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Charge transparencies and amplification properties of Integrated Micromegas detectors Maximilien Chefdeville NIKHEF, Amsterdam RD51, Amsterdam 17/04/2008

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Overview InGrid, an integrated Micromegas Charge transparencies –Electron collection –Ion backflow Amplification properties in Ar-iC 4 H 10 mixtures –Measuring Fano factors with the Timepix chip –Mean energies per ion pair –Gas gain –Energy resolution and gain fluctuations

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InGrid, an integrated Micromegas for pixel readout gas detectors Solve alignment / pillar Ø / pitch issues of Micromegas pixel detectors by integrating the grid onto the chip Wafer post-processing –Grid geometry fits the chip –Pillar Ø ~ 30 μm Very good grid flatness –Gain homogeneity –Very good resolution 2 cm Ø 11.7 % 5.9 keV in P10 pillar 2 cm Ø

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Electron collection studies

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Electron collection Micromegas basics –Funnel of field lines at the hole entrances –Compression factor is equal to the field ratio FR: S A = S D.E D /E A = S D / FR –For FR>FR*, all field lines are transmitted to the amplification region SASA SDSD E Drift E Amplif. Ion drift lines Obviously, FR* depends on the grid optical transparency Dependence on the hole pitch and the hole diameter Also, the electrons don’t follow exactly the field lines Dependence on the gas mixture

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Electron collection Measurements – 55 Fe 5.9 keV source –Prototypes: μm hole pitch μm hole diameter –Pocket MCA Amptek –Constant grid voltage, vary E D Lowering of the gain with E D Grid geometry study: –Ar 5% iC 4 H 10 –FR* ↓ with the grid optical transparency Gas study: –Ar/CO 2 5/95 10/90 20/80 and “pure” Ar –FR* ↑ with the electron temperature

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Ion backflow studies

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Ion backflow in Micromegas First studies performed by Saclay/Orsay I. Giomataris, V. Lepeltier, P. Colas Nucl. Instr. and Meth. A 535 (2004) 226 Intrinsic low BF as most of the field lines in the avalanche gap end on the grid Number of ions arriving on the grid depends on: –Shape/size of the field line funnel –Ion formation positions Shape/size of the field line funnel –Grid geometry –Ratio of the Amplification to Drift fields Ion formation positions –Longitudinally: Townsend coefficient –Transversally: Electron diffusion Ion drift lines Electron avalanches E Drift E Amplif.

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Experimental set-up X-ray gun up to 12 keV photons, 200 μA –Operated at 9 keV energy (50 μA) 10 keV photo e- range ~ 1 cm in Ar –Collimator is 2 cm thick with a 3 mm Ø hole Guard electrode 1 mm above the grid –Adjustable voltage Cathode/Anode current measurements –Voltage drop through 92 MΩ resistor Z input = 1 GΩ, ΔI = 1 pA –Voltage drop through 10 MΩ resistor Z input = 100 MΩ, ΔI = 100 pA Reversed polarities: –Cathode at ground, grid and anode at positive voltages –No field between detector window and cathode Gas mixture: Ar:CH 4 90:10

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Experimental set-up Electronics Voltmeters X-tube Gas chamber Collimator

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Detector geometries 4 different hole pitches 20, 32, 45 and 58 μm 20 & 32 μm pitch grids have pillars inside holes 45 & 58 μm pitch grids have pillars between holes 3 different amplification gap thicknesses –45, 58 and 69 μm ± 1 μm –Operated at 325, 350 and 370 V –Amplification fields of 72, 60 and 53 kV/cm Gains of 200, 550 and 150 Diffusion coef. of 142, 152 and 160 μm/√cm Avalanche width of 9.5, 11.6 and 13.4 μm

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Measurements in Ar:CH 4 90:10 Vary field ratio FR from 100 to 1000 –Drift field from ~ 500 V/cm down to few ~ 50 V/cm –At high FR (low Drift field), primary e- loss due to field distortions Stop at FR ~ 1000 Fit curve with BF = p 0 /FR p1

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Measurements with 45 μm gap InGrids Gain ~ 200 σ t = 9.5 μm 20 μm pitch p1 = μm pitch p1 = μm pitch p1 = μm pitch p1 = 1.19 BF = p 0 /FR p1 At given field ratio and ion distribution, the backflow fraction ↓ with the pitch

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Measurements with 58 μm gap InGrids Gain ~ 500 σ t = 11.6 μm 20 μm pitch p1 = μm pitch p1 = μm pitch p1 = μm pitch p1 = 1.21 BF = p 0 /FR p1 BF < 1 ‰ At given field ratio and ion distribution, the backflow fraction ↓ with the pitch

