Efficiency Calibration of HPGe Gamma Spectrometry of LAN system

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P-4a-264 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 2
P-4a-264 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. 3
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Efficiency Calibration of HPGe Gamma Spectrometry of LAN system

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