Initial development of a silicon scatterer detector for low-energy gamma rays in whole gamma imaging

Whole gamma imaging (WGI) is our original concept to combine PET and Compton imaging techniques so as to utilize all measured gamma rays for imaging by inserting a scatterer detector ring in a PET detector ring. Previously, we developed the first WGI prototype, in which GAGG scintillation detectors were used for the scatterer, to show a proof-of-concept for higher energy (over 500 keV) gamma rays such as from 137Cs and 44Sc. On the other hand, however, silicon (Si) detectors would be a better choice than the GAGG detectors for lower energy gamma rays due to the better energy resolution and higher efficiency of Compton scattering for the Si detectors. Therefore, in this work, we have been developing a Si scatterer for the next generation WGI for a wide range of target energies. The scatterer consisted of double-sided strip silicon detectors (DSSDs) and readout frontend circuits with ASICs. The size of a detector active area was 43.2 mm × 43.2 mm. Strip pitch was 900 μm. The number of strips was 48 channels for each side. The thickness of a DSSD chip was 600 μm. The DSSD chips were connected to the readout circuit board with two ASIC chips which contained 48 channel pre-amplifiers and time-over-threshold circuits. At first, we evaluated energy resolutions of the Si detector for low energy gamma rays from Ba-133, Am-241 and Co-57 sources. In all energy spectra, the respective total absorption peaks could be resolved. The energy resolutions of 16.8%, 18.1% and 17.0% were obtained for the 59.5 keV, 81 keV and 122 keV gamma rays, respectively. The performance of the WGI could be improved by using the Si absorber at the current stage of development based on the previous simulation. However, the energy resolution of the Si scatterer obtained in the experiment was slightly worse than the theoretical value. Therefore, further optimization is needed in order to obtain the best performance.The 2019 IEEE Nuclear Science Symposium (NSS) and Medical Imaging Conference (MIC