Goddard February 2003 R.Bellazzini - INFN Pisa 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 X-Ray Polarimetry with Micro Pattern Gas Detectors Goddard February 2003 R.Bellazzini - INFN Pisa 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 Goddard February 2003 R.Bellazzini - INFN Pisa GEM electric field Polarization information is derived from the tracks of the photoelectron, imaged by a finely subdivided gas detector. X-Ray Polarimetry with Micro Pattern Gas Detectors V V -840 V Ground pixel GEM 20 ns E X photon (E) PCB conversion gain collection Goddard February 2003 R.Bellazzini - INFN Pisa Dependence of polar angle of photo-electron in Ne Goddard February 2003 R.Bellazzini - INFN Pisa Slowing down: Elastic scattering: 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. Most of the energy is released at the end of the path. Stopping power/Scattering 1/Z Basics of photoeffect in gases Goddard February 2003 R.Bellazzini - INFN Pisa Read out plane Gas electron Multiplier GEM pitch: 90 m GEM holes diameters: 45 m, 60 m Read out pitch: 260 m Absorption gap thickness: 6 mm 512 electronic channels from a few mm 2 active area are individually read out The overall detector assembly and read-out electronics Goddard February 2003 R.Bellazzini - INFN Pisa The anode charge collection plane The micro-pattern read-out plane Goddard February 2003 R.Bellazzini - INFN Pisa Bragg peak Auger electron Large-angle scattering 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. Real photoelectrons tracks from unpolarized radiation Ne/DME 80/20 gas mixture 5,0 keV photoelectron 870 eV Auger electron Goddard February 2003 R.Bellazzini - INFN Pisa 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 M 2 transforms under a rotation of angle and we can impose: In this way we obtain the mean direction of the track. Goddard February 2003 R.Bellazzini - INFN Pisa The large average number of fired pixel per event allows a good track reconstruction. Most clusters are sensibly far from a spherical shape (M 2 max /M 2 min ~ 1), which is crucial for angular reconstruction. Real data, 5.9 keV unpolarized radiation from 55 Fe source Basic reconstruction algorithm Goddard February 2003 R.Bellazzini - INFN Pisa 5.9 KeV unpolarized source 5.4 KeV polarized source Modulation factor = (Cmax Cmin)/ (Cmax + Cmin) 50% at 6 KeV MDP scales as:for bright sources for faint sources Basic reconstruction algorithm Goddard February 2003 R.Bellazzini - INFN Pisa Conversion point reconstruction: algorithm Barycentre Reconstructed convertion point Distance proportional to the square root of major principal second momentum. 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: The initial part of the track is characterized by a lower ionization density and this asymmetry can be exploited to reconstruct the conversion point. Goddard February 2003 R.Bellazzini - INFN Pisa 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). Basic algorithm Improved algorithm Angular reconstruction Goddard February 2003 R.Bellazzini - INFN Pisa 8.0 keV 100% polarized radiation, basic reconstruction algorithm. Same events, analyzed exploiting reconstructed conversion point. Modulation factor rises up from 24% to 30%. Angular reconstruction Goddard February 2003 R.Bellazzini - INFN Pisa Position of barycentres with respect to the reconstructed conversion point. 5.9 keV unpolarized radiation 5.4 keV 100% linearly polarized radiation No rotation of the detector is needed! Conversion point reconstruction Goddard February 2003 R.Bellazzini - INFN Pisa Conversion point reconstruction 5.9 keV unpolarized radiation 5.4 keV 100% linearly polarized radiation Goddard February 2003 R.Bellazzini - INFN Pisa 500 m 1 mm Imaging capabilities Basic reconstruction algorithm Conversion point reconstruction Goddard February 2003 R.Bellazzini - INFN Pisa Monte Carlo simulation I Simulation of primary ionization distribution. Auger electron Bragg peak Transverse diffusion toward the GEM (5 mm in Ne). Sampling onto readout plane (100 m pitch). Goddard February 2003 R.Bellazzini - INFN Pisa 5.0 keV photoelectrons tracks in Ne (100% linearly polarized, collimated photons beam). Monte Carlo simulation II Goddard February 2003 R.Bellazzini - INFN Pisa 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. Monte Carlo simulation III Goddard February 2003 R.Bellazzini - INFN Pisa Tested prototype simulation Modulation factor as a function of photon energy (basic algorithm). Modulation factor as a function of photon energy (conversion point algorithm). Monte Carlo simulation Experimental data Monte Carlo simulation Experimental data Goddard February 2003 R.Bellazzini - INFN Pisa 100 mm pitch detector simulation Modulation factor as a function of photon energy for several absorption gap thickness (100 m readout pitch). Complete energy scan for 1 cm absorption gap. Goddard February 2003 R.Bellazzini - INFN Pisa Present and optimized configuration for astrophysical applications Goddard February 2003 R.Bellazzini - INFN Pisa PCB read-out anodes Next technological step VLSI pixel chip from digital X-ray camera Goddard February 2003 R.Bellazzini - INFN Pisa 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 .. Goddard February 2003 R.Bellazzini - INFN Pisa 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. Conclusions