Some high-efficiency two-crystal Ge(Li)Nal(Tl) pair spectrometers

Abstract

Two types of two-crystal pair spectrometers employing Ge(Li) principal and NaI(Tl) auxilliary detectors are described. In one case, the germanium detector is surrounded by a 20 cm in dia. by 30 cm long optically uniform annular NaI(Tl) crystal coupled to seven photomultiplier tubes. An excellent line shape is obtained by operating the photomultipliers in a special way making use of the fact that light distributions following the absorption of Compton and annihilation quanta are different. In another pair spectrometer, which is essentially simpler, a small annular crystal detects the annihilation quanta emitted from the main crystal in backward directions. The intrinsic peak efficiencies of both spectrometers are almost 20% of the intrinsic double-escape peak efficiency of the germanium detector.