FPGA-Based Back-End Electronics for the Imaging X-Ray Polarimetry Explorer Mission

The Imaging X-Ray Polarimetry Explorer (IXPE) is the next NASA small explorer mission, due to be launched with a Falcon 9 rocket on May 2021. IXPE will perform polarization measurements in the 2–8 keV band, complementing the unprecedented polarimetric capabilities with moderate-to-good imaging, spectroscopy, and timing. The scientific payload consists of three identical telescopes, each including a mirror module assembly and a detector unit at the focal plane. The core of each detector unit is a Gas Pixel Detector (GPD). We designed the GPD back-end electronics based on a radiation-tolerant FPGA for data acquisition and processing, event sequencing, and on-line data compression. Two custom digital serial interfaces-the Command and Control Interface (CCI) and the Science Data Interface (SDI)-implement the communication of the units with a central on-board computer. We designed and manufactured a comprehensive test equipment, based on a commercial FPGA hosted on a custom VME board, to emulate the functionality of the on-board computer that will operate the detector in flight. In this paper, we shall discuss the basic architectural choices behind the design of the GPD backend electronics, as well as the tests with X-rays performed in the lab using the aforementioned test equipment.