Gamma-ray imaging detectors

This thesis describes the development of detectors that are required to image hard X-ray and low energy γ-ray emissions (∼10 keV-1 MeV). These instruments may, on the one hand, be used to observe some of the most energetic processes occurring in our universe, such as supernovae and black holes, or to image auroral hard X-ray emissions from high altitudes in the polar atmosphere. They may also be applied in medical imaging as an important clinical diagnostic procedure, as well as in other fields. The scientific requirements for the performance characteristics of the next generation of γ-ray imaging devices are described and different types of instruments suitable for γ-ray imaging are reviewed. The particular value of making detectors consisting of scintillation crystals coupled to position-sensitive photomultiplier tubes (PSPMT) is discussed in the context of these requirements. Some gamma cameras have been developed using a single, continuous, scintillation crystal viewed by a 5 inch diameter PSPMT. A spatial resolution of about 3 mm FWHM and an energy resolution of better than 14% FWHM could be achieved at 140 keV. The promising performance of these devices should encourage its use in portable gamma cameras for medical imaging. Position-sensitive phoswich detectors have also been developed for γ-ray astronomy. Two broad-band imaging detectors for use in the energy range from about 15 keV to 1 MeV have been investigated. In particular, a compact design, which has a weight of about 100 kg, could provide a continuum (15-500 keV) sensitivity of about 4x10^-7 ph cm^-2s^-1keV^-1 for 3σ statistical significance during 106 seconds observation time. These devices could form the basis for a future γ-ray astronomy satellite mission. A ratio pulse-shape discrimination technique has been developed for the recognition of the location of γ-ray interaction in a phoswich detector. The results show that this technique can work well for photon interactions even at energies below 17 keV both on NaI(T1)/CsI(T1) and CsI(Na)/GSO phoswich configurations. The simplicity of the circuit and the advantage for multi-component discrimination demonstrate its potential use in many applications.