A novel gaseous X-ray polarimeter: data analysis and simulation Astronomical telescopes and instrumentation, Waikoloa 2002 A novel gaseous X-ray polarimeter: data analysis and simulation A research by: R. Bellazzini, E. Costa, F. Angelini, L. Baldini, A. Brez, G. di Persio, L. Latronico, M. M. Massai, N. Omodei, L. Pacciani, P. Soffitta, G. Spandre. Waikoloa, 28 August 2002 Luca Baldini

The Micro Pattern Gas Detector The MPGD exploit the dependence of photoemission direction on photon polarization. The photoelectron track is reconstructed by the means of a finely segmented readout plane. Waikoloa, 28 August 2002 Luca Baldini

Some events… 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 events, 5.9 keV unpolarized radiation from 55Fe source). Large-angle scattering Auger electron Bragg peak Waikoloa, 28 August 2002 Luca Baldini

Basic reconstruction algorithm I The basic algorithm for the reconstruction of the photoemission direction is based on the determination of the principal axes of charge distribution. Major axis Minor axis Waikoloa, 28 August 2002 Luca Baldini

Basic reconstruction algorithm II 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). Waikoloa, 28 August 2002 Luca Baldini

Basic reconstruction algorithm III 5.9 keV unpolarized radiation 5.4 keV 100% linearly polarized radiation Cuts on the shape can be applied in order to optimize the sensitivity of the instrument: Waikoloa, 28 August 2002 Luca Baldini

Absorption point reconstruction I The initial part of the track is characterized by a lower ionization density and this asymmetry can be exploited to reconstruct the conversion point. dE/dx x Principal axis xb Reconstructed conversion point ~ (M2max)1/2 Barycentre Waikoloa, 28 August 2002 Luca Baldini

Absorption point reconstruction II 5.4 keV unpolarized radiation entering the detector through a 50 mm collimator. 1D projection Standard deviation of gaussian fit: Barycentres Reconstructed absorption points Waikoloa, 28 August 2002 Luca Baldini

Absorption point reconstruction III 500 mm 1 mm Waikoloa, 28 August 2002 Luca Baldini

Absorption point reconstruction IV Position of barycentres with respect to the reconstructed conversion point. 5.9 keV unpolarized radiation 5.4 keV 100% linearly polarized radiation Waikoloa, 28 August 2002 Luca Baldini

Angular reconstruction I The reconstruction of the absorption 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 Waikoloa, 28 August 2002 Luca Baldini

Angular reconstruction II 8.0 keV 100% polarized radiation, basic reconstruction algorithm. Same events, analyzed exploiting reconstructed conversion point. Modulation factor rises up from 24% to 30%. Waikoloa, 28 August 2002 Luca Baldini

Monte Carlo simulation I Transverse diffusion toward the GEM (5 mm in Ne). Simulation of primary ionization distribution. Auger electron Bragg peak Sampling onto readout plane (100 mm pitch). Waikoloa, 28 August 2002 Luca Baldini

Monte Carlo simulation II 5.0 keV photoelectrons tracks in Ne (100% linearly polarized, collimated photons beam). Waikoloa, 28 August 2002 Luca Baldini

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. Waikoloa, 28 August 2002 Luca Baldini

Tested prototype simulation Modulation factor as a function of photon energy (exploiting reconstructed conversion point). Modulation factor as a function of photon energy (basic algorithm). Experimental data Experimental data Monte Carlo simulation Monte Carlo simulation Waikoloa, 28 August 2002 Luca Baldini

100 mm pitch detector simulation Modulation factor as a function of photon energy for several absorption gap thickness (100 mm readout pitch). Complete energy scan for 1 cm absorption gap. Waikoloa, 28 August 2002 Luca Baldini

Conclusions Some data collected during tests presented. Algorithms used throughout analysis described in details. Monte Carlo simulation in good agreement with experimental data. Still room for great improvements. Waikoloa, 28 August 2002 Luca Baldini