1. Development of high resolution Micro-Pattern Gas Detectors (MPGD) with wide readout pads Madhu Dixit TRIUMF & Carleton University Saha Institute for Nuclear Physics Kolkata, 22 January 2010

2. 22 January 2010 Madhu Dixit 2 Development of High Resolution Macro Micro-Pattern Gas Detectors? with wide readout pads

3. 22 January 2010 Madhu Dixit Charge track measurement - Some Preliminaries 1) Primary ionzation in drift or conversion gap P(n) = n e - /n! = average no. primary clusters 3) Electrons drift with Gaussian diffusion Towards gas amplification stage track 2) Number of electrons per cluster fluctuates (cluster size distribution) Fischle et al (A301, 202 (1991) Gain stage 3

4. 22 January 2010 Madhu Dixit MPGDs need narrow pads for good resolution 2-4 mm conversion gap No. primary clusters ≈ 3-4 But resolution usually better due to small track angles, delta rays, diffusion e.g. COMPASS GEMs & Micromegas; w = 400  m:  x  70  m Conversion gap For a 90° track & all clusters aligned & correlated, the resolution would be box like: 4

5. 22 January 2010 Madhu Dixit 5 Without diffusion, imprecise centroid for wide pads Micro Pattern Gas Detector Proportional wire Anode padsCathode pads Induced cathode signal determined by geometry Direct signal on the MPGD anode pad width w Precise centroid determination possible with wide pads Pad width limits the MPGD-TPC resolution Not so for the proportional wire/cathode pad readout However, unavoidable ExB effects limit wire/pad TPC resolution

6. MPGDs in charged particle tracking applications 22 January 2010 Madhu Dixit Many reasons to use MPGDs rather than wire/pad system But, MPGDs need sub-mm pads for < 100 μm resolution Large high resolution (~100 μm ) MPGD systems proposed for many new applications For too many channels, $$, rad. length, electronics heat load!! ILC TPC, 1.5 M channels, even with 2 mm x 6 mm pads T2K TPC: 100,000 channels, even with 7 mm x 9 mm pads to get only ~ 500 μm Super LHC ATLAS Micromegas muon chambers will have to cover several hundred square meters Can one achieve high resolution with wide MPGDs pads? 6

7. The classical TPC could do it with geometry! 22 January 2010 Madhu Dixit ExB cancels track angle effect TPC wire/pad readout 100 µm But for the ExB effect, the conventional proportional wire TPC could achieve excellent resolution (~7 mm pads for ALEPH TPC at LEP) 7

8. 22 January 2010 Madhu Dixit 8 Disperse track charge after gas gain to improve centroid determination with wide pads. For the GEM, large transverse diffusion in the high E-field field in transfer and induction gaps provides a natural mechanism to disperse the cluster charge. Measurements with prototype GEM-TPCs indicate that ~ 1 mm wide pads may be needed for TPC operation in high magnetic fields. Explore other concepts to disperse the charge Improving MPGD TPC resolution without resorting to narrower pads Charge dispersion - a geometrical pad signal induction mechanism making position sensing insensitive to pad width.

9. Finding the avalanche position on a proportional wire 22 January 2010Madhu Dixit Charge division on a proportional wire Solution for charge density (L ~ 0) Telegraph equation Deposit point charge at t=0 Telegraph equation 2-D generalization Generalize charge division to charge dispersion in 2D Solution for charge density in 2-D Finding the avalanche location on a MPGD resistive anode surfaceAvalanche Amp 1 Amp 2 x1x1x1x1 x1x1x1x1 A geometrical way to use wide MPGD pads with precision 9

10. Charge dispersion in a MPGD with a resistive anode 22 January 2010 Modified GEM anode with a high resistivity film bonded to a readout plane with an insulating spacer. 2-dimensional continuous RC network Point charge at r = 0 & t = 0 disperses with time. Time dependent anode charge density sampled by readout pads. Equation for surface charge density function on the 2-dim. continuous RC network:  (r,t) integral over pads  (r) Q r / mm mmns

11. Equivalent circuit for currents in a MPGD with an intermediate resistive anode 22 January 2010 Madhu Dixit Resistive anode foil Signal pickup pads Current generators Pad amplifier 11

12. Simulating the charge dispersion phenomenon 22 January 2010 Madhu Dixit The charge dispersion equation describe the time evolution of a point like charge deposited on the MPGD resistive anode at t = 0. A full simulation done including the effects of: Longitudinal & transverse diffusion in the gas. Intrinsic rise time T rise of the detector charge pulse. The effect of preamplifier rise and fall times t r & t f. And for particle tracks, the effects of primary ionization clustering. 12

13. 22 January 2010 Madhu Dixit 13 x high, x low, y high, y low define pad boundaries & Charge density function for a point charge: The simulation for a single charge cluster Physics effects included in simulation in two parts: 1) as effects which depend on spatial coordinates x & y, or; 2) as effects which depend on time. 1) The spatial effects function includes charge dispersion phenomena & transverse size w of the charge cluster due to transverse diffusion. Q pad (t) is the pad signal from charge dispersion when a charge Nq e of size w is deposited on the anode at t = 0; (1)

14. 22 January 2010 Madhu Dixit 14 I(t) includes rise time, longitudinal diffusion & charge preamplifier fall time t f &rise time t r as time effects. (2) (1) and (2) are convoluted numerically for the model simulation.

