Efficiency Calibration of HPGe Gamma Spectrometry of LAN system

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Efficiency Calibration of HPGe Gamma Spectrometry of LAN system
Qian Jianfu, Shen Tingyun, Nan Qing Liang, Liu Boxue, Zhu Yanguo
ABSTRACT
In gamma-spectrometry efficiency determinations for germanium detector are very important for analysis
environmental samples. Monte Carlo simulation technique and experimental method for determining the total
efficiency of volume source and detect have been previously described for gamma spectrometric measurement
systems. This paper has described the necessary and use for building LAN (Local Analysis Net) system for
HPGe gamma spectrometry has been described. A new method to calculator the total efficiency combination
theoretical to experimental has been introduced for any axially symmetric arrangement of source and detector
with 152 Eu point source. That is to computer the total efficiency of different energy with the known activity on
the detector. Factors are derived using the Monte Carlo accounting routine. You may get the volume efficiency
for HPGe spectrometry inputting your experimental value of point sources and geometric size of detector
through LAN system. The derived procedures are simple. It may subtract the cost and decentralized data of every
gamma spectrometry system. The total efficiency that are given using the method agree very well with values
established by experiment of 241 Am、 109 Cd、 137 Cs、 54 Mn、 65 Zn standard sources over four detectors. The
measuring errors are 3.8% except that individual value is 7.4%.
Keywords: HPGe gamma spectrometry, Monte Carlo method, efficiency calibration, summing correction
1.PREFACE
HPGe gamma detector is used for analysis environmental samples in which calibration is very important. It
needs large amount of labor power, comparatively long time and rich experience. If the source is far from the
detector, the gamma spectrum analysis software now available can be competent to calibration. But, in order to
obtain higher detecting efficiency, we usually put the source within 10 mm from the detector, At such a short
distance, the multiplying effects of many emission particles are tremendous, and thus affects the calculation of
total efficiency and the accuracy of calibration. To acquire the function ship between efficiency and energy in
wider energy range, various single energy standard sources must be prepared. About 13 standard sources are now
wed worldwide, among which, five nucleus’ lifetimes are short. Since efficiency calibration is required to carry
on once one week or two weeks, so spectrometry users have to pay much money for these single-energy sources.
On the basis of earlier studies, we have done research on utilizing multi-energy 152 Eu point source in efficiency
calibration. Users only need to do some adding correction, and then the efficiency of volume source will be
worked out.
2.WAYS
In order to eliminate the deviation between theoretical calculations and experimental results, we have
adopted three detectors’ measuring efficiency values. With comparison to theoretical calculation and some
adjustment or correction, the relatively accurate efficiency of volume source can be acquired. The relevant data
of the four HPGe detectors is listed in Table 1.
Table 1. Geometric Data of Detectors
Detectors Radius (mm) Height (mm)
Relative efficiency
(%)
1 24.9 59.2 15
2 32.65 72.5 50
3 30.2 82.2 45
4 28 56.3 30
Table 2. Data about Source
Source Radius (mm) Height (mm) mixture
152 Eu 3.75 5.7
Volume source Single energy
241 Am、 54 Mn、
3.75 5.7
source
137 Cs、 65 Zn、 57 Co
Point source
152 Eu
Single energy
source
241 Am、 54 Mn、
137 Cs、 65 Zn、 57 Co
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To make it easy operational, we have written a program to compute the peak efficiency, including Monte
Carlo method. Users only need to measure the spectrum data of
152 Eu and
137 Cs point source in two different
positions (one is where coincidence summing effects can be neglected, for example, 25cm far away; another is at
the same height with the volume source), next input the corresponding values according to the flow chart, and
then users can get the summing correction factor of their 152 Eu point source. Following the flow chart, input the
relevant parameters of source and detector to obtain theoretical efficiency of 152 Eu point source. In the end, the
efficiency curve of
152 Eu volume source at every energy can be established through transmission method.
The flow chart of calibrating volume source:
Measure spectrum data of 152 Eu and 137 Cs point source at 25cm and 7cm away
Calculate the value of ku、 Ku(Cs)
Calculate the coincidence adding factor of users detector’s 152 Eu point source
Compute theoretical efficiency values of volume source and point source with program
Establish transmission efficiency carve of 152 Eu point source
Insert efficiency value at cared energy
2.1THEORETICAL CALCULATION
The theoretical efficiency of point source and volume source are obtained through Monte Carlo method,
which is also called Random Sampling or Statistical Experiment method. It transfers the solving of problems
into seeking for the expected value or statistical average value, that is to say, simulating physical experiment
with mathematical means. According to the flow chart, input the needed data and compute the interaction
efficiency between photons and crystal after the emission photons arrive at the crystal.
