Method of and apparatus for monitoring radioactivity concentration of gas

A radioactivity monitoring system capable of exactly detecting the concentration of .sup.131 I in a gas even in a low concentration is disclosed in which a sample gas is passed through a filter cartridge filled with an iodine absorbing material to accumulate iodine, radiation is counted on both entrance and exit sides of the filter cartridge, the absorption distribution of iodine absorbed in the filter cartridge is estimated from the ratio between respective counts obtained on the entrance and exit sides, and one of the counts is corrected by the estimated absorption distribution of iodine to calculate the radioactivity concentration of .sup.131 I in the gas.

BACKGROUND OF THE INVENTION 
The present invention relates to a method of and an apparatus for 
continuously monitoring the concentration of a specified radioactive 
element in a gas, and more particularly to a method of and an apparatus 
for monitoring the radioactivity concentration of a gas wherein a 
radioactive element in the gas is accumulated on an absorption material 
and then the radioactivity of the accumulated element is measured, so that 
even an extremely low radioactivity concentration can be detected. 
An article entitled "Continuous Monitoring of Radioactive Iodine Emission" 
by J. G. Wilhelm and H. Mahnau, which was presented at the 13th AEC Air 
Cleaning Conference, 1974, discloses a monitoring apparatus in which a 
sample gas is introduced into a filter cartridge filled with an iodine 
absorption material, to accumulate .sup.131 I in the sample gas on the 
absorption material, .gamma.-rays emitted from the accumulated .sup.131 I 
are detected by a scintillation counter disposed adjacently to the filter 
cartridge, and both the frequency of detecting .gamma.-rays and the volume 
of the sample gas passing through the filter cartridge are monitored to 
know the radioactivity concentration of .sup.131 I in the sample gas. 
In such an apparatus, it is an important problem how many portions of 
.sup.131 I isotopes can be collected by the absorption material when the 
sample gas passes through the filter cartridge. The absorbing power of the 
iodine absorption material depends upon such factors as temperature, 
humidity and velocity of the gas passing through the filter cartridge. 
Accordingly, the ratio of the amount of iodine collected by the filter 
cartridge to the amount of iodine in the gas passing through the filter 
cartridge varies with the above-mentioned factors. Therefore, the 
radioactivity concentration of a gas which is calculated on the basis of 
the radioactivity of accumulated iodine, contains unavoidable errors. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a method of and an 
apparatus for monitoring the radioactivity concentration of a gas which 
can exactly monitor the radioactivity concentration, even when the 
collection efficiency of a filter cartridge for a radioactive element is 
varied. 
Another object of the present invention is to provide a method of and an 
apparatus for monitoring the radioactivity concentration of a gas wherein 
an accurate value of the radioactivity concentration is obtained without 
being corrected by measured values of the temperature, the humidity and 
the like of the gas. 
A further object of the present invention is to provide a method and an 
apparatus suitable for the monitoring of the radioactivity concentration 
of residual .sup.131 I in a gas. 
The above and other objects of the present invention can be attained by a 
monitoring method according to the present invention which comprises the 
steps of passing a sample gas in a direction through a filter cartridge 
filled with an absorption material for a specified radioactive element to 
accumulate the radioactive element in the filter cartridge, counting 
radiation on both entrance and exit sides of the filter cartridge, 
estimating the absorption distribution of the radioactive element absorbed 
in the filter cartridge on the basis of the ratio between a pair of counts 
obtained on both sides of the filter cartridge, and correcting one of the 
counts by the estimated absorption destribution of absorbed radioactive 
element to obtain the radioactivity concentration of the gas. 
According to the present invention, the estimation of the absorption 
distribution of the radioactive element absorbed in the filter cartridge 
and the correction of the count on the basis of the above estimation are 
carried out in the following manner. 
When the sample gas has passed through the filter cartridge in one 
direction, if the absorption coefficient of the absorption material is 
kept constant, the absorption distribution of the absorbed radioactive 
element in the above-mentioned direction is given by the following 
equation: 
EQU A(x.sub.i)=e.sup.-.mu.x.sbsp.i -e.sup.-.mu.(x.sbsp.i.sup.+.DELTA.x) ( 1) 
where A is the amount of the absorbed radioactive element (in relative 
value), .mu. an absorption coefficient, x.sub.i a depth of the absorption 
material (the depth from the entrance to the i-th part of the absorption 
material when the absorption material is divided into n parts in the 
direction from the entrance thereof to the exit), .DELTA.x.sub.a small 
thickness of the absorption material in the above direction. 
