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X-ray photoelectric p olarimetry with the G as Pixel Detector

X-ray photoelectric p olarimetry with the G as Pixel Detector. Paolo Soffitta IASF-Rome/INAF.

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X-ray photoelectric p olarimetry with the G as Pixel Detector

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  1. X-ray photoelectric polarimetry with the Gas Pixel Detector Paolo Soffitta IASF-Rome/INAF Workshop on X-ray Polarimetry 2011-12-7 Tsinghua University Beijing

  2. Modern polarimeters dedicated to X-ray Astronomy exploit the photoelectric effect resolving most of the problems connected with Thomson/Bragg polarimeter. The exploitation of the photoelectric effect was tempted very long ago, but only since five-ten years was it possible to devise photoelectric polarimeters mature for a space mission. The photo-electric effect is very sensitive to photon polarization Heitler W.,The Quantum Theory of Radiation An X-ray photon directed along the Z axis with the electric vector along the Y axis, is absorbed by an atom. The photoelectron is ejected at an angle θ (the polar angle) with respect the incidentphotondirection and at an azimuthal angle φ with respect to the electricvector. If the ejected electron is in ‘s’ state (as for the K–shell) the differential cross sectiondepends on cos2 (φ),thereforeitispreferentiallyemitted in the direction of the electricfield. Being the cross sectionnull for φ = 90o the modulationfactor µ equals 1 for anypolar angle. By measuring the angular distribution of the ejected photelectrons (the modulation curve) it is possible to derive the X-ray polarization. β =v/c Workshop on X-ray Polarimetry2011-12-7 Tsinghua University Beijing

  3. The cross section with respect to θ shows that the photoelectronisejectedpreferentiallyat 90° with respect to the direction of the incidentradiationwhileat high energyitisslightlybentforward. Workshop on X-ray Polarimetry 2011-12-7 Tsinghua University Beijing

  4. Basics of photoelectriceffect in materials. Once ejected, the photoelectron interacts with the atoms of the medium 1) It is slowed down by the electrons of the surrounding atoms that ionizes, creating along the path a stream of charges, the ‘track’, without sensitive variation in the direction 2) it is scattered by the atom nuclei with sensitive change in direction. 3) If there is an electric drift fields the track blurs diffusing as √Drift-length. Slowing down: Most of the energy is released at the end of the path. Elastic scattering: Stopping power/Scattering  1/Z Most of the slowing down with the creation of secondary charges and elastic scattering happens when the energy of the photoelectron is low, therefore at the end of the path. The tracks start straight and ends as a skein. The information about photoemission direction which brings memory of the X-ray polarization resides therefore in the initial part of the track. Workshop on X-ray Polarimetry 2011-12-7 Tsinghua University Beijing

  5. The photoelectron range in gas is long enough to be efficiently imaged. In Silicon at 10 keV the range is only 1 m. Range of photoelectron in gases. Workshop on X-ray Polarimetry 2011-12-7 Tsinghua University Beijing

  6. GEM electric field X photon (E) conversion GEM gain collection pixel PCB E a 20 ns X-raypolarimetry with a Gas Pixel Detector The principle of detection To image the track IASF-Rome/INAF and INFN-Pisa developed the Gas Pixel detector. A photon crosses a Beryllium window and it is absorbed in the gas gap, the photoelectron produces a track. The track drifts toward the multiplication stage that is the GEM (Gas Electron Multiplier) which is a dielectric foil metallized on both side and perforated by microscopic holes (30 um diameter, 50 um pitch)and it is then collected by the pixellated anode plane that is the upper layer of an ASIC chip. Costa et al., 2001. Polarization information is derived from the angular distribution of the emission direction of the tracks produced by the photoelectrons. The detector has a very good imaging capability. Workshop on X-ray Polarimetry 2011-12-7 Tsinghua University Beijing Costa et al., 2001

  7. The first generation: the PCB approach Multilayer metalized kapton foils and vertical vias to fan-out the signal from each pixel. The fan-out which connects the segmented anode (collecting the charge) to the front end electronics is the real bottleneck! Technological constraints limit the maximum number of independent electronics channels (~ 1000 @ ~ 200 m pitch). Crosstalk between adjacent channels (signals traveling close to each other for several cm). Not negligible noise (high input capacitance to the preamplifiers.). Workshop on X-ray Polarimetry 2011-12-7 Tsinghua University Beijing

  8. The overall detector assembly and read-out electronics Workshop on X-ray Polarimetry 2011-12-7 Tsinghua University Beijing