15. 22 January 2010 Madhu Dixit 15 Proof of concept tests with a collimated point x ray source Point source ~ 50 µm collimated 4.5 keV x rays. Aleph TPC preamps.  Rise = 40 ns,  Fall = 2  s. DAQ - 500 MHz Tektronix digital scope.

16. 22 January 2010 Madhu Dixit 16 GEM pad response function (PRF) for a point source Simulation compared to the measured PRF Measured PRF deviates from simulation due to RC nonuniformities. (Solid line) Pad 23 Pad 24 Pad 22 Scan across width Ionization from 50  m collimated x-ray spot. 2x6 mm 2 pads PRF – is a measure of signal amplitude as a function of cluster position. (Ar+10%CO2)

17. 22 January 2010 Madhu Dixit 17 Charge dispersion signal for a GEM Simulation versus measurement (Ar+10%CO2) (2 x 6 mm 2 pads) Collimated ~ 50  m 4.5 keV x-ray spot on pad centre. Simulated primary pulse is normalized to the data. Difference = induced signal (not included in simulation) studied previously:MPGD '99 (Orsay) Primary pulse normalization used for the simulated secondary pulse

18. GEM TPC charge dispersion simulation (B=0) Cosmic ray track, Z = 67 mm Ar+10%CO 2 22 January 2010 Madhu Dixit Centre pulse used for normalization - no other free parameters. 2x6 mm 2 pads Simulation Data 18

19. Tracking with the charge dispersion signal 22 January 2010 Madhu Dixit Unusual highly variable charge pulse shape. Pulses on charge collecting pads: Large pulses with fast rise-time. The decay time depends on the system RC, the pad size & the initial charge cluster location. Pulses on other pads: Smaller pulse heights & slow rise & decay times determined by the system RC & pad location with respect to the track. Both rise time and pulse height contain position information How to define an “amplitude” which uses pulse shape information? 19

20. 22 January 2010 Madhu Dixit 20 The pad response function (PRF) for a track The PRF is a measure of pad signal “amplitude” as a function of track position relative to the pad. We use pulse shape information to optimize the PRF. The PRF can, in principle, be determined from simulation. However, system RC nonuniformities & geometrical effects introduce distort the PRF and introduce bias in position measurement. The position bias can be corrected by calibration. PRF and bias determined empirically using a subset of data which was used for calibration. The remaining data used for resolution studies.

21. Track pad response function (PRF) 22 January 2010 Madhu Dixit A knowledge of PRF enables one to deduce track parameters from observed pulses A knowledge of PRF enables one to deduce track parameters from observed pulses Our first ideas to compute the PRF “amplitude” – integrate pad pulses over variable width windows following a recipe. 21

22. 22 January 2010 Madhu Dixit 22 a 2 a 4 b 2 & b 4 can be written down in terms of  and  & two scale parameters a & b. Track PRFs with GEM & Micromegas readout The PRFs are not Gaussian. The PRF depends on track position relative to the pad:PRF = PRF(x,z) PRF can be characterized by its FWHM  (z) & base width  (z). PRFs determined from the data have been fitted to a functional form consisting of a ratio of two symmetric 4th order polynomials.

23. The pad response function (PRF) measures pad signal amplitude as a function of track position 22 January 2010 Madhu Dixit relative amplitude 15.7 cm 6 < z < 7cm 8 < z < 9cm 10< z < 11cm 12 < z < 13cm 14 < z < 15cm 4 < z < 5cm 2 < z < 3cm 0 < z < 1cm x pad -x track (mm) TPC Ar/iC4H10 95/5 23

24. 22 January 2010 Madhu Dixit 24

25. COSMo TPC cosmic ray tests at 5 T Nov-Dec 2006 22 January 2010 Madhu Dixit 25

26. B = 5 Tesla Cosmic ray tests at DESY Transverse diffusion = 19 microns/sqrt(cm) 22 January 2010 Madhu Dixit 26

27. Bias correction 22 January 2010 Madhu Dixit 27

28. An better way to analyze charge dispersion data The original PRF “amplitude” dependent on TPC operational parameters Develop a standard way not requiring fine tuning Tested several new ideas with simulated data Apply and test new algorithm to reanalyze DESY 5 T magnetic field results. Criteria: PRF can be applied consistently and easily over a wide range of TPC operating conditions. Observed resolution function is Gaussian. New Measured resolution is as good or better then obtained previously A simple fixed window integration works the best! 22 January 2010 Madhu Dixit 28

29. Resolution comparison: Old Method Vs average(700ns) 22 January 2010 Madhu Dixit New method New method 5663/17669 Flat 40 µ m Resolution Independent of z. Old Method Flat 50 µm Resolution Old Method 3652/17669 Flat 50 µm Resolution Independent of z over 15 cm. 29

30. Micromegas with a resistive anode gain with charge dispersion 22 January 2010 Madhu Dixit Argon/Isobutane 90/10 Resistive anode suppresses sparking & improves Micromegas HV stability 30

31. Summary 22 January 2010 Madhu Dixit 31 Charge dispersion makes MPGD position sensing independent of pad width and high resolution can be achieved independent of pad width: At 5 T, an unprecedented ~ 40  m resolution for 2 x 6 mm 2 pads for drift distances up to 15 cm. With the charge dispersion, large TPCs like T2K could achieve better resolution with existing channel counts Channel counts could be reduced for large high resolution systems such as Micromegas muon chambers proposed for ATLAS at SLHC Significant reduction of detector cost and complexity possible with charge dispersion. 31