Calibration of argon gas neutron activation sampling devic

Day, W. C.

Selden, R. F.

English

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HEALTH P H Y S I C S  
Oficial Journal of the Health Physics Society 
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Health Phwfis Pergamon Press 1963. Vol 9, pp. 921-925)) Printed in Northern Ireland 
' K 9 ) / 9 6 3  
-- 
CALIBRATION OF A FISSION GAS MONITOR* 
WALTER C. DAYt and ROBERT F. SELDE y Y 9 - J '  
Lewis Research Center, National Aeronautics and Space Adminis ration, 
Cleveland, Ohio 
(Received 17 July 1961; in revisedform 8 April 1963) // 9 c /  
Abstract-A method for calibrating beta-sensitive gas-monitoring equipment is presented 
which avoids precise mixing of concentrated active gases with diluent air This method depends 
upon ntutron activation of the argon in P- 10 proportional counting gas L d  passage ofthe active 
gas s u e v e l y  through the monitor chamber and a proportional counter chamber. The latter 
serves for the determination of specific activity of the monitor gas. The method is satisfactory 
for specific activities corresponding approximately to maximum permissible concentration 
(MPC) levels. 
INTRODUCTION 
IN MONITORING for gross airborne activities, the 
separate determination of particulate and 
gaseous activities is frequently desirable. The 
particulates can be determined by conventional 
filter techniques, which in conjunction with 
suitable detectors serve for the separate deter- 
mination of gross alpha, beta and gamma 
activities. I n  most situations, true gaseous alpha 
contaminants are not ofinterest. Because of these 
considerations, the calibration method discussed 
herein is limited to gaseous beta-gamma activi- 
ties. The monitor chambers discussed employ 
a chamber volume of roughly 1 ft3 and thin 
window GM or scintillation detectors mounted 
internally. The calibration method is not lim- 
ited to such arrangements however. These 
detectors were selected because their response 
reflects disintegrations rather than emitted 
energy. 
Most gaseous activities, such as mixed fission 
gases and their daughters, emit both beta and 
gamma radiation, but not in any fixed ratio for 
all isotopes. Thesc contaminants are so complex 
and time dependent that it is seldom,feasible to 
identift the separate activities and their relative 
+Work pafbrmed at NASA Lewis Research 
Center during 1960 and early 1961. 
?Now located at the Lawrence Radiation 
Laboratory, Univerdty of California, Livermore, 
California. 
1 
1 
1 
NASA L&& ~esearch center. 
4 iJT%bC 
contribution to the total air contamination. An 
estimate of their total contribution must suffice 
in most instances. The concomitant gamma 
radiation is customarily ignored because of the 
generally lower detection efficiencies involved 
plus the fact that beta emission usually accom- 
panies disintegration. 
In principle, the response of monitoring 
systems can be estimated analytically on the 
basis of geometrical and electrical considerations, 
detection efficiencies, average beta absorption, 
etc.§(1s2) In most practical situations, however, 
the required corrections and assumptions are 
such that some sort of calibration is necessary in 
order to obtain a reasonably good estimate of 
monitor response. This calibration has to be a 
compromise in view of the range of beta energies 
exhibited by the fission gases. The use of argon- 
41 for this purpose seems to be as satisfactory a 
choice as any other. 
The low-energy betas originating in the more 
remote parts of the monitor chamber areless 
likely to impinge on the detector surfaceand 
produce a count than are those of greater energy. 
This diEculty can be minimized by reducing the 
physical dimensions of the monitor chamber but 
this also reduces the amount of activity which can 
actuate the detector. As a consequence the 
chamber has to be of reasonable size in order to 
5 While intended for gauuna radiation some 
geometric relations shown could be used for beta 
radiation, with proper assumptiom. 
921 
922 CALIBRATION OF A FISSION GAS MONITOR 
get statistically significant results at the low 
MPC levels of interest. Beta particles having 
energies in excess of 0.2 MeV can penetrate a 
total air distance of about 1 ft. A portion of 
those betas having lower energies are also de- 
tected depending upon their point of origin in 
the chamber. On the basis of these qualitative 
considerations, the calibration factor should not 
show marked variation with energy for those beta 
activities having maximum energies of 0.6 MeV 
or more, inasmuch as such betas have average 
energies exceeding 0.2 MeV. Computed calibra- 
tion factors for right cylindersc2) indicate that 
this conclusion is approximately valid. An 
inspection of a typical tabulation of the beta 
energies for the fission gases and their daughters 
reveals that many, but by no means all, show 
maximum energies in excess of 0.6 MeV.(3) 
CALIBRATION METHOD 
The usual method of calibrating air monitors 
depends upon the admixture of known amounts 
of some gaseous activity to a known volume of 
air with a later determination of the monitor 
response with this contaminated air. This 
approach permits the use of gaseous activities 
having known beta energies. Thus, any effects 
of beta energy upon the calibration can be 
readily studied within the limits imposed by the 
availability of suitable reference activities. 
