in radiological half-lives, these membranes preferentially
discriminate against thoron and are used where thoron interference might
pose problems with radon measurements.
The filter cup is covered with a micropore paper filter which,
according to the manufacturer, allows gaseous radon to enter the cup but
blocks the passage of radon daughters. This configuration is preferred
where radon is to be measured alone and thoron is not significant.
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Figure 1
552756
.
552741!
n
552754
n
Four Configurations of nuclear track detectors
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The bare badge configuration is simply the detector itself mounted on
a 6.5 cm square card. This configuration is sensitive to radon and radon
daughter products, but only responds to plate-out on the detector surface
as compared to the open cup which responds to radioactivity on the walls
of the cup as well.
The cup detectors are supplied by the manufacturer nested in
aluminized plastic sleeves along with filter material, snap-ring filter
retainers, data forms, etc. The bare badges are shipped individually
packaged in glassine envelopes. After exposure, the detectors are
returned to the manufacturer where they are processed and read. Exposure
data is then returned to the customer.
Radon Exposure Chamber
The EERF radon chamber was constructed on site of 1/2-inch plywood
supported by an aluminum frame (Fig. 2). The overall dimensions of the
chamber are 1.2 x 1.2 x 2.4 meters. A door allows placement and removal
of measuring instruments. A passbox allows loading of small instruments,
detectors, and filters without disturbing the environment in the chamber.
A rubber glove port allows manipulation of devices in the chamber, and
twelve sampling ports with flow rate meters allow direct air sampling from
inside the chamber.
The EERF chamber is a continuous flow design. The radon-222 source
used in the chamber consists of selecting one of three gas-washing bottles
containing 25 uCi, lOOnCi and 400uCi respectively, of radium-226 in
solution. The appropriate source is selected for the chamber
concentration desired. Each ud* of radium-226 generates approximately 126
pCi of radon-222 per minute. The radon is harvested from the radium
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Figure 2
EERF Radon Calibration Facility
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solution by air bubbled through the solution at about 1 liter per minute.
The radon enriched air flows through traps and filters to remove any
inadvertent particulate radium carry-over, and then into a 20-liter aging
bottle which allows 20 minutes of radon daughter in-growth time before
release into the chamber. Final chamber concentrations of radon are
maintained by dilution with room air, which is normally controlled between
50 and 200 liters per minute. Condensation nuclei are added to the
dilution air supply by a voltage-controlled nichrome heating wire. An
automatic air humidifier keeps relative humidity at approximately 80
percent. Diffuser panels inside each end of the chamber accomplish
homogeneity of radon concentration in the chamber. Having passed through
the chamber, the radon exhausts through a wall into a large empty room
which serves as a buffer against turbulence. From this room it is
exhausted to the outside by a fan.
Exposure Measurements
Concentrations of radon-222 in the chamber were determined by
twice-daily grab samples in scintillation cells calibrated with known
quantities of radium-226. Radon was also monitored continuously with
calibrated flow-through radon detectors with integrated count totals
printed out hourly.
Working levels were determined by twice-daily grab samples of
filtered air with concentrations calculated according to a Modified
Tsivoglou technique (Th70) or an alpha spectroscopy technique (Ma69). The
alpha counter used is calibrated with a reference source of known
activity. In addition, the EERF hosted an intelaboratory radon/daughter
measurement conference in September, 1981, in which representatives from
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the Bureau of Mines, Environmental Measurements Laboratory, Oak Ridge
National Laboratories, Argonne National Laboratory, Mound Laboratory and
Lawrence Berkeley Laboratory participated in intercal ibration
measurements. All EERF radon exposure data have been corrected to the
calibrated values obtained during the exercise. These corrected radon
values, then, are the "true" radon values used in this study against which
the values reported by Terradex are compared. Stability and statistical
characteristics of our radon data are presented in the Results section of
this report.
Condensation nuclei concentrations were monitored with an Environment
One Model Rich-100 Condensation Nuclei Counter and the data recorded on a
strip chart recorder.
Temperature and humidity were measured with standard gauges mounted
inside the chamber, with values recorded manually each day.
Exposures Protocol
For each exposure, the desired concentration of radon was selected
and allowed to stabilize in the chamber as determined by a flow-through
detector and scintillation cell measurements. Detectors for the
experiment were then introduced into the chamber through the passbox and
suspended on a three-dimensional string matrix such that approximately .3
meters of free space surrounded each detector (Fig. 3). The detectors
were left in the chamber long enough to accumulate the desired exposure
and were then removed, packaged according to the manufacturer's
instructions, and shipped to the manufacturer for processing.
