Gamma Spectrum Enhancement in a Metal Loaded Plastic Scintillator using PSD Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/tx31qp75z

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  • Photoelectric and Compton interactions are some of the common interaction mechanisms we observe when gamma radiation interacts with matter. These interactions are highly depended on the energy of the radiation and the atomic number of the detector material. Lower the energy of the photon and higher the atomic number of the detector; higher is the probability of a photoelectric events occurring. Higher the energy of the photon and lower the atomic number of the detector; higher is the probability of a Compton interaction. Conventional scintillation detectors like NaI(Tl) have a high Z atom and are useful in giving us spectroscopy information. From a manufacturing stand point we can’t manufacture huge blocks of NaI detectors and from an economic stand point, inorganic scintillators are expensive. These short comings can be overcome by using plastic scintillators, which are both easy to manufacture and costs effective. Traditionally plastic scintillators have not been good at detecting photo peaks because of the high number of Compton events the photon undergoes while it interacts with plastic (Carbon and Hydrogen; Low Z). Efforts at Sandia national lab have resulted in a plastic scintillator being produced that is made of polystyrene with a high Z material loaded in the plastic base. Therefore, from a spectroscopic point of view we observe a photo peak when we expose the detector to gamma photons. This material carries with it the ease of manufacturing coupled with low cost economics while providing us spectroscopic information. On comparing the spectroscopy ability of inorganic and organic scintillators, inorganic scintillators outperform organic (plastic) scintillators in terms of resolution and absolute efficiency. This study aims at understanding difference in pulse shapes between low Z and high Z materials. This understanding of differences can be used to perform pulse shape discrimination (PSD) between pulses from different materials. These differences can be employed to preferentially reject pulses from low Z materials while preserving counts under the photo peak thereby providing us a Compton suppressed spectrum. This correction helps us in obtaining an enhanced spectrum with few counts under the Compton continuum. This work explores the methods and algorithms used to perform pulse shape discrimination. Three single photon radionuclides; 198Au, 137Cs and 54Mn were used to provide us with a wide range of gamma photon energies. Multi photon measurements were also carried out with 22Na, 198Au-137Cs and 137Cs and 60Co. The three algorithms used to perform the discrimination include the classic charge integration method, pulse gradient analysis and the frequency gradient analysis. In the interest of reducing cost of the detection system, the ability to perform PSD at reduced sampling was also studied. The detector yielded the best resolution of 9.23% for the 662 keV peak of 137Cs. Preliminary results indicate that the algorithms were successful in suppressing the Compton continuum and preserving a large portion of the counts under the photo peak. In terms of energy dependence of the algorithm charge integration is the least energy dependent followed by pulse gradient analysis and frequency gradient analysis is the most energy dependent algorithm. 198Au has shown an improvement in the FOM by a factor of 1.7, followed by 137Cs that has shown a FOM improvement by 2.3 times, the corrected spectrum from 54Mn also shows an improvement in the FOM by around 1.8. Studies have also been carried out to perform PSD at reduced sampling frequency of 1.25 GHz and 833 MHz, initial results indicate that discrimination algorithms were successful at reduced sampling rates.
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