Charge spreading and position sensitivity in a segmented planar germanium detector

The size of the charge cloud collected in a segmented germanium detector is limited by the size of the initial cloud, uniformity of the electric field, and the diffusion of electrons and holes through the detector. These factors affect the minimize size of a practical electrode structure and consequently the position sensitivity. We have completed measurements on a germanium strip detector with a finely collimated gamma ray beam to measure these properties. Preliminary results indicate that the electrons and holes spread by <65 μm over 9 mm drift distance. The experiment was conducted using a germanium strip detector with 25 boron implanted strips on one face and 25 orthogonal lithium drifted strips on the opposite face. The size of the charge cloud is measured by scanning the fan beam between neighboring strips. Results of these experiments suggest that full charge collection on a single strip would be achieved for a majority of events with strip pitch down to ≤300 μm in a 10 mm thick device. Concerns of degradation in the energy resolution due to charge sharing between strips is important for strip pitch much finer than this. There is no evidence of charge loss in hole collection by the boron implanted strips. Applications of a fine pitch germanium strip detector include an imaging focal plane behind a hard X-ray mirror (NASA Constellation mission), or a gamma-ray polarimeter for nuclear physics, amongst others.