The use and sensitivity of the multiwire proportional counter to detect Kr-85 in ambient air are examined. Data also cover monitoring beta and X-ray emitting radio nuclides at low activity levels. Results show the counter to have excellent properties for monitoring Kr-85 down to 0.0004 pCi cu/cm

Gregory, J. C.

English

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https://ntrs.nasa.gov/search.jsp?R=19760011322 2020-03-22T17:07:47+00:00Z
Semi-Annual Report 1976
Contract No. NAS8-31170
(NASA-CR-144205) SPECIAL-GEOMETRY
	 N76-18410
PROPCRTIONAL COUNTEFS AND TECHNIQUES FOE
DETECTION OF LOW-LEVELS OF X-RAYS AND
	
N	 3'^ U
BETA-PARTICLES Semiannual Progress Report,
	 Unclas
1 Aug. 1975 - 1 Feb. 1976 (Alabama Univ.,
	 G3/35 18563
SPECIAL-GEOMETRY PROPORTIONAL COUNTERS
AND TECHNIQUES FOR DETECTION OF LOW-LEVELS OF X-RAYS
AND S-PARTICLES
by
John C. Gregory
Semi-Annual Progress Report
1 August 1975 - 1 February 1976
Submitted to
National Aeronautics and Space Administration
George C. Marshall Space Flight Center
Marshall Space Flight Center, Alabama 35812
Submitted by
The University of Alabama in Huntsville
School of Graduate Studies and Research
P.G. Box 1247
Huntsville, Alabama 35807
-r
1
February 1976
SUMMARY
Part of the program of development of a planetary x-ray diffracto-
meter at UAH has involved the devising of arrays of detectors of
special geometries to increase the counting efficiency of a Martian
instrument for signal photons.
The multiwire, multiplane proportional counter described here was
designed and built to test various modes of increasing efficiency by
using coincidence and anticoincidence techniques. Another application
of these techniques was developed during the work and is described
here. The S-detection application and the Martian x-ray application involve
pulling low level signals out of gamma- and singly charged particle
background and are conveniently tackled by proportional counter
methods as described here.
The use of the inert radioactive gas Kr-85 for tracer studies, leak
detection, etc., will become more widespread as the sensitivity of
available detectors increases. A multiwire multiplane proportional
counter system for beta detection has been developed with a measured
minimum detectable concentration for a 3-minute count of 3 x 10 -3
pCi cm-3
 of Kr-85 in a semi-infinite cloud. Larger proportional
counters operated for counting times of several hours can reduce this
limit to below 1 x 10-4 pCi cm 3 . These detectors should be useful in
monitoring low levels of the radioactive inert gases and of x-ray
emitting nuclides.
f.
I
{
INTRODUCTION
In the last ten years, a number of applications of krypton-85 as
a fluid-flow detection medium have been developed. Some examples in
the literature1 -4
 describing applications and advantages of the radio-
active gas tracer techniques and a rather complete bibliography 5 of
krypton-85 uses are referenced. The technique has been used to detect
leaks in hermetic seals, for oil field porosity studies, blood flow
studies, and detecting stress cracks in turbine blades. The use of
the krypton-85 tracer has been considered for general leak detectionl,6
in much the same way that helium detection is now applied. Acceptance
by industry and other users has suffered from a lack of detectors of
sufficient sensitivity to detect levels of radio-krypton low enough to
satisfy concern about statutory limits 7 for practical applications.
Recently we have used the technology of construction of large area
proportional counter hodoscopes, originally developed for accelerator
experiments, to build a number of detectors with optimum-sensitivity for
low concentrations of kr ypton-85 in ambient air. In addition to
laboratory applications where krypton-85 and other beta emitting gases
are used, the counters have application in monitoring beta and x-ray
emitting radio nuclides at low activity levels in the environment.
A comprehensive discussion of the calibration and testing of a
:variety of different krypton-85 detectors commercially available in 1970
in the U.S. was given by Smith, Cochran and Shleien 8 , and in 1972 in
Germany by Maushart9.
a r
4DETECTOR SENSITIVITY TO KRYPTON-85
The sensitivity of a detector to a particular radiation may be
expressed in terms of the statistical error in the signal, S, in counts,
obtained when the detector of area A is exposed for time t seconds to
a concentration of radioactivity of activity C (in this case in pico-
Curies per cm 3).
S = FSACt	 where F s
 is the efficiency of calibration
factor for the particular radiation and
deteitor 3 combination in counts cm 2s-ipCi- cm .
Similarly, the sensitivity to background radiations given by;
B = FBARt	 where Fg is a composite calibration
factor for the background radiation
and R is the background activity.
The limit of detection of the - radioactivity is set at some number,
N, of standard deviations of the background counting rate a(B)~ 3B.
SL = Na (B) = N 3 B.
This limit corresponds to a minimum detectable concentration,
CL , where
C	 N F
BARt -
	N ^_FB ­. R
FSAt	 Fs	 A • t
Thus to increase the sensitivity of any measurement to the lowest
