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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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Presentation on theme: "Charge transparencies and amplification properties of Integrated Micromegas detectors Maximilien Chefdeville NIKHEF, Amsterdam RD51, Amsterdam 17/04/2008."— Presentation transcript:

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

2 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

3 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 % FWHM @ 5.9 keV in P10 pillar 2 cm Ø

4 Electron collection studies

5 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

6 Electron collection Measurements – 55 Fe 5.9 keV source –Prototypes: 20-58 μm hole pitch 10-45 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

7 Ion backflow studies

8 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.

9 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

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

11 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

12 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

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

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

15 Measurements with 70 μm gap InGrids BF = p 0 /FR p1 Gain ~ 150 σ t = 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

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

17 Primary statistics and amplification properties

18 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

19 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/ )

20 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

21 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

22 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 ~ 3-4.10 -2 for Ar/iC 4 H 10 mixtures Exponential fluctuations: n 0 = 116.4 ± 2.8 B = 5.15.10 -2 ± 0.52.10 -2 Polya fluctuations with m=2: n 0 = 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

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

24 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) 026.9220 125.1235 2.525.7230 526.2225 1027.8212 2032.9179

25 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

26 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 %

27 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 %

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


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