Transcript ppt

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
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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
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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)
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Low gain (<104)
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 (105-106)
– Low input fluxes
– Typical count rates 0.1 – 106 cps
MCP
Pair
(can be as high as 108 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
Charge distribution on strips
Charge cloud
Ideal electron amplifier (MCP)
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Substrate
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Conductive film
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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
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
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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)
V1
Istrip
V2
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
V1
Istrip
V2
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
V1
Istrip
V2
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)
V1
Istrip
V2
Existing technology
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Substrate
– No geometrical distortions
Definitely needs
– Small pores (ultimate limit of spatial resolution)
improvement
– 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)?
Relatively
– Repeatable
good
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)
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
Definitely needs
– Accurately controlled resistance in a wide range improvement
(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
Relatively
good
– Can be baked without changing its properties
(required for tube production)?
– Repeatable
Existing technology
• Emissive film
Definitely needs
improvement
– 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
Relatively
good
– Efficient charge replenishment
– Good photoelectron sensitivity (no need for a
separate photocathode)
Pulsed operation: event counting
– Resistance of the pore
V1
Istrip
• 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 ~1015 
Pore current Istrip ~1pA
Positive wall charge builds up on the pore walls,
mostly at the bottom where the amplification is the highest.
Typical pore capacitance 10-18 F
Recharge time ~ RC = 1 ms
V2
Only portion of that charge replenishes the
wall positive charge through tunneling
Rough estimate of MCP stable resistance and local count rate
– Assuming 8” MCP can sustain 60oC operation
V1
Istrip
V2
– QRad ~ 3.5 Watt for 20 cm MCP
– VMCP~1 kV => IStrip ~ 3.5 mA => RMCP~286 M
(radiative heat dissipation only)
– Assume we can sustain 10x lower resistance through
heat conduction on the spacers - RMCP~30 M
– 20 cm diameter MCP, with 20 m pores on 24 m
centers has ~63E6 pores
– RPore ~ 1.9E15 , IPore ~ 0.5 pA
– 10% of strip current can be extracted as charge =>
Iout ~ 0.05 pA/pore
– Assume output charge value of 106e/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 (?)
The ageing effect is not localized to only
illuminated area
A.S. Tremsin et al., Proc. SPIE 2808 (1996) pp.86-97.
• 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~105
Rate >10 MHz/cm2
Accumulated dose
~0.01 C/cm2
Uniform
flat field illumination
No preconditioning of the detector was performed
Normalized by initial
flat field
MCP gain reduction effect: ageing under irradiation
Preconditioning is required for stable gain operation!
It is always done during standard tube manufacturing process.
14 mm
Uniform flat field
image (neutrons)
Resolution mask image
Gain~105
Rate ~ 3 MHz/cm2
Accumulated dose
~0.001 C/cm2
No preconditioning of the detector was performed
Almost uniform
flat field illuminaiton
UV photons
MCP gain reduction effect: ageing under irradiation
Different applications may require
completely different preconditioning procedure:
14 mm
Uniform flat field
image (neutrons)
Rate of scrubbing
Input
current
Resolution
mask
image
Almost uniform
5 the scrubbing
Gain/voltage
Gain~10at
flat field illuminaiton
Rate ~ 3 MHz/cm2
Accumulated dose
detectors
are2 usually
~0.001 C/cm
UV photons
High gain
scrubbed at
low gain to allow more uniform scrub along the pore
No preconditioning of the detector was performed
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
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.
B. Pracek, M. Kern, Appl. Surf. Sci. 70/71 (1993) 169
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
V1
– negative coefficient of resistance
– poor heat dissipation in MCP detectors
Istrip
V2
– 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
R. Colyer et al., Proc.
SPIE 7185-27 (2009)
MCP thermal runaway
A.S. Tremsin et al., Proc. SPIE 2808 (1996) pp.86-97.
A.S. Tremsin et al., Nucl. Instr.Meth. 379 (1996) pp.139-151.
Conduction layer and thermal stability of MCPs
A.S. Tremsin et al., Rev. Sci. Instr. 75 (2004) pp.1068-1072
• 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
1.E+11
Resistance (Ohms)
R@100V
R@200V
R@300V
1.E+10
1.E+09
1.E+08
-20
0
20
40
T(C)
R  R0 (1   T (T  T0 ))
 T  0.036C 1
A.S. Tremsin et al., Rev. Sci. Instr. 75 (2004) pp.1068-1072
Can bulk conductive substrate be an alternative to
conduction layer?
Reduced ion feedback
V1
V1
E
ions
Istrip
V2
Istrip
V2
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
•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
1
0.9
Si MCP Scrub
0.8
Relative MCP gain
•Both thermal coefficient T and voltage-dependent
coefficient V of the conduction film should be very
small
(corrected for the
input flux variation)
0.7
0.6
0.5
0.4
Scrubbing interruption
points
0.3
0.2
0.1
0
0
•Both increase of gain and gain reduction are equally
bad. The low/high gain can be compensated by
accelerating voltage
0.005
0.01
0.015
0.02
2
Charge extracted (C/cm )
0.025
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
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•
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