A Well-Type Ge(Li) Detector

The feasibility and usefulness of a coaxial Ge(Li) spectrometer as a well-type detector for sum spectra of coincident gamma rays is demonstrated. The value of the welltype geometry has previously been established in nuclear gamma spectroscopy with the use of (NaI(Tl) detectors. In this work a 16 cm3 hollow cylindrical germaniun detector has been made and mounted in such a way that access to the center of the detector is achieved from outside the cryostat. The detector is a fully depleted structure in which a hole of 1 cm diameter was ultrasonically cut, and a non-injecting contact applied to the inside surface. The measured resolution of the detector for a 1173 keV gamma ray sum peak from 144Pm was 2.6 keV f.w.h.m, which was mainly limited by statistics and preamplifier noise. The detection of sum lines arising from 2 or more coincident gamma rays is demonstrated by using the radioactive source of 144pm. With the source inside the well, for which the counting solid angle is nearly 4π, the sum peak of three lines at 1791 keV (696 + 618 + 477) was observed with an energy resolution of 3.5 keV f.w.h.m, and a peak to background ratio greater than 15:1. For the source outside the detector at a distance of 10 cm, this sum line was not observed above background for comparable counting statistics. Comparison is also made to results obtained from a 3" well-type NaI(Tl) crystal.