Toward Simultaneous 2D ACAR and 2D DBAR: Sub-Pixel Spatial Characterization of a Segmented HPGe Detector Using Transient Charges

Sub-pixel spatial characterization of an Ortec HPGe double-sided strip detector was measured in preparation for its planned use in a positron annihilation spectrometer (PAS) to simultaneously measure the two-dimensional Angular Correlation (2D ACAR) and Doppler Broadening (2D DBAR) of Annihilation Radiation. Sr-85 514-keV photons were finely collimated onto the center of 25, equally spaced sub-pixels within a single 5-mm by 5-mm intrinsic pixel, using a collimator with an aperture diameter of ${0.15}\pm {0.05}~{\rm mm}$. The location of each full-energy recorded event was determined by analyzing the relative sizes of coincident transient charges on charge collection strips adjacent to the strip recording full-energy events. Interpolation, using ratios of the coincident transient charge sizes, was used to determine two-dimensional locations of full-energy events. Radial location data was fit to a function which describes a Gaussian point spread function uniformly distributed over a circular aperture. The standard deviation of the point spread function, 0.22 mm, is a measure of the spatial resolution of this detector system. The relative efficiency for detection of 514-keV photons across the intrinsic pixel was also measured.