10
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Figure 3
Detectors Deployed in Exposure Chamber
I
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The exposure protocol required two series of runs, each series
consisting of one run for 10 days at 100 pCi/liter, one for 20 days at 50
pCi/liter, and one for 40 days at 25 pCi/liter, thus delivering the same
total exposure in three different exposure patterns. The entire series of
runs for all three exposures was repeated and constituted the second
series of runs. In addition, linearity of detector response was
determined by sequentially removing selected detectors from the exposure
chamber during one of the exposure runs.
Once radon concentration in the chamber had stabilized and been
determined, exposure duration was altered to deliver the cumulative
exposure of as near to 1000 pCi/1-day as possible. This was designed to
test the accuracy with which the exposure was delivered and measured, an
essential component of a calibration study. During each run, the chamber
contained various numbers of detectors in each of the four configurations,
including for each configuration at least one detector from two production
batches, Batch 8 and Batch 10.
At the time the study was designed, two assumptions were made that
had to be changed midway through the study: detector reliability was good
and lot-to-lot variability was small. Both of these assumptions tended to
keep the number of required data points quite small in the initial study
design. After the initiation of the first series of runs, the
manufacturer announced an approximate 30 percent difference in detector
response between Batch 8 and Batch 10. In addition, preliminary analysis
of the data from the first series of runs indicated greater than expected
variability. For these reasons it was deemed prudent to expand the data
base during the second series of runs, and, consequently, more detectors
12
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of each kind were exposed. Table 1 summarizes the numbers of detectors of
each type used in each series of exposure runs, Table 2 summarizes the
exposures achieved for the various stages of this study.
The final design-related item of note involved the selection of
detector area to be read. The manufacturer offers three standard
22 2
choices: 1 mm , 5 mm , or 15 mm , with respectively increasing
sensitivity due to statistical resolution. It was expected that
difficulty in maintaining exposure levels would dominate the overall
2
error, and for that reason the sensitivity associated with the 5 mm
2
service was judged adequate. All detectors used in this study had 5 mm
read.
13
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Table 1. Number of Detectors Used
A. 1st Series
Lot 1 (Batch 8)
Lot 2 (Batch 10)
Configuration(a)
M
0
Nominal Exposure
Length (days)
40 1111
20 1111
10 1111
Configuration a)
Nominal Exposure
Length (days)
40
20
10
MOB
1111
1111
1111
B. 2nd Series
Lot 1 (Batch 8)
Lot 2 (Batch 10)
Configuration(a)
M
0
Nominal Exposure
Length (days)
40 4442
20 4442
10 4442
Configuration (a)
M
Nominal Exposure
Length (days)
40
20
10
4444
4444
4444
F = Filter cup.
M = Membrane cup,
0 = Open cup.
B = Bare Card.
Table 2. Actual Delivered Exposures
Purpose
Rn Concentration (pCi/1) Duration (days)
Total Exposure
(pCi/liter-day)
Calibration
Calibration
Calibration
Calibration
Calibration
Calibration
Linearity
Linearity
Quality Control
110.00
98.84
55.12
50.91
26.90
26.09
110.70
109.80
110.00
9
10
18
20
38
38
3
6
9
990.0
988.4
992.2
1018.2
1022. 2
991.4
332.1
658.8
990.0
14
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Results
Chamber Stability
Exposure characteristics throughout the study were excellent.
Flow-through samples with integrated hourly count totals in conjunction
with twice-daily scintillation cell grab samples were used to verify
concentrations in the chamber. Average concentrations and one standard
deviation indicated less than five percent variation in radon
concentration over all exposure runs. Details of quality assurance
procedures for the EERF radon chamber are available separately (EPA82).
Radon concentration data produced by the EERF flow-through detector radon
concentrations were used with track densities reported by the manufacturer
to calculate calibration factors.
Calibration Factors
A summary of all calibration factors determined during the study are
shown in Table 3. For purposes of clarity it is noted again here that
each series is composed of three runs each (10, 20, and 40 day
exposures). The average calibration factor for all detectors of each type
and batch in each series is compared to the manufacturer's reported
calibration factors in Table 4.
The data in Table 4 indicate that the variation in calibration
factors for the first series of runs ranged from a percent relative
standard deviation of 6.9 percent (bare configuration, Batch 10) to 31.2
percent (membrane configuration, Batch 10).
Variation in the second series of runs was also higher, ranging from
12.2 percent "relative standard deviation (bare configuration, Batch 10) to
34.1 percent relative standard deviation for the open configuration, Batch
15
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Table 3. Summary of Calibration Factors (Tracks/mm per pCi /I iter-day
Nomi nal
Exposure
Length
Configuration/
Batch
F8
F10
B8
BIO
M8
M10
08
010
Series
.0295
.0283
.0728
.0956
.0121
.0135
.0610
.0656
10 Days
Series
.0207
.0584
.0490
.0734
.0114
.0195
.0324
.1046
+ .
