x-ray polarimetry with micro pattern gas detectors samba – trieste (italy) 27/29 may 2002...

X-Ray Polarimetry with Micro Pattern Gas Detectors

SAMBA – Trieste (Italy) 27/29 May 2002 R.Bellazzini - INFN Pisa

A Photoelectric Polarimeter based on

a Micro Pattern Gas Detector for X-Ray Astronomy

Ronaldo BellazziniINFN - Pisa



Polarization from celestial sources may derive from:

•Emission processes themselves:

cyclotron, synchrotron, non-thermal bremmstrahlung (Westfold, 1959; Gnedin & Sunyaev, 1974; Rees, 1975

•Scattering on aspherical accreting plasmas: disks, blobs, columns.

(Rees, 1975; Sunyaev & Titarchuk, 1985; Mészáros, P. et al. 1988)

•Vacuum polarization and birefringence through extreme magnetic fields

(Gnedin et al., 1978; Ventura, 1979; Mészáros & Ventura, 1979)

Why X-ray Astrophysical Polarimetry?



Polarization from Supernova Remnants: The Crab case

Crab-Nebula shows the same degree and angle of polarization from radio to X-rays and this is a signature of synchrotron

emission.

Radio

(VLA)

Optical

(Palomar)

Infrared

(Keck)

X-rays

(Chandra)



X-ray polarimetry offers a definitive test of strong field gravity near very compact sources: Black Hole binaries, Neutron Stars and microquasars...Unlike spectral data, polarization data are strongly affected by general relativistic effects.

For example:A BH is surrounded by an optically thick and geometrically thin accretion disk. Heigher energy photons come from smaller disk radii. As a consequence, as the photon energy increases from 1 to 10 KeV, the plane of linear polarization will swing smoothly trough an angle of ˜27° for a 9 Solar Mass BH and 40º for an extreme Kerr BH (for an inclination of 41º).This effect is due to the strong gravitational bending of light rays.



Simulated view of an accretion disk around a black hole as it appears to a distant observer

- Light bending makes visible the bottom part of a disk.

- Doppler boosting produce an increased intensity of one-sidePolarimetry would add to energy and time two further observable

quantities, the amount and the angle of polarization, constraining any model and interpretation: a theoretical/observational breakthrough.” P. Meszaros et al. 1988

Accreting X-ray Pulsar



ro2

5Z4137mc2

h

72

4 22sin 2cos

1 cos 4

Heitler W.,The Quantum Theory of Radiation

The photo-electric effect is very sensitive to photon polarization

Polarization information is derived from the track of the photoelectrons imaged by a finely subdivided gas detector



Dependence of polar angle of photo-electron in Ne



22

22

))2/((

/

screen

kin

sin

EZ

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.

kinEx

E 112

J

E

AE

Z

dS

dE 166.1log500.78

Basics of photoeffect in gases



Projection of MC photoelectron tracks



The micro-pattern gas detector scheme



The overall detector assembly and read-out electronics



The anode charge collection plane



Microscope picture of the GEM structure

Microscope picture of the pixelized read-out



cathode

GEM foil

Read-out PCB

-3000 V

- 1380 V

-900 V

0 V

Read-out PCB

cathode

GEM foil

Electric field structure

X ray

gas mixture:

- Ne / Ar / Kr …..- Methane/ Ethane C02, DME…...

20 ns

E

X photon (E)

PCB

GEM

pixel

conversiongain

collection



5.9 KeV electrons



Angular distribution5.9 KeV unpolarized source 5.4 KeV polarized source

Modulation factor = (Cmax – Cmin)/ (Cmax + Cmin) ˜ 50% at 6 KeV

AT

BS

S

nnMDP

2

1 MDP scales as: 1

for bright sources

for faint sources 1

02cos BAC



Scatter plot of the baricenters relative to the reconstructed impact point

5.9 KeV unpolarized source 5.4 KeV polarized source

No rotation of the detector is needed!



5.9 KeV unpolarized source 5.4 KeV polarized source



The path of the electron has been seriously affected by Coulomb scattering on a nucleus so that the mean direction of the track (red line) is quite different from the starting direction. A better reconstruction is made after identification of the conversion point and removal of the final part of the track

Modulation factor dependance on the track length used in the reconstruction.

Real photoelectron track



Red line – direction of the photoelectrons using the baricenter informationGreen line – reconstructed direction using impact point



Imaging capability

Baricenters Impact points



Modulation factor as a function of X-ray energy (no cuts on the events) Simulation of a MPGD with 80/20 Neon-DME mixture, 100 m pith readout plane and 1 cm absorption thickness



Present Prototype(2-10 keV)

Improved configuration (3.5-10 keV)

Drift/Absorption Gap 6 mm 30 mmDrift field 3000 V/cm 1500 V/cmGas filling and pressure (Ne 80% - DME 20%)

1 Atm( Ne 40%-DME 60%)

4 AtmGas Gain 5000 2500Transverse diffusion in drift 80 m <100 mGEM thickness 50 m copper clad

kapton foil50 m copper clad

kapton foilGEM hole geometry 40 m diameter

60 m pitch40 m diameter

60 m pitchGEM Voltage 400 V 600 VDetection efficiency at 5.4 keV 3.8 % 91 %Read-out pixel size 200 m 50 mNumber of pixels 512 40000Read-out plane technology Multilayer advanced PCB VLSITrack lenght/pixel size (6 keV) 6 6Sensitivity to Her X1 T = 400 s ; MDP = 10 % T = 20 s ; MDP = 10 %Sensitivity to 3C-273 T = 2.2e5 s ; MDP = 2 % T= 4e4 s ; MDP = 1 %Sensitivity to MCG-6-30-15 T = 5e5 s ; MDP = 2% T = 1e5 s ; MDP = 1 %Gain in the integration timeover SXRP (strong sources)

5 (over Thomson)15 (over Bragg)


Gain in the integration timeover SXRP (faint sources)



Present and optimized configuration for astrophysical applications



PCB read-out anodes

Next technological step

VLSI pixel chip from digital X-ray camera



XEUS: a telescope comprising a Detector Spacecraft that receives cosmic X-rays focused by a Mirror Spacecraft f.lying exactly 50 m in front of it



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.

x-ray polarimetry with micro pattern gas detectors samba – trieste (italy) 27/29 may 2002...

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