Investigating the Position-Sensitive Capabilities of Segmented Germanium Detectors using Gamma Imager

Abstract

In recent years, many efforts have been devoted towards the development of advanced gamma-spectroscopy detectors and sophisticated pulse-processing electronics. A crucial breakthrough in the domain of gamma-spectroscopy has been achieved after the development of highly segmented germanium detec tors. Detector segmentation is essential in determining gamma-ray interaction location using Pulse Shape Analysis (PSA). The experimental dataset of traces obtained using various standard gamma sources provided a more realistic track ing algorithm to determine energy and 3-D point of interaction for each event. The PSA technique has been used to achieve energy and 3-D point of gamma interaction depending upon the amplitude of the pulse and its shape at each electrode. The prime feature of locating a point of interaction is that the moving charge induces a charge on the electrode, and based on the properties of elec trical segmentation, an image charge is produced on the neighboring/adjacent segment. Due to the complexity of signals/pulses in the segmented detectors, obtaining their full 3-Dimensional (3-D) characterization is necessary. Therefore, dedicated gamma scanning systems/imagers have been developed employing various novel approaches for gamma-ray interaction location determi nation. The primary aim of this thesis has been to develop a one-shot scanning system based on a collimation-free scanning technique. The thesis comprises two sets of investigations and developments: 1. In the first set of investigations, the performance test of a planar segmented germanium detector has been performed at GSI Germany. The primary motivation of this work is to prepare the detector deployed as an implan tation detector in future DEcay SPECtroscopy (DESPEC) experiments to be performed at the Facility for Antiproton and Ion Research (FAIR) in Germany. The detector under study is a double-sided orthogonal strip detector comprised of ten strips per two opposite sides in horizontal and vertical directions. For its scanning, an existing GSI scanner facility has been used. The GSI scanner consists of a position-sensitive scintillator de tector, i.e., LYSO scintillator coupled with a photomultiplier tube with a mesh of 16 X and 16 Y anodes, and a 22Na standard gamma source. The principle of scanning is positron annihilation correlation and the analysis of pulse shape comparison scan. Further, 241Am source scanning has been performed at steps of 1 mm using a lead (Pb) collimator having a hole of diameter 1 mm. The data analysis has been carried out to determine the performance of the planar germanium detector towards the incoming gamma-rays. The gamma interaction depth has been studied by calculat ing the rise-time of traces stored for each gamma interaction point inside the detector volume, providing ≈ 1 mm resolution along the depth. The position resolution of the detector in lateral directions, determined using the amplitude difference of the transient charges, has also been found to be ≈ 1 mm. 2. Considering the ongoing efforts of the Indian gamma-spectroscopy commu nity to get an AGATA-like array in India, we initiated the development of a gamma imager at IIT Ropar in collaboration with GSI, Germany. The aim of the ongoing R&D is to develop a one-shot scanning technique and employ the device for scanning highly segmented germanium detec tors to achieve the gamma interaction locations, electric field distribution, and defects. The prime aim is to develop a gamma imager with simple yet high-end electronics, higher gain, and compact hardware inclusion as an advancement over the existing gamma scanning/imager systems. In this thesis, a position-sensitive detector, i.e., LYSO monolithic scintillator crystal of diameter 7 cm and thickness 3 mm coupled with the matrix of 96 silicon photomultipliers of 3 mm x 3 mm dimensions, has been devel oped. The testing/characterization of the detector has been performed at GSI, Germany, to understand detector properties and determine its posi tion resolution. In the first step, the test was performed with 24 detector channels employing a 22Na standard gamma source. However, the second test was performed using a coincidence setup between the existing GSI scanner and the new imager. The preliminary test results have been an alyzed and discussed for detector position resolution, using differences in amplitudes at the neighboring segments.