The Development of a Segmented N-Type Germanium Detector, and Its Application to Astronomical Gamma-Ray Spectroscopy

Extensive calculations and simulations have shown that the instrumental background in a coaxial germanium gamma-ray detector flown at balloon altitudes or in space, can be substantially reduced by segmenting the outer contact. The contact is divided into horizontal strips around the side of the detector, giving it many characteristics similar to those of a stack of planar detectors. By choosing different segment coincidence requirements in different energy ranges, one can obtain a factor of ~ 2 increase in sensitivity to spectral lines between 70 keV and 1 MeV, compared with an unsegmented detector. The reverse electrode configuration (using n-type germanium), with the p contact outside, is preferred for this application due to its thin dead layer and resistance to radiation damage in space. A small two-segment n-type coaxial detector has been developed and tested. To our knowledge, this is the first n-type germanium coaxial detector to be successfully made with a segmented cathode. Plans are to use the experience gained from the development of this prototype detector to contruct a large multi-segment device.