UC Berkeley

The work presented in this thesis reflects a small piece in the complex mosaic of radiation detector developments. Advanced concepts in signal processing are being studied to enhance efficiency and spatial resolution of gamma-ray imaging systems. The ultimate goal of the work presented here is using signal processing, specifically signal decomposition (SD) algo- rithms, to get better imaging efficiency and angular resolution of the gamma-ray imaging system.

Position sensitive semiconductors (PSD) are important tools for gamma-ray detection and imaging. Substantial progress has been made in the development of these detectors, such as demonstration of 3D position readout, and demonstration for Compton imaging applications. One approach of position-sensitive readout is the use of high purity germanium Double Sided Segmented Detectors (DSSD).

Compton Imaging is now an established gamma ray imaging modality for energies rang- ing from about 200 keV to several MeV. Fundamentally, the performance of a Compton imager is limited by Doppler broadening. However, the current performance of Compact Compton Imaging systems is limited by intrinsic detector properties such as position and energy resolution and the ability to resolve individual interactions, as well.

We have developed and benchmarked signal processing techniques to improve the position resolution to be significantly better than given by the voxel size. Specifically, we have developed Signal Decomposition (SD) algorithms, which are based on physics models of the charge creation and transport processes and mathematical techniques such as singular value decomposition to infer the energy and three-dimension position of individual gamma-ray interactions.

The experimental results presented in this dissertation demonstrate the performance of a Compton Camera system in the second generation of the Compact Compton Imager (CCI2) configuration. This system utilizes two planar double sided segmented HPGe detectors. The segment size of these detectors is 2 mm. Using SD we were able to achieve a spatial

resolution of about 0.5 mm resulting in about 800k spatial voxels in both HPGe double-sided strip detectors (DSSD) characterized by an area of about 60 cm2 and a thickness of 1.5 cm and read out by only 76 individual and discrete preamplifiers. The increase in granularity significantly improves the imaging resolution and efficiency, which is the ultimate goal.

The best performance of the CCI2 system to date with respect to angular resolution has been achieved by the SPEctroscopic Imager for gamma-Rays (SPEIR) imaging system. Compared to the SPEIR system, angular resolution due to the SD algorithm is improved by several degrees and an increase in imaging efficiency of a factor of two has been demonstrated. Applying SD to the best quality events(single pixel events in both detectors), the angular of 3 degrees was achieved and corresponding imaging efficiency of 0.3%.

In order to benchmark the performance of the CCI2 system with SD algorithm, GEANT4 simulations were performed. An angular resolution of 3 degrees corresponds to 0.75 mm posi- tion resolution in all three dimensions, according to the simulations. Significant benefits can be attributed to signal decomposition algorithms. However, there is room for improvement in the algorithm itself in order to achieve 2.5 degrees angular resolution, which would be the ultimate goal for the CCI2 system.