Gamma Ray Efficiency Comparisons for Si(Li), Ge, CdTe and HgI2 Detectors

Gamma-ray efficiency measurements were made for Si(Li), Ge, CdTe and HgI2 spectrometers and detectors of comparable sizes for gamma ray energies from 60 keV to 3.85 MeV. Full energy efficiency measurements of cooled Si(Li) and Ge spectrometers and a room temperature CdTe spectrometer show that the efficiencies improve in going from silicon (Z= 14) to germanium (Z= 32) to cadmium telluride (Z= 50) in agreement with the photoelectric cross-sections which vary as Z5. Because of its shorter carrier drift lengths, the CdTe spectrometer must be operated at the highest practical voltages. The detection efficiencies, based on all pulses above a threshold set by noise, were measured for Si(Li), CdTe and HgI2 detectors at room temperature. A 30 keV threshold and small photoelectric cross-section combine to give an efficiency minimum for the Si(Li) detector for 100 keV < Eγ < 200 keV. Both CdTe and HgI2 detectors iave no such minimum. In addition the higher electronic stopping powers of HgI2 and CdTe in comparison with Si reduce edge effects - an important factor in high energy γ-ray efficiency.