  9. Ithasbeendeveloped a CMOS pixel read-out ASIC Bellazzini et al., NIMA, 2004 pixel pitch 80 µm in an hexagonal array, comprehensive of preamplifier/shaper, S/H and routing (serial read-out) for each pixel number of pixels: 2101 The pixellated (2101 pixels) top layer of the ASIC chip is the collection plane. The bottom layers (5 layers total ) provide a complete analogue chain separated for each pixel with a preamplifier/shaper/sample and hold and serial readout. Workshop on X-ray Polarimetry 2011-12-7 Tsinghua University Beijing

  10. The read-out system Workshop on X-ray Polarimetry 2011-12-7 Tsinghua University Beijing

  11. From 2k to 22k pixels Bellazzini et al. NIMA 2006 Workshop on X-ray Polarimetry 2011-12-7 Tsinghua University Beijing

  12. ASIC features 105600 pixels 50 μm pitch • Peaking time: 3-10 ms, externally adjustable; • Full-scale linear range: 30000 electrons; • Pixel noise: 50 electrons ENC; • Read-out mode: asynchronous or synchronous; • Trigger mode: internal, external or self-trigger; • Read-out clock: up to 10MHz; • Self-trigger threshold: 2200 electrons (10% FS); • Frame rate: up to 10 kHz in self-trigger mode • (event window); • Parallel analog output buffers: 1, 8 or 16; • Access to pixel content: direct (single pixel) or serial • (8-16 clusters, full matrix, region of interest); • Fill fraction (ratio of metal area to active area): 92%) The chip is self-triggered and low noise. The low noise allows for detect single electron in the track with a small gain of the GEM. Also it is not necessary to readout the entire chip since it is capable to define the sub-frame that surrounds the track. The dead time downloading an average of 1000 pixels is 100 time lower with respect to a download of 105 pixel. The needed power for the chip is 0.5 W. For space application we use a Peltier to arrive at 2 W. Bellazziniet al., NIMA 2006 (b) Workshop on X-ray Polarimetry 2011-12-7 Tsinghua University Beijing

  13. Workshop on X-ray Polarimetry 2011-12-7 Tsinghua University Beijing

  14. A sealed X-ray polarimeter (Bellazzini et al., NIMA 2007). Different filling gas GEM Ne-DME 80-20 1 Bar 1 cm drift 50 μm pitch 50 thick (CERN) He-DME 20-80 1 Bar 1 cm drift 50 μm pitch 50 thick(CERN) DME 100 0.8 Bar1 cm drift 80 μm pitch 100 thick (SciE.) Ar-DME 60-40 2 Bar 2 cm drift 80 μm pitch 100 thick(SciE.) A custom and very compact DAQ system to generate and handle command signals to/from the chip (implemented on Altera FPGA Cyclone EP1C240), to read and digitally convert the analog data (ADS5270TI Flash ADC) and to store them, temporarily, on a static RAM, has been developed. By using the RISC processor NIOS II, embedded on Altera FPGA, and the self-triggering functionality of the chip, it is possible to acquire the pedestals of the pixels in the same chip-defined event window (region of interest) immediately after the event is read-out. The readout of the pedestals is user-defined and can be performed once as well as several times. Workshop on X-ray Polarimetry 2011-12-7 Tsinghua University Beijing

  15. Be Window Factor of Safety Verification • Per GSFC-STD-7000 standard the Beryllium design factors of safety are (for static loads): • yield stress ≥ 1.4 • ultimate stress ≥ 1.6 • 50μm thick Beryllium window maximum static loads: 1bar • Beryllium data • (as supplied by manufacturer Analytical Oy, Finland): • Beryllium tensile yield strength = 340 MPa • Beryllium tensile ultimate strength = 450 MPa XPOL Beryllium Window verification by analysis analyses with ANSYS™ software • Model parameters: • mesh of 24756 elements (type SHELL181, CONTA174, TARGE170, SURF154) and 12842 nodes • analysis results to count for the membrane behavior of the thin beryllium foil • Analysis results: • maximum beryllium yield stress @ 1.5bar = 249MPa • maximum beryllium strain @ 1.5bar = 7.2 10-4 • yield stress factor of safety > 2 Workshop on X-ray Polarimetry 2011-12-7 Tsinghua University Beijing

  16. Be Window Factor of Safety Verification XPOL Beryllium Window verification by test Test facility : Mitutoyo BHN506 coordinate measuring machine (CMM), with optical head, 5μm of resolution Location: INFN-Pisa 100k class clean room measured deformations along a window vertical mid-plane at various internal differential pressures, 0-1.6 bar Verification conclusions: XPOL beryllium window can sustain the on orbit limit loads with a factor of safety that exceeds the GEVS standard Workshop on X-ray Polarimetry 2011-12-7 Tsinghua University Beijing Be window strain test results vs analysis result

  17. Thermal Tests The Fe55 source illuminates the whole sensitive area. Workshop on X-ray Polarimetry 2011-12-7 Tsinghua University Beijing

