Paolo Soffitta IAPS/INAF Polarimetria X Enrico Costa, Sergio Fabiani, Fabio Muleri, Alda Rubini, … (IAPS/INAF) Ronaldo Bellazzini Alessandro Brez Michele Pinchera, Massimo Minuti, Gloria Spandre, … (INFN-Pisa) Riunione Nazionale Astronomia X 15-16/11/2012 P. Soffitta

Heitler W.,The Quantum Theory of Radiation The photo-electric effect is very sensitive to photon polarization Modern polarimeters dedicated to X-ray Astronomy exploit the photoelectric effect resolving most of the problems connected with Thomson/Bragg polarimeter. The exploitation of the photoelectric effect was tempted very long ago, but only since five-ten years was it possible to devise photoelectric polarimeters mature for a space mission. An X-ray photon directed along the Z axis with the electric vector along the Y axis, is absorbed by an atom. The photoelectron is ejected at an angle θ (the polar angle) with respect the incident photon direction and at an azimuthal angle φ with respect to the electric vector. If the ejected electron is in ‘s’ state (as for the K–shell) the differential cross section depends on cos 2 (φ), therefore it is preferentially emitted in the direction of the electric field. Being the cross section null for φ = 90 o the modulation factor µ equals 1 for any polar angle. β =v/c By measuring the angular distribution of the ejected photelectrons (the modulation curve) it is possible to derive the X-ray polarization. Riunione Nazionale Astronomia X 15-16/11/2012 P. Soffitta Costa et al. Nature 2001

The photoelectron range in gas is long enough to be efficiently imaged. In Silicon at 10 keV the range is only 1  m. Range of photoelectron in gases. Riunione Nazionale Astronomia X 15-16/11/2012 P. Soffitta Soffitta at al, NIM A 2001 Costa et al., Nature 2001

GEM electric field Polarization information is derived from the angular distribution of the emission direction of the tracks produced by the photoelectrons. The detector has a very good imaging capability. pixel GEM 20 ns a E X photon (E) PCB conversion gain collection The principle of detection X-ray polarimetry with a Gas Pixel Detector Costa et al., 2001 A photon cross a Beryllium window and it is absorbed in the gas gap, the photoelectron produces a track. The track drifts toward the multiplication stage that is the GEM (Gas Electron Multiplier) which is a kapton foil metallized on both side and perforated by microscopic holes (30 um diameter, 50 um pitch) and it is then collected by the pixellated anode plane that is the upper layer of an ASIC chip. To efficiently image the track at energies typical of conventional telescopes IASF-Rome and INFN-Pisa developed the Gas Pixel detector. The tracks are imaged by using the charge. Costa et al., 2001, Bellazzini et al.2006, 2007

ASIC features pixels 50 μm pitch Peaking time: 3-10  s, externally adjustable; Peaking time: 3-10  s, externally adjustable; Full-scale linear range: electrons; Full-scale linear range: electrons; Pixel noise: 50 electrons ENC; Pixel noise: 50 electrons ENC; Read-out mode: asynchronous or synchronous; Read-out mode: asynchronous or synchronous; Trigger mode: internal, external or self-trigger; Trigger mode: internal, external or self-trigger; Read-out clock: up to 10MHz; Read-out clock: up to 10MHz; Self-trigger threshold: 2200 electrons (10% FS); Self-trigger threshold: 2200 electrons (10% FS); Frame rate: up to 10 kHz in self-trigger mode Frame rate: up to 10 kHz in self-trigger mode (event window); (event window); Parallel analog output buffers: 1, 8 or 16; Parallel analog output buffers: 1, 8 or 16; Access to pixel content: direct (single pixel) or serial Access to pixel content: direct (single pixel) or serial (8-16 clusters, full matrix, region of interest); (8-16 clusters, full matrix, region of interest); Fill fraction (ratio of metal area to active area): 92%) Fill fraction (ratio of metal area to active area): 92%) The chip is self-triggered and low noise. It is not necessary to readout the entire chip since it is capable to define the sub-frame that surround the track. The dead time downloading an average of 1000 pixels is 100 time lower with respect to a download of 10 5 pixel.

Riunione Nazionale Astronomia X 15-16/11/2012 P. Soffitta 1) The track is recorded by the PIXel Imager 2) Baricenter evaluation 3) Reconstruction of the principal axis of the track: maximization of the second moment of charge distribution 4) Reconstruction of the conversion point: major second moment (track length) + third moment along the principal axis (asymmetry of charge release) 5) Reconstruction of emission direction: pixels are weighted according to the distance from conversion point. Tracksreconstruction Tracks reconstruction Bellazzini, SPIE 2003, Pacciani, SPIE 2003

The overall detector assembly and read-out electronics Riunione Nazionale Astronomia X 15-16/11/2012 P. Soffitta Bellazzini, IEEE, 2002

A new prototype with an extended GEM for better drift field uniformity Mixture fillingHe 20% + DME 80% 1 bar Gas cell thickness 1 cm GEM50 um pitch, 50 um thick, 88 x 88 mm OLD NEW Riunione Nazionale Astronomia X 15-16/11/2012 P. Soffitta Same window, same ASIC but a much larger GEM, with the addition of a large Guard Ring and field forming frames.