This method is particularly desirable where a 
single contaminant is of interest in the monitor- 
ing system. 
There are some disadvantages in using this 
method, however. These include: 
(1) The specific activity of the added con- 
taminant must be determined separately or else 
depend upon the supplier’s evaluation. The 
time, cost and equipment required for this 
independent determination may be objection- 
able. 
(2) The specific activity of the added contam- 
inant may be well above MPC levels and thereby 
constitute some hazard if the concentrated 
contaminant is accidentally released. 
(3) Some chemical processing to derive the 
active gas may be required except where active 
inert gases are employed. 
(4) Precise volume measurements are required 
in the actual dilution. With high initial specific 
activities this may present a problem. 
(5) Intimate mixing with the air volume is 
required. 
(6) The contaminants generally have to be 
obtained offsite. 
(7) Appreciable absorption of active con- 
taminants on the container walls may occur for 
other than the inert gases. 
The method herein suggested depends upon 
the thermal neutron activation of the argon in 
P-10 proportional counter gas (90 per cent 
argon, 10 per cent methane) such that the result- 
ing specific activity lies in the range of interest. 
Some details for two activation schemes are 
included in the next section. This P-10 mixture 
is readily obtainable commercially and is 
usually available wherever proportional count- 
ers are in use. After activation, this gas is passed 
successively through an ordinary proportional 
counter chamber of known volume and the 
monitor chamber being calibrated. The monitor 
chamber can employ any of the detectors pre- 
viously mentioned or it can be operated as an 
ionization chamber. The specific activity of the 
P-10 gas is determined simply and directly by the 
ratio of the net count rate of the proportional 
counter and the product of its volume and the 
dpm-pc constant. Thus the specific activity of 
the gas in the proportional counter (PC) is given 
by 
PC net counts/min 
PC volume in cm3 x 2.2 x lo6 dpm/pc 
The only assumption involved is that all beta 
disintegrations in the counter volume are re- 
corded as counts irrespective of where these dis- 
integrations occur therein. Obviously, this condi- 
tion will not be altogether valid for those gas 
systems where any separation of daughter 
particulate activities or any adsorption of 
gaseous activities on the counter walls can 
occur. Any such separation effects will influence 
the gas monitor also. 
Once this measure of specific activity and the 
corresponding monitor response has been ob- 
tained, the calibration factor for a particular 
monitor chamber is readily computed. This 
should be valid for all other specific activities, 
provided that the monitor counting system is not 
overloaded and the beta particles have the 
requisite energy. Overloading is unlikely at  the 
activity levels of practical concern. 
WALTER C. D.4Y and ROBERT F. SELDEN 923 
About 99 per cent of the argon-41 undergoes 
coincident beta and gamma decay, the maxi- 
mum beta energy being 1.2 MeV and the photon 
energy being 1.3 MeV.(s) The remaining dis- 
integrations yield a beta particle having a 
maximum energy of 2.48 MeV. Most of the 
betas, therefore, have sufficient energy to effect a 
monitor count for the suggested chamber sues, 
irrespective of the point of origin in the monitor 
chamber. The gamma contribution to the 
monitor response should be s m a l l  for a thin 
scintillator and is likely to be trivial in the case of 
a thin-window Geiger detector. As hr as the 
proportional counter is concerned, the argon41 
acts essentially as a pure beta emitter. Its half- 
life (1 10 min) is short enough to permit calibra- 
tions over a fair range of activities if this is 
desired. 
Obviously the specific activity in both the 
monitor and the proportional counter chambers 
must be the same in order to obtain a proper 
calibration constant. Ordinarily, there is con- 
siderable disparity in size between the propor- 
tional and the gas monitor chambers, these being 
about 60 and 14,000 ml in the present instance. 
The greater this disparity, the greater the uni- 
formity in specific activity must be in order to 
have a representative sample in the proportional 
chamber. Reversing the flow direction through 
the proportional and monitor chambers provides 
a good check on the uniformity of the specific 
activity in each. Prior flushing with inert P-10 
gas is advisable for complete air removal. 
Another satisfactory procedure which requires 
considerably less total activity is to first fill both 
chambers with inert P-10 gas. The active P-10 
gas is then added to the monitor chamber such 
that its gas pressure is somewhat above atmos- 
pheric. After thorough mixing (e.g. with an 
internal fan) some of this active mixture is bled 
through the proportional counter chamber for 
flushing purposes. The calibration is then made 
as before. A suitable correction for the higher 
pressure in the monitor chamber can be made if 
desired. 
