Gas Pixel Detectors for Photoelectric X-ray Astronomical Polarimetry

The gas pixel detector is a true 2D imaging device that has been designed and built to image the projection of the photoelectron track, produced by an X-ray photon, onto a pixelated sensitive plane. It is a sealed gas cell with a thin beryllium window, a gas electron multiplier, and a multilayer self-triggering ASIC CMOS chip (named XPOL) which provides the analog charge image. This device has been designed and built for sensitive X-ray polarimetry starting in 2001 and to date is still the sole application used. The design is basically unchanged since 2007 when the first sealed detector was made available. In the subsequent years, the performances of the detector were studied in detail. The gas pixel detector was, then, eventually, selected for flight on a NASA SMEX mission in 2017. In this chapter, we describe its characteristics and the drivers of its design and its historic evolution. Moreover, we describe its major weaknesses and strengths indicating the path for future evolutions as gas detectors for X-ray polarimetry and its possible use in all the possible configurations for space missions.