A. Bamberger1 Experimental study of double GEM readout using MediPix2 chip A. Bamberger, M. Debatin, J. Ludwig, M. Titov, N. Vlasov

A. Bamberger2 Typical geometry: 5 µm Cu on 50 µm Kapton 70 µm holes at 140 mm pitch Thin metal-coated polymer foil chemically pierced by a high density of holes (technology developed at CERN) F. Sauli, Nucl. Instrum. Methods A386(1997)531 F. Sauli,

A. Bamberger3 Advantages of GEM DRIFT TRANSFER positive ion feed back minimized high rates small rate of discharges for highly ionising particles...see later F. Sauli, 2002 IEEE Proceedings

A. Bamberger4 Properties gas amplification up to 6000 easily achievable with Ar/CO 2 sufficient for min. ionizing particles in gas thicknesses of few mm S1S2S3 S4 Induction gap e - I+I+

A. Bamberger5 Comparison µMEGAS and GEM replottet from NIM A 424 (1999) 321, NIM A 479 (2002) 294 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

A. Bamberger6 Double GEM 10·10 cm² 28 channels readout electronics resistive chain for HV <4000 V semitransparent drift electrode

A. Bamberger7 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. Bamberger8 Overall view pocket for MediPix2 board and cable

A. Bamberger9 Double GEM drif t elektrode readout elektrode GEM 1 GEM2 thickness of drift field 6 mm transfer gap 2 mm induction gap 2 mm resistors for protection ΔV GEM = 350 – 400 V, E D, E T, E I ~ 2.5 kV/cm subject to further optimisation Gas: Ar/CO 2

A. Bamberger10 Homogeneity and energy resolution for 55 Fe photons homogeneity < ±5% energy resolution of photo- electrons of 5.9 keV: FWHM 28% strip readout

A. Bamberger11 New readout electrode configuration with 2x2 cm 2 before after movie shows 4x4 matrix with source HCAL readout !

A. Bamberger12 Inserting MediPix2 into the GEM stack gap for separation of electrodes crosses MediPix2

A. Bamberger13 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, E ind = 3.5 kV/cm readout of MediPix2 normal functioning over many hours

A. Bamberger14 Readout with MUROS2 the parameters: lower threshold between e - upper threshold ~ 10 times higher HV 3900 V, 410 V across GEM Collimated 55 Fe source used: 4 mm opening at a distance of 35 mm source colli- mator MediPix2

A. Bamberger15 Short term shot of 55 Fe photons 14 mm estimates blob size: 10 x10 pixles = 550x550 µm 2 at gain ~

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

A. Bamberger17 MediPix2 exposed 30 min to source w/o collimator: Boundary of GEM electrodes steep slopes

A. Bamberger18 Some considerations for the resolution Basics: 1.transverse diffusion of Ar/CO2: μm/cm 2.size of energy deposition of 5.9 keV photon 300 – 500 μm 3.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. Bamberger19 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. Bamberger20 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. Bamberger21 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. Bamberger22 The effort would be in vain without the help of F. Sauli M. Campbell E. Heijne X. Llopart A. Zwerger MANY THANKS !