Characterization of Charge Sharing in the FOXSI Sounding Rocket Hard X-ray Detectors Using the Advanced Light Source at Berkeley

The FOXSI sounding rocket experiments represent the first ever solar-dedicated direct-focusing hard X-ray (HXR) instruments. The most recent flight (FOXSI-3) occurred in September 2018, utilizing Wolter-1 style concentric-shell optics and both silicon (Si) and cadmium telluride (CdTe) double-sided strip detectors. The angular resolution of the optics (5" FWHM, equivalent to 50um at the detector bench) is finer than the strip pitch of the Si and CdTe detectors (75um and 60um, respectively), meaning that the resolution of the instrument at sounding rocket focal lengths is detector-limited [1]. In order to further improve characterization of the spatial and spectral properties of the FOXSI-3 detectors, experiments were recently completed using Beamline 3.3.2. at the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory. Using this beamline, a fine (2um x 2um or 5um x 5um) monoenergetic x-ray beam was scanned across detector strips and inter-strip regions in steps much smaller than the strip pitch. Such scans were performed for both Si and CdTe detectors at a number of x-ray energies between 5.5-12 keV. These experiments were particularly motivated by the desire to measure the incidence of charge sharing, which occurs when a single incident photon causes signals to be registered in multiple adjacent strips. Precise measurement of charge sharing behavior is important to optimize characterization of detector efficiency. Additionally, with a strong understanding of charge sharing behavior in a detector, spatial resolution could be improved by sub-strip localization of photon impacts. Here, we present initial analysis of results from these ALS experiments, including investigation into charge sharing properties in both the Si and CdTe FOXSI-3 detectors. We also discuss the implications of these results for the use of these detectors in the future FOXSI-4 mission, which will utilize yet-higher-resolution HXR optics.

[1] Furukawa,K.,et al. "Development of 60um pitch CdTe Double-sided Strip Detectors for the FOXSI-3 sounding rocket experiment." NIMPR, Section A: HSTD11, 2017.