The Next Stage of X-ray Speed-Reading: Developing a SiSeRO Active Pixel Matrix

The most recent NASA Astrophysics Biennial Technology Report identifies "fast, low-noise, megapixel X-ray imaging arrays" and "rapid readout electronics for X-ray detectors'' as high priority technology development gaps for future PCOS strategic astrophysics missions. A future X-ray flagship observatory of the type envisaged by the Astro2020 Decadal Survey, and X-ray Probe mission concepts such as AXIS, provide examples of strategic missions requiring large (megapixel) detectors capable of supporting high frame rates, while still achieving Fano-limited noise. At soft X-ray energies (below 0.5 keV), where a rich discovery space awaits, lower noise levels than those provided by the current state-of-the art detectors are needed. CCD technology has been the workhorse of X-ray astrophysics for decades. While highly successful, neither classical CCDs nor competing technologies such as monolithic and hybrid CMOS sensors can provide the combination of X-ray quantum efficiency, noise and frame rate required. In previous work, we have developed a novel X-ray CCD output stage concept dubbed the Single-electron Sensitive ReadOut (SiSeRO) and demonstrated proof-of-principle operation with very encouraging results. This includes a repetitive non-destructive readout capability offering the prospect of sub-electron noise performance, and to improve and simplify the gain calibration of such systems. Fabricated in the proven and scalable MIT Lincoln Laboratory process technology, SiSeROs are a direct descendant of CCDs and, as such, inherit much of their technological maturity and many of their performance advantages. We here propose to build the first active pixel matrix based on SiSeRO devices. The architecture, with a SiSeRO for every pixel, would remedy the two main weaknesses of X-ray CCDs. First, as no large distance charge transfer will occur, there should be no degradation due to radiation-induced displacement damage. Second, a matrix would allow the capability to combine full frame, low-noise readout with high speed region-of-interest readout in the same observation, without any loss of photons. This would enable wide-field observations where specific sources could be monitored with ~100 microsecond cadence, providing precise timing information and avoiding pileup. In order to develop and demonstrate this detector technology, we aim to perform extensive device simulations and layouts in year 1, followed by fabrication in year 2, and testing and characterization in year 3.

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