Further, the probability .eta..sub..gamma. (x.sub.i) of radiation which is 
emitted from the radiation source distributed at a depth x.sub.i of the 
absorption material and passes through the radiation detector disposed on 
the entrance side and the probability .eta..sub..gamma. (x.sub.n-i) of the 
above radiation which passes through the radiation detector disposed on 
the exist side depend upon the respective shapes of each radiation 
detector and the absorption material, and the distance between each 
radiation detector and the absorption material. 
By combining the distribution A(x.sub.i) given by equation (1) with the 
above-mentioned probability, the efficiency K.sub.1 at the radiation 
detector on the entrance side and the efficiency K.sub.2 at the radiation 
detector on the exit side are given by the following equations: 
##EQU1## 
According to the present invention, K.sub.1, K.sub.2 and K.sub.1 /K.sub.2 
are previously calculated for various values of .mu.. When the monitoring 
of radioactivity of a gas is actually carried out, a value of absorption 
coefficient .mu. at the time when a sample gas passes through the filter 
cartridge is estimated from the ratio N.sub.1 /N.sub.2 of a count N.sub.1 
obtained by the detector on the entrance side to a count N.sub.2 obtained 
by the detector on the exit, since the ratio N.sub.1 /N.sub.2 is equal to 
the above-mentioned ratio K.sub.1 /K.sub.2. That is, the absorption 
distribution of radioactive element in the filter cartridge is estimated. 
Then, the efficiency K.sub.1 at the detector on the entrance side is 
calculated on the basis of the estimated absorption distribution. The 
efficiency K.sub.1 thus obtained is used to calculate the radioactivity 
concentration of the sample gas which is given by the following equation: 
##EQU2## 
where C.sub.o is the radioactivity concentration of the sample gas 
(.mu.Ci/cm.sup.3), .gamma..sub.k the counting efficiency of the radiation 
detector depending upon the radiation energy, T.sub.1 the counting time 
(sec), V the volume of the sample gas passing through the filter cartridge 
(cm.sup.3), and N.sub.1 the count obtained by the radiation detector on 
the entrance side of the filter cartridge.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows an embodiment of a monitoring apparatus according to the 
present invention which is suitable for the monitoring of radioactive 
iodine in nuclear power stations. 
Referring now to FIG. 1, a large number of filter cartridges 2 for 
accumulating iodine are arranged in a turnable 1. A sample gas starting 
from an exhaust duct 11 of a reactor building is led through a sampling 
pipe 12 to a first one of the filter cartridges 2, passes through the 
filter cartridge, and then goes back to the exhaust duct 11 through an air 
pump 6 and an integrating flow meter 7. After a predetermined amount of 
gas has flowed from the duct 11 to the duct 11 through the above-mentioned 
flow path, the turntable 1 is rotated to place the filter cartridge, 
through which the predetermined amount of gas has passed, between a pair 
of radiation detectors 4A and 4B. FIG. 2 is an enlarged view of the 
detectors 4A and 4B. The .gamma.-rays emitted from .sup.131 I accumulated 
in the filter cartridge are counted by both of the upper and lower 
radiation detectors 4A and 4B. Each of these radiation detectors is the 
so-called scintillation counter, which is made up of a cylindrical crystal 
of NaI(Tl) and a photomultiplier. In FIG. 2, reference numerals 5A and 5B 
designate radiation ray shields. The sample gas is passed through a second 
one of the filter cartridges in a period when the counting of radiation is 
conducted. When the predetermined amount gas has passed through the second 
filter cartridge, the turntable 1 is again rotated to count the radiation 
emitted from the second filter cartridge. 
Signals from the radiation detectors 4A and 4B are sent to pulse-height 
analyzers 8A and 8B through lines 13A and 13B, respectively. In each of 
the pulseheight analyzers 8A and 8b, the amount of .gamma.-rays emitted 
from .sup.131 I is measured through pulse-height analysis. The results of 
analysis are sent through lines 14A and 14B to a signal processor 9, in 
which, in accordance with the process shown in FIG. 3, the overall 
efficiency of the monitoring apparatus is determined, and the 
radioactivity and the radioactivity concentration of the sample gas are 
calculated. 
