Berkeley Lab

Positron Annihilation Coincidence Scanning System for Characterization of Germanium Detectors

PI: Kai Vetter
Staff: Quinn Looker, Micah Folsom

Understanding pulse shapes in new detector types is critical in calibration and understanding detector response.  Particularly, P-type Point Contact (PPC) detectors show great potential in pulse shape analysis to distinguish single-site and multi-site interactions, yet many effects contribute to the overall pulse shape.  A detailed understanding can be obtained through studying interactions in a localized volume of the detector, but traditionally this requires long experimental campaigns involving a collimated beam, sometimes relying on coincidence with a second detector.  However, a positron annihilation coincidence system has been demonstrated that allows an entire Ge detector to be scanned in hours, with a simple rotation potentially providing three-dimensional position information.  A Na-22 source is placed between a LYSO scintillator coupled to a multi-anode PMT and the Ge detector to be scanned.  The LYSO detector provides position information for one annihilation photon, and a second annihilation photon detected in the Ge in coincidence is assumed to be traveling in the opposite direction on the same line.  This system is designed to characterize pulse shapes in PPC detectors originating from the bulk, transition layer near the Li contact, volume near the surface-current-blocking ring (or “ditch”), and undepleted “bubble” in the center.