activities, we maximize the exposure, A. t, and design our detector for
optimum Fs . Usually this also increases F B ; but, since the sensitivity
increases as Fs / B the net result is beneficial. Design features
may be incorporated deliberately to reduce FB.
Emissions from krypton-85 are:
s, Emm - 0.67 MeV, E = 0.49 MeV, frequency 99.6%
y, E = 0.514 MeV, branching ratio 0.4%
Some detectors (e.g., scintillators) make use of the y, but are
intrinsically inefficient because of the low branching ratio. They may
also have higher sensitivity to background y's. Gas-filled a-detectors
may either inhale the radioactive gas or incorporate thin windows to
admit the 6's. The mean range of krypton-85 Ws is 20 cm of ambient air
or 25 mg cm 2 of plastic material.
Multiwire proportional counters compare favorably with Geiger
counters for this application for several reasons. They are readily
fabricated in le-ge area arrays, they may be readily stacked in layers
to provide coincidence and anticoincidence for improvement in the
racio Fs / 3 ' FB , and they posses energy resolution which may also be used
to improve background rejection.
J	 _ ^y
EXPERIMENTAL
The multiwire proportional counter with which the measurements
described below were made is shown in Figures 1 and 2. Anode wires are
50 pm gold-plated tungsten; anode spacin e, and gap-width is 0.5 cm.
Three anode planes are shown, with four cathode planes. 	 The
upper two cathode planes were formed from a single continuously wound
150 pm dia. stainless steel wire.	 Wire separation was 0.1 cm. 	 The
cathode plane separating the second and third anode planes was a
0.125 cm- thick aluminum plate, the thickness of which exceeds the
maximum range of krypton-85 B's.
Three signal outputs were brought out of the counter, one for each
anode plane.	 All the anodes in each plane were connected.	 The outer-
most wires of the upper two planes were connected to the bottom
plane to form an anticoincidence shield.	 The active area (central
portion) was 7.5 x 15 = 112.5 cm 2
Electronics consisted of three preamplifiers, post amplifiers, and
discriminators io , plus coincidence and anticoincidence logic. 	 The
following logic requirements could be set: 	 1) a particle must trigger
both upper plane discriminators, and it must not trigger the bottom
plane (U + M - 1,)-.	 or 2) anticoincidence between upper and	 lower planes
(U	 -	 1.) .
Calibrations with krypton-85 were performed by placing the counter
In a large hermetically sealed steel box 68 cm x 68 cm x 24 cm. 	 Known
volumes of a standard 24 PCi cm -3 mixture of krypton-85 in N, were
titrated into the box and mixed with the air inside with a pump. The
physical arrangement used here to represent an 'infinite cloud' of
krypton-85 in air will clearly give conservative estimates of the
minimum detectable limit, but no corrections were made for this
factor.
Table I
Laboratory Background Measurement
Signal Count Rate (200 s) -1 	^^
Upper plane. U 933
Middle plane, M 870
Lower plane, L 1,924
U a d L in anti-
coincidence (U -L) 334
-	 U and L in anti-,
plus U and M in 206
coincidence
(U+M - L)
The background counting rates in the laboratory are shown in
Table I for each plane separately and with the counter in two logic
modes.
It may be seen that the simple anticoincidence mode (U - L) reduces
the background to one-third of its normal value, while the added
requirement of coincidence with M reduces it to one -fifth.
The performance of the counter in two logic modes to an 'infinite
cloud' of krypton-85 is shown in Table II.
Table II
Counts Per 200 s (Mean of 8 Readings)
Logic Mode U+ M- L U- L
0.07 pCi cm 3 689 1,102
Background 185 284
i	 J
Although the ratio of source counts to background counts is only
slightly improved by dropping the middle plane coincidence requirement,
the ratio F./ g increases considerably.
The results of several series of measurements of count rate at
different concentrations of krypton-85 are shown in Figure 3. The count
rate, S, is the number of counts in 200 s attributable to the radio-
active source and is obtained by subtracting the mean background from
the total count rate.
Our laboratory background in the mode (U - L) used to collect the
data in Figure 3 was typically measured at 1.5 counts 9-1 or 0.013 cts
cm-2 s-1 . This is lower than the background rate with the most
sensitive 4 n Geiger detector measured by Smith, et al .,viz 0.021 cts
cm-2 s-1 . Furcher improvement would be expected with passive shielding
or a double window (47) proportional detector.
From the curve in Figure 3, the calibration factor, Fs , was
determined to be 0.49 counts cm-2 s- ' pCi- 1 cm S. CL may also be
derived from Figure 3 for a 200 s count with this detector for any
desired level confidence. At the 20 level for a 200 s count,
CL - 3.1 x 10-3 pCi C!2-3. Minimum detectable concentrations for
other times and confidence levels are given in Table III.
Table III
CL(Kr-85) Minimum Detectable Limits (pCi cm 3 ) for Proportional
Counters of Different Areas,
Various Counting Times and Levels
of confidence
r
Confidence Level._.__-___^__ _a ^
	
