Signal interpolation in germanium detectors for improved 3-D position resolution

A technique has been implemented for improving the 3-D position resolution in a germanium strip detector. By using the signals induced on multiple electrodes, the position of the drifting charges can be interpolated to a resolution smaller than the width of the strips. Interpolation allows for a desired position resolution to be achieved with the fewest number of electronic channels. Applications include portable and space-based instruments where power, cooling, and mass are at a premium. Measurements were made on a fully instrumented 19×19 planar germanium strip detector. This system is used as a Compton imager and detects gamma-ray sources anywhere in a 4-pi field of view. It is shown that, using interpolation, the point spread function for a 662 keV point source improves from 25° to 10° FWHM under specified conditions. However, it is shown that many interactions occur too close together to be effectively interpolated. This and other practical limitations are discussed. In addition, an electrostatic model has been developed and shown to be in good agreement with measurements. This model was used to optimize detector design, predicting signal to noise ratios, and check the calibration of the signals.