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

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 RubiniIstituto 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

In polarimetry sensitivity is a matter of photons

Source detection > 10 photonsSource spectral slope > 100 photonsSource polarization > 100.000 photons

MDP is the Minimum Detectable Polarization

MDP =

M is the modulation factorA is the telescope effective areae Is the detector efficiencyF(E) is source intensity (ph/s/cm2/kev)Bdiff is the diffused backgroundBint is the internal (unresolved) background

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

BackgroundThe 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/cmcm-long tracks from background can still be in range.


The IPC of SXRP

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


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


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-4Xenon-Argon 1.25 atm 107 1.47-55 3.69 c/s 6.4E-4

(*) Calculated at posteriori from total and differential counts



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



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

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

20-3

0 m

m

Active area

Guard ring

MEP-GEM90 prototype section:


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.


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 :


• 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.


• Larger detector section with respect to the active plane. To reduce the impact of background produced by the walls.

OLDNEW

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 Fe55

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 Fe55.

Residual modulation of events excluding the Fe55 source.


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)


XIPE backgroundXIPE 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.


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


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 mmDiffused background contribution :7.1 10-10 c/sInternal 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.


End of Presentation


GPD New Mechanical Drawing


The Gas Pixel Detector

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

Documents

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