1. 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

2. 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

3. 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 % 5.9 keV in P10
2 cm Ø

4. Electron collection studies

5. 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

6. 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

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

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
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

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

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: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

13. 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

14. 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

15. 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

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 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

19. 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>)

20. 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

21. 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

22. 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 ~ for Ar/iC4H10 mixtures
Exponential fluctuations:
n0 = ± 2.8
B = ±
Polya fluctuations with m=2:
n0 = ± 2.6
B = ±
Mean energy per ion pair
W = 3000 /114.6 = ± 0.5 eV

23. At 350V…
RMSt = 6.25 %
η = 0.93
RMSη = 2.56 %
RMSp = 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)
26.9
220 1
25.1
235
2.5
25.7
230 5
26.2
225 10
27.8
212 20
32.9
179

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 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 %

27. 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 %

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