Imaging Well Detectors: a New Gas Electron Tracker for Compton Telescopes Phil DeinesJones, NASAGSFC

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Imaging Well Detectorsa New Gas Electron
Tracker for Compton Telescopes?Phil
Deines-Jones, NASA/GSFC

• What are well detectors?
• How do we make them?
• Applications
• X-ray imaging
• 2-D electron tracking
• 3-D electron tracking

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Development of the Well Detector

• Successful implementation by several groups
• Bartol, et al. 96, Chaplier, et al. 99 (1d,
  drilled pcb material)
• Bellazinni, et al. 99 (1d,etched polyimide)
• Deines-Jones et al. 00 (2d, UV laser ablated
  polyimide)
• Imaging results presented last November

Bartol, F. et al., J. Physics III, France, 6, 337
(1996) Bellazzini, R., et al., NIM A423, 125
(1999) Chaplier, G. et al., NIM A426, 339
(1999) Deines-Jones, P. et al., to be published,
NIM A
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Features of Well Detectors
New type of gas proportional counter developed by
several groups

• Naturally pixelized, genuine imaging
• Inexpensive to manufacture in large area
• Rugged - large plated electrodes on polymer
  substrates
• Stable even at very high gas gains
• Natural Time Expansion Chamber (TEC) drift field
  independent of gas gain

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Well Detector Concept
400 ?m
Simulation of a single electron avalanche in a
well.
An array of wells forms a well detector.
A close relative of the Gas Electron Multiplier
(GEM)
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Well Detector Manufacturing at LHEA

• Anodes and cathodes are patterned on
  acrylic-coated polyimide at a commercial flexible
  printed circuit shop.
• Wells are then drilled at Goddard by UV laser
  ablation.
• LHEA machining innovations give high yield
• Machine-vision alignment
• Funnel-shaped well
• Machining in oxygen flow

Cross-section of a funnel well
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Applications Readout Schemes

• X-ray imaging ? crossed-strip
• Large-area 2-D electron tracking ? multiplexed
  pixel readout
• 