Extremely Low-noise, High Frame-rate X-ray Image Sensors for Strategic Astrophysics Missions

X-ray imaging sensors for future strategic missions will require relatively small pixels (≤16 μm) with relatively large depletion depths (≳100 μm), with excellent soft X-ray performance at high (20-100 frames/sec) readout rate. Achieving physics-limited performance in these devices requires a large number of parallel, fast output amplifiers to achieve the required frame rate and very low readout noise to achieve the required spectral resolution and soft X-ray quantum efficiency. Low noise is critical because charge packets produced by low-energy photons will diffuse to be collected in multiple pixels, some of which contain very small (a few dozen electrons or less) amounts of charge. Reliable detection and accurate measurement of such small charge packets is essential for both good spectral resolution and low-energy quantum efficiency, and requires very low noise (~2.5 electrons RMS or less). Current flight technology achieves read noise levels of a few electrons RMS at pixel rates of 50 - 100 kHz per output. To mature the detector technology required for future strategic missions we propose to extend our current Strategic Astrophysics Technology work, which has demonstrated a single-channel pJFET CCD output amplifier with < 3 electrons RMS noise operating at 2 MHz. We propose to demonstrate a multi-output CCD with noise < 2.5 e- RMS per read, and with technical characteristics (output rate, output pitch, pixel size and depletion depth) and spectral resolution meeting the requirements of the AXIS X-ray probe, a mission concept conforming to the recommendations of Astro2020. To do this we will i)fabricate a version of our current test CCD equipped with 16 of our best current pJFET amplifiers and test it with a multi-channel ASIC readout under development in our current program; ii) optimize the design of our existing pJFET amplifier for even lower noise and iii) further develop and test a new, high-responsivity amplifier design, the Single-electron Sensitive ReadOut (SiSeRO), which in our recent measurements of proof-of-concept devices shows the potential for sub-electron noise. We will achieve the latter two objectives by means of two additional versions of the multi-output CCD, one each equipped for development of pJFET and SiSeRO amplifiers, respectively. Our access to fabrication facilities at MIT Lincoln Laboratory and the design, device simulation, and test capabilities of our team allow all of these elements to be addressed efficiently within the scope of our proposed program. Successful completion of this work will raise the technology readiness of this sensor architecture very close to TRL 4, and facilitate rapid achievement of TRL 4 in a follow-up program.

The Strategic Astrophysics Technology program (SAT) supports focused development efforts for key technologies to the point at which they are ready to feed into major missions in the three science themes of the Astrophysics Division: Exoplanet Exploration, Cosmic Origins, and the Physics of the Cosmos. This program is specifically designed to address middle technology readiness level (TRL) "gaps" between levels 3 and 6: the maturation of technologies that have been established as feasible, but which are not yet sufficiently mature to incorporate into flight missions without introducing an unacceptable level of risk. NASA does not require a data management plan for proposals to SAT.

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