Table 3. The K value of three measuring system
Energy(Kev) NO.1 NO.2 NO.3 The average value of K
121.78 0.8372 0.8049 0.8887 0.8436
244.69 0.9209 0.9198 0.9718 0.9375
344.28 0.9798 0.9703 1.0067 0.9856
443.98 0.9511 0.9656 0.9897 0.9688
778.9 0.9665 0.9863 0.9846 0.9791
964 1.021 1.045 1.0037 1.0232
112.13 0.9722 0.9355 0.9891 0.9656
1408.03 1.0153 1.0447 1.0149 1.025
Table 4. The standard Laboratory coincidence Adding Factors
Energy 121.8 224.7 344.3 443.9 778.9 964 1112.1 1408.0 661.6
(Kev)
Ku 8.831 8.262 8.256 7.976 8.007 7.957 7.976 7.916 8.457
Ccu 1.094 1.17 1.129 1.109 1.037 1.037 1.037 1.042
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2.2 COMBINING THEORETICAL CALCULATION WITH EXPERIMENTS
Combining the theoretical calculation with experimental established efficiency, we can get:
K 1 =ε vm /ε pm (1)
K 1 =ε vc /ε pc (2)
K = K 1 /K 2 (3)
where, ε vm is the actually measured peak efficiency of volume source, ε pm is the actually measured peak
efficiency of point source, ε vc is the calculated peak efficiency of volume source, and ε pc is the calculated
peak efficiency of point source.
In user’s measuring system, obtaining volume source’s efficiency requires to carry on the fourth formula
ε v =ε pm * K 2 * K (4)
where, εv is the peak efficiency of volume source calculated with transmission method.
From the fourth formula we can see, users can get the comparatively accurate efficiency of volume source
if combining the actually measured volume efficiency with the k value provided in the program.
3.EXPERIMENTS
We have carried on research and calculation in three p-type HPGe gamma spectrometry systems which
have different parameters and efficiency, and the relative peak efficiency at the energy of 1.332Mev is 15%、
20% and 45%respectibely. The distance between the point source and the front end of the detector is 7cm.
The volume source is a plastic, cylinder-shaped, basic substance source, which is placed at the front and of
the detector. The adopted radioactive nucleus are 241 Am、 109 cd、 57 co、 137 cs、 54 Mn、 65 Znand 152 Eu,and we’ve
collected the spectrum data of every single-energy point source and volume source listed above. A66 spectrum
analysis software from American’s ORTEC Company has been applied to analyses peak parameters.
Considering the adding correction factor of 152 Eu when ding analysis, we have achieved a set of k values. In the
end, the efficiency curve has been testified on the fourth detector (the relative efficiency is 30% at the energy of
1.332Mev) and the results are listed in figure 5. Except for the deviation of ±7.4^at the energy point of 122.06
Kev, deviations at another five energy points are all within the range of ±3.8%.
Table 5. Comparison of volume source’s activity on the four detector
nucleus Energy
standard source activity As measured activity Am
(Bq)
(Bq)
(Am-As)/As
241 Am 59.54 4825.241 4619.567 -0.04262
109 Cd 88.03 2586.093 2625.295 0.01516
57 Co 122.06 103.37 111.0257 0.07406
57 Co 136.2 103.37 107.392 0.03891
137 Cs 661.66 692.93 698.8345 0.00852
54 Mn 834.83 211.515 215.599 0.01931
65 Zn 1115.52 271.39 269.622 -0.00651
4.CONCLUSION
Calibrating peak efficiency of volume source in the way described above can solve the problem of in
sufficient single energy. Some computers can be connected to form a pc net for exchanging information and
sharing resources. With the measured efficiency value of 152 Eu point source (having been carried on coincidence
adding correction), considering the k value provided by the net’s general station and doing simple simulation
calculation with Monte Carlo method, we can achieve the efficiency calibration of volume source easily. The
statistical error of peak efficiency is 1%. It takes 90 minutes to run the program on 386 computer, so minutes on
486 computer and 15 minutes on 586 computer.
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PC work station
Pc work station
PC (net)general station or VAX
Providing services for files and disks
Gamma
system
spectrometry
Other parts of
the net
Gamma spectrometry
system
5.DISCUSSION
For the energy beyond 136kev, the measured values agree well with the transmission values of total
efficiency, while at the energy of 122kev, the error ix 7.8%. The reason is complicated and many-sided, such as
the statistical counting error and inaccuracy of the standard source’s activity provided.
Precise calculation of the total efficiency of cylinder-shaped volume source, as well as fast running and
other advantages, depend heavily on the personal computer. Besides, the self-absorption factor and adding
correction factor are rather important in efficiency calibration. In environmental samples, the self-absorptions of
sample’s density within wide range are quite different, but the method we’ve introduced is really used. Users
only need to measure the standard point source and some fixed data, such as detector’s parameters and trans
mission parameter. So, this method is suitable for point source and other volume sources.
If possible, we will evaluate the method on more users' detectors, and that maybe provide better result for
our work..
REFERENCES
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