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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.

This work marks the first use of a fully digital trigger system and new CAEN V1724 digitisers to create a Compton camera from two semiconductor double sided strip detectors. The system was designed to be able to identify and locate gamma ray emitting radionuclide within an energy range of 60 to 1408keV. Compton images were produced at AWE Aldermaston and the University of Liverpool across an energy range of 80 to 1408keV, using point sources, extended sources and also including special nuclear materials. The image at 80keV is the lowest recorded energy for a Compton image using a two detector cryogenically cooled Compton camera. GAMOS simulations have been used to check the experimental data and provide evidence that indicates if pulse shape analysis was applied to the experimental data the image resolution would be improved by up to 4mm at 662keV.

Gamma-ray spectroscopy is an essential tool in the study of nuclear phenomenon. The study of exotic nuclei and nuclear states have been used to expand the nuclear chart as well as understand the origin of the universe. Large volume high purity germanium arrays, very high beam intensities and more recently exotic beams have lead to new understanding of nuclear physics. The Advanced Gamma Tracking Array (AGATA) aims to utilise high purity germanium (HPGe) detectors in order to achieve a dramatic increase in efficiency over current spectrometers. The work detailed in this thesis shows a highly detailed characterisation of two AGATA asymmetric capsules of the same shape in order to test and compare performances. Detector A004 was acceptance tested and scanned at the University of Liverpool in February 2010. Detector A006 was scanned between April and September 2010. Resolution, efficiency and charge collection parameters have been studied, comparing these two detector. The results of the comparison show an excellent agreement between the performance of the two detectors. The depletion rate of the detectors has also been compared quantitatively indicating the dependence of depletion on impurity concentration of the capsules. An experimental pulse shape database was generated for detector A006. The sensitivity of the detector response was quantified indicating the regions of the detector of high variation in pulse shape response. The AGATA detector library, which used to simulate the detector response for pulse shape analysis, is compared with this database. The position resolution achieved between the two data sets has been calculated. The combination of the detailed comparison and characterisation of the detector with the study of the simulate database will provide the AGATA collaboration with useful information to improve detector simulations in the further for pulse shape analysis.

The Advanced GAmma Tracking Array (AGATA) project is an European collabora- tion project that is aiming to construct a next generation gamma-ray spectrometer for nuclear-structure applications. The final array will consist of 180 highly-segmented HPGe detectors mounted into 60 triple clusters forming a full 4π shell. AGATA will utilise pulse-shape analysis and gamma-ray tracking to achieve high photopeak efficiency and peak-to-total. Central to the AGATA spectrometer is the Nouvelle Ac- quisition temps-Réel Version 1.2 Avec Linux (NARVAL) data-acquisition system that will perform pulse-shape analysis and gamma-ray tracking calculations in real time (online). The system is comprised of separate blocks of actions, called actors. Each one containing a self-sufficient piece of code designed for a specific action. AGATA data can be downloaded and replayed through an offline version of NARVAL, known as the NARVAL-emulator. With a new format and techniques there is need to define new methodology to analyse the data. Discussed in the first part of this work will be processing of AGATA data and how by utilising the actors of the NARVAL-emulator different output formats can be chosen. In the next part of this work, the Compton polarimetry capability of an AGATA triple cluster will be investigated by utilising pulse-shape analysis and the detector’s granularity. The basic design of a Compton polarimeter consists of two detectors; one acting as a scatterer and the second as a absorber. The same principles apply to an AGATA triple cluster with each sector act- ing as the scatterer and absorber. The pulse-shape analysis algorithm identifies the positions and energies of the interaction points. Data was obtained from the heavy- ion fusion-evaporation of 110Pd (32S,4n) 138Sm at 135 MeV was used to investigate the Compton polarimetry capability of an AGATA triple cluster. This work utilised the phenomenon that the scattering cross-section for gamma rays is larger in the direction perpendicular to its electric field vector than parallel. This is described by the differ- ential Compton scattering cross-section for polarised gamma rays. A useful event to determine linear polarisation occurs if the incident gamma ray is Compton scattered from the first interaction and then fully absorbed at the second interaction. The sum of the energies deposited in the two interactions gives the energy while the azimuthal scattering angle contains the linear polarisation information. To determine the linear polarisation of a gamma ray, a polarisation sensitivity and asymmetry measurement can be made. Such measurements were made in this work using gamma rays from 138Sm of known linear polarisations. The polarisation sensitivity method utilised the incident and scattered gamma-ray energy, in addition to the azimuthal scattering an- gle information produced from the pulse-shape analysis algorithm. The experimental polarisation sensitivities ranged from 5.0±0.3% at 139.7 keV to 16.4±0.1% at 1287 keV. A asymmetry measurement was carried by determining the number of gamma rays that scattered perpendicular and parallel to the reaction plane. A positive asym- metry would indicate an electric transition and a negative a magnetic one. It was found that there was partial consistency with expectation. From the asymmetry and polarisation sensitivity measurement an experimental linear polarisation for each gamma ray was determined. The result should be in the range −1≤P(θγ)≤+1 with the sign propagating through from the asymmetry measurement. Finally a coinci- dence efficiency and figure of merit was measured and are given as 32.6±1.0% and 3.9±0.2%, respectively at 346.9 keV. Overall it was concluded that there was limited Compton polarimetry capability and proposed reasons for this were discussed.