While the chiefvirtue of this method lies in its 
simplicity, there are some disadvantages: 
(1) The calibration is limited to argon41 
activity. Hence, the magnitude of beta energy 
effects must be separately evaluated analytically 
or by adding other gaseous activities. 
(2) The P-10 gas density is greater than that of 
air in the ratio of about 38 : 29. The linear beta 
absorption coefficient in P-10 gas is thus greater 
than for air. 
These are not believed to be serious objections 
with respect to monitoring the inert fission gases 
and their daughters as air contaminants. As 
already indicated, the exact nature and composi- 
tion of these activities will be unknown. The 
complexities in mode of decay and rate of decay 
of such activity mixtures precludes an exact 
specific activity evaluation by any type of gas 
monitor. As a consequence, corrections for 
monitor response at various beta energies are 
unwarranted. Furthermore the proper MPC 
values are just as uncertain, thus necessitating 
the use of mixture contamination limits. 
By the same token, correcting the calibration 
constant for the difference in density between 
P-10 gas and air is also of questionable value. 
This is especially true in view of the fact that this 
constant, as obtained by the calibration method 
suggested above, necessarily gives conservative 
values for air activity. Owing to the smaller beta 
absorption coefficient for air, a given activity 
level in air must give a larger monitor response 
than will the same level in P-10 gas. 
APPARATUS AND PROCEDURE 
Diagrams of the apparatus used in two 
typical calibrations are presented in Fig. 1 (a) and 
(b). The chamber and detector details are 
indicated for each case. Both chambers have an 
internal volume of roughly 0.5 fP. The steel 
walls of the cylindrical chamber are 3 in. thick. 
The resulting background with the 4 Geiger 
detectors was about 70 counts/min. The cham- 
ber is provided with a s m a l l  i n t d  fhn to 
achieve adequate mixing. The spherical cham- 
ber has a thin sheet-aluminium wall. Its back- 
ground, for the particular detector adjustments 
used, was about 145counts/min. Sice no 
provision for internal mixing was made, it was 
necespary to depend upon adequate flushing 
with active P-10 gas. Both chambers were well 
flushed with inert P-10 gas More Calibration. 
For the cylindrical monitor a Po& source 
having an emission rate of about 4 x 108 
neutrons/sec was used to activate the P-10 gas. 
Polyethylene tubing 0.25 in. 0.d. wound on a 
thimble to give a coil length of about 8 in. and a 
924 CALIBRATION OF A FISSION GAS MONITOR 
TUBES. 1.4 TO 2 
mglcrnB WINDOW 
THICKNESS, 1.5 
in. WINDOW DIA.7 
WINDOWLESS. 
GAS FLOW 
PROPORTIONAL 
COUNTER 
VOLUME - 60 rnl 
FOR ACTIVATING P-IO 
GAS WITH A PoBe MONITOR 
CHAMBER, 
loin ~d , 
NEUTRON SOURCE 
loin LONG 
I A l  CYLINDER 
(BI SPHERE 
FIG. 1. Schematic diagram showing calibration 
apparatus. 
thickness of about 3 in. was used as the modera- 
tor. This coil was placed inside a container 
designed for 2000 psig. P-10 gas to be activated 
was admitted at  500 psig, both to the interior 
and exterior sides of this polythylene coil. The 
PoBe source was placed in the thimble and left 
there for several half-lives. After activation, the 
gas in the tubing and some of that outside, was 
passed into the monitor chamber to give a final 
pressure somewhat above atmospheric. After 
mixing, this active P-10 mixture was bled 
through the proportional counter chamber and 
its specific activity determined. 
In the case of the spherical chamber, the P-10 
gas was activated in an aluminium tank 4 in. in 
diameter and 16.5 in. long at about 50 psig, 
using the thermal flux (about IO7 neutrons/ 
(cmz)(sec)) available on the exterior of the 
NASA Zero Power Homogeneous Reactor 
Facility. Higher specific activities were attain- 
able by this method. 
RESULTS AND DISCUSSION 
Calibration data for the cylindrical and 
spherical monitor chambers are presented in 
Figs 2 and 3, respectively. 
In order for the monitor response to be 
exactly proportional to the specific activity at all 
activity levels in the monitor chamber, all cali- 
bration points when plotted on rectilinear paper 
PROPORTIONAL COUNTER NET counts/rn.n 
FIG. 2. Comparison of proportional counter and 
cylindrical gas monitor count rates 
REPRESENT SUCCESSIVE COUNTS TAKEN ._ C AFTER EACH PURGE OF THE PROPORTIONAL 
COUNTER AND MONITOR CHAMBER WITH 
IRRADIATED P-IO GAS / 
TWO SUCCESSIVE 
ACTIVATIONS 
ARE INCLUDED / r 
/ 
/I' 
IVI I 5 1 1 " ' '  8 8 a ' ' ~ ~ ' ~ ~  ' ' ' " "  
IO 102 io3 
PROPORTIONAL COUNTER NET counls/rnin 
FIG. 3. Comparison of proportional counter and 
spherical gas monitor count rates. 