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Measurements with 70 μm gap InGrids BF = p 0 /FR p1 Gain ~ 150 σ t = 13.4 μm 32 μm pitch p1 = μm pitch p1 = μm pitch p1 = 1.28 BF < 1 ‰ At given field ratio and ion distribution, the backflow fraction ↓ with the pitch

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Summary of the measurements At given field ratio, the backflow fraction ↓ with the ion distribution width and ↑ with the hole pitch

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Primary statistics and amplification properties

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Measuring Fano factors with Gridpix Detect individual electrons from 55 Fe R 2 = (F+b)/N + (1-η)/ηN F: Fano factor √b: single e- gain distribution rms (%) η: detection efficiency N: number of primary e- Raw spectrum Access to F if efficiency η is known b=0 Measure the primary statistics Mean energy per ion pair W Fano factor F

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Detection efficiency Electron detected if its avalanche is higher than the pixel threshold threshold Detection efficiency: η = ∫ t ∞ p(g).dg Exponential fluctuations: p(g) = 1/. exp (-g/ ) η(g) = exp (-t/ ) “Polya” fluctuations: parameter m=1/b with √b the relative rms p(m,g) = m m /Γ(m). 1/. (g/ ) m-1. exp (-m.g/ ) p(2,g) = 4. 1/. g/. exp (-2.g/ ) η(2,g) = (1+2.t/ ). exp(-2.t/ )

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Experimental setup Gas chamber –Timepix chip 15 μm SiProt + 50 μm InGrid –10 cm drift gap –Cathode strips and Guard electrode –Ar 5 % iC 4 H 10 55Fe source placed on top –Collimated to 2 mm Ø beam –Difficult to align precisely Ideally, gain & threshold homogeneous –Pixel to pixel threshold variations Threshold equalization provides uniform response –Gain homogeneity should be OK thanks to: Amplification gap constant over the chip (InGrid) Amplification gap close to optimum Imperative: have enough diffusion to perform counting –Long drift length, look at escape peak –However: SiProt layer induces charge on neighboring pixels 500 V/cm chip guard strips 55 Fe 5.9 & 6.5 keV

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Event selection Suppress noise hits –Operate chip in TIME mode 10 μs active time count clock pulses of 10 ns –Cut hits 4σ t away from the mean time –Cut hits 4σ x,y away from the mean x,y Select large diffusion events –Measure the number of clusters as a function of spread (σ t 2 ) for increasing grid voltages Effective number of electron from double Gaussian fit 320 V 340 V

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Collection efficiency Data points: n e (V g ) = η(V g ).n 0 Analytical form of η(g) known for exponential and Polya fluctuations Use gain parameterization: g(V g ) = A.exp(B.V g ) A depends on the absolute gain B ~ for Ar/iC 4 H 10 mixtures Exponential fluctuations: n 0 = ± 2.8 B = ± Polya fluctuations with m=2: n 0 = ± 2.6 B = ± Mean energy per ion pair W = 3000 /114.6 = 26.2 ± 0.5 eV

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At 350V… RMS t = 6.25 % η = 0.93 RMS η = 2.56 % RMS p = 5.70 % F = 0.35

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Extending knowledge of W to other Ar/iso gas mixtures W constant above 1 keV –No matter the X-ray energy, same energy fraction is spent in ionization Generate measurable primary currents (X-tube) –Primary current depends on W and absorption coefficient Start with Ar and progressively introduce iso –Check that the absorption does not change when introducing isobutane Iso fraction (%)W (eV)Np (5.9 keV)

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Gas gain curves in Ar/iso mixtures Gas gain measured with an 55Fe source –Penning transfers from Ar excited states to isobutane molecules –Cooling of the electrons at increasing isobutane concentration

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Energy resolution in Ar/iso mixtures General trend –Resolution improves from gains of few 10 2 to few 10 3 –Degradation above few 10 3 Minima of resolution: Ar 2.5 % iC 4 H 10 : 16 % FWHM Ar 20 % iC 4 H 10 : 14 % FWHM FWHM Ratio = 1.14 √W ratio = 1.13 Gain fluctuations F = 0.35 R = 6% RMS b = 0.5 (m=2) √b ~ 71 %

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Conclusions Charge transparencies –Electron collection efficiency well understood –Ion backflow fractions in agreement with ones measured with standard Micromegas Counting electrons from 55 Fe with Timepix in Ar 5% iC 4 H 10 –W = 26.2 eV –F = 0.35 Gas gain fluctuations obey a Polya distribution with m=2, i.e. relative fluctuations of 71 %

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Thanks for your attention Thanks to all people from the NIKHEF/Twente/Saclay pixel collaboration

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