Development of small pixel CZT detectors for future hard X-ray missions

The NuSTAR (Nuclear Spectroscopic Telescope Array) mission launched in 2012, and it has successfully deployed the first orbiting telescopes to focus light in the high-energy X-ray range (3 - 79 keV), providing a wealth of new information about the sources of high-energy X-rays. Follow-up missions such as the proposed HEX-P, BEST, and FORCE could perform a deeper black hole census providing a more refined measurement of black hole spins, allowing for greater knowledge about supermassive black holes. These missions are motivated by recent breakthroughs in hard X-ray mirror technologies where mirrors made of monolithic silicon segments and mirrors made directly or through replication of shells demonstrate the feasibility of making hard X-ray mirrors with angular resolutions of 5-10 arc seconds Half Power Diameter (HPD) compared to NuSTAR’s 1 arc minute HPD. Such a high angular resolution requires matched detectors (higher pixel density) to fully benefit from the achievable improved spatial resolution. In the above framework, the development of the HEXID ASIC, embedding is a novel pixelated front-end suitable for reading out a finely segmented CZT sensor, is presented. The required large dynamic range (from 2 keV to 180 keV) and low input noise (ENC < 20 e⁻) together with a small pixel size (150 µm) pose several design challenges in chip implementation. The chosen architecture of the front-end circuit and in-pixel processing blocks, together with the readout architecture of the registered signals and other adopted design solutions, driven by the quoted requirements, will be reviewed.