1. 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

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

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

10. 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

11. 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

12. 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

13. 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

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

15. 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

16. 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

17. 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

18. 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