Monte Carlo Application for the use of Detector Response Function on Scintillation Detector Spectra

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Title: Monte Carlo Application for the use of Detector Response Function on Scintillation Detector Spectra
Author: Speaker, Daniel P
Advisors: Robin P. Gardner, Committee Chair
Hany S. Abdel-Khalik , Committee Member
Robert White, Committee Member
Abstract: The DRF is the pulse height distribution for an incident radiation, and is also a PDF which has the properties of always being greater than or equal to zero and also integrates to unity. The application of the DRF on a simulated spectrum results in the benchmarking of the simulation results with experimental results. The results are the nice Gaussian shapes that are caused by the statistical fluctuations in the energy and collection efficiency of the detector. To find the perfect simulation of the DRF is impossible due to the fact that the detector might have imperfections, where electrons can essentially become trapped and not be collected. One must rely on empirical models of nonlinearity and simulation data to do this. This is what CEAR’s DRF code g03 does. The time consuming task of a code like g03 is the time it takes to simulate the Monte Carlo simulation, in particular the electron transport of it. G03 couples rigorous gamma ray transport with very simple electron transport. By this methodology the non-linearity and the variable flat continua part of the DRF is accounted for. There are some problems and upgrades that needed to be addressed, for instance the difference in the valley region between the Photopeak and Compton Edge and parts of the Compton Continuum. This Monte Carlo simulation also simulates the detector as a bare crystal. It was found that this could account for as much of a reduction of as much 5 percent of the incident energy. And also distort the response function in the lower energy range of the function. For this MCNP was employed to simulate the difference between the bare and covered crystal. The MCNP simulation also included a surface current tally for electrons and photons on the interface between the can and the crystal, and also the interface between the side of the crystal and the can. From the results of the simulation of the can and no can simulation for the pulse height spectra are different. It here when it was determined to add a patch to make the simulation of the detector response function more accurate. This causes a sizeable difference in valley region, which can be explained as many different photopeaks in the valley region, due to Compton scatters in the can. Also one can distinguish between the plots and conclude that the side of the can contributes to the continuum due to the backwards continuum which starts around 0.2 MeV. The way that this will be added is different for the place where in the can contributes and type. For the electrons in the front and the side, the spectrum will be run through a program that will calculate the energy deposited and this will be added directly to the spectrum. The photons on the side will be run though MCNP with an f8 tally which will be in turn added to the spectrum. The photons from the front, and perhaps the most significant, will be added by g03 having a spectrum of incident photons on the crystal instead of the way it is done now with a monoenergetic energy. Then a patch will be added to make the code more accurate.
Date: 2009-08-07
Degree: MS
Discipline: Nuclear Engineering

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