Development of a GridPix-based X-ray Polarimeter

Abstract

Measuring the polarisation of an electromagnetic wave can be a useful tool to study the emission source of the wave and the interactions of the wave during its propagation. While in the optical spectrum a measurement can be done with foils that filter polarisation directions, a measurement in the X-ray range requires precisely manufactured crystals. These crystals serve as polarisation filters but work only in a narrow energy band. There are several applications in material science and X-ray astronomy, that utilise the polarisation of X-rays Therefore, a detector that can directly measure this property would be beneficial for these applications. <br /> Such a detector concept was simulated, built, and tested within the scope of this thesis. It exploits the fact that that low energetic X-ray photons mainly interact via the photo electric effect and the resulting photo electrons are emitted in a direction that is correlated with the polarisation direction. Thus, tracking of these electrons can be used to measure the polarisation. The gas medium of the detector ensures sufficiently long tracks, allowing for a precise track reconstruction via a GridPix — a combination of a pixelised readout ASIC and an integrated amplification stage. <br /> In this thesis, two different ASICs were used — the Timepix and its successor the Timepix3. For the latter a new readout and control system was developed. X-ray polarimeters were build with both ASICs and their performance was compared. Therefore the energy resolution, position resolution, rate capability, and polarisation reconstruction were analysed with a range of detector parameters like gas mixtures and photon energies. Furthermore, the spurious modulation was studied. <br /> It was demonstrated that the detector concept works within an energy range between 2 keV and 17 keV, while achieving FWHM energy resolutions up to 20 % and position resolutions of the photon absorption point up to 40 µm. Furthermore, the Timepix3-based detector showed a significantly higher rate-capability than the Timepix-based detector. The Timepix3 also enabled recording events in three space dimensions, allowing for a 3D reconstruction of events. It was shown that this can reduce the required number of photons to detect a polarisation with a given confidence level by about 10 %.