  18. Fe55 source PT100 on window PT100 on frame Thermo-vacuum test Workshop on X-ray Polarimetry 2011-12-7 Tsinghua University Beijing

  19. Vibration Tests The three axes setups are shown. For each axis we have performed a sine sweep between 20 and 2000Hz at 2oct/min and a random test 3dB for 75s over the predicted random vibration environment of the Pegasus rocket. In all the random tests the item was randomly vibrated to an overall 3grms(IXPE proposal).Subsequently a successful vibration test has been performed at 11.4 g for launcher with ESA rocket. No resonance were founded between 20 and 2000 Hz (the foreseen one is at 3000 Hz). Workshop on X-ray Polarimetry 2411-12-7 Tsinghua University Beijing

  20. Survival test with Fe ions. GPD was exposed to a total dose of 1.7 104 Fe ions corresponding to the total dose in a Low Earth Orbit of 40 years of irradiation. Bellazzini et al., proceed. of X-ray Polarimetry Workshop, 2010 Workshop on X-ray Polarimetry 2011-12-7 Tsinghua University Beijing

  21. IASF-Rome facility for the production of polarized X-rays Close-up view of the polarizer and the Gas Pixel Detector Facility at IASF-Rome/INAF keV Crystal Line Bragg angle 1.65 ADP(101) CONT 45.0 2.01 PET(002) CONT 45.0 2.29 Rh(001) Mo Lα 45.3 2.61 Graphite CONT 45.0 3.7 Al(111) Ca Kα 45.9 4.5 CaF2(220) Ti Kα 45.4 5.9 LiF(002) 55Fe 47.6 6.4 Si(400) Fe 45.5 8.05 Ge(333) Cu Kα45.0 9.7 FLi(420) AuLα45.1 17.4 FLi(800) MoKα44.8 Aluminum and Graphite crystals. Capillary plate (3 cm diameter) Spectrum of the orders of diffraction from the Ti X-ray tube and a PET crystal acquired with a Si-PiN detector by Amptek. A CdTe detector is also avilable PET (Muleri et al., SPIE, 2008) Workshop on X-ray Polarimetry 2011-12-7 Tsinghua University Beijing

  22. In order to characterize completely the GPD as a polarimeter, we devised a mechanical system based on linear and rotary stages connected to a controller which in turn is connected with a PC via ethernet. The linear and rotary stages are manufactured by Newport such as the XPS controller. A lab-view software controls the movements and the acquisition. We move the detector and the beam is fixed. • The stage permit : • X-Y displacement of the detector for XY mapping. • X-Y displacement of the X-ray beam for alignment of the beam with the rotation axis. • Rotation of the detector to change polarization direction. • Inclination of the detector (Large inclination and small inclination). • Vertical displacement of the detector. • Rail for manual linear displacement of the X-ray beam for maintenance. Workshop on X-ray Polarimetry 2011-12-7 Tsinghua University Beijing

  23. Track reconstruction 1) The track is recorded by the Polarimeter. 2) Baricenterevaluation. 3) Reconstruction of the principal axis of the track: maximization of the second moment of charge distribution. Real track 4) Reconstruction of the conversion point: major second moment (track length) + third moment along the principal axis (asymmetry of charge release). 5) Reconstruction of emission direction: pixels are weighted according to the distance from conversion point. It brings memory of the polarization. Workshop on X-ray Polarimetry 2011-12-7 Tsinghua University Beijing

  24. Position reconstruction capability Matrix of 0.6 mm holes diam.2 mm apart. Workshop on X-ray Polarimetry 2011-12-7 Tsinghua University Beijing

  25. Not only MonteCarlo: Our predictions are based on data Eachphotonproduces a track. From the track the impact point and the emission angle of the photoelectronisderived. The distribution of the emission angle is the modulation curve. Muleri et al. 2007 Impact point The modulation factor measured 2.6 keV, 3.7 keV and 5.2 keV has been compared with the Monte Carlo previsions. The agreement is very satisfying. By rotating the polarization vector the capability to measure the polarization angle is shown by the shift of the modulation curve. Present level of absence of systematic effects (5.9 keV). (Bellazzini et al. 2010). Soffitta et al., 2010 Workshop on X-ray Polarimetry 2011-12-7 Tsinghua University Beijing

  26. We changed the position of the detector in a matrix of 5 x 5 locations with a step of 2.25 mm. We used a Ti X-ray tube and CaF2 crystal at 4.5 keV. At the input we placed a 1/40 capillary plate and 1/100 capillary plate at the beam output adding at the output a diaphragm of 500 m of diameter. We acquired about 30000 events in 2 hour in each position. Modulation factor as a function of the beam position. Polarization angle at different location of the detector. Workshop on X-ray Polarimetry 2011-12-7 Tsinghua University Beijing

  27. More energies, more mixtures Pure DME (CH3)2O Modulation curve at 2.0 keV μ = 13.5% We performed measurement at more different energies and gas mixtures. (Muleri et al., 2010).

