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Micromegas TPC addendum on measurements

CCAST. Micromegas TPC addendum on measurements. P. Colas, Saclay Lectures at the TPC school, Tsinghua University, Beijing, January 7-11, 2008. METHOD FOR MEASURING THE DRIFT VELOCITIES. Setup for drift velocity measurements. LASER.

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Micromegas TPC addendum on measurements

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  1. CCAST Micromegas TPCaddendum on measurements P. Colas, Saclay Lectures at the TPC school, Tsinghua University, Beijing, January 7-11, 2008 MiPGD TPC resolution and gas

  2. METHOD FOR MEASURING THE DRIFT VELOCITIES MiPGD TPC resolution and gas

  3. Setup for drift velocity measurements LASER MiPGD TPC resolution and gas

  4. Electron and ion drift velocities are obtained from the time they take to cross the 3mm drift gap Mesh signal Drift electrode signal Dt Dt Time (ns) Time (ms) MiPGD TPC resolution and gas

  5. MiPGD TPC resolution and gas

  6. SETUP USED FOR ION BACKFLOW AND AGING STUDIES X-ray source Mesh current measuremt on the HV supply 1500 lpi micromegas 500 lpi MiPGD TPC resolution and gas

  7. Data taken in the 4 GeV p beam at KEK in June 2005 Gas: Ar+5% isobutane ‘Multi-Prototype TPC’ (Ron Settles) to test various technologies Saclay-Orsay Micromegas endplate MiPGD TPC resolution and gas

  8. 384 pads, 2.3x6.3 mm2 Read out by ALEPH electronics Max drift 26 cm permanent monitoring by reading the mesh signal of a 55Fe source JTPC online event display (D. Karlen) 55Fe beam MiPGD TPC resolution and gas

  9. Drift velocity measurement Using a beam at 45 deg. Look at time distribution on one pad. Max time gives drift time over 26.08+-0.02 cm (add trig. delay) • Cross-check of gas purity and MC simulation 1 cm scint. cathode Vdrift (Ar+5%iso, E=220V/cm) = 4.181 +- 0.034 cm/ms In agreement with Magboltz : 4.173 +- 0.016 MiPGD TPC resolution and gas

  10. Measurement of the diffusion coefficient at B=0, 0.5 and 1T 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) in m/√cm Good agreement between the two methods and good agreement with Magboltz. MiPGD TPC resolution and gas

  11. Study of the resolution (theory) • At large drift distance, transverse diffusion dominates: resol ~ CD√z/√Neff • Neff different from Ntot because of • Ionisation fluctuations 1/<1/N> • Gain fluctuations: x <G2>/<G>2 • At small distance, hodoscope effect: not enough charge spreading by diffusion to encompass more than 1 pad Ex: for 60 e- total, Neff=21.2±2.7 MiPGD TPC resolution and gas

  12. Resolution measurement B=0 r.m.s. of the residuals (√swithswo) 2 methods for the track: global likelihood fit or c2 fit Note: bias at small z - the track is reconstructed close to the middle of the central pad (hollow points) MiPGD TPC resolution and gas

  13. Resolution measurement B=0.5 and 1T MiPGD TPC resolution and gas

  14. Scaling 1/√12 (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 Neff, the optimal resolution is about 10% of the pad size. 1/√(12.Neff) MiPGD TPC resolution and gas

  15. Extrapolation to ILC-TPC MiPGD TPC resolution and gas

  16. CONCLUSIONS • 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 • A good agreement is found with beam test results • For Linear Collider applications, standard 2x6mm2 pads will not give the target resolution. Ongoing developments will be necessary: charge spreading or pixel readout MiPGD TPC resolution and gas

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