The influence of anisotropic electron drift velocity on the signal shapes of closed-end HPGe detectors

This study is concerned with the anisotropy of the electron drift velocity in germanium crystals at high electric fields and low temperature. and its influence on the charge collection process in n-type, high-purity germanium (HPGe) detectors of closed-end, coaxial geometry. The electron trajectorie s inside HPGe detectors are simulated using a phenomenological model to cal culate the dependence of the drift velocity on the angle between the electr ic field and the crystal orientation. The resulting induced currents and pu lse shapes for a given detector geometry and preamplifier bandwidth are com pared to experiment. Experimentally, the dependence of the pulse shapes on the conductivity anisotropy in closed-end HPGe detectors was observed. The experimental data on pulse shapes were obtained by sampling preamplifier si gnals of an encapsulated. hexaconical EUROBALL detector, which was irradiat ed by collimated Na-22 and Am-241 sources. The crystal orientation was meas ured by neutron reflection. Qualitative agreement between the simulated and experimental pulse shapes was found. A variation in the charge collection time of up to 50 ns was observed For different drift directions of the carr iers relative to the crystal orientation. Furthermore? a deflection of the trajectories from a straight radial drift direction of about 20 degrees was predicted by the simulations. These two main effects of charge carrier dri ft velocity anisotropy in coaxial Ge detectors are expected to play an impo rtant role in the development of gamma-ray tracking detectors. (C) 2000 Els evier Science B.V. All rights reserved.