ESA selected XIPE Mission Focal Plane Optimization
IDPASC Portugal - PHD Programme 2016
2016 / 2017
Experimental Particle Physics | Astrophysics
Rui Curado da Silva
Universidade de Coimbra
Laboratório de Instrumentação e Física Experimental de Partículas
XIPE mission was one of the three 2015 M4 call selected missions for phase A by ESA (http://www.esa.int/Our_Activities/Space_Science/Three_candidates_for_ESA_s_next_medium-class_science_mission). After phase A period, aimed at studying technical and scientific aspects of the three concepts, one mission will be selected in June 2017 to be launched by 2026. XIPE mission focal plane detectors are photoelectric polarimeters based on the Gas Pixel Detector (GPD) design , these devices are composed by Gas Electron Multipliers (GEM) and multi-pixel anode readout. The filling gas mixture defines the ultimate intrinsic polarimetric sensitivity of the instrument, i.e. the modulation factor and the detector efficiency, which depends on gas parameters. Various mixtures for GPDs have already been studied, either by simulation or experimentally, for X-rays energy bands of 2-8 keV and 4-30 keV. The best results point to He-DME or Ne-DME at 1 atm for 2-8 keV range and Ar/DME at 2 atm for 4-30 keV range. However, gas mixtures can be further optimized for XIPE mission [2, 3, 4]. The objective of this thesis is the optimization of the GPD gas mixture both by simulation and experimental work, using a custom-made FORTRAN Monte-Carlo code [5,6] and an experimental setup capable of measuring the electron distribution on the anode readout produced by the detected X-rays, in order to measure the transverse spreading rms values for the electron clouds in several gas mixtures. Through this research work it will be choose the best trade-off gas mixture, between lowest electron diffusion in the gas and the highest possible electron drift speed. The best gaseous mixture solution will allow better reconstruction of photoelectrons emission direction and therefore a better degree and angle of polarization determination. The solution obtained must meet XIPE scientific requirements: MDP (Minimum Detectable Polarization) < 10% for 100 ks and 1 mCrab .  P. Soffitta, et al., “XIPE: the X-ray Imaging Polarimetry Explorer”, Exp. Astron. vol. 36, no. 3, pp. 523-567, 2013.  R. Bellazzini et al., “Photoelectric X-ray Polarimetry with Gas Pixel Detectors”, Nucl. Instr. and Meth. A, vol. 720, pp. 173-177, 2013.  F. Muleri, et al., “Low energy polarization sensitivity of the Gas Pixel Detector”, Nucl. Instr. and Meth. A, vol. 584, pp. 149-159, 2008.  L. Pacciani et al., “The sensitivity of a photoelectric X-ray polarimeter for Astronomy: The impact of gas mixture and pressure”, Proc. SPIE, vol. 4843, pp. 394-405, 2003.  T. H. V. T. Dias et al., "Monte Carlo simulation of x-ray absorption and electron drift in gaseous xenon", Phys. Rev. A, vol. 48, no. 4, pp. 2887-2902, 1993.  T. H. V. T. Dias et al., "Full-energy absorption of x-ray energies near the Xe L- and K-photoionization thresholds in xenon gas detectors: simulation and experimental results", J. Appl. Phys., vol. 8, pp. 2742-2753, 1997.  A. Derevianko and W. R. Johnson, "Non-dipole effects in photoelectron angular distributions for rare gas atoms", At. Data Nucl. Data Tables, vol. 73, pp. 153-211, 1999.