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Title: Improvement of the performance of broad energy germanium detectors for gamma-ray spectroscopy
Author: Ali, N. A.
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2017
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The project presented in this thesis aims at improving the performance of High Purity Germanium (HPGe) detectors through the optimisation of their peak-to-Compton (P/C) response, which in turn will improve the sensitivity of these detectors. This can benefit different applications of HPGe detectors such as dosimetry, radioactive waste characterisation and environmental sample counting. In order to achieve this goal, the application of Pulse Shape Analysis (PSA) to gamma-ray spectroscopy measurements was carried out with a Canberra unique Broad Energy Germanium (BEGe) detector using a calibration source and an environmental sample in a Marinelli counting geometry. The first stage of the PSA technique applied in this work was the storage of a data set containing 1024 samples of the charge signal induced for each event, the Moving Window Deconvolution (MWD) derived energy and the time stamp information. This has been accomplished using a full digital acquisition system, which makes use of the Caen DT5780 digitiser with 100MHz sampling rate and 14bit dynamic range. Once the data have been stored, they were investigated offline using the MTSort software in order to extract the required information. The offline analysis, which is the second stage of PSA in this work, involved the calculation of the pulse height of the stored signals, which reflects the energy deposited inside the detector, using the baseline average window determination method. In addition to this, the considered risetime parameters t30 and t90 (the times taken for the pulse to rise from 10% to 30% and 10% to 90% of its maximum height) were measured for all recorded pulse shapes. As the interactions occur in different positions within the detector and hence different charge trajectories, there was a variation in the calculated risetime, producing a distribution in time. To account for these effects, risetime filtering was employed. In this, matrices of t90 versus t30 distributions were obtained for the different gamma radiations emitted by the calibration source. This allowed the P/C ratio to be optimised by preferentially selecting risetime regions of interest on the t90 vs. t30 maps, where full-energy photopeaks events are more likely to be located on these risetime distributions. Setting risetime regions of interest on these maps is referred in this work as risetime filters (or gates). Energy spectra have been successfully reconstructed from the selected events using each risetime map. A detailed spectroscopic analysis has been carried out for the created energy spectra to assess the efficacy of the risetime filters applied. This has been performed through quantitatively measuring the P/C ratios for full-energy photopeaks in the postfiltered energy spectra. In addition, the ratio efficiency of photopeaks has been measured using the post-filtered energy spectra. The methodology and results of the successful application of PSA to the recorded charge signals have been presented. The results have shown that the optimised P/C ratio obtained after the successful application of PSA has increased, for example, 33% for 241Am, 37% for 137Cs, and 38% for 40K. The main parametric PSA algorithms that have successfully improved the P/C response can now be implemented in the Caen firmware to be used for detector applications such as environmental sample counting or nuclear waste essays.
Supervisor: Boston, Andrew ; Page, Robert Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral