A Total Absorption Ge(Li) Gamma Ray Spectrometer

Abstract

A configuration of a large Ge(Li) detector has been developed which suppresses single, local interactions, particularly Compton scatterings. The peripheral n+ contact of a coaxial detector is split into two or more segments, and events are recorded from the central p contact if signals are coincidentally present on two segments of the n+ contact. A sum signal is derived from the central p contact through a high resolution preamplifier, and individual localization signals are taken from each segment of the n+ contact through simple voltage-sensitive preamplifiers. The maximum sensitivity of the system lies between 300 and 1300 keV where the Compton crosssection is dominant and many Compton scattered γ-rays are not sufficiently energetic to escape. The total absorption mode achieves about 20% of the photopeak efficiency of the detector operated in the normal singles mode. An increase in the ratio of the full energy peaks to the Compton continuum of 10:1 is found for 60Co γ-rays, and it is significantly greater for γ-rays of less than 1 MeV. The use of internal detector summing of the linear signal eliminates the need for complicated external preamplifier gain adjustment and stability. Capacitively coupled, voltage-sensitive preamps on each n+ segment add negligible noise and reduce the sum signal-to-noise ratio only to the extent that the input shunt capacitance used reduces the sum signal level by about 10%. Thus, a 30 cc split n+ detector yields 3.9 keV 60Co resolution in the coincidence mode compared with 3.5 keV when both n+ segments are grounded.