• x-ray polarimetry

X-ray astronomy has a vast history of successful missions and observations that, nevertheless, include almost no polarimetric data.
The Imaging X-ray Polarimetry Explorer (IXPE) is a mission set to be launched in April 2021 that aims to fill that gap. Its heart is the Gas Pixel Detector (GPD), a gas polarimeter that exploits photoelectric effect of X-rays in a gas to detect their angle of polarization. The GPD detects the path of the photoelectron via a matrix of 300-by-352 hexagonal pixels; then, a reconstruction algorithm infers its initial emission direction, whose distribution is peaked along the direction of polarization.
As part of my personal contribution, I have used a Monte Carlo simulator of the detector to study how different gas mixtures affect the instrument output. Also, I have taken part in two sets of measurements, for whom I have carried out the analysis. The first study concerns the recombination of electrons inside the gas cell of the GPD, an effect that causes a loss of the signal carried by such electrons; the analysis found the magnitude of that effect and how polarimetry measurements are affected by it. The second study is about the response of the GPD as a function of temperature; the stability of the detector has been tested and proved with a wide range of quantities it outputs, from 15 °C to 40 °C.
All the performance tests reported in the thesis prove that the GPD is a stable and reliable detector, and is suited for a space X-ray polarimetry mission.