The iodine absorption material, with which the filter cartridge 2 is 
filled, has a porous carrier which carries therein silver particles or 
silver compounds. Examples of the iodine absorption material are disclosed 
in, for example, U.S. Pat. No. 3,838,554. The iodine absorption material 
of this kind can efficiently absorb iodine which is borne by air in the 
form of CH.sub.3 I-molecule or I.sub.2 -molecule. In this embodiment, the 
iodine absorption material in the filter cartridge has a diameter of 76 mm 
and a height of 16 mm. Incidentally, each of the radiation detectors 4A 
and 4B includes an NaI(Tl) crystal having a diameter of 76.2 mm and a 
height of 76.2 mm. 
FIG. 3 is a flow chart for showing the processing performed in the signal 
processor 9. In a step 16, a count N.sub.1 with respect to radiation on 
the entrance side of the filter cartridge and a count N.sub.2 on the exit 
side are measured. This is carried out by taking in, through the lines 14A 
and 14B, the results of analysis from the pulse-height analyzers 8A and 
8B. In a step 17, the ratio N.sub.1 /N.sub.2 is calculated. In a step 18, 
a value of absorption coefficient .mu. which corresponds to the calculated 
value of N.sub.1 /N.sub.2, is read out of a data table which has been 
stored in the processor 9. 
The correspondence between the value of the ratio N.sub.1/N.sub.2 and the 
value of the absorption coefficient .mu. has been previously calculated 
and determined on the basis of the following principle. 
As is well known, the absorption coefficient .mu. is defined as the 
reciprocal of a distance (in an absorption material) necessary for the 
amount of an element (namely, iodine in this embodiment) in a gas to be 
reduced to 1/e of an original amount when the gas passes through the 
absorption material. Accordingly, in this embodiment, the distribution of 
iodine absorbed by the absorption material in the filter cartridge 2 is 
expressed by the previously-mentioned equation (1), that is, 
EQU A(x.sub.i)=e.sup.-.mu..sbsp.i -e.sup.-.mu.(x.sbsp.i.sup.+.DELTA.x) 
The absorption coefficient .mu., as is shown in FIG. 4A, depends upon the 
humidity, the temperature and the velocity of the gas passing through the 
absorption material. Two absorption coefficients .mu..sub.A and .mu..sub.B 
shown in FIG. 4A correspond to the distribution C.sub.A and C.sub.B of 
absorbed iodine shown in FIG. 4B, respectively. As is shown in FIG. 4B, 
the distribution of absorbed iodine is greater in inclination as the 
absorption coefficient .mu. is larger. 
While, the probability .eta..sub..gamma. of the radiation which is emitted 
from the radiation source distributed on a thin disc and passes through a 
cylindrical radiation detector is given by a triple integral. The 
probability .eta..sub..gamma. with respect to the radiation detector used 
in this embodiment was calculated for various distances between the 
radiation sources and the detector. The results of calculation are shown 
in FIG. 5. 
The probability .eta..sub..gamma. (x.sub.i) of the radiation which is 
emitted from the radiation source distributed at a depth x.sub.i (cm) of 
the absorption material from the entrance side thereof and passes through 
the radiation detector 4A and the probability .eta..gamma.(x.sub.n-i) of 
the same radiation which passes through the radiation detector 4B are 
obtained from the graph shown in FIG. 5. The efficiencies K.sub.1 and 
K.sub.2 given by combining the absorption distribution of the absorbed 
iodine with the probability of the radiation passing through the detector 
are expressed by the previously-mentioned equations (2) and (3), that is, 
##EQU3## 
where K.sub.1 is the efficiency at the radiation detector 4A, and K.sub.2 
that at the radiation detector 4B. 
The ratio K.sub.1 /K.sub.2 is equal to the ratio (N.sub.1 /N.sub.2) of the 
count N.sub.1 obtained on the side of the detector 4A to the count N.sub.2 
on the side of the detector 4B. 
By calculating the efficiency K.sub.1 and K.sub.2 on the basis of equations 
(2) and (3) for various values of .mu., the values of K.sub.1 /K.sub.2 or 
N.sub.1 /N.sub.2 corresponding to these values of .mu. are obtained. 
FIG. 6 is a graph showing the relation between .mu. and N.sub.1 /N.sub.2, 
which is obtained for the radiation detectors 4A and 4B and the filter 
cartridge 2 used in the embodiment. 
