X-ray single photon detection with XPOL-III

XPOL-III is a newly developed CMOS ASIC simultaneously working for collecting charge and processing signals inside Gas Pixel Detectors (GPD). Starting from the architecture of the XPOL ASIC and its successful operation in the IXPE space mission, we implemented specific design changes aiming at increasing the rate capability and the uniformity of response. XPOL-III includes more than 100k pixels at 50 μm pitch in a total active area of about 15 × 15 mm2. Each pixel acts as a charge-collecting anode and is connected to its charge-sensitive amplifier, followed by a shaping circuit and a sample-and-hold system. The chip, like its predecessor, provides self-triggering capability, with automatic localization of the region of interest (ROI) to be readout for each single photon. A new programmable definition of the margin pixels around the ROI was implemented to reduce readout time. Other improvements include the sensitivity of the trigger electronics and an increase in the maximum speed for the serial event readout. In this work we describe the design of this new ASIC and the results of its preliminary tests, in particular in the context of the gas detector application, in which imaging the photoelectron track emitted by single X-ray absorption allows us to measure beam polarization together with timing, imaging, and spectroscopy.