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

‘ i ’ (di h l C /k ) i i l

• ‘rim’ (dia hole Cu/kapton) critical
• 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 2nd RD51 workshop in Paris

GEM Production GEM Production

Recent Recent Status of Status of Development Development

100um RIKEN/SciEnergy GEM (thick-foil and fine-pitch) i h 80

x750

80um pitch 80um hole 40um thickness 100um

Remove copper

thickness 100um

by wet etching Irradiate CO2 laser Remove remaining edge from the other side from the other side

Gain Curve (RIKEN GEM) Gain Curve (RIKEN GEM)

Recent Recent Status of Status of Development Development

GEM test setup and parameters – Thick-foil and fine-pitch GEM

( ) ( )

(single layer) – HV supplied through a resister chain chain – Ed=2.5kV/cm, Ei=4~5kV/cm, ⊿VGEM=300~600V Fe-55 spectrum

5.9keV

– 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

– X-ray from Fe-55 (5.9keV)

To keep good spatial resolution and keep discharge point at high gain.

Our GEM is most suitable for Cosmic X-ray

Polarimeters.

Gain instability (RIKEN GEM) Gain instability (RIKEN GEM)

Recent Recent Status of Status of Development Development

y ( ) y ( )

No increase and decrease just after HV on.

ve gain relativ

• F. Simon (IEEE, 2006)
• T. Tamagawa(IEEE,2007)

3 hours

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 G GemGrid 2

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

3 4

Measurements on Si-rich Silicon Nitride (Si3N4) C l i t D/O 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!) (ρ D/O) (ε O/D) = ρ ε (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 di h t ti f il Edge discharge protection foil 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)

ill i di id l f i h 1000 h i d i i i ill i di id l f i h 1000 h i d i i i

Fill individual frames with 1000 short triggered acquisitions

Fill individual frames with 1000 short triggered acquisitions

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

• f potential by hitting e- and/or ions until E // surface

GEM hole

equalizing with water region of Stronger effects for good insulator 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