Overview of MCP requirements: resistive layer and SEE A. S. Tremsin, O. H. W. Siegmund Space Sciences Laboratory University of California at Berkeley Berkeley, CA LAPD collaboration meeting, October 15-16, 2009, Argonne National Laboratory

Fixed MCP properties MCP manufacturing –Specified geometry is selected –Certain MCP resistance is targeted –Good SEE emission layer –Metallization –Preliminary (simple) testing –Storage/transportation

Resistive and emission layers: preconditioning MCP manufactured and shipped First inspection and operation Gain, uniformity, hotspots Conformality to each other Preconditioning: scrubbing Real use

Resistive and emission layers: preconditioning Would be nice to have MCPs being ready for use as shipped

MCP preconditioning As manufactured MCPs require substantial preconditioning –Geometrical and resistive conformality (MCP stacks) –Outgasing (sealed tubes) –Gain stabilization (high counting rate applications) –Hot spots (can be reduced by self-scrubbing) Most of these are defined by the resistive and emissive layer properties Present technology: MCP substrate defines both geometry and functional properties (through resistive/emissive layers)

Novel MCP technology Separate substrates characteristics from the MCP operational properties –Nano-engineered films Synkera with AAO Arradiance with glass and plastic substrates LAPD collaboration Tune resistive/thermal/outgasing/lifetime properties separately Large selection of materials

Two distinct modes of MCP operation Current amplification (e.g. image intensifiers) –Low gain (<10 4 ) –Moderately to high input fluxes –Usually frame-based readouts (CCD, CMOS) –Limited dynamic range –Timing resolution is limited to readout frame rate Event counting –Moderately to high gain for single particle detection ( ) –Low input fluxes –Typical count rates 0.1 – 10 6 cps (can be as high as 10 8 with low noise readouts, e.g. Medipix) –Both spatial and temporal information on each detected event –More sensitive to gain reduction from ageing, ion feedback

Ideal electron amplifier (MCP) Substrate –No geometrical distortions –Mechanically robust in large formats –Compatible with large processing temperatures –Low outgasing/contaminating films deposited above –Small pores (ultimate limit of spatial resolution) –Cheap –Easy to manufacture Conductive film –Accurately controlled resistance in a wide range (small format MCP/ large format / large/small pores) –Thermal coefficient of resistance is positive (self regulating/avoiding thermal runaway) –Does not require high deposition temperatures –Vacuum compatible –Can be baked without changing its properties (required for tube production) –Repeatable Emissive film –High secondary electron emission coefficient (high gain, low operational voltage, smaller L/D/ number of plates –Stable under electron bombardment –Can be baked without changing its properties (required for tube production) –Low outgasing –Efficient charge replenishment –Good photoelectron sensitivity (no need for a separate photocathode) V1V1 V2V2 I strip

Ideal electron amplifier (MCP) Substrate –No geometrical distortions –Mechanically robust in large formats –Compatible with large processing temperatures –Low outgasing/contaminating films deposited above –Small pores (ultimate limit of spatial resolution) –Cheap –Easy to manufacture V1V1 V2V2 I strip

Ideal electron amplifier (MCP) Conductive film –Accurately controlled resistance in a wide range (small format MCP/ large format / large/small pores) –Thermal coefficient of resistance is positive (self regulating/avoiding thermal runaway) –Does not require high deposition temperatures –Vacuum compatible –Can be baked without changing its properties (required for tube production) –Repeatable V1V1 V2V2 I strip

Ideal electron amplifier (MCP) Emissive film –High secondary electron emission coefficient (high gain, low operational voltage, smaller L/D/ number of plates –Stable under electron bombardment –Can be baked without changing its properties (required for tube production) –Low outgasing –Efficient charge replenishment –Good photoelectron sensitivity (no need for a separate photocathode) V1V1 V2V2 I strip

