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The background of the gas pixel detectors and its impact on imaging X-ray polarimetry

The background of the gas pixel detectors and its impact on imaging X-ray polarimetry. Paolo Soffitta , Riccardo Campana , Enrico Costa, Sergio Fabiani , Fabio Muleri and Alda Rubini Istituto di Astrofisica e Planetologia Spaziali /INAF, Rome, Italy

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The background of the gas pixel detectors and its impact on imaging X-ray polarimetry

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  1. The background of the gas pixel detectors and its impact on imaging X-ray polarimetry Paolo Soffitta, Riccardo Campana,Enrico Costa,Sergio Fabiani, Fabio Muleriand AldaRubini Istituto di Astrofisica e PlanetologiaSpaziali/INAF, Rome, Italy Ronaldo Bellazzini, Alessandro Brez, Massimo Minuti, Michele Pincheraand Gloria Spandre IstitutoNazionale di FisicaNucleare INFN-Pisa, Pisa Italy SPIE Astronomical Telescope + Instrumentation Conference, Amsterdam 1- 6 July 2012

  2. In polarimetrysensitivityis a matter of photons MDP is the Minimum Detectable Polarization MDP = • M is the modulation factor • A is the telescope effective area • Is the detector efficiency • F(E) is source intensity (ph/s/cm2/kev) • Bdiff is the diffused background • Bint is the internal (unresolved) background Source detection > 10 photons Source spectral slope > 100 photons Source polarization > 100.000 photons

  3. Background • The Background of gas proportional counters is smaller than that of Silicon Detectors because : • Use of anticoincidence system • Use of pulse shape discrimination in proportional counters. It is a syntetic parameter that derives from the shape of the track. The GPD instead sees the track therefore we can select tracks much better. However : A minimum ionizing particle releases about 150 keV in 400 m Silicon Detector, therefore a discrimination system is the most efficient back-ground rejection system in Silicon. In gas a minimum ionizing particle releases about 1.5 MeV/g cm-2 that : in Ar is 2.7 keV/cm in DME is 3.6 keV/cm cm-long tracks from background can still be in range. SPIE Astronomical Telescope + Instrumentation Conference, Amsterdam 1- 6 July 2012

  4. The IPC of SXRP Internal wiring of SXRP IPC from top to bottom : cathode plane, anticoincidence plane, Wedge and Strip plane, anode plane, cathode plane, 4 Field Forming rings • The detector for SXRP were actually using the following system to eliminate the background : • Back Anticoincidence • Pulse shape discrimination • Lateral anticoicidence with W&S frame. SPIE Astronomical Telescope + Instrumentation Conference, Amsterdam 1- 6 July 2012

  5. Measured Instrumental background for experiments with large area gas detector HEAO1 A2 orbit LEO : 445 km, 22.75 deg Feroci M., et al., Nuclear Instruments and Methods in Physics Research A 371 (1996) 538-543 Tennant A.F. Technical Memorandum 85101 http://nssdc.gsfc.nasa.gov/nmc/experimentDisplay.do?id=1977-075A-02 http://heasarc.gsfc.nasa.gov/docs/journal/heao1-a2_5.html Mixture Area Energy Band total counts c/s/kev/cm2 (*) ------------------------------------------------------------------------------------------------------------------- Ar CH-4 1 Atm 800 cm2 1.5-20 keV 1.9 ct/s 1.1 E-4 Xenon 1 Atm 800 cm2 2-60 keV 5.5 ct/sec 1.0 E-4 (*) Calculated from the geom. area the energy band and the total counts SPIE Astronomical Telescope + Instrumentation Conference, Amsterdam 1- 6 July 2012

  6. OSO 8 (Wisconsin exper.) Orbit : LEO 550 km 32.95 deg Bunner, A. ApJ, 220: 261-271, 1978 Miscela Area (*) Banda conteggitotali c/s/kev/cm2 ------------------------------------------------------------------------------------------------- Methan 0.5 atm 106 0.13-3.65 0.6 c/s 1.61E-3 Neon 1.25 atm 106 0.75-6.0 0.088 c/s 1.58E-4 Xenon-Argon 1.25 atm 107 1.47-55 3.69 c/s 6.4E-4 (*) Calculated at posteriori from total and differential counts SPIE Astronomical Telescope + Instrumentation Conference, Amsterdam 1- 6 July 2012

  7. Measured Instrumental background for experiments with large area gas detector EXOSAT ME Orbit : Elliptical (355 km - 191570 km) 72.75 deg EXOSAT AO3 p. 35 Miscela Area Energy band Total counts c/s/kev/cm2 (*) ----------------------------------------------------------------------------------------------------------------- Argon 2 Atm 1500 cm2 2-10 keV 3 c/s/kev 2E-3 Xenon 2 Atm 1500 cm2 5-50 keV 12 c/s/keV 8E-3 (*) Calculated at posteriori from total counts and area SPIE Astronomical Telescope + Instrumentation Conference, Amsterdam 1- 6 July 2012

  8. Measured Instrumental background for experiments with large area gas detector RXTE Orbit : LEO 580 km 23 deg Jahoda et al.Astrophysical Journal Supplement Series, 163:401–423, 2006 Miscela Area Energy Band conteggitotali c/s/kev/cm2 (*) ------------------------------------------------------------------------------------------------- Xenon-CH4 1.05 Atm 2-60 1562 (PCA2) fig.24 Jahoda 2.6 E-4 90-10 (*) Calculated at posteriori from total back counts, CBX fraction and area from fig. 24 of Jahoda et al., 2006 SPIE Astronomical Telescope + Instrumentation Conference, Amsterdam 1- 6 July 2012

