ASIX: Single-photon, energy resolved X-ray imaging with 50 μm hexagonal hybrid pixel

Massimo Minuti^*

• National Institute of Nuclear Physics, Ministry of Education, Universities and Research, Rome, Italy

The Analog Spectral Imager for X-rays is a technology demonstrator of a small-pixel Hybrid Pixel Detector (HPD) designed for applications such as X-ray diffraction, synchrotron-based material science, and soft X-ray astrophysics requiring energy-resolved imaging. The ASIX architecture aims at mitigating the adverse effects of charge sharing, typical of small-pixel devices. In contrast to other frame-based photon counters or multi-threshold devices, ASIX employs, along with a 50 μm pixel, an ultra-low-noise (< 30 e− ENC), fully analog, asynchronous, single-photon readout, targeting 10 μm spatial resolution and 350 eV FWHM at 8 keV within the same exposure. This paper describes the ASIX imager architecture and reports on the development and testing of two Minimum Viable Products (MVPs), developed by coupling XPOL-III, a readily available 180-nm CMOS readout ASIC, to a 300 μm thick silicon sensor with 50 μm pixels and to a 750 μm thick CdTe sensor with 100 μm pixels, respectively. The MVPs achieved estimated energy resolution of 780 eV FWHM at 17.5 keV (CdTe), and 620 eV FWHM at 9.7 keV (silicon) and spatial resolution of 20 μm (CdTe) and 7 μm (silicon). These results confirm our preliminary models predicting the feasibility of simultaneous high energy and spatial resolution in such a small-pixel devices, thus securing the ASIX specifications. Finally, the paper highlights the technology gaps that ASIX would potentially fill in both terrestrial and space applications.