Calibration of 3D positioning in a Ge cross-strip detector

In preparation for the Nuclear Compton Telescope, a novel gamma-ray telescope designed for balloon-borne astrophysical observations, we have calibrated, the 3D positioning capabilities of a prototype 2 mm pitch cross-strip Ge detector. To accurately position in the third dimension (depth) we use the relative timing difference in charge collection on the anode and cathode, a sensitive measure of depth in the detector. In order to calibrate the depth determination in terms of the collection time difference, we have developed a statistical calibration technique which involves illuminating opposite sides of the detector with photons of known energy and requiring self-consistency of the measured mean free path of the photons on both sides. Requiring this to occur simultaneously for several different photon energies ensures that there will be no energy dependence of the calibration (within our sensitivity range). We can then check for consistency with the known mean free paths in germanium for each photon energy, as well as with our detailed simulations of the detector performance. We present the result of our prototype detector calibration as well as demonstrate the excellent agreement between these calibrations and our simulations.