Measuring soft X-ray quantum efficiency: CMOS sensors, proportional counters, and a methodology for determining the transmission of X-ray transparent windows

The overarching goal of this research program is to develop and characterize technology to enable impactful X-ray astrophysics missions on small satellite (SmallSat) platforms. Within this scope, there are two distinct needs this research seeks to address: the need for greater capability and accessibility of space-based detectors for lower energy (soft) X-rays, and the need for absolutely calibrated instruments to establish the X-ray brightnesses (absolute fluxes) of standard candles (celestial objects) for use in recalibrating the major X-ray observatories in orbit.

The proposed SmallSat-based Cal X-1 mission seeks to establish the absolute X-ray fluxes (photons/second/area) of candidate X-ray standard candles with great enough certainty that major orbital X-ray observatories can use their own observations of these objects to improve upon their existing calibrations. To address Cal X-1's need for a very stringently calibrated detector for the ground-based calibration of their flight instruments, we designed, built, and tested a pathfinder proportional counter and readout pipeline. For calibration purposes, we constructed a high-vacuum soft X-ray beamline incorporating a complementary metal oxide semiconductor (CMOS) sensor for high resolution X-ray spectroscopy. Finally, we developed and implemented a novel method for determining the transmission of X-ray transparent windows from observations of multi-energy fluorescence spectra.

With the additional goal of expanding the range and accessibility of available detectors for space-based soft X-ray observation, we made use of the Intermediate Energy X-ray (IEX) beamline at Argonne National Laboratory to determine the soft X-ray detection efficiency (quantum efficiency) of a commercially available Sony IMX290LLR-C backside-illuminated CMOS imaging sensor.