Existing technology Substrate –No geometrical distortions –Small pores (ultimate limit of spatial resolution) –Mechanically robust in large formats –Compatible with high processing temperatures –Low outgassing, not contaminating films deposited above –Cheap –Easy to manufacture Conductive film –Accurately controlled resistance in a wide range (small format MCP/ large format / large/small pores) –Thermal coefficient of resistance is positive (self regulating/avoiding thermal runaway) –Does not require high deposition temperatures –Vacuum compatible –Can be baked without changing its properties (required for tube production)? –Repeatable Emissive film –High secondary electron emission coefficient (high gain, low operational voltage, smaller L/D/ number of plates) –Stable under electron bombardment –Can be baked without changing its properties (required for tube production)? –Low outgassing –Efficient charge replenishment –Good photoelectron sensitivity (no need for a separate photocathode) Definitely needs improvement Relatively good

Existing technology Substrate –No geometrical distortions –Small pores (ultimate limit of spatial resolution) –Mechanically robust in large formats –Compatible with high processing temperatures –Low outgassing, not contaminating films deposited above –Cheap –Easy to manufacture Definitely needs improvement Relatively good

Existing technology Conductive film –Accurately controlled resistance in a wide range (small format MCP/ large format / large/small pores) –Thermal coefficient of resistance is positive (self regulating/avoiding thermal runaway) –Does not require high deposition temperatures –Vacuum compatible –Can be baked without changing its properties (required for tube production)? –Repeatable Definitely needs improvement Relatively good

Existing technology Emissive film –High secondary electron emission coefficient (high gain, low operational voltage, smaller L/D/ number of plates) –Stable under electron bombardment –Can be baked without changing its properties (required for tube production)? –Low outgassing –Efficient charge replenishment –Good photoelectron sensitivity (no need for a separate photocathode) Definitely needs improvement Relatively good

–Resistance of the pore Limited number of counts per pore per second next event with the same gain can only occur after the wall charge is replenished Typical event transit time ~100 ps Typical pore resistance ~10 15  Pore current I strip ~1pA Positive wall charge builds up on the pore walls, mostly at the bottom where the amplification is the highest. Typical pore capacitance F Recharge time ~ RC = 1 ms Only portion of that charge replenishes the wall positive charge through tunneling V1V1 V2V2 I strip Pulsed operation: event counting

–Assuming 8” MCP can sustain 60 o C operation –Q Rad ~ 3.5 Watt for 20 cm MCP –V MCP ~1 kV => I Strip ~ 3.5 mA => R MCP ~286 M  (radiative heat dissipation only) –Assume we can sustain 10x lower resistance through heat conduction on the spacers - R MCP ~30 M  –20 cm diameter MCP, with 20  m pores on 24  m centers has ~63E6 pores –R Pore ~ 1.9E15 , I Pore ~ 0.5 pA –10% of strip current can be extracted as charge => I out ~ 0.05 pA/pore –Assume output charge value of 10 6 e/pulse, 10 pores involved in each pulse => 33 events/pore/s With these assumptions: typical local count rate will be limited to ~100 events/pore/s However, we observed 10x better performance locally: charge is shared by the neighboring pores (?) V1V1 V2V2 I strip Rough estimate of MCP stable resistance and local count rate

A.S. Tremsin et al., Proc. SPIE 2808 (1996) pp The ageing effect is not localized to only illuminated area

Ageing of microchannel plates Gain reduction is due to changes in the conduction/emission films and/or their interfaces

MCP gain reduction effect: ageing under irradiation Flat field image Long integration image Gain~10 5 Rate >10 MHz/cm 2 Accumulated dose ~0.01 C/cm 2 Uniform flat field illumination Normalized by initial flat field No preconditioning of the detector was performed

MCP gain reduction effect: ageing under irradiation 14 mm Uniform flat field image (neutrons) Resolution mask image Gain~10 5 Rate ~ 3 MHz/cm 2 Accumulated dose ~0.001 C/cm 2 Almost uniform flat field illuminaiton UV photons No preconditioning of the detector was performed Preconditioning is required for stable gain operation! It is always done during standard tube manufacturing process.

