January 10, 2008MiPGD TPC resolution and gas1 Micromegas TPC addendum on measurements P. Colas, Saclay Lectures at the TPC school, Tsinghua University, Beijing, January 7-11, 2008

January 10, 2008MiPGD TPC resolution and gas2 METHOD FOR MEASURING THE DRIFT VELOCITIES

January 10, 2008MiPGD TPC resolution and gas3 Setup for drift velocity measurements LASER

January 10, 2008MiPGD TPC resolution and gas4 Time (  s) Time (ns) tt tt Mesh signal Drift electrode signal Electron and ion drift velocities are obtained from the time they take to cross the 3mm drift gap

MiPGD TPC resolution and gas 5 January 10, 2008

MiPGD TPC resolution and gas6 X-ray source 1500 lpi micromegas 500 lpi Mesh current measurem t on the HV supply SETUP USED FOR ION BACKFLOW AND AGING STUDIES

January 10, 2008MiPGD TPC resolution and gas7 Data taken in the 4 GeV  beam at KEK in June 2005 Gas: Ar+5% isobutane ‘Multi-Prototype TPC’ (Ron Settles) to test various technologies Saclay-Orsay Micromegas endplate

January 10, 2008MiPGD TPC resolution and gas8 beam 55 Fe 384 pads, 2.3x6.3 mm2 Read out by ALEPH electronics Max drift 26 cm permanent monitoring by reading the mesh signal of a 55 Fe source JTPC online event display (D. Karlen)

January 10, 2008MiPGD TPC resolution and gas9 Drift velocity measurement Cross-check of gas purity and MC simulation Cross-check of gas purity and MC simulation Using a beam at 45 deg. Look at time distribution on one pad. Max time gives drift time over cm (add trig. delay) 1 cm scint. cathode V drift (Ar+5%iso, E=220V/cm) = cm/  s In agreement with Magboltz :

January 10, 2008MiPGD TPC resolution and gas10 Measurement of the diffusion coefficient at B=0, 0.5 and 1T Magnetic field 0 T 0.5 T 1 T Global likelihood 488 ± ± ± 7 PRF width (stat.only) 475 ± ± ± 18 Magboltz in  /√cm 2 methods: global likelihood fit of the track width to all pad charges (shown here), or width of the PRF (slope at large distance is unbiased) Good agreement between the two methods and good agreement with Magboltz.

January 10, 2008MiPGD TPC resolution and gas11 Study of the resolution (theory) At large drift distance, transverse diffusion dominates: resol ~ C D √z/√N eff N eff different from N tot because of -Ionisation fluctuations 1/ -Gain fluctuations: x / 2 At small distance, hodoscope effect: not enough charge spreading by diffusion to encompass more than 1 pad Ex: for 60 e- total, N eff =21.2±2.7

January 10, 2008MiPGD TPC resolution and gas12 Resolution measurement B=0 r.m.s. of the residuals (√  with  wo ) 2 methods for the track: global likelihood fit or  2 fit Note: bias at small z - the track is reconstructed close to the middle of the central pad (hollow points)

January 10, 2008MiPGD TPC resolution and gas13 Resolution measurement B=0.5 and 1T

January 10, 2008MiPGD TPC resolution and gas14 Scaling (use dimensionless quantities scaled by the pad width w) The resolution dependance on z has two regimes: At large z the asymptotic behaviour follows the diffusion limit At low z the effect of finite pad size dominates. For typical values of N eff, the optimal resolution is about 10% of the pad size. 1/√12 1/√(12.N eff )

January 10, 2008MiPGD TPC resolution and gas15 Extrapolation to ILC-TPC

January 10, 2008MiPGD TPC resolution and gas16 CONCLUSIONS A clear understanding of the basic limitations on the resolution of a Micromegas TPC with standard pad readout has been obtained A clear understanding of the basic limitations on the resolution of a Micromegas TPC with standard pad readout has been obtained The role of ionisation statistics, gas gain fluctuations and finite pad size have been clearly assessed, opening the way to optimization The role of ionisation statistics, gas gain fluctuations and finite pad size have been clearly assessed, opening the way to optimization A good agreement is found with beam test results A good agreement is found with beam test results For Linear Collider applications, standard 2x6mm 2 pads will not give the target resolution. Ongoing developments will be necessary: charge spreading or pixel readout For Linear Collider applications, standard 2x6mm 2 pads will not give the target resolution. Ongoing developments will be necessary: charge spreading or pixel readout