The APA pixel readout ASIC for an APD based fast 2D X-ray hybrid pixel detector

The XNAP project develops a two-dimensional counting pixel X-ray detector based on silicon avalanche photodiodes (APDs) with time resolution capabilities in the nanosecond range. Today, such a time scale is only accessible with single elements or linear arrays of fast devices. In order to read out APD arrays in the 1 k pixel range, a mixed-signal ASIC named APA was developed, which is bump-bonded via an interposer to the sensor to form a hybrid detector. The ASIC prototypes and different types of APD arrays with divided anode and divided cathode design were evaluated on 4 × 4 pixels and tested using 5.9 keY, 8 keY and 14.4 keY X-rays. We report on the system design and present first encouraging results on energy selectivity, position resolution, timing and maximum count rate.