MCP gain reduction effect: ageing under irradiation 14 mm Uniform flat field image (neutrons) Resolution mask image Gain~10 5 Rate ~ 3 MHz/cm 2 Accumulated dose ~0.001 C/cm 2 Almost uniform flat field illuminaiton UV photons No preconditioning of the detector was performed Different applications may require completely different preconditioning procedure: Rate of scrubbing Input current Gain/voltage at the scrubbing High gain detectors are usually scrubbed at low gain to allow more uniform scrub along the pore

What has changed in conduction/emission layers? Lower gain - SEE is reduced –Is it due to change in the bulk properties of the emission layer (impurities/electron traps migration or redistribution)? –Is it surface contamination? scrubbing at different pressures should lead to different ageing curves –Changes in the interface with the conduction layer?

SEE surface of lead-glass MCPs A.M.Then, C.G. Pantano, J. Non-crystalline Solids 120(1990) 178

SEE surface of lead-glass MCPs: ageing B. Pracek, M. Kern, Appl. Surf. Sci. 70/71 (1993) 169 Concentration of K atoms (likely due to ion diffusion process) is greatly increased on the surface after ageing. Also small increase of carbon contamination was observed.

SEE surface of lead-glass MCPs: ageing A.M.Then, C.G. Pantano, J. Non-crystalline Solids 120(1990) 178

SEE surface of lead-glass MCPs: ageing A.M.Then, C.G. Pantano, J. Non-crystalline Solids 120(1990) 178

Resistance coefficient of MCP: thermal runaway –negative coefficient of resistance –poor heat dissipation in MCP detectors –certain gain required to detect individual events Limited local count rate: fixed amount of charge extracted from local area Tradeoff between gain (resolution) and local count rate V1V1 V2V2 I strip R. Colyer et al., Proc. SPIE (2009)

MCP thermal runaway A.S. Tremsin et al., Proc. SPIE 2808 (1996) pp A.S. Tremsin et al., Nucl. Instr.Meth. 379 (1996) pp

Conduction layer and thermal stability of MCPs A.S. Tremsin et al., Rev. Sci. Instr. 75 (2004) pp Need very good control of the resistance value of the conduction layer. Not only as manufactured but also through the entire tube production process.

Si MCP thermal coefficient Different manufacturing process, no lead glass, alkali metal doping; still similar value of TCR A.S. Tremsin et al., Rev. Sci. Instr. 75 (2004) pp

Thermally stable MCPs Conduction film with positive thermal coefficient: MCP self regulation increased local count rate saturation increased strip current increase Joule heat temperature increases resistance increases locally reduced heat generation MCP substrate with excellent thermal conduction and spacers will help to improve heat dissipation

Can bulk conductive substrate be an alternative to conduction layer? V1V1 V2V2 I strip V1V1 V2V2 Reduced ion feedback E ions T. W. Sinor et al., Proc. SPIE 4128 (2000) 5. Making bulk-conductive glass microchannel plates Jay J.L. Yi, Lihong Niu, Proc. SPIE 68900E-1 (2008) Much more heat will be generated as very small fraction of strip current will be used for charge replenishment

Stable conduction and emission films Both thermal coefficient  T and voltage-dependent coefficient  V of the conduction film should be very small Do not change properties under electron bombardment A stable SEE layer with low emission is better than high SEE film which changes as device operates Both increase of gain and gain reduction are equally bad. The low/high gain can be compensated by accelerating voltage

Improved interface between conduction and emission films Currently only ~10% of strip current can be extracted as output current, the rest of it is only generating extra heat Will be very good if that fraction of useful current can be increased. Pore saturation mechanism is very important.

Conduction and emission film requirements Conduction film –Accurately controlled resistance in a wide range (small format MCP/ large format / large/small pores) –Thermal coefficient of resistance is positive (self regulating/avoiding thermal runaway) or close to zero –Does not require high deposition temperatures –Vacuum compatible –Can be baked without changing its properties (required for tube production)? –Repeatable Emissive layer –High secondary electron emission coefficient (high gain, low operational voltage, smaller L/D/ number of plates) –Stable under electron bombardment –Can be baked without changing its properties (required for tube production) –Low outgassing –Efficient charge replenishment Compatible with large format MCP plates visible photocathodes tube sealing Stable Cheap Repeatable