Gain fluctuations and Neff Analysis of March 2010 data Neff measurement Gain fluctuation...

Gain fluctuations and Neff

Analysis of March 2010 dataNeff measurementGain fluctuation measurements with TimePix

Beijing, 30/03/2010 1P. Colas - FJPPL-FCPPL-LCTPC Analysis meeting

FJPPL-FCPPL-LCTPC Analysis meeting

B=0 data : Drift velocity measurements

Vdrift = 7.698 +- 0.040 cm/µs at E=230 V/cm (Magboltz : 7.583+-0.025(gas comp.))The difference is 1.5+-0.6 %


B=0 data : Drift velocity measurementsDrift Velocity in T2K gas and compared to Magboltz simulations for P=1035 hPa, T=19°C and 35 ppm H20


Beijing, 30/03/2010 P. Colas - FJPPL-FCPPL-LCTPC Analysis meeting 4

Pad Response Functions, z ~ 5 cm

CLPResistive ink

B=1T data : comparison of resistive ink and Carbon-loaded polyimide

Bias


Residuals


Z=5cm

Z=35cm

Z=50cm

MEAN RESIDUAL vs ROW number

Z-independent distortions

Distortions up to 50 microns for resistive paint

Rms 7 micron for CLP film


Resolution vs Drift Distance

zN

C

eff

D2

220

2 /NDOF = 15.4 / 11

CLP module


Fits for Z>30 and z<30 give consistent results


Relaxed cuts : less than 5 hits (>30 ADC) outside 10 central pad lines

Reject multiple tracks

Stability with cutsStability with cutsStability with cutsStability with cuts

10Beijing, 30/03/2010 P. Colas - FJPPL-FCPPL-LCTPC Analysis meeting

Stability of Neff with gainStability of Neff with gainStability of Neff with gainStability of Neff with gain

11Beijing, 30/03/2010 P. Colas - FJPPL-FCPPL-LCTPC Analysis meeting


Neff is the same for resistive inkNeff is the same for resistive ink

Resistive ink module

RESULTS• 0 = 53 µm : confirms previous results. 1/57

times the pad size. No hodoscope effect down to 3cm drift.

• Neff = 42.5 ± 0.5 (stat) ± 1.5 (CD) at P=1035 hPa, with CD = 95.5±1.6 µm/√cm(B=0.98 T). – To be compared to

• <1/N>-1 from Heed (H. Schindler) : 47.1 (in these conditions)

• Thus <G2>/<G>2 = 1.11 ± 0.04 (polya ~8±2.3 !)

2

2

/1

1

G

G

NNeff


Expectation from Heed (H.Schindler)

2

2

/1

1

G

G

NNeff


1/N

5 GeV electronsPad length 6.84 mm

Discussion• Neff is close to the maximum value allowed by

ionization statistics. • Penning effect in the drift volume could

increase Neff

• Difficult to imagine un-controlled systematics that lead to overestimate Neff

• Maybe CD is overestimated but unlikely. Usually measurements agree within few %


Expectations for 1/<1/N>Pitch 6.84 mm (mM@DESY)

Pitch 5.4 mm (GEM@DESY)

5 GeV e- 47.1 34.9

320 MeV µ 32.1 23.8


Caveat : does not scale with pad length! (would be false by 7% between 5.4 and 7mm)So it makes no sense to give it by unit length!

P=1035 hPaT=19°CT2K gas

Pitch 6.84 mm (mM@DESY)

Pitch 5.4 mm (GEM@DESY)

5 GeV e- 61.9 49.0

320 MeV µ 46.0 36.5

Expectations for Ntot


Ar:CO2 / 90:10CD = 223 µ/√cm

Neff ~ 26(cosmics, 6mm pads)

Unpublished result

Unpublished result

Beijing, Feb.6, 2007 P. Colas - Micromegas TPC 18

4 GeV/c + beam, B=1T (KEK)

eff

dx N

zC 2

20

Effect of diffusion: should become negligible at high magnetic field for a high gas

Madhu Dixit LCWS10, Beijing 19

Transverse spatial resolution Ar+5%iC4H10 E=70V/cm DTr = 124 µ/cm (Magboltz) @ B= 1T

2 mm x 6 mm pads

σ0=(27±2)μmNeff=35.9±1.3

σ0=(50±2)μmNeff=21.1±1.1

KEK B=1T 4 GeV π+ beam - resolution comparison Old algorithm vs new fixed window PRF algorithm

Preliminary

Beijing, Feb.6, 2007 P. Colas - Micromegas TPC 20

Gain = 4700 Gain = 2300

Neff=25.2±2.1 Neff=28.8±2.2

At 4 T with this gas, the point resol° is better than 80 µm at z=2m

• B=0.5 T• Resolution at 0 distance ~50 µ even at low gain

ACFA LCWS 2007, Cosmics (COSMO)

Dependence of resolution with data taking conditions


Relaxed fiducial cutsZ=5cm

Same, with hard fiducial cuts


Peaking time (ns)

Res

olut

ion

(µm

)CLP module

Dependence of resolution with data taking conditions

Res

olut

ion

at z

=5c

m (

µm

)

Vmesh (V)


Avalanche statistics

Polya : not the exact solution, but provides a simple parametrization of the avalanche size G.