Turning now back to FIG. 3, the processing performed in the signal 
processor 9 is again explained. In the step 18, the value of absorption 
coefficient .mu. corresponding to the value of N.sub.1 /N.sub.2 calculated 
in the step 17 is obtained from the above-mentioned relation between .mu. 
and N.sub.1 /N.sub.2. That is, the absorption distribution of absorbed 
iodine in the filter cartridge 2 is estimated from the ratio between the 
counts obtained on both sides. In a step the 19, the efficiency K.sub.1 at 
the radiation detector 4A is calculated using the estimated distribution 
of absorbed iodine. In more detail, the value of .mu. obtained in the step 
18 is substituted for .mu.'s in the equation (2)to calculate the 
efficiency K.sub.1. In a step 20, using the efficiency K.sub.1 and the 
counting efficiency .gamma..sub.k of the radiation detector depending upon 
radiation energy, the overall detection efficiency .eta..sub.o is 
calculated which is given by the following equation: 
EQU .eta..sub.o =K.sub.1 .multidot..gamma..sub.k (4) 
In a step 21, the radioactivity of the sample gas having passed through the 
filter cartridge is calculated on the basis of the following equation, 
using the overall detection efficiency .eta..sub.o and the count N.sub.1 : 
##EQU4## 
where N.sub.o is the radioactivity of sample gas (.mu.Ci), and T.sub.1 the 
counting time (sec). 
In a step 22, the signal from the integrating flow meter 7 is sent through 
the signal line 15 to the processor 9, and the radioactivity concentration 
of the sample gas is calculated which is given by the following equation: 
EQU C.sub.o =N.sub.o /V (6) 
where C.sub.o is the radioactivity concentration of sample gas 
(.mu.Ci/cm.sup.3), and V the volume of sample gas having passed through 
the filter cartridge. 
In a step 23, the results obtained are outputted. Thus, the processing 
shown in FIG. 3 is completed. 
As explained above, according to this embodiment, the distribution of 
iodine absorbed in the filter cartridge is estimated from the ratio 
between the counts obtained on both the entrance and exit sides of the 
filter cartridge, and the detection efficiency of the radiation detector 
is obtained on the basis of the estimated absorption distribution and used 
to calculate the radioactivity concentration of .sup.131 I in the gas. 
Accordingly, an accurate value for the radioactivity concentration is 
obtained independently of a change in the absorption coefficient, and 
therefore this embodiment is suitable for the monitoring of exhaust of 
.sup.131 I into air which is required to detect .sup.131 I in an extremely 
low atmospheric concentration. However, the present invention is not 
restricted to the monitoring of .sup.131 I, but is applicable to various 
cases where a radioactive element in a gas is accumulated on an absorption 
material to detect the radioactivity concentration of the gas. 
In the embodiment shown in FIGS. 1 to 3, a pair of radiation detectors are 
employed. However, a monitoring apparatus according to the present 
invention can include only a single radiation detector. FIG. 7 is a 
sectional view showing an embodiment of such a monitoring apparatus. The 
embodiment shown in FIG. 7 differs from that shown in FIG. 2 in that the 
lower radiation detector 4B is eliminated and a mechanism for turning the 
filter catridge upside down is added in place of the detector 4B. In other 
words, after a predetermined amount of gas has passed through a filter 
cartridge 2, the filter cartridge is moved to the under side of the 
radiation detector 4A and the radiation is counted for a predetermined 
period. The filter cartridge 2 is then taken out by a mechanism 25 for 
rising and falling the filter cartridge. The taken-out filter cartridge 2' 
is then turned over by another mechanism 26 for turning the filter 
cartridge upside down, and brought back to the original position, beneath 
the radiation detector 4A to count the radiation again for a predetermined 
period. Incidentally, reference numerals 27 and 28 designate motors for 
rising and falling the filter cartridge and for turning the filter 
cartridge upside down, respectively. When the time of the first counting, 
the time of the second counting, the time necessary for turning over the 
filter cartridge, the count obtained in the first counting, and the count 
obtained in the second counting are given by T.sub.1, T.sub.2, T.sub.3, 
N.sub.1, and N.sub.2 ', respectively, the count N.sub.2 ' in the second 
counting is corrected taking the decay of radioactivity into 
consideration, as is given by the following equation: 
EQU N.sub.2A =N.sub.2 '/e.sup.-0.693(T.sbsp.1.sup.+T.sbsp.3.sup.)/T.sbsp.1/2(7) 
where N.sub.2A is the corrected value of the count in the second counting, 
and T.sub.1/2 the half-life of the radioactive element. 
The absorption coefficient .mu. is estimated from the ratio of N.sub.1 
/T.sub.1 to N.sub.2A /T.sub.2, and then the same processing as shown in 
FIG. 3 is performed. That is, the radioactivity concentration of gas can 
be exactly monitored in the same manner as the embodiment shown in FIGS. 1 
to 3.