HIGH-EFFICIENCY CSI(TL)HGI2 GAMMA-RAY SPECTROMETERS

CsI(TI)/HgI2 gamma-ray spectrometers have been constructed using 0.5'' diameter detectors which show excellent energy resolution: 4.58% FWHM for 662 keV 137 Cs gamma-ray photons [1]. Further efforts have been f ocused on optimization of larger size (greater than or equal to 1'' di ameter) detector structures and improvement of low noise electronics. In order to take full advantage of scintillation detectors for high en ergy gamma-rays, larger scintillators are always preferred for their h igher detection efficiencies. However, the larger capacitance and high er dark current caused by the larger size of the detector could result in a higher FWHM resolution. Also, the increased probability of inclu ding nonuniformities in larger pieces of crystals makes it more diffic ult to obtain the high resolutions one obtains from small detectors. T hus for very large volume scintillators, it may necessary to employ a photodiode (PD) with a sensitive area smaller than the cross-section o f the scintillator. Monte Carlo simulations of the light collection of various tapered scintillator/PD configuration were performed in order to find those geometries which resulted in the best light collection. According to the simulation results, scintillators with the most favo rable geometry, the conical frustum, have been fabricated and evaluate d. The response of a large conical frustum (top-2'', bottom-1'', 2'' h igh) CsI(TI) scintillator coupled with a 1'' HgI2 PD was measured. The energy resolution of the 662 keV peak was 5.57%. The spectrum shows m uch higher detection efficiency than those from smaller scintillators, i.e., much higher peak-to-Compton ration in the spectrum.