1 3  
WALTER C .  DAY and 
must lie on a straight line which passes through 
the origin. In other words, 
Monitor response in net counts/min 
= k x specific activity 
PC net counts/min 
PC volume x 2.2 x lo6 dpm/pc = k x  
where k is the desired calibration constant. 
In Figs 2 and 3, the logarithm of the monitor 
net count rate is plotted against the logarithm of 
the proportional counter net count rate usinglog- 
log paper. The equation for this line is accordingly 
log(monitor counts/min) = log(PC countslmin) + log k - iog(PC volume) - Iog(2.2 x 103 
The last three terms on the right side of this 
equation are constants for a given calibration 
system. Only one of these, namely log k, is 
determined by the monitor chamber and its 
detector. As k varies from one monitor system to 
another, the lines representing the calibration 
data should properly lie parallel to one another 
and to the dotted line through the even decade 
points. This property of log-log plots is of some 
service in drawing the best calibration line. 
The straight lines through the calibration data 
points in Figs 2 and 3 are drawn in accordance 
with this principle of parallelism. Except for 
three points plotted for low activities in Fig. 2, 
most of the experimental data in both figures 
agree quite well with the calibration lines shown. 
The actual error magnitudes at the 95 per cent 
level are shown at three activity levels in Fig. 2. 
Except at the very low activity levels, most of the 
points lie within the expected deviations &om 
the calibration line selected for representing the 
experimental data. 
The proportional wunter net count rate 
corresponding to an activity level of lom6 p c / d  
is indicated in Fig.s 2 and 3 for orientation 
purposes. The minimum practical activity level 
r is usable may be assum- 
their net count rate equals 
und count rate. In  the 
case of the cylindrical monitor (Fig. 2) this limit 
is about lo-' yc/&l. For the spherical monitor 
this limit is about 4 x 10-7pc/ml. These do 
not necessarily represent the ultimate limits but 
rather those for which these particular systems 
are wnsidered satis&ctory. 
The data plotted in Fig. 3 represent several 
additions of activated P-10 g a s  to the monitor 
ROBERT F. SELDEN 925 
chamber, some of which involved activation of 
separate batches of P-10 gas. The fact that all 
points lie so close to the calibration line must 
indicate that the added activity was well dis- 
tributed throughout the sphere. If this were not 
so it is difficult to see how the activity in the 
proportional counter chamber could be so rep- 
resentative of that in the sphere in all cases. N o  
special provision for internal mixing in the 
spherical chamber was employed in this case 
other than that provided by the incoming 
activated gas st ream.  
The validity of the assumption stated earlier 
that all disintegrations in the proportional 
counter chamber were wunted has not been 
exhaustively investigated. The available litera- 
ture has not been of much help in this respect. 
HOGREBE(4) does claim that the counting losses 
near the wall of a cylindrical chamber of about 
the same diameter are of the order of a few per 
cent. Some qualitative tests with a C-14 point 
source in a proportional chamber of about four 
times the diameter show that losses in excess of 
10 per cent are unlikely. It is believed that the 
losses in the proportional counter chamber used 
in these calibrations were substantially less. 
Errors of this magnitude are certainly acceptable 
in terms of the intended use of these monitor 
chambers. 
SUMMARY 
Monitoring equipment for the measurement 
of gaseous activities in air ordinarily requires 
some sort of calibration. A simple method using 
conventional equipment is described, and some 
typical calibration data presented. The method 
depends upon the neutron activation of the argon 
in P-10 proportional counter gas and the deter- 
mination of its specific activity in such a counter. 
The same gas is also passed through the monitor 
chamber and its response noted. 
REFERENCES 
1. V. V. B0-v and V. A. BAZHENOV, Some 
Metbods of Measuring !-Active Gases. Metrology 
of RadhucMs, p. 327. International Atomic 
Energy Agency, Austria (1960). 
2. T. ROQCWU, 111, Reactor ShicMing Design M d ,  
Chap. 9. TID 7004 (1956). 
3. D. STROMINGER, J. M. HOLLANDER and G. T. 
SEABORG, Rev. Mod. Phys. 3@, 2 (1958). 
4. K. HOGRF~BE, The Absolute Measurement of the 
Radioactivity of Gases. Metrology of Radio- 
nuclci&s, p 301. International Atomic Energy . 
Agency, Austria (1960). 


Calibration of a fission gas monitor

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