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 Muleri and Alda Rubini Istituto di Astrofisica e Planetologia Spaziali/INAF, Rome, Italy Ronaldo Bellazzini, Alessandro Brez, Massimo Minuti, Michele Pinchera and Gloria Spandre SPIE Astronomical Telescope + Instrumentation Conference, Amsterdam 1- 6 July 2012 Istituto Nazionale di Fisica Nucleare INFN-Pisa, Pisa Italy

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

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

4. The IPC of SXRP SPIE Astronomical Telescope + Instrumentation Conference, Amsterdam 1- 6 July 2012 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. 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

5. SPIE Astronomical Telescope + Instrumentation Conference, Amsterdam 1- 6 July 2012 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 cm 2 1.5-20 keV 1.9 ct/s 1.1 E-4 Xenon 1 Atm 800 cm 2 2-60 keV 5.5 ct/sec 1.0 E-4 (*) Calculated from the geom. area the energy band and the total counts

6. SPIE Astronomical Telescope + Instrumentation Conference, Amsterdam 1- 6 July 2012 OSO 8 (Wisconsin exper.) Orbit : LEO 550 km 32.95 deg Bunner, A. ApJ, 220: 261-271, 1978 Miscela Area (*) Banda conteggi totali 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

7. Measured Instrumental background for experiments with large area gas detector SPIE Astronomical Telescope + Instrumentation Conference, Amsterdam 1- 6 July 2012 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 cm 2 2-10 keV 3 c/s/kev 2E-3 Xenon 2 Atm 1500 cm 2 5-50 keV 12 c/s/keV 8E-3 (*) Calculated at posteriori from total counts and area

8. Measured Instrumental background for experiments with large area gas detector SPIE Astronomical Telescope + Instrumentation Conference, Amsterdam 1- 6 July 2012 RXTE Orbit : LEO 580 km 23 deg Jahoda et al.Astrophysical Journal Supplement Series, 163:401–423, 2006 Miscela Area Energy Band conteggi totali 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

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

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.

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

14. SPIE Astronomical Telescope + Instrumentation Conference, Amsterdam 1- 6 July 2012 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. He-DME new design. The Fe 55 collimated source on the corner allows for preventing time-out of the electronics. OLD DESIGN New design Projection on Y and X of a slice excluding Fe 55. Residual modulation of events excluding the Fe 55 source.

15. An example of expected background. SPIE Astronomical Telescope + Instrumentation Conference, Amsterdam 1- 6 July 2012 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)

16. XIPE background SPIE Astronomical Telescope + Instrumentation Conference, Amsterdam 1- 6 July 2012 XIPE is capable of imaging point of point and extended source. Energy 3 keV Characteristics XIPE Energy 3 keV HPW optics 14.7 arcsec HPW gas 10.0 arcsec HPW gas + optics 19.3 arcsec Simulating the PSF of the optics and the effect of the inclined penetration we arrived at the expected overall HPW including both effects.

17. Impact of background Point source : PSF 20’’ => 170  m diameter Source counting rate (Crab = 115 c/s) SPIE Astronomical Telescope + Instrumentation Conference, Amsterdam 1- 6 July 2012 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

18. A Dim extended source SPIE Astronomical Telescope + Instrumentation Conference, Amsterdam 1- 6 July 2012 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.

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

20. GPD New Mechanical Drawing

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