  28. A detector more tuned on hard X-rays. For NHXM multilayer optics, from simulation we devised a 3-cm thick GPD filled with 3-atm of Ar-DME 80-20. The GPD built and tested was, as a first step, 2-cm and 2-atm. The MEP prototype in the IASF-Rome facility. MEP detector isworkingapparentlywell. Itis a goodProportionalCounter. Unfortunatelyitbrokesoonafterthistesting. Anywayweforesawfurtherchanges. A larger detector for better control of the electricfield and to exclude background produced on the walls.

  29. Guard Ring Workshop on X-ray Polarimetry 2011-12-7 Tsinghua University Beijing

  30. The XPOL-GEM90 prototype assembly (INFN-PI-XPOL-01400-001)consists of 3 sub-assemblies: • GEM90-Drift_assembly • INFN-PI-XPOL-01430-001 • GEM90-GEM_assembly • INFN-PI-XPOL-01420-001 • GEM90-PCB_V2_assembly • INFN-PI-XPOL-01410-001 XPOL-GEM90 Prototype Assembly SciEnergy GEM 50 μm thick 50 μm pitch X Y PCB-Ref-SYS Workshop on X-ray Polarimetry 2011-12-7 Tsinghua University Beijing

  31. He-DME 20-80 1Bar-1-cm. 55Fe unpolarized source. Large case He-DME 20-80 GPD below an 55Fe source. First-Light of the Ar-DME polarimeter (70-30) 2-Bar 2-cm (with large case), Fe55unpolarized source. Ti polarized Kα line polarized. Workshop on X-ray Polarimetry 2011-12-7 Tsinghua University Beijing

  32. New Improvement foreseen for GPD. - Reduction of ASIC dead time. A factor of 20 in reduction of the dead time will allow to arrive at a dead time of 10 μs: with 5000 c/s the dead time is 5 %.In the present ASIC chip this level is reached with 250 c/s (approximately the expected counting rate for the Crab in mission like NHXM (˜600 cm2). -Tiling of ASIC chip. Workshop on X-ray Polarimetry 2011-12-7 Tsinghua University Beijing

  33. The missions where the GPD was proposed either are waiting after a phase A completed or were not selected or evolved in missions without anymore a polarimeter on-board. POLARIX Costa et al., ExpAst 2010 IXO NHXM Bookbinder, SPIE, 2010 Tagliaferri et al, ExpAst 2010

  34. Implementation of X-ray polarimetry with GPD in proposed missions: - POLARIX (ASI small mission, fasa A completed) 3 Jet-X optics (3,5 m FL, 20 ‘’ HED 450 cm2 @ 2 keV, HEW=(20’’)) 3 GPD (1-cm, 1-Atm, He-DME 20-80) MDP 12 % in 105 s for 1 mCrab source (2-10 keV) 3.8 % in 105 s for 10 mCrab source (2-10 keV) - NHXM (Proposed ESA M3 Mission not selected) 1 of 4 Multi-layer optics (Pt-C) (10 m FL, 600 cm2 @ 8 keV) 2 GPD : 1-cm, 1-Atm, He-DME (LEP) (2-10 keV); 3-cm 3-Atm Ar-DME (MEP) (6-35 keV) MDP: LEP 9.7 % in 105 s for 1 mCrab source (2-10 keV) 3.1 % in 105 sfor 10 mCrab source (2-10 keV) MEP13 % in 105 s for 1 mCrab source (6-35 keV) 4.1 % in 105 for 10 mCrab source (6-35 keV) In study (HEP, Compton scattering) MDP 7.2 % for 10 mCrab in 105 s (20-80 keV) Costa, et al., Exp Ast 2010 Tagliaferri et al. , Exp. Ast. 2010; Soffitta et al. SPIE 2010 - IXO (ESA/NASA/JAXA Large Mission Evolved in Athena with no polarimeter on-board) Area= 2.5 m2 FL = 20 m HEW= 5’’ XPOL: MDP 1 % 1 mCrab 105 s.

  35. Measurement in New Prototypes • Modulation factor for different energies and positions. • Sensitivity on the polarization angle for different positions. • Level of absence of systematic effects improved. • Position resolution. • Energy resolution. Workshop on X-ray Polarimetry 2011-12-7 Tsinghua University Beijing

  36. End Workshop on X-ray Polarimetry 2011-12-7 Tsinghua University Beijing

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