Z = G / <G>


= 0 : exponential distribution

K. Fujii


Micromegas + TimePix

TimePixREADOUTREADOUT

MICROMESHMICROMESH

DRIFTDRIFT

DRIFTDRIFT

SPACESPACE

EEDD ~ 0.7 kV/cm ~ 0.7 kV/cm

EEAA ~ 80 kV/cm ~ 80 kV/cm

InGrid (Nikhef-Twente)

2.5 cm

50 µm

Fe 55 source

Chamber operated with an Ar+5% isobutane mixture

Beijing, 30/03/2010 26

1) Measure Time Over Threshold (linear with charge above 5 ke-) for single isolated pixels : direct access to avalanche charge distribution.

2) See electrons from an X-ray conversion one by one (55Fe) and count them. Efficiency vs gain sensitive to parameter

P. Colas - FJPPL-FCPPL-LCTPC Analysis meeting

threshold 3) (not repeated here, see RD51 in Crete) Measure fluctuations of primary ionization and derive gain fluctuations from energy resolution.

Gain fluctuations from Time Over Threshold

Select isolated clusters with only 1 pixel. These are single electron avalanches (~10 µ rms radius).TOT is linear with number of electrons seen by the amplifier above 5 ke- : Ne = 167 TOT – 6700(red curve, corresponding to the threshold setting of our data taking)Valid up to 30 000 electrons.

This gives a direct access to avalanche size.

Number of electrons


TOT

(in 2

8 ns

tim

e bi

ns)

10000 20000 30000

U in (Volts)


Z from Time Over Threshold distributions

Unfortunately, the tails are dominated by TOT resolution effects.

Z = G/<G> = (167*TOT-6700)/G(V)

G(V) from a measurement using a source.

Avalanche size distribution from TOT


Polya fits above Zmin = 5000/<G> (region of linearity of TOT) are goodHowever theta values are not reliable (very correlated with the gain measurement and the TOT scale. There is a discrepancy between the average number of electrons and the gain: this is a possible effect from the protection layer and from the shaping by electronics.

HV mesh Gain fitted

310 V 2900 5.3±1.3

330 V 6000 3.4±0.1

350 V 12600 4.2

We do not regard these fitted values as measurements of theta. They point to a value of ~4 but with very large systematic errors (factor of 2?)

Monte Carlo simulation


ELECTRON COUNTING

Gas : Ar+5% isobutane


In our setup, we use the Chromium K-edge to cut the K line (Center for X-Ray Optics)

Monte Carlo simulation. Shows that we need enough drift distance to separate the clusters. Also shows that the escape peak is better contained than the photopeak.


Data Vmesh = 350 V

NUMBER OF CLUSTERS


Drift distance (z) cut performed using the diffusion : sqrt(rmsx 2 + rmsy 2 ) > 28 pixels (cluster separation)Cloud center within a window around the chip center (containment)


Use escape peak (only one line, better contained)

Then correct for collection efficiency (96.5 +- 1 % from MC, in this range of field ratios : 80-90)Convert U_grid into gain/threshold (threshold = 1150 e-)


Collection efficiency from simulation : 96.5±1 %

Gain measurements (from a 80x80 mm2 copper mesh with the same gap 50 µm, gas : Ar+5% isobutane)


Prediction from R. Veenhof et al., Data (in red) from D. Attié et al.(see also D. Arrogancia et al. 2009)

2µ thickness

1µ thickness

~1 at moderate gain (few 1000). Maybe higher at gains above 5000Exponential behaviour (=0) strongly excluded, as well as


Determination of W and F


The background is totally negligible (time cut taking 30 time buckets around the electron cloud among 11000)The probability for merging two clusters is small, with the rms cuts. The probability for loosing electrons by the containment cuts is small. Attachment also is negligible.The main inefficiency comes from collection : 96.5+-1 % from simulation.

Using the escape peak:

W= 2897 eV / 120.4 = 24.06 +- 0.25 eV

This translates to 245+-3 electrons for the 5.9 keV line, larger than what is usually admitted for pure Ar (227). Photoelectric effect on the mesh is not excluded. This could also be a Penning effect in the conversion region.

The Fano factor could be derived from the rms of the escape line (6.8 e- ) but needs large corrections from inefficiencies.


Next steps

• Measure space resolution for a given length of charged track with TimePix or Octopuce data, and cross-check Ntot from Heed with data


CONCLUSIONS


InGrid, Microbulk, and TimePix are new detectors which allow to study the conversion and avalanche processes with unprecedented accuracy.

Time Over Threshold measurements give access to direct measurement of the fluctuations, provided absolute gain and TOT calibration can be better controlled.

The onset of single electron efficiency with Micromegas gain allows the exponential fluctuations to be excluded and favours Polya fluctuations with close to 1 at moderate gain and reaching a few units at gains of 10 000.

To measure Fano fluctuations will require an improved setup with a longer drift and better controled field.

Special thanks to R. Veenhof, J. Timmermans and Y. Bilevych

Thanks to

D. ATTIÉ1), M. CAMPBELL2 ), M. CHEFDEVILLE3) , E. DELAGNES1) , K. FUJII4) , I.GIOMATARIS1) , H. VAN DER GRAAF5) , X. LLOPART2) , M. LUPBERGER1) , H. SCHINDLER2) ,J. SCHMITZ6), M. TITOV1)

1) CEA/Irfu Saclay, 2) CERN, 3)LAPP Annecy, 4)KEK, 5)Nikhef, 6)Twente U.


Gain fluctuations and Neff Analysis of March 2010 data Neff measurement Gain fluctuation...

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