  9. The MEP-GEM90 design consists of 3 sub-components: • MEP-GEM90-Drift_assembly • MEP-GEM90-GEM_assembly • MEP-GEM90-ASIC_Board_assembly 20-30 mm □ 90 mm MEP-GEM90 Prototype adhesive • New GEM (P50L18R88) layout: • MEP-GEM90 prototype section: Guard ring Active area

  10. Background Rejection for the Gas Pixel Detector • Pulse shape discrimination : It cannot be applied to the ASIC because of the pixel anode pattern readout. • Back Anticoincidence cannot be applied because the ASIC is glued to the package and fixed to the bottom case. However converted electrons within the transfer gap are not multiplied therefore they do not contribute to the background. Only converted electrons/photons that pass the GEM are multiplied and detected as background. SPIE Astronomical Telescope + Instrumentation Conference, Amsterdam 1- 6 July 2012

  11. Possible background rejection in the Gas Pixel Detector. • Upper and lower threshold. • Window maximum size. It is similar to the pulse shape discrimination for background tracks parallel to the detector plane • Contiguity of the track. In DME at 10 keV the stopping power is 20 MeV/cm2/g or 40 keV/cm or (200eV/pixel) well above the energy to create a pair electron-ion (30 eV). A minimum ionizing electron instead looses 1.8 MeV/cm2/g or 3.6 keV/cm (18 eV/pixel) therefore the tracks can be discontinuous. An example of tracks discontinuous are given below : SPIE Astronomical Telescope + Instrumentation Conference, Amsterdam 1- 6 July 2012

  12. Pulse shape discrimination with the GEMS: • By reading out the GEM signal the rise time is sensitive to the track length in • direction perpendicular to the GEM plane (this method could be orthogonal to the window size discrimination). Being in DME the drift velocity 1cm/s tracks derived by X-rays can have a rise time of less then 150 ns (the GEMS channels length is only 50 m) to be compared with background tracks of 1 s. • Side coincidence: • (1) By a proper design of the guard-ring with perforated holes similar to the GEM it is possible to use a side-veto. • (2) Using the external frame of the ASIC chip as an anticoincidece. Decreasing Field of View expected by using a frame of tha ASIC chip as large as the expected range of X-ray photoelectrons. (ASIC 1.5 x 1.5 cm ; Focal Length 3.5 m) • Larger detector section with respect to the active plane. To reduce the impact of background produced by the walls. SPIE Astronomical Telescope + Instrumentation Conference, Amsterdam 1- 6 July 2012

  13. Larger detector section with respect to the active plane. To reduce the impact of background produced by the walls. OLD NEW SPIE Astronomical Telescope + Instrumentation Conference, Amsterdam 1- 6 July 2012

  14. OLD DESIGN Env. Back Background spatial distribution of a GPD 2-cm; 2-bar filled with an ArDME gas mixture. Effects on the modulation curve of the background suggesting that most of the background comes from the walls. Projection on Y and X of a slice excluding Fe55. New design Residual modulation of events excluding the Fe55 source. He-DME new design. The Fe55 collimatedsource on the corner allows for preventing time-out of the electronics. SPIE Astronomical Telescope + Instrumentation Conference, Amsterdam 1- 6 July 2012

  15. An example of expected background. Example polarimeters on board XIPE • XIPE : • Two existing JETX- telescopes (as the SWIFT XRT). • Two focal plane GPD filled with He-DME 2080. (plus two polarimeter filled with ArDME mixture for solar flares) SPIE Astronomical Telescope + Instrumentation Conference, Amsterdam 1- 6 July 2012

  16. XIPE background XIPE is capable of imaging point of point and extended source. Simulating the PSF of the optics and the effect of the inclined penetration we arrived at the expected overall HPW including both effects. SPIE Astronomical Telescope + Instrumentation Conference, Amsterdam 1- 6 July 2012

  17. Impact of background Point source : PSF 20’’ => 170 m diameter Source counting rate (Crab = 115 c/s) The position resolution of the GPD is smaller than the PSF of the JET-X optics convolved with the inclined penetration. • Back-ground : • Diffused = 2.3 10-12 c/s • Internal = 3.9 10-7 c/s • Background = 3.5 nCrab SPIE Astronomical Telescope + Instrumentation Conference, Amsterdam 1- 6 July 2012

  18. A Dim extended source Sgr B2 is a faint extended (3 arcmin x 3 arcmin) molecular cloud in the Galactic Center Region may be echoing the past activity of Sgr A*. The reflected X-rays from SgrA* can be highly polarized. Expected counting rate from SgrB2 : 6 10-4 c/s Extension 3’ x 3’ or 3 mm x 3 mm Diffused background contribution :7.1 10-10 c/s Internal background contribution : 1.2 10-4 c/s For XIPE observing schedule making a mistake on the background of a factor of three-four can affect only the sensitivity for SgrB2. For the other point/extended source a mistake of a factor of 10-100 is allowed without impact on sensitivity. SPIE Astronomical Telescope + Instrumentation Conference, Amsterdam 1- 6 July 2012

  19. End of Presentation SPIE Astronomical Telescope + Instrumentation Conference, Amsterdam 1- 6 July 2012

  20. GPD New Mechanical Drawing SPIE Astronomical Telescope + Instrumentation Conference, Amsterdam 1- 6 July 2012

  21. The Gas Pixel Detector SPIE Astronomical Telescope + Instrumentation Conference, Amsterdam 1- 6 July 2012

  22. SPIE Astronomical Telescope + Instrumentation Conference, Amsterdam 1- 6 July 2012

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