IAPS facility for polarized X-rays keV Crystal Line Bragg angle 1.65 ADP(101) CONT PET(002) CONT Rh(001) Mo L α Graphite CONT Al(111) Ca K α CaF 2 (220) Ti K α LiF(002) 55 Fe Si(400) Fe K α Ge(333) Cu K α FLi(420) Au L α Fli(800) Mo K α 44.8 Facility at IASF-Rome/INAF Close-up view of the polarizer and the Gas Pixel Detector Capillary plate (3 cm diameter) Aluminum and Graphite crystals. Spectrum of the orders of diffraction from the Ti X-ray tube and a PET crystal acquired with a Si-PiN detector by Amptek (Muleri et al., SPIE, 2008) PET Riunione Nazionale Astronomia X 15-16/11/2012 P. Soffitta Muleri et al., SPIE 2008

Not only MonteCarlo: Our predictions are based on data The modulation factor measured 2.6 keV, 3.7 keV and 5.2 keV has been compared with the Monte Carlo previsions. The agreement is very satisfying. Each photon produces a track. From the track the impact point and the emission angle of the photoelectron is derived. The distribution of the emission angle is the modulation curve. By rotating the polarization vector the capability to measure the polarization angle is shown by the shift of the modulation curve. Muleri et al Soffitta et al., 2010 Present level of absence of systematic effects (5.9 keV). Bellazzini 2010 Impact point Riunione Nazionale Astronomia X 15-16/11/2012 P. Soffitta OLD Design He-DME Mixture

OLD (narrow) design : more energies, more mixtures We performed measurement at more different energies and gas mixtures. Pure DME (CH 3 ) 2 O μ = 13.5% Modulation curve at 2.0 keV (Muleri et al., 2010).

Spurious 5.9 keV 125 kcounts: Modulation factor:~50% Spurious modulation measured: ~0.54% Spurious polarization measured: ~1% MDP 99% with  =50% and 125 kc: ~ 2.3% Riunione Nazionale Astronomia X 15-16/11/2012 P. Soffitta New Design (Larger) He-DME mixture Muleri et al., 2012

Energy resolution 6.4 keV Energy (keV)  E/E % % % % Riunione Nazionale Astronomia X 15-16/11/2012 P. Soffitta

Modulation factor EnergyModulation factor / / / keV with a cut on low energy tail of Pulse Height Riunione Nazionale Astronomia X 15-16/11/2012 P. Soffitta

Old design Ar-DME mixture New Design He-DME mixture Background Riunione Nazionale Astronomia X /11/2012 P. Soffitta Soffitta et al., SPIE 2012

Riunione Nazionale Astronomia X /11/2012 P. Soffitta

The position resolution of the LEP GPD. Soffitta NIM A 2012 The HEW of the GPD is nine times smaller than that of the JET-X optics when blurred by the inclined penetration (20 ‘’).

Ar-DME filled detector Riunione Nazionale Astronomia X 15-16/11/2012 P. Soffitta Fabiani et al., SPIE 2012 Muleri et al. SPIE 2007

Riunione Nazionale Astronomia X 15-16/11/2012 P. Soffitta

The New ASIC for X-ray polarimetry A new version of the ASIC CMOS chip has been produced The main characteristics are : A smaller pixel but a larger number to get the same effective area (1.5 cm x 1.5 cm) More uniform pedestals : A low threshold can be achieved (200 electrons) therefore with a very small gain the single electron in the photoelectron track can be reached. More stable pedestals. No need to readout the pedestal for each event. Smaller fiducial region (less number of pixel per track). The dead time of this ASIC is much smaller and compatible with an X-ray optics like that of IXO.

Riunione Nazionale Astronomia X 15-16/11/2012 P. Soffitta Triggered by the effective area at high energy up to 80 keV of the mirror foreseen for NHXM but exploiting the heritage of previews works on Compton Polarimetry. We re-started such activity. Angular depandance of Compton effect. Costa et al. NIM 1995 Soffitta et al., SPIE 2010 Compton Polarimetry

Riunione Nazionale Astronomia X 15-16/11/2012 P. Soffitta Fabiani S. et al., submitted to AstroParticle Physics 2012 At 90 deg of scattering angle and at 20 keV, the deposited energy is only 750 eV. We wanted to answer to the question : Are we experimentally able to detect such a small energy from an organic scintillator ? With what tagging efficiency ? How changes the tagging efficiency with respect to energy ? The answer to this question allows for determining reliable astrophysical sensitivity evaluation based on real-data and not only to Monte Carlo simulation.

Riunione Nazionale Astronomia X 15-16/11/2012 P. Soffitta

We measured in the lab without using a synchrotron facility, the tagging efficiency as the ratio between the coincidence rate and the rate detected by the absorber once background subtracted and windowed in energy. Such measurement have been performed with a Cd 109 source (22 keV) and Am 241 source (60 keV).

Riunione Nazionale Astronomia X 15-16/11/2012 P. Soffitta

By using GEANT 4 and a Monte Carlo specifically developed at this purpose we evaluated the tagging efficiency as a function of energy by using the two measured values at 22 keV and 60 keV. The sensitivity estimation on the right performed for a configuration similar to that of the experimental laboratory set-up is based on an experimental measurement of the efficiency.

Riunione Nazionale Astronomia X 15-16/11/2012 P. Soffitta Fine

Riunione Nazionale Astronomia X /11/2012 P. Soffitta In order to determine the tagging efficiency as a function of energy we simulated the experimental set-up by using GEANT 4. We obtained the coincidence spectrum by using GEANT 4 and an in-house Monte Carlo to simulate the detector response at (22 keV, Cd 109 ) and (59.5 keV, Am 241 ) comparing those with real data. We determined therefore the common energy threshold. At this point we inverted the procedure, we evaluated coincidence spectrum for different threshold and applying the commpn threshold we evaluated the tagging efficiency as a function of energy.