some thoughts on charging-up effects HvdG, Nikhef RD51 Workshop, - PowerPoint PPT Presentation

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

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

GEMs: - often cascade of 3 GEMs used to limit gain per GEM to ~40 - ‘rim’ (dia hole Cu/kapton) critical ‘ i ’ (di h l C /k ) i i l - shape of hole wall critical - charge up effects

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

Recent Recent Status of Status of Development Development GEM Production GEM Production RIKEN/SciEnergy GEM 100um (thick-foil and fine-pitch) pitch 80um i h 80 80um hole 40um x750 thickness 100um thickness 100um Remove copper by wet etching Irradiate CO 2 laser Remove remaining edge from the other side from the other side

Recent Recent Status of Status of Development Development Gain Curve (RIKEN GEM) Gain Curve (RIKEN GEM) ( ( ) ) GEM test setup and parameters – Thick-foil and fine-pitch GEM (single layer) – HV supplied through a resister chain chain Fe-55 spectrum – Ed=2.5kV/cm, Ei=4~5kV/cm, ⊿ V GEM =300~600V 5.9keV – Gas: Ar+CO 2 (30%) flow – Readout by 1cm x 1cm pad Gain measurement gain=3x10 4 – Gain vs applied voltage – X-ray from Fe-55 (5 9keV) – X-ray from Fe-55 (5.9keV) To keep good spatial resolution and keep discharge point at high gain. O ur GEM is most suitable for Cosmic X-ray Polarimeters.

Recent Recent Status of Status of Development Development Gain instability (RIKEN GEM) Gain instability (RIKEN GEM) y ( y ( ) ) No increase and decrease just after HV on. ve gain relativ 3 hours F. Simon (IEEE, 2006) T. Tamagawa(IEEE,2007) time (s) � No gain increase in short measurements � This is not for a special batch of GEMs but for all GEMs we 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

GemGrid 1 G GemGrid 2 G

Bulk high-resistivity materials hydregenated amorphous silicon Si rich silicon nitride (Si 3 N 4 ) 3 4

Measurements on Si-rich Silicon Nitride (Si 3 N 4 ) C l Column resistance: ρ D/O i t D/O Potential surface measurable: gain drop factor 2 at dV = 17.5 V With known current: bulk resistivity ρ measurable: ~ 1 – 50 10 13 Ohm cm Surface time constant: Column resistance x (virtual colum capacitance) = ( ρ D/O)*( ε O/D) = ρ ε ( ρ D/O) ( ε O/D) = ρ ε (independant of layer thickness D!) (independant of layer thickness D!)

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

conductivity of kapton Micromegas on pillars Ed Edge discharge protection foil di h t ti f il discharges + vibrations

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

Charge-up effects After (rapid) ramping of HV: - polarisation: reduction of E-field in insulator (bulk) volume 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

GEM hole

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

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