X-Ray Polarimetry with Micro Pattern Gas Detectors

1. X-Ray Polarimetry with Micro Pattern Gas Detectors A new X-Ray Polarimeter based on the photoelectric effect for Black Holes and Neutron Stars Astrophysics(part. II: The Instrument)Ronaldo BellazziniINFN - Pisa

2. Photoelectric cross section The photoelectric effect is very sensitive to photon polarization! Simple analytical expression for photoemission differential cross section (k-shell photoelectron in non-relativistic limit): If we project on the plane orthogonal to the propagation direction…

3. -2400 V -1280 V GEM electric field X photon (E) -840 V conversion GEM gain Ground collection pixel PCB a E 20 ns X-Ray Polarimetry with Micro Pattern Gas Detectors Polarization information is derived from the tracks of the photoelectron, imaged by a finely subdivided gas detector.

4. Dependence of polar angle of photo-electron in Ne

5. Basics of photoeffect in gases Slowing down: Most of the energy is released at the end of the path. Elastic scattering: Stopping power/Scattering  1/Z Elastic scattering is responsible of a progressive randomization of photoelectron direction; most of the information about photoemission direction resides in the initial part of the track.

6. The overall detector assembly and read-out electronics Gas electron Multiplier Read out plane GEM pitch: 90 mm GEM holes diameters: 45 mm, 60 mm Read out pitch: 260 mm Absorption gap thickness: 6 mm 512 electronic channels from a few mm2 active area are individually read out

7. The micro-pattern read-out plane The anode charge collection plane

8. Large-angle scattering Auger electron Bragg peak Real photoelectrons tracks from unpolarized radiation The initial part of the track, with a low ionization density, evolves into a clear Bragg peak, while the photoelectron direction is randomized by Coulomb scattering. • 5,0 keV photoelectron • 870 eV Auger electron Ne/DME 80/20 gas mixture

9. Basic reconstruction algorithm Reconstruction algorithm is based on the determination of the two (orthogonal) principal axes of charge distribution. Major principal axis Minor principal axis We know how M2 transforms under a rotation of angle q and we can impose: In this way we obtain the “mean direction” of the track.

10. Basic reconstruction algorithm The large average number of fired pixel per event allows a good track reconstruction. Most clusters are sensibly far from a spherical shape (M2max/M2min ~ 1), which is crucial for angular reconstruction. Real data, 5.9 keV unpolarized radiation from 55Fe source

11. 5.9 KeV unpolarized source 5.4 KeV polarized source MDP scales as: for bright sources for faint sources Basic reconstruction algorithm Modulation factor = (Cmax – Cmin)/ (Cmax + Cmin) ˜50% at 6 KeV

12. Reconstructed convertion point Barycentre Conversion point reconstruction: algorithm The initial part of the track is characterized by a lower ionization density and this asymmetry can be exploited to reconstruct the conversion point. • Why conversion point reconstruction? • To improve angular reconstruction. • To improve imaging capabilities. • The algorithm is based on the determination of the third momentum (along the major principal axis) of charge distribution: Distance proportional to the square root of major principal second momentum.

13. Basic algorithm Improved algorithm Angular reconstruction The reconstruction of the conversion point can be exploited to improve angular accuracy, rejecting the final part of the track, which is blurred by Coulomb scattering (real events, 8.0 keV polarized radiation).

14. Angular reconstruction 8.0 keV 100% polarized radiation, basic reconstruction algorithm. Same events, analyzed exploiting reconstructed conversion point. Modulation factor rises up from 24% to 30%.

15. Conversion point reconstruction Position of barycentres with respect to the reconstructed conversion point. 5.4 keV 100% linearly polarized radiation 5.9 keV unpolarized radiation No rotation of the detector is needed!

16. Conversion point reconstruction 5.4 keV 100% linearly polarized radiation 5.9 keV unpolarized radiation

17. 500 mm 1 mm Imaging capabilities Conversion point reconstruction Basic reconstruction algorithm

18. Transverse diffusion toward the GEM (5 mm in Ne). Simulation of primary ionization distribution. Sampling onto readout plane (100 mm pitch). Bragg peak Auger electron Monte Carlo simulation I

19. 5.0 keV photoelectrons tracks in Ne (100% linearly polarized, collimated photons beam). Monte Carlo simulation II

20. Monte Carlo simulation III 5.0 keV photoelectrons tracks in Ne (100% linearly polarized, collimated photons beam). Modulation factor, as evaluated from charge released within a certain distance from conversion point.

21. Experimental data Experimental data Tested prototype simulation Modulation factor as a function of photon energy (conversion point algorithm). Modulation factor as a function of photon energy (basic algorithm). Monte Carlo simulation Monte Carlo simulation

22. Complete energy scan for 1 cm absorption gap. 100 mm pitch detector simulation Modulation factor as a function of photon energy for several absorption gap thickness (100 mm readout pitch).

23. Present and optimized configuration for astrophysical applications

24. Next technological step PCB read-out anodes VLSI pixel chip from digital X-ray camera

25. According to Nature….. “ the work is highly significant for high energy astrophysics and astronomy in general. X-ray polarimetry is a unique probe of particle acceleration in the universe. It will provide a new tool for studying the fascinating and poorly understood jet sources. The instrumentation described here will very likely revolutionize this area of study …..”

26. Conclusions The performances obtained with the tested prototype have resulted much better than those of any actual traditional X-ray Polarimetry. In its improved configuration the MPGD target performance is the detection of 1% polarization for 1 mCrab sources. This sensitivity will allow polarimetry measurements to be made on thousands of galactic and extragalactic sources: a real breakthrough in X-ray astronomy.