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

Overview InGrid, an integrated Micromegas Charge transparencies Electron collection Ion backflow Amplification properties in Ar-iC4H10 mixtures Measuring Fano factors with the Timepix chip Mean energies per ion pair Gas gain Energy resolution and gain fluctuations

InGrid, an integrated Micromegas for pixel readout gas detectors pillar 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 % FWHM @ 5.9 keV in P10 2 cm Ø

Electron collection studies

Electron collection Micromegas basics SD Ion drift lines Funnel of field lines at the hole entrances Compression factor is equal to the field ratio FR: SA = SD.ED/EA = SD / FR For FR>FR*, all field lines are transmitted to the amplification region SA SD EDrift EAmplif. 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

Electron collection Measurements Lowering of the gain with ED 55Fe 5.9 keV source Prototypes: 20-58 μm hole pitch 10-45 μm hole diameter Pocket MCA Amptek Constant grid voltage, vary ED Lowering of the gain with ED Grid geometry study: Ar 5% iC4H10 FR* ↓ with the grid optical transparency Gas study: Ar/CO2 5/95 10/90 20/80 and “pure” Ar FR* ↑ with the electron temperature

Ion backflow studies

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 Grid geometry Ratio of the Amplification to Drift fields Longitudinally: Townsend coefficient Transversally: Electron diffusion Ion drift lines EDrift EAmplif. Electron avalanches

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 Zinput = 1 GΩ, ΔI = 1 pA Voltage drop through 10 MΩ resistor Zinput = 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:CH4 90:10

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

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

Measurements in Ar:CH4 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 = p0/FRp1

Measurements with 45 μm gap InGrids BF = p0/FRp1 Gain ~ 200 σt = 9.5 μm 20 μm pitch p1 = 1.01 32 μm pitch p1 = 0.90 45 μm pitch p1 = 0.96 58 μm pitch p1 = 1.19 At given field ratio and ion distribution, the backflow fraction ↓ with the pitch

Measurements with 58 μm gap InGrids BF = p0/FRp1 Gain ~ 500 σt = 11.6 μm 20 μm pitch p1 = 1.08 32 μm pitch p1 = 1.02 45 μm pitch p1 = 1.01 58 μm pitch p1 = 1.21 BF < 1 ‰ At given field ratio and ion distribution, the backflow fraction ↓ with the pitch

Measurements with 70 μm gap InGrids BF = p0/FRp1 Gain ~ 150 σt = 13.4 μm 32 μm pitch p1 = 1.14 45 μm pitch p1 = 1.13 58 μm pitch p1 = 1.28 BF < 1 ‰ At given field ratio and ion distribution, the backflow fraction ↓ with the pitch

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

Primary statistics and amplification properties

Measuring Fano factors with Gridpix Detect individual electrons from 55Fe Measure the primary statistics Mean energy per ion pair W Fano factor F Raw spectrum b=0 R2 = (F+b)/N + (1-η)/ηN F: Fano factor √b: single e- gain distribution rms (%) η: detection efficiency N: number of primary e- Access to F if efficiency η is known

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

Experimental setup Gas chamber 55Fe source placed on top Timepix chip 15 μm SiProt + 50 μm InGrid 10 cm drift gap Cathode strips and Guard electrode Ar 5 % iC4H10 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 55Fe 5.9 & 6.5 keV 500 V/cm chip guard strips

Event selection Suppress noise hits Select large diffusion events Operate chip in TIME mode 10 μs active time count clock pulses of 10 ns Cut hits 4σt away from the mean time Cut hits 4σx,y away from the mean x,y Select large diffusion events Measure the number of clusters as a function of spread (σt2) for increasing grid voltages Effective number of electron from double Gaussian fit 320 V 340 V

Collection efficiency Data points: ne(Vg) = η(Vg).n0 Analytical form of η(g) known for exponential and Polya fluctuations Use gain parameterization: g(Vg) = A.exp(B.Vg) A depends on the absolute gain B ~ 3-4.10-2 for Ar/iC4H10 mixtures Exponential fluctuations: n0 = 116.4 ± 2.8 B = 5.15.10-2 ± 0.52.10-2 Polya fluctuations with m=2: n0 = 114.6 ± 2.6 B = 3.35.10-2 ± 0.32.10-2 Mean energy per ion pair W = 3000 /114.6 = 26.2 ± 0.5 eV

At 350V… RMSt = 6.25 % η = 0.93 RMSη = 2.56 % RMSp = 5.70 % F = 0.35

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)   26.9 220 1 25.1 235 2.5 25.7 230 5 26.2 225 10 27.8 212 20 32.9 179

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

Energy resolution in Ar/iso mixtures General trend Resolution improves from gains of few 102 to few 103 Degradation above few 103 Minima of resolution: Ar 2.5 % iC4H10: 16 % FWHM Ar 20 % iC4H10: 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 %

Conclusions Charge transparencies Electron collection efficiency well understood Ion backflow fractions in agreement with ones measured with standard Micromegas Counting electrons from 55Fe with Timepix in Ar 5% iC4H10 W = 26.2 eV F = 0.35 Gas gain fluctuations obey a Polya distribution with m=2, i.e. relative fluctuations of 71 %

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