The Low Energy Polaimeter based on the Gas Pixel Detector

1. The Low Energy Polaimeter based on the Gas Pixel Detector Paolo Soffitta IAPS/Roma Italy

2. The Gas Pixel Detector GEM electric field • A beryllium window • A conversion/drift gap with parallel electric field. The gas is diluited so that photoelectrons leave a ionization track. The electrons are drifted by the electric field • A Gas Electron Multiplier (GEM) amplifies the track without changing the shape • The electrons are collected from a multi-pad plane with pads of 50 µm pitch on an hexagonal pattern. • The pad-plane, acting as the anode, is the top layer of a VLSI chip. Each pad is independently read s-photoelectrons are ideal analyzers of polarization X photon (E) conversion GEM gain collection pixel ASIC E a 20 ns A new design for GPD - SPIE2012 - Bellazzini, Costa et al.

3. Tracksreconstruction 1) The track is recorded by the PIXel Imager 2) Baricenter evaluation 3) Reconstruction of the principal axis of the track: maximization of the second moment of charge distributionn 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 the conversion point. A new design for GPD - SPIE2012 - Bellazzini, Costa et al.

4. The core: an ASIC performing the functions of anode, FEE, event pre-selection • 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%) A new design for GPD - SPIE2012 - Bellazzini, Costa et al.

5. A new prototype with an extended GEM for better drift field uniformity NEW OLD Same window, same ASIC but a much larger GEM, with the addition of a large Guard Ring and field forming frames. A new design for GPD - SPIE2012 - Bellazzini, Costa et al.

7. 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

8. 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

9. Modulation factor with a cut on low energy tail of Pulse Height 2.6 keV A new design for GPD - SPIE2012 - Bellazzini, Costa et al.

10. Spurious modulation @ 5.9 keV 125 kcounts: Modulation factor: ~50% Spurious modulation measured: ~0.54% Spurious polarization measured: ~1% MDP 99% with m=50% and 125 kc: ~ 2.3% A new design for GPD - SPIE2012 - Bellazzini, Costa et al.

11. MEASUREMENT WITH THE GPD Energy resolution 6.4 keV Position resolution 4.5 keV FWHM 82 mm A new design for GPD - SPIE2012 - Bellazzini, Costa et al.

12. MONTE CARLO SIMULATIONS OF THE GPD Core and wings Reconstructed position Core : 83.5 % (from simulations) The core is 84 % of the total counts from simulations. True interaction point

13. Quantum Efficiency included the Beryllium window for the gas mixture filling He-DME (20-80) We are limited by the transmission through the USB the limit is 20-30 c/s.

14. Resources : • Weight GPD + Electronics (total)= 1850 g • Dimensions of the electronic BOX (with foot) : 142 mm (+ 110 for extensions for V.D. and connectors) 192 mm (+ 40 for sealed gas filling tube) 80 mm (H) • Geometrical active area GPD = 15 mm x 15 mm • POWER : HV = (400 V , 870 V, 2650 V negative) CAEN N470 LV = 5 Volt, 1A • Control & Data Transmission : USB connector to PC Windows

15. Center GPD

16. Possible meaurements at Panter • Measurements of the overall position resolution of the GPD + JET-X optics. • Measurement of the overall modulation factor. • Measurement (upper limits) on the residual modulation Methods : Alignment detector optical axis by inclining in two direction and taking the beam with the maximum symmetry. 2) Monochromatise the X-ray tube with suitable crystals (with known R/R) at a known Bragg angle. Collect fluorescence photons excited by fluorescence X-ray lines with larger energies (and excluding the reflected lines by means of the spectral capability of the GPD

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