Examining Signal Decomposition in Ge Tracking Detectors through Source-Based Coincidence Measurements

The performance of a gamma-ray tracking detector, such as those used in the GRETINA spectrometer [1], is dependent on its ability to accurately locate multiple interaction points in the Ge crystal. Interactions are located by observing both net and induced charge as a function of time on the detector’s segmented contact. As multiple interactions are likely, linear combinations of basis signals, a set of simulated signals with unit charge deposited on a grid that spans the detector volume, are fit against the observed signal yielding the interaction positions. While the location of the primary interaction point was found to be good ($\sigma_{pos} \le 2$mm) the location of secondary, lower energy interactions appear less reliable. To investigate this issue, we carried out a series of source-based coincidence measurements. These employed a collimated source and a secondary detector by which we could select single interaction events. Given these events originate from known positions, we can take them in combination to directly test the efficacy of the signal decomposition procedure. We will present a description of the method and preliminary results with a GRETINA quad detector. [1] S. Paschalis, I.Y. Lee, et. al., Nucl. Inst. and Meth. in Phys. Res. A, 709 (2013) 44