2a`^
Test Coutater, 200 a 	 ^ 3.1 x 10-3
Test Counter, 4 hr.	 3.8 x 10'4
0.25 m 2 , IoT, Counter, 200	 4.7 x 10-"
0.25 m 2 , 47, Counter, 4 hr.! 6.0 x 10'S
3a
4.9 x 10'3
5.8 x 10-"
7.3 x 10-"
9.0 x 10'5
Multiwire proportional counters may be made arbitrarily large. We
have had several years reliable experience with such detectors of active
area 50 cm x 50 cm, though we have never used them for radionuclide
monitoring. Since these. detectors have similar electrode dimensions
and operating characteristics, we should expect them to possess similar
values of Fs . Assuming this, calculated minimum detectable concentrations
of krypton-85 using the 0.25 m  counter with windows on both sides are
shown in Table III. Four hours is used as the longest counting time.
r
A multiwire multiplane proportional counter has been shown to have
excellent properties for monitoring concentrations of krypton-85 in air
down to 4 x 10-" pCi cm-3 . Larger but still portable detectors could
reduce this minimum detectable limit to - 8 x 10_5 pCi cm- 3 . Detectable
limits of this order will probably remain the practical limit, since
further increases in sensitivity will require detectors of much larger
area and complexity. Passive shielding could be used to increase the
sensitivity somewhat by reducing the background rate. Such measurements
would be practically confined to the laboratory.
Proportional counters of this type may have a wider application in
the monitoring of low levels of a and x-ray emitting nuclides.
1. "A Survey of Leak Detection for Aerospace Hardware," F. E. Wells,
Materials Evaluation, May 1969, p. 97.
2. "Radioactivi ty by the Whiff," M. S. Stocknoff and F. 5cornavacca,
Industrial Research, March 1975.
3. "Radiotracer Propellant Gage," Isotopes and Radiation Technola¢v,
Vol. , No. 4 (1968).
4. "Radioactive Gas Penetrant System." W. C. Eddy, Proceedings of the
Eith h Sympolium on Non Destructive Evaluation in Aerospace,
Weapons Systems and Nuclear Applications, San Antonio, April 1971.
5. "Krypton-85; A Review of the Literature and an Analysis of
Radiation Hazards," W. P. Kirk, EPA Office of Research and
Monitoring, Washington, DC 20460 (1972).
6. "Using Radio Krypton for Detection of Leaks in Space Vehicles,"
MSFC Report IN-R-QUAL 66-6.
7. Standards for Protection Against Radiation:	 10 CRF 20.
B.	 "Calibration. and Initial Field Testing of Kr-85 Detectors for
Environmental Monitoring," D. G. Smith, J. A. Cochran, and
B. shleien, BRH/NERHL 7--4, November 1970, U.S. Department of HEW,
Bureau of Radio`ogica:i Health, Rockville, Maryland 20852.
9. "Rauh- and Abluftuberwachung an Kerntechnischen Einrichtungen,"
1973, BF-54-W-1-1237, BF Vertriebs Gmbh fur Messtechnik, D-7500
Karlsruhe 31 (R. Maushart).
10. Rizzo, A; T. A. Parnell; and G. A. Guenther, IEEE Transactions on
Nuclear Science, NS-19, 3 (1972).
Figure 1:	 Diagram of multiwire multiplane proportional
counter showing configuration.
Figure 2:	 Photograph of test counter.
Figure 3:	 Calibration curve of the test counter for Kr-85
activities in the range 0.005 -1.0 pCi cm
-3
made in the presence of normal room background.
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Special-geometry proportional counters and techniques for detection of low-levels of X-rays and beta-particles

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