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some thoughts on charging-up effects HvdG, Nikhef RD51 Workshop, Paris, 2008

some thoughts on charging-up effects HvdG, Nikhef RD51 Workshop, Paris, 2008. Micro Strip Gas Counters: hard to operate: discharges, ruining electrodes ageing ! Very strong electric field in insulator’s volume & surface !. GEMs: often cascade of 3 GEMs used to limit gain per GEM to ~40

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some thoughts on charging-up effects HvdG, Nikhef RD51 Workshop, Paris, 2008

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  1. some thoughts on charging-up effects HvdG, Nikhef RD51 Workshop, Paris, 2008

  2. Micro Strip Gas Counters: hard to operate: • discharges, ruining electrodes • ageing • ! Very strong electric field in insulator’s volume & surface !

  3. GEMs: • often cascade of 3 GEMs used to limit gain per GEM to ~40 • ‘rim’ (dia hole Cu/kapton) critical • shape of hole wall critical • charge up effects

  4. Overview of MPGD development in JAPAN Atsuhiko OCHI Kobe University, JAPAN 13 Oct. 2008 2nd RD51 workshop in Paris

  5. x750 Recent Status of Development GEM Production RIKEN/SciEnergy GEM (thick-foil and fine-pitch) pitch 80um hole 40um thickness 100um Remove copper by wet etching 100um 80um Irradiate CO2 laser Remove remaining edge from the other side

  6. Fe-55 spectrum 5.9keV Recent Status of Development Gain Curve (RIKEN GEM) • GEM test setup and parameters • Thick-foil and fine-pitch GEM • (single layer) • HV supplied through a resister chain • Ed=2.5kV/cm, Ei=4~5kV/cm, ⊿VGEM=300~600V • Gas: Ar+CO2(30%) flow • Readout by 1cm x 1cm pad • Gain measurement • Gain vs applied voltage • X-ray from Fe-55 (5.9keV) gain=3x104 To keep good spatial resolution and keep discharge point at high gain. Our GEM is most suitable for Cosmic X-ray Polarimeters.

  7. Recent Status of Development Gain instability (RIKEN GEM) No increase and decrease just after HV on. 3 hours relative gain • No gain increase in short measurements • This is not for a special batch of GEMs but for all GEMs we • produced • Possible reasons; • Less charging-up due to cylindrical hole shape • Less polarization of Liquid Crystal Polymer F. Simon (IEEE, 2006) T. Tamagawa(IEEE,2007) time (s)

  8. GemGrid 1 GemGrid 2

  9. Bulk high-resistivity materials hydregenated amorphous silicon Si rich silicon nitride (Si3N4)

  10. Measurements on Si-rich Silicon Nitride (Si3N4) Column resistance: ρ D/O Potential surface measurable: gain drop factor 2 at dV = 17.5 V With known current: bulk resistivity ρ measurable: ~ 1 – 50 1013 Ohm cm Surface time constant: Column resistance x (virtual colum capacitance) = (ρ D/O)*(ε O/D) = ρ ε(independant of layer thickness D!)

  11. Resistive Plate Chambers (RPCs) • essential: high-resistivity material • - quenches sparks • - sufficient charge compensation current • Traditional: insulator + dope (Sardinian oils…?) • New: high-resistivity bulk (ceramic) material: higher counting rates • Compare graphite covered mylar foil

  12. conductivity of kapton Micromegas on pillars Edge discharge protection foil discharges + vibrations

  13. Slow increase of grid voltage until good single electron efficiency • Vg = -400 V Gain ~ 8000 • Nice cosmic & alpha tracks recorded in TOT mode • Measure of drift time with the time mode • Triggered setup (3 scintillators and lead plates) • Fill individual frames with 1000 short triggered acquisitions

  14. Charge-up effects • After (rapid) ramping of HV: • polarisation: reduction of E-field in insulator (bulk) volume • In homogeneous field with insulator // to field: nothing With E component perp. on insulating surface: modification of potential by hitting e- and/or ions until E // surface

  15. GEM hole

  16. equalizing with water Stronger effects for good insulator region of worse insulation

  17. Very preliminary: Use as little as possible insulating surfaces // strong E fields Even more preliminary: As for gain: GEMs perform les than (corresponding) Micromegas

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