+" .
+ .
T .
+_ .
T .
+" .
0025%
0198%
0014%
0071%
0011%
0032%
0022%
0425%
20
1st
Series
.0257
.0340
.0496
.0832
.0116
.0173
.0368
.0573
Days
2nd
Series
.0201 + .
.0547 + .
.0417 + .
.0686 + .
.0135 + .
.0225 + .
.0349 + .
.1019 + .
0009%
0104%
0036%
0059%
0011%
0035%
0025%
0227%
1st
Series
.0321
.0441
.0576
.0882
.0153
.0249
.0634
.0699
40 Days
2nd
Series
.0193
.0438
.0582
.0821
.0133
.0276
.0349
.1032
+
T
T
T
T
T
7
.0015%
.0049%
.0009%
.0037%
.0021%
.0054%
.0041%
.0329%
Numbers in the 1st series are single data points and appear without error terms.
Calibration factors are average of all data points in each category. Error term equals one standard deviation.
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Table 4. Manufacturer Calibration Factors
1st Series
2nd Series
(N=3)
F8(b)
F10
B8
M10
08
0
B8
BIO
.0291 +
.0355 T
.0130 +
.0186 T
.0537 +
.0643 +
.0600 +
.0890 +
.0032
.0080
.0020
.0058
.0147
.0064
.0118
.0062
(11%)
(22.5%)
(15.4%)
(31.2%)
(27.3%)
(9.9%)
(19.6%)
(6.9%)
.0200
.0523
.0127
.0232
.0341
.1032
.0496
.0747
(N=12)
+
T
T
T
T
+"
T
±
.0019
.0157
.0019
.0058
.0034
.0352
.0090
.0091
(9
(30
(14
(25
(9
(34
(18
(12
Reported
Manufacturer
.7%)
.0%)
.9%)
.0%)
•9%)
.1%)
•1%)
•2%)
.0372
.0484
.0223
.0290
.0562
.0731
.0891
.1158
+
T
T
T
T
T
T
+"
by
(A181)
16%
16%
26%
26%
22%
22%
20%
20%
(a' Midway t.hrouah t.hp st.urlv. the manufarturpr reDorted an aoDroximate
30 percent increase in Batch 10 sensitivity which is reflected in
the Batch 10 calibration factors.
(b) p = filter configuration
B = Bare configuration
M = Membrane configuration
0 = Open cup configuration
Error terms = one standard deviation.
17
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10. Of the 16 average calibration factors determined, vary significantly
from the manufacturer's reported values, the variability distributed
mostly among configurations and batches, in the second series.
In comparing the first series averages with the second series
averages, the first series was higher (range 2 percent to 57 percent) in 5
of 8 cases. The first series averages were lower (range 20 percent to 38
percent) in the remaining 3 cases.
Lot-to-Lot Variability
Batch 10 detectors showed an increased sensitivity in all cases, thus
confirming the manufacturer's report. In most cases, however, the degree
of sensitivity increase was substantially greater than the reported 30
percent.
Linearity
Linearity of detector response over the range of total exposure was
seen for both the filter and open configurations. Figs. 4-7 show this
linearity and the linear regression analysis of each batch (lot) and
configuration. Filter detectors showed a slope of .03955 (coefficient of
correspondence = .99805) and .03055 (coefficient of correspondence =
.9350) for Batch 8 and Batch 10, respectively. The open configuration
detectors showed slopes of .06309 (coefficient of correspondence = .98110)
and .07200 (coefficient of correspondence = .98890) for Batch 8 and 10,
respectively. The error bars on the data points represent one standard
deviation. Each data point is the average of 2 detectors. The linearity
portion of the study was done with a minimum of data points but was
designed to reveal any anomolous non-linearities which could have affected
other areas of the study. No such anonalies were found.
18
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50
40
Tracks/mm2
30
20
10
Figure 4
100 300 500 700 900
pCi/l-Days
Linearity Plot for Batch 8 Filter Detectors
19
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50
40
Tracks/mm2
30
20
10
Figure 5
100 300 500 700 900
pCi/l-Days
Linearity Plot for Batch 10 Filter Detectors
20
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Figure 6
Tracks/mm3
10
100 300 500 700 900
pCi/l-Days
Linearity Plot for Batch 8 Open Detectors
21
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Figure 7
80
70
60
50
40
Tracks/mm2
30
20
10
100 300 500 700 900
pCi/l-Oays
Linearity Plot for Batch 10 Open Dosimeters
22
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Quality Control
Twenty Batch 8 filter configuration detectors exposed together during
the first run were divided into two groups (5 and 15), each submitted for
processing and read out at a different time. Approximately six months
separated the processing of the two groups.
