Experimental study of the response characteristics of large Ge(Li) detectors

Abstract

The relative efficiencies of a cylindrical Ge(Li) detector having an estimated active volume of 77 cm 3 were measured in the full-energy, single-escape and double-escape peaks produced by γ rays of energies 2–12 MeV. These relative efficiencies at 2–10 MeV are expressed theoretically in terms of the absolute efficiencies calculated for all interactions except coherent scattering, the probability that a 0.511 MeV annihilation photon escapes from the active volume of the crystal, and an empirical reduction factor which is energy dependent. The escape probability is constant at 0.676±0.006. The γ-ray absorption coefficient for all interactions producing the full-energy peak is derived by comparison with the known pair-production coefficient and tabulated for γ rays of energies 2–10 MeV. The above Ge(Li) detector and a second one having an estimated active volume of 114 cm 3 were scanned horizontally with a fine pencil of γ rays from a 10 mg radium needle. The relative angular efficiencies measured in the full-energy peaks at 1120, 1764, 2204 and 2447 keV were used to evaluate the angular attenuation factor Q k for k = 1, 2, 4 and 6. The experimental and calculated factors agree if the effective outer radius of the detector is taken to be 87% of the nominal value. Radiography was found useful in checking the dimensions and axial position of one of the detectors in its aluminum housing.