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Heitler W.,The Quantum Theory of Radiation

A new X-Ray Polarimeter based on the photoelectric effect for Black Holes and Neutron Stars Astrophysics (part. II: The Instrument) Ronaldo Bellazzini INFN - Pisa. The photo-electric effect is very sensitive to photon polarization. Heitler W.,The Quantum Theory of Radiation.

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Heitler W.,The Quantum Theory of Radiation

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  1. 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. The photo-electric effect is very sensitive to photon polarization Heitler W.,The Quantum Theory of Radiation Polarization information is derived from the track of the photoelectrons imaged by a finely subdivided gas detector

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

  4. Basics of photoeffect in gases The photoelectron is slowed by ionizing collisions with outer electrons of the atoms of the medium. The energy loss increases with decreasing kinetic energy (Bethe law for low energy). Electrons are also scattered by charges in the nuclei with no significant energy loss . This follows the screened Rutherford law : While scattering crucially depends on the atomic number, slowing down is only moderately dependent. The photoelectron leaves in the absorber a string of electron/ion pairs, marking the path from its creation to the stopping point. We call this cluster a “track”: in the initial part of this track resides the information on the original electron direction and thence the key to derive the polarization of the photon. This dependence is preserved if the track is projected onto a plane perpendicular to the radiation.

  5. Projection of MC photoelectron tracks

  6. The micro-pattern gas detector scheme

  7. The overall detector assembly and read-out electronics

  8. The anode charge collection plane

  9. Microscope picture of the GEM structure Microscope picture of the pixelized read-out

  10. X photon (E) conversion GEM gain collection pixel PCB a E 20 ns X ray gas mixture: - Ne / Ar / Kr ….. - Methane/ Ethane C02, DME…... Electric field structure

  11. 5.9 KeV electrons

  12. 5.9 KeV unpolarized source 5.4 KeV polarized source MDP scales as: for bright sources for faint sources Angular distribution Modulation factor = (Cmax – Cmin)/ (Cmax + Cmin) ˜50% at 6 KeV

  13. 5.9 KeV unpolarized source 5.4 KeV polarized source Scatter plot of the baricenters relative to the reconstructed impact point No rotation of the detector is needed!

  14. 5.9 KeV unpolarized source 5.4 KeV polarized source

  15. Red line – direction of the photoelectrons using the baricenter information Green line – reconstructed direction using impact point

  16. Imaging capability Baricenters Impact points

  17. Present and optimized configuration for astrophysical applications

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

  19. 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 …..”

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