Germanium spectrometers with internal junctions

Abstract

For fabricating a spectrometer from a high purity Ge crystal with an internal junction two contact configurations are possible: p νπn or p πνn, where ν and π represent the high purity n and p portions respectively. The two configurations are compared theoretically and experimentally with respect to depletion voltages, pulse risetimes, charge collection efficiencies and γ-ray pulse height resolutions. It is shown that the p νπn configuration has advantages in depletion voltage, charge collection, and timing properties as a result of the more favorable electric field. Spectrometers with depletion depths of 3 cm at 2000 V have been made from material having a net concentration gradient of dopants of 5 × 10 −9 cm −4. The charge trapping in the Ge limited the high energy (∼1 MeV) γ-ray resolutions to about 0.4% for a 3 cm p νπn device. For spectroscopy of high energy charged particles, however, these diodes combine a thin entrance window (<50 μg/cm 2) with a large stopping power and should be very useful.