The calibration factor for the first group was .0359 +_ .0016 and the
calibration factor for the second group was .0224+^ .0057, a decrease of
about 38 percent.
Conclusions
The results of this evaluation tend to support the conclusion that
detectors exposed together and processed together will yield good
statistics. The calibration factors seen in Table 3 indicate that there
is no apparent affect of length of exposure on detector response. If not
processed together, the detectors exhibit a rather large and randomly
distributed variability as shown in Table 4. If coupled with a precise
determination of appropriate calibration factors (in one's own calibration
chamber), the detectors should prove useful over the range of exposures
used in this study. Also, our data indicate linearity of detector
response.
It was recognized at the beginning of the study that the detectors
respond not only to Rn-222 but also to radon daughter product alpha
particles (Po-214, Po-218). The bare detector configuration and probably
to some extent the open cup configuration would be expected to exhibit
some variability in response due to daughter product plate-out on detector
23
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surfaces. Working level equilibrium conditions during exposures would
then impact the response of those detectors. No attempt was made to
rigidly control the equilibrium conditions during exposure runs but random
grab samples throughout the study consistently indicated an approximate 50
percent equilibrium condition.
The unpredictable variations exhibited between exposure runs,
however, underscore the necessity for using the detectors with stringent
calibration and quality control techniques.
The differences in lot-to-lot sensitivity are significant and must be
considered when use of the detectors may involve more than one production
lot.
Differences in many factors - chamber equilibrium, exposure levels
used, cup manufacture, detector processing and reading - could conceivably
explain differences between first and second series of runs. It is
perhaps worth noting that the results in this study are similar to results
previously reported by the manufacturer in that results within a run were
very consistent, but results between runs were not. Further study should
be designed to delineate the source of variability in these detectors,
particularly at lower exposures levels.
24
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REFERENCES
AT81 Alter, H. Ward and Fleischer, Robert I., 1981, Passive
Integrating Radon Monitor for Environmental Monitoring, Health
Physics, 40 693 1981.
ANL74 Argonne National Laboratory, 1974, An Instant Working Level Meter
with Automatic Individual Radon-Daughter Readout. Final Report
for U.S. Bureau of Mines Contract No. H0122106, August 1974.
EPA82 Environmental Protection Agency, 1982, Quality Assurance Project
Plan for the EERF Radon Calibration Facility, Montgomery, Alabama.
Ge76 George, A.C., 1976, Scintillation Flasks for the Determination of
Low Level Concentrations of Radon. Proceedings of Ninth Midyear
Health Physics Symposium, Denver, Colorado, February 1976.
Ge77 George, A.C. and Breslin, A.J., 1977. Measuring Radiation in and
Around Uranium Mills. Workshop of Methods for Measuring
Radiation in and Around Uranium Mills, ed. E.O. Harward, Atomic
Industrial Forum, Inc., Program Report, Vol. 3 1977.
Ha72 Harley, J.H., ed., EML Procedures Manual, U.S. Department of
Energy Report HASL-300, updated annually 1972.
Ha69 Harley, N.H. and Pasternack/B.S., 1969, The Rapid Estimation of
Radon Daughter Working Levels When Daughter Equilibrium is
Unknown. Health Physics, 17, 109 1969.
25
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REFERENCES-Continued
Ku56 Kusnetz, H.L., 1956, Radon Daughters in Mine Atmospheres - A
Field Method for Determining Concentrations. Am. Ind. Hyg.
Assoc. J., _17, 85 1956.
Lu57 Lucas, H.F., 1957, Improved Low-Level Alpha Scintillation Counter
for Radon. Review Sci. Instrum., 28, 680 1957.
Ma69 Martz, D.E., Ho-lleman, D.F., McCurdy, D.E., and Schiager, K.J.,
1969, Analysis of Atmospheric Concentrations of RaA, RaB and RaC
by Alpha Spectroscopy. Health Physics, 17, 131 1969.
Sc74 Schiager, K.J., 1974, Integrating Radon Progeny Air Sampler. Am.
Ind. Hyg. Assoc. J., 35, 165 1974.
Th70 Thomas, J.W., 1970, Modification of the Tsivoglou Method for
Radon Daughters in Air. Health Physics, 18, 113 1970.
Th72 Thomas, J.W., 1972, Measurement of Radon Daughters in Air.
Health Physics, 23, 783 1972.
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