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Experimental study of double GEM readout using MediPix2 chip

Experimental study of double GEM readout using MediPix2 chip. A. Bamberger, M. Debatin, J. Ludwig, M. Titov, N. Vlasov. GEM (Gas Electron Multiplier). Thin metal-coated polymer foil chemically pierced by a high density of holes (technology developed at CERN). Typical geometry:

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Experimental study of double GEM readout using MediPix2 chip

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  1. Experimental study of double GEM readout using MediPix2 chip A. Bamberger, M. Debatin, J. Ludwig, M. Titov, N. Vlasov A. Bamberger

  2. GEM (Gas Electron Multiplier) Thin metal-coated polymer foil chemically pierced by a high density of holes (technology developed at CERN) Typical geometry: 5 µm Cu on 50 µm Kapton 70 µm holes at 140 mm pitch F. Sauli, Nucl. Instrum. Methods A386(1997)531 F. Sauli, http://www.cern.ch/GDD A. Bamberger

  3. DRIFT TRANSFER • positive ion feed back minimized • high rates • small rate of discharges for highly ionising particles...see later Advantages of GEM F. Sauli, 2002 IEEE Proceedings A. Bamberger

  4. I+ e- Induction gap e- S1 S2 S3 S4 Properties • gas amplification up to 6000 easily achievable with Ar/CO2 • sufficient for min. ionizing particles in gas thicknesses of few mm A. Bamberger

  5. Comparison µMEGAS and GEM Discharge probability with α-particles Spatial resolution in laboratory tests µMEGAS: 15 µm GEM: 40 µm BUT: many orders of magnitude NIM A 477 (2002) 23 NIM A 425 (1999) 262 replottet from NIM A 424 (1999) 321, NIM A 479 (2002) 294 A. Bamberger

  6. semitransparent drift electrode Double GEM 10·10 cm² 28 channels readout electronics resistive chain for HV <4000 V A. Bamberger

  7. Some features of the apparatus • all essential elements within the gas tight box: compact, easy handling i.e. tilting is possible • Noise reduction due to short leads • breaking gas volume/flow for changes turned out to be an affordable disadvantage (recovery within a few hours) • Multi electrode analog readout (L3 muon amplifiers 7x4): important for checking gas gain A. Bamberger

  8. Overall view pocket for MediPix2 board and cable A. Bamberger

  9. thickness of drift field 6 mm • transfer gap 2 mm • induction gap 2 mm Double GEM Gas: Ar/CO2 resistors for protection drif t elektrode GEM 1 GEM2 readout elektrode ΔVGEM= 350 – 400 V, ED , ET , EI~ 2.5 kV/cm subject to further optimisation A. Bamberger

  10. strip readout Homogeneity and energy resolution for 55Fe photons • homogeneity < ±5% • energy resolution of photo- electrons of 5.9 keV: FWHM 28% A. Bamberger

  11. New readout electrode configuration with 2x2 cm2 before after HCAL readout ! movie shows 4x4 matrix with source A. Bamberger

  12. Inserting MediPix2 into the GEM stack gap for separation of electrodes crosses MediPix2 A. Bamberger

  13. Close-up of the arrangement • surface of MediPix2 level with readout plane • „ring“-like electrode helps to detect possible discharges near MediPix2 due to cross talk, • „dummy“ MediPix2 with bonds showed no obvious discharging up to 4000 V, Eind= 3.5 kV/cm • readout of MediPix2 normal functioning over many hours A. Bamberger

  14. source colli-mator MediPix2 Readout with MUROS2 • the parameters: lower threshold between 2000 - 3000 e- • upper threshold ~ 10 times higher • HV 3900 V, 410 V across GEM • Collimated 55Fe source used: 4 mm opening at a distance of 35 mm A. Bamberger

  15. 14 mm 14 mm Short term shot of 55Fe photons estimates blob size: 10 x10 pixles = 550x550 µm2 at gain ~ 3 103 A. Bamberger

  16. 180 s exposed sample and displacement of colim. source source with collimator moved by 2 mm structure of joint between GEM electrodes seen A. Bamberger

  17. MediPix2 exposed 30 min to source w/o collimator: Boundary of GEM electrodes steep slopes A. Bamberger

  18. Some considerations for the resolution Basics: • transverse diffusion of Ar/CO2: 150 - 200 μm/cm • size of energy deposition of 5.9 keV photon 300 – 500 μm • defocussing effect GEMs Comments: • drift space is 6 mm cone like, slanted tracks reveal 1.) • dispersion of edge due to electrode boundary reveals 3.) (two bounderies of the doube GEM setup involved !) • the „hit over threshold“ feature complicates the disentangling 7 mm no source A. Bamberger

  19. Further investigations • oberservation of min. ion. tracks • quantify broadening due to drift volume • use 50µm pitched GEMs • reduce transfer and induction gap (1 mm), (are bonding loops above the chip a problem?) • use gas mixtures with other nobel gasses A. Bamberger

  20. Consideration for low photon energy spectroscopy: Conversion in gas or in Si • at low energies (few keV) signal/noise dominated either by statistical fluctuations of primary clusters (GEM/μMEGAS) or by the „baseline“ fluctuation (Si converter with coupled electronics like MediPix) • σ/N = 0.13 (5.9/E)0.5 for double GEM • σ/N = 200/1639 (5.9/E) for Si (σ = 200 e-) • break-even-energy at 5.3 keV Therefore it is favorable to use gas based amplifiers below a few keV A. Bamberger

  21. Summary • extremely robust operation of GEMs (no faulty GEM, or visible change of hole during operation during 2 month observed) • HV-stable condition for operation of a „naked“ MediPix2 (~week) with a double GEM. No broken MediPix sofar! • acurate position resolution seems to be achieveable A. Bamberger

  22. The effort would be in vain without the help of • F. Sauli • M. Campbell • E. Heijne • X. Llopart • A. Zwerger MANY THANKS ! A. Bamberger

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