High efficiency CsI(Tl)/HgI2 gamma ray spectrometers

CsI(Tl)/HgI₂ gamma-ray spectrometers have been constructed using 0.5' diameter detectors which show excellent energy resolution: 4.58% FWHM for 662 keV ¹³⁷Cs gamma-ray photons [1]. Further efforts have been focused on optimization of larger size (≥1' diameter) detector structures and improvement of low noise electronics. In order to take full advantage of scintillation detectors for high energy gamma-rays, larger scintillators are always preferred for their higher detection efficiencies. However, the FWHM resolutions of larger PDs may be higher as a result of larger capacitances and dark currents. For very large volume scintillators, it may necessary to employ a PD with a sensitive area smaller than the cross-section of the scintillator. Monte Carlo simulations of the light collection for 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 favorable geometry, the conical frustum, have been fabricated and evaluated. The response of a large conical frustum (top-2', bottom-1', 2' high) CsI(Tl) scintillator coupled with a 1' HgI₂ PD was measured. The energy resolution of the 662 keV peak was 5.57%. The spectrum shows much higher detection efficiency than those from smaller scintillators, i.e., much higher peak-to-Compton ratio in the spectrum.