Applications of Pulse Shape Analysis Techniques for Segmented Planar Germanium Detectors

University dissertation from Stockholm : KTH

Abstract: The application of pulse shape analysis (PSA) and ?-ray tracking techniques has attracted a great deal of interest in the recent years in fields ranging from nuclear structure studies to medical imaging. These new data analysis methods add position sensitivity as well as directional information for the detected ?-rays to the excellent energy resolution of germanium detectors. This thesis focuses on the application of PSA on planar segmented germanium detectors, divided into three separate studies. The pulse shape analysis technique known as the matrix method was chosen due to its ability to treat events with arbitrary number and combinations of interactions within a single detector. It has been applied in two experiments with the 25-fold segmented planar pixel detector -- imaging and polarization measurements -- as well as in a simulation of upcoming detectors for DESPEC at NuSTAR/FAIR.In the first experiment, a point source of 137Cs was imaged. Events where the 662 keV ?-rays scattered once and were then absorbed in a different segment were treated by the PSA algorithm in order to find the locations of these interactions. The Compton scattering formula was then used to determine the direction to the source. The experiment has provided a robust test of the performance of the PSA algorithm on multiple interaction events, in particular those with interactions in adjacent segments, as well as allowed to estimate the realistically attainable position resolution. In the second experiment, the response of the detector to polarized photons of 288 keV was studied. The polarization of photons can be measured through the observation of the angular distribution of Compton-scattered photons, Hence the ability to resolve the interaction locations had once again proven useful.The third study is focused on the performance of the proposed planar germanium detectors for the DESPEC array. As these detectors have not yet been manufactured at the time of this writing, a set of data simulated in GEANT4 was used. The detector response was calculated for two of the possible segmentation patterns -- that with a single pixelated contact and one where both contacts are segmented into mutually orthogonal strips. In both cases, PSA was applied in order to reconstruct the interaction locations from this response. It was found that the double-sided strip detector can achieve an over-all better position resolution with a given number of readout channels. However, this comes at the expense of a small number of complex events where the reconstruction fails. These results have also been compared to the performance of the 25-fold pixelated detector.