Fast Timing ASIC for Germanium Strip Detectors

Large-area, tracking, semiconductor detectors with excellent spatial and spectral resolution enable exciting new access to soft (0.2-5 MeV) gamma-ray astrophysics. The improvements from semiconductor tracking detectors come with the burden of high density of strips and/or pixels that require high-density, low-power, spectroscopy quality readout electronics. CMOS ASIC technologies are a natural fit to this requirement and have led to high-quality readout systems for all current semiconducting tracking detectors. The Compton Spectrometer and Imager (COSI) at University of California Berkeley and the Gamma-Ray Imager/Polarimeter for Solar flares (GRIPS) at Goddard Space Flight Center utilize germanium cross-strip detectors and are on the forefront of NASA's Compton telescope research with funded missions of long duration balloon flights. A germanium strip detector ASIC requires excellent energy resolution and precise timing of the signals at the anode and cathode of the device to allow the depth of the interaction within the crystal to be determined. The previous APRA program produced an ASIC that meets the minimum science requirements for the missions and retired the primary risk of adding a timing circuit to the ASIC while maintaining excellent energy resolution. The current ASIC uses on analog shaper for both the energy and timing which leads to compromise shaping times that reduce instrument sensitivity. We propose to design an updated ASIC that has two shapers per channel, a fast shaper to produce accurate timing measurements and a slow shaper for germanium quality energy resolution. Dr. De Geronimo has other ASIC designs with this capability and the design can copy the current shaper for reuse in the fast shaper. COSI and GRIPS both target detectors with ~0.5 mm strip pitch so that the strip pitch is not the dominant term in the angular resolution of the instrument. At this pitch, charge sharing between neighboring strips is the predominant type of event. Using a reasonable energy threshold of 12 keV per strip, it would be common to see shared charge events in the neighboring strips that is less than the threshold. The current ASIC does not read out channels that are under threshold so the measured energy can differ from the actual interaction energy by up to two times the threshold. COSI avoided this issue with their discrete analog electronics by reading out neighboring strips even if they were under threshold. We propose to port Dr. De Geronimo's neighboring strip readout scheme from other ASIC designs to the germanium front-end ASIC. This should be a relatively risk-free change as it only modifies the digital backend of the chip without needing to modify the analog chain. Making these modifications to the current germanium front-end ASIC will produce an ASIC designed for future NASA missions with all of the functionality of the current COSI discrete electronics. It will enable instruments using this ASIC to have sensitivities approaching the limits of germanium detectors.

The Astrophysics Research and Analysis program (APRA) supports suborbital and suborbital-class investigations, development of detectors and supporting technology, laboratory astrophysics, and limited ground based observing. Basic research proposals in these areas are solicited for investigations that are relevant to NASA's programs in astronomy and astrophysics, including the entire range of photons, gravitational waves, and particle astrophysics. The emphasis of this solicitation is on technologies and investigations that advance NASA astrophysics missions and goals.

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