Doctorate in Applied and Engineering Physics  

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Thesis Proposal for the

Doctorate Program in Applied and Engineering Physics (DAEPHYS)

Starting in the Academic Year 2016/2017

Proposal 024

Title:ESA selected XIPE Mission Focal Plane Gas Mixture Optimization
Objectives (recommended length: 2000-3000 char):
XIPE mission was one of the three 2015 M4 call selected missions for phase A by ESA ( 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 [1], 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 must meet XIPE scientific requirements: MDP < 10% for 100 ks and 1 mCrab [1].

[1] P. Soffitta, et al., “XIPE: the X-ray Imaging Polarimetry Explorer”, Exp. Astron. 36, no. 3, pp. 523-567, 2013.
[2] R. Bellazzini et al., “Photoelectric X-ray Polarimetry with Gas Pixel Detectors”, NIM A, vol. 720, pp. 173-177, 2013.
[3] F. Muleri, et al., “Low energy polarization sensitivity of the Gas Pixel Detector”, NIM A, vol. 584, pp. 149-159, 2008.
[4] L. Pacciani et al., “The sensitivity of a photoelectric X-ray polarimeter for Astronomy: The impact of gas mixture and pressure”, SPIE, vol. 4843, pp. 394-405, 2003.
[5] 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.
[6] 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", JAP, vol. 8, pp. 2742-2753, 1997.
[7] A. Derevianko and W. R. Johnson, "Non-dipole effects in photoelectron angular distributions for rare gas atoms", At. Data Nucl. Data Tables, vol. 73, ppp. 153-211, 1999.
Framework (recommended length: 500-2000 char):
The XIPE mission was one of the three 2015 M4 call selected missions for phase A by ESA [13]. 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 might be the first X-ray polarimetry launched into space, opening a whole new observational window. The XIPE scientific payload is composed of a mirror assembly and focal plane detectors.
The focal plane instrument is based on GPD photoelectric X-ray polarimeters. The GPD measures photons linear polarization by reconstructing the emission direction of the ejected photoelectrons. It consist of: a gas cell with a thin 50 μm Beryllium entrance window, an absorption/drift gap, a charge amplification stage and a multi-anode read-out which is the pixelated top metal layer of a CMOS ASIC. The emission direction is used to measure the polarization because the photoelectron is preferentially ejected in the direction of the absorbed photon polarization. For polarized photons, the modulation curve amplitude of the azimuthal emission directions is proportional to the degree of polarization and its maximum is aligned with the polarization direction. The proposed GPD baseline is a mixture 20% He - 80% DME at 1 atm, sensitive in the 2-8 keV energy range with a gas absorption/drift thickness of 10 mm.
As XIPE Instrument Team partner, our group, has the task of optimize the GPD gas mixture. The PhD student will develop its research work in team with 4 researchers, one postdoc and a master student.
In case XIPE will not be selected in June 2017, XIPE partners are motivated to propose an improved version of this mission concept for future ESA calls.
Tasks (recommended length: 1000-3000 char):
In order to optimize XIPE focal plane gas mixture both simulation and experimental analysis will be performed. The PhD research work will consist in a set of 3 tasks.

Task 1
The PhD student will develop a Monte-Carlo simulation code based in an early-stage custom-made FORTRAN based code [5, 6] that was developed for Xe gas filled detectors irradiated by unpolarized X-rays. The new code takes into account the energy- and shell-dependent angular differential cross-sections for photoionization by polarized X-rays. This code is not yet validated by polarimetric measurements. Herein, the student will develop this code for noble gases like He or Ne, as well as quenching additive gases like DME or iso-C4H10 gases, including the respective cross-sections, furthermore he will implement the first order non-dipole corrections to the dipole approximation of the cross-sections [7]. It should record the growth of the electron cloud produced in the gas, reproducing in detail the X-ray photoionization events and the cascade decay of the residual atomic ions (including the emission of photoelectrons, Auger electrons and fluorescence X-rays), following electrons’ slowdown in the gas to sub-ionization energies. This mixture solution will allow optimal reconstruction of photoelectron emission direction and therefore a higher polarimetric modulation factor (Q), allowing finest degree and angle of polarization determination. Celestial source polarized X-ray emission profiles (e.g. the emission spectra of the Crab nebula) will be included in the simulation code providing incoming photon energy. The absorption of polarized X-rays along the conversion/drift region of the detector will be also considered. Together with Q, the detection efficiency is a determinant parameter to achieve a fine polarimetric sensitivity (MDP < 10% for 100 ks and 1 mCrab).

Task 2
Experimentally, the PhD student will participate in the development of gas testing system that will include a gas electron multiplier and a strip anode readout. The student will measure the transverse spreading of the electron clouds produced by X-rays absorbed in the drift region of the GEM based detector. The induced electron distributions on the anode readout will be studied for several gas mixtures (He, Ne gases with the quench gases DME, iso-C4H10, etc.), transport electric fields in the GPD regions and for different drift- and induction-regions thicknesses.

Task 3
Data comparison between task 1 and task 2 will be useful in the optimisation of simulation code and experimental setup. Successive iterations of comparisons between simulation and experimental data will be performed until the optimal gas mixtures will be determined with the best confidence level. The selected gas mixtures will be tested in a GPD at the INAF, Italy or at ESRF in Grenoble under a polarized beam, in order to assess that the chosen gas mixture potentially meet the mission requirements.
Research centre/lab or R&D unit hosting the thesis project:
Laboratório de Instrumentação e Física Experimental de Partículas
University to which the thesis project will be presented:
UC - Universidade de Coimbra
DAEPHYS Scientific Domain in which the project fits:
Relation of the project to the Scientific Domains of DAEPHYS:
This project addresses to more than one Daephys scientific domain: instrumentation, nuclear and radiation physics techniques and metrology. Most of the work of the PhD student will be performed on radiation detector development and Monte Carlo simulation code development, that include nuclear and radiation physics techniques in order to optimize gas detector performances.
Candidate profile:
The candidate must have a solid formation in radiation physics, preferably with a multidisciplinary profile with experience in radiation detectors and in Monte Carlo programming.
Does this proposal involve more than one University?:
Synergies between the two Universities participating in the proposal:
DAEPHYS strongly encourages the presentation of thesis projects in co-supervision by researchers from two of the universities participating in the Program. In this field, explain the benefits resulting from the proposed co-supervision and the involvement of elements from the two universities, e.g. building critical mass teams, profiting from existing infrastructures or advanced equipments, profiting from expert technical know-how, etc. If the proposal involves only one University, write n/a.
(recommended length: 500-1000 char)
Does this proposal involve a company?:
Proposals involving a company:
DAEPHYS strongly encourages the presentation of thesis projects involving a company, preferably a high-tech company. These proposals have to: 1) be centered on a technological problem in which the partner company has been (or plan / would like to be) involved; 2) have a co-supervisor on the enterprise; 3) include part of the project to be carried out in the company.
(recommended length: 500-1000 char)



Name:Rui Miguel Curado da Silva
Institution:Laboratório de Instrumentação e Física Experimental de Partículas
link to CV or indication of ORCID ID:



Name:Jorge Manuel Maia Pereira
Institution:Laboratório de Instrumentação e Física Experimental de Partículas
link to CV or indication of ORCID ID:


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