| LaBr3 detector is a new kind of scintillator detector.LaBr3 detector has good energy resolution, high detection efficiency, good linear response,excellent preventing ability, high light output capacity, and fast attenuation,because of the outstanding advantages, it is widely used in measurement of gamma spectrum. However, two kinds of radionuclide 138 La and 227 Ac exist in LaBr3 crystal,they will have an impact on the measurement of natural radionuclides. 138 La has two gamma transitions.There will be High background counts in the range of 1.7 MeV ~ 2.8 MeV when 227 Ac and it’s Progeny decays. High background counts will have a significant impact on the measurement related to peaks of 238 U and 232 Th. Gama energy spectrum is complicated, there are many factors affect the gamma energy spectrum: gamma spectrometer instrument performance, background interference and statistical fluctuation etc. The background is an important component part of gamma energy spectrum. Radionuclide content and activity and the corresponding net peak area of characteristic peak have a direct relationship, The background subtraction will affect the characteristic peaks of net peak area.Therefore if we want to get accurate effective net peak of the sample and reduce the influence of background,the background subtraction is necessary. In this paper, we pursue research in the background deduction of LaBr3.The results were as follows:1、This article has carried on the following three experiments. Experiment one : measuring three types LaBr3 crystal by ORTEC Detective – DX HPGe detector in lead chamber,and then calculate the activity of 138 La in the unit volume crystal. The types of the crytals are Saint Gobain BrilLanCe 380 76S76, Hua Kailong KLB5075, Hua Kailong KLB3838.Experiment two : measuring three types LaBr3 crystal and NaI crystal in incomplete closed lead chamber.And then, calculate the activity of 138 La in the unit volume crystal and analysis the influence on gamma energy spectrum.Experiment three :using LaBr3 detector(Hua Kailong KLB5075) to measure background model, model of U, Th, K to get gamma energy spectrum.2、Activity calculation of 138 La. We obtain the efficiency calibration curves of HPGe detector by efficiency calibration without a radioactive source.According to the efficiency Calibration curves,calculating the detection efficiency of HPGe detector for the gama ray of 789 keV or 1436 keV in three types of LaBr3 detector.Then we calculate the activity of 138 La in the unit volume crystal according to the detection efficiency and the area of the peaks.3、Activity calculation of 227 Ac. Because of the limited laboratory conditions, the lead chamber can not be completely closed when LaBr3 detector is in it, so the external interference can not be completely shielded.The NAI detector is measured in the semi closed lead chamber for the deduction of the contribution of the natural background at 227 Ac to its counter count. The detection efficiency of the two detectors with the same energy gamma ray is different, and the ratio of the detection efficiency of the same energy gamma ray to the same energy gamma ray is obtained by efficiency calibration without a radioactive source. The peak area of the energy band of the NAI detector in 227 Ac is converted to the peak area of the lanthanum bromide detector, deducting the area after conversion when the peak area of the 227 Ac is calculated.4、Because of the advantages of MCNP5 for particle transport simulation, we use MCNP5 to simulate the background spectra of LaBr3.In particular, the beta spectrum is added to the background spectrum of LaBr3 in this articel, with this, the background spectrum is in agreement with the measured spectrum.5 、 Firstly smoothing the spectrum curve,then using SNIP method,fourier transform method, linear method to subtract the background.Also use several methods of correction method: using Gaussian function to decompose the peaks of 40 K and 138 La, so that we can get more accurate peak area of 40K; according to the calculated peak area of 228 Ac,we modifiy the method of 232 Th peak deduction.Finally we compare all of the methods and choose the best.The results show that the contents of U,Th,K are not reliable without background deduction.For 40 K, deducting the contribution of 138 La after the SNIP and fourier transform is the best. For 238 U, the effect of fourier transform is the best. For 232 Th, the effect of direct deduction of the fixed 227 Ac peak area is the best.6、This paper is based on LabVIEW software development platform to finish software design.Software interface design is including document management, energy calibration, smooth lines, background subtraction, help and exit the system. |
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