Study and characterization of the Gas Electron Multipliers for the IXPE focal plane

Astronomical polarimetry allows to study the physical properties of a great variety of sources, as polarization is intrinsically linked to the geometry and magnetic field configuration of the emitting source. Polarimetry is a common tool in the optical and radio bands but it is almost unexplored in the X-ray band, where the only high significant polarization measurement was performed more than 40 years ago for the Crab Nebula. This lack of measurements is due to the difficulty in developing conventional X-ray polarimeters (based on Thomson scattering and Bragg reflection) with a good sensitivity, along with the intrinsic difficulty of X-ray polarimetry, characterized by low amplitude signals and requiring significantly more data with respect to spectroscopy or imaging. In recent years, the advent of a new generation of high-sensitivity polarimeters based on the photoelectric effect renewed interest in the field and in 2017 NASA selected IXPE — the Imaging X-ray Polarimetry Explorer — to be the next mission dedicated to X-ray polarimetry. IXPE — a small satellite scheduled for a launch in 2021 in Low Earth Orbit — will provide a breakthrough in astrophysics and fundamental physics, finally adding polarization to the X-ray source properties currently measured (energy, time, and location). IXPE will perform accurate X-ray polarimetric measurements for several categories of cosmic X-ray sources that are likely to be X-ray polarized in order to study their emission mechanism, configuration of the magnetic field and internal geometry. This includes extended sources such as pulsar wind nebulae and supernova remnants, magnetized compact objects, microquasars and active galactic nuclei. The observation of these systems, featuring strong gravitational and magnetic fields, will also allow to study effects of fundamental physics that are not accessible on Earth. The IXPE payload is composed of three X-ray telescopes, operating in the 2-8 keV energy range, with identical mirror modules and identical polarization- sensitive imaging detectors at their foci: the Gas Pixel Detectors (GPD). The GPD exploits the photoelectric effect in gas — the dominant interaction process in the IXPE energy band — in order to measure the X-ray polarization of cosmic sources. In the case of polarized X-rays, photoelectrons are preferentially emitted along the polarization direction, thus polarization can be measured by imaging the photoelectron track of each event. The GPD, developed by the IXPE Italian team, is designed as a proportional gas detector with an Application-Specific Integrated Circuit (ASIC) as a finely pixelized collecting anode and a Gas Electron Multiplier (GEM) as amplification stage. The ASIC pixels pitch and the GEM holes pitch are small enough to achieve a good sampling of the photoelectron tracks: in the current generation both the readout and amplification stage have a 50 μm pitch. The direction of emission of the photoelectron is determined by means of a track-reconstruction software. The main focus of my thesis is the study and characterization of the GEMs for the GPDs that will be on-board IXPE. GEM foils for the GPD are currently produced by the company SciEnergy in Japan, with the collaboration of RIKEN using an innovative laser etching technique. These GEMs are tested at the INFN laboratories in Pisa with the aim of studying and characterizing these devices in terms of operating voltage, gain, uniformity and temporal stability. A characterization of the GEMs in terms of the geometric properties of the holes is also required and carried out. As part of my personal contribution, I have actively taken part in the definition of the GEM test procedures and in the drafting of the corresponding documentation, curing the aspects of quality assurance of these devices. I also performed a large fraction of the GEM tests on different incoming batches and the corresponding data analysis, providing my contribution to the development of the GPD-related software. The results obtained by means of the GEM tests allowed for a fine tuning of the GEM production process, contributing to the final design of the detector. Moreover, on the basis of the tests results, I took part in the selection of GEM foils to be assembled in flight-model GPD prototypes. Gain uniformity represents one of the main selection criteria when choosing a GEM for a GPD assembly. In order to have a good energy resolution and high polarization sensitivity of the GPD it is indeed important to have a uniform gain over the entire area of the detector. Gain non-uniformity can be corrected by means of a calibration matrix, mapping one-to-one the size of the ASIC, that is applied before the photoelectron track is identified and reconstructed. As part of my contribution, I developed a new algorithm for gain calibration at pixel scale. This algorithm is already incorporated in the off-line track reconstruction software and used for tests on flight-model GPDs. It will be further developed and used for the analysis of flight data. In this thesis, Chapter 1 presents a review of X-ray polarimetry, including the basic definitions, the description of the physical processes giving rise to polarization in the X-ray band and the experimental techniques, along with historical remarks on past observations. Chapter 2 is focused on the IXPE mission, including a description of its scientific objectives and a technical overview. The GPD is fully described in Chapter 3 in terms of its structure, principle of operation and performance as polarimeter. The track-reconstruction algorithm is also presented. The following two chapters are focused on the main theme of this thesis, that also represents my contribution to the IXPE project: in Chapter 4 the GEM tests are described and the obtained results are presented, while in Chapter 5 the developed algorithm for gain calibration at pixel scale is presented