Xác nhận hiệu lực mô hình mô phỏng đặc trưng hệ phổ kế gamma đầu dò bán dẫn siêu tinh khiết GMX35P4-70 với chương trình MCNP5 và GEANT4

Trong công trình này, chúng tôi sử dụng hai chương trình mô phỏng Monte Carlo MCNP5 và GEANT4 để mô phỏng hệ đầu đò HPGe kí hiệu GMX35P4-70, sau đó nghiên cứu đặc trưng phổ và tính toán hiệu suất đỉnh.
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Xác nhận hiệu lực mô hình mô phỏng đặc trưng hệ phổ kế gamma đầu dò bán dẫn siêu tinh khiết GMX35P4-70 với chương trình MCNP5 và GEANT4 TẠP CHÍ KHOA HỌC ĐHSP TPHCM Truong Thi Hong Loan et al. _____________________________________________________________________________________________________________ VALIDATION FOR MONTE CARLO SIMULATION OF CHARACTERISTICS OF GAMMA SPECTROMETER USING HPGe GMX35P4-70 DETECTOR BY MCNP5 AND GEANT4 CODES TRUONG THI HONG LOAN*, VU NGOC BA** , TRUONG HUU NGAN THY**, HUYNH THI YEN HONG*** ABSTRACT The study used two Monte Carlo simulation codes of MCNP5 and GEANT4 to simulate HPGe detector of GMX35P4-70, then its response spectra and peak efficiencies characteristics were evaluated. The results show that when increasing the inner dead layer thickness of the detector from 1.8mm to 2.2mm, there is a better fit of the response spectra and the peak efficiencies characteristics compared with the measured ones. In general, it is useful to use two these input files to simulate response spectra and calculating the peak efficiency of GMX detector for determination of radionuclide distribution in the soil by in situ or laboratory gamma-ray spectrometry. Keywords: GMX detector, Monte Carlo, MCNP5, Geant4. TÓM TẮT Xác nhận hiệu lực mô hình mô phỏng đặc trưng hệ phổ kế gamma đầu dò bán dẫn siêu tinh khiết GMX35P4-70 với chương trình MCNP5 và GEANT4 Trong công trình này, chúng tôi sử dụng hai chương trình mô phỏng Monte Carlo MCNP5 và GEANT4 để mô phỏng hệ đầu đò HPGe kí hiệu GMX35P4-70, sau đó nghiên cứu đặc trưng phổ và tính toán hiệu suất đỉnh. Kết quả cho thấy khi thay đổi bề dày lớp chết từ 1.8mm đến 2.2mm đáp ứng phổ mô phỏng và hiệu suất đỉnh phù hợp với thực nghiệm hơn. Từ đó có thể sử dụng mô hình mô phỏng để tính toán hiệu suất hoặc cung cấp đáp ứng phổ cho việc phân tích hoạt độ phóng xạ sử dụng hệ phổ kế gamma trong phòng thí nghiệm hay tại hiện trường. Từ khóa: GMX detector, Monte Carlo, MCNP5, GEANT4. 1. Introduction Monte Carlo method is based on the seeding of the random number to sample in a set. It was first used by Metropolis (1947) [15]. This method has a very important role in computational physics. There are so many authors who have used the Monte Carlo method to solve problems in the nuclear physics by writing and developing the codes as MCNP [15], Geant [2], EGSnrc [8]. Thereby some authors have applied the codes for evaluation of response spectra of detector and have compared the results with * Ph.D., University of Science Ho Chi Minh City; Email: tthloan@hcmus.edu.vn B.Sc., University of Science Ho Chi Minh City *** M.Sc.,University of Science Ho Chi Minh City ** 27 TẠP CHÍ KHOA HỌC ĐHSP TPHCM Số 3(81) năm 2016 _____________________________________________________________________________________________________________ experimental spectra [4], [6], [11], [12], [13], [14]. Rodenas et al [10], Ngo Quang Huy et al [9] have used MCNP code to evaluate the dead layer thickness of the HPGe detector based on comparison of the simulated efficiencies and empirical ones. Ashrafi et al [1], Berndt et al [3] have done to scan the detector to have detailed data of detector configuration which is used for simulation. Hau et al [5] have used MCNP code for studying Compton scattering and HPGe detector benchmark with previously validated Cyltran model. Thereby Monte Carlo method has a very important role to study the spectra characteristics of the HPGe detector. In this work we studied spectra characteristics of HPGe GMX35P4-70 detector by using MCNP5 and GEANT4 codes, the change of sensitive volume of Germanium crystal after a long time of use due to the increased thickness of the inner dead layer. In order to do that, comparison of the simulated spectra response and the empirical ones for point sources of radioactive isotopes at 25cm from detector surface were carried out. It takes our care for peak efficiencies, the valley area, Compton edge, and energy range from 20 keV up to 60 keV in the simulated spectra. 2. Materials and Methods The studied GMX35P4-70 HPGe detector has its diameter of 55.8 mm, height of 78.1 mm, core hole diameter of 8.6 mm, core hole depth of 69.6 mm, beryllium window thickness of 0.5mm. Reference sources of 241Am, 137Cs, 54Mn, 57Co, 60Co, 22Na isotopes of 1µCi (3%) at 25cm from the detector surface were used for spectra response measurements and simulation. In this work two MCNP5 and Geant4 codes were used to simulate photon transports in studied detector. The information of configuration and materials of the detector which based on data from Ortec industries were used in the input file of detector simulation. The codes was done under Linux operating system with personal computer using i3 core. Number of particle history was selected for efficiency errors below 0.1%. FWHM values were obtained from fitted empirical FWHM values to energies as follows: FWHM  0.00074340 5  0.00063323 24 E  0.86152781 78E 2 3. (1) Results and Discussion 3.1. Simulation of GMX spectrometer using MCNP5 and GEANT4 codes To determine accurately radioactivity of gamma emitted isotopes for HPGe detector, the peak efficiency of detector need to be exactly known. The peak efficiency curve of the detector is dependent on incident gamma energies. However there are no available enough the reference point sources for efficiency calibration, especially in the energy ranges below 120 keV or above 1.5 MeV. It is necessary to use analytical or Monte Carlo method to estimate the peak efficiencies. In this case, Monte Carlo simulation becomes useful and important. In this work, MCNP5 and GEANT4 codes were used to simulate the HPGe detector and to have a validation of the simulated 28 Truong Thi Hong Loan et al. TẠP CHÍ KHOA HỌC ĐHSP TPHCM _____________________________________________________________________________________________________________ spectra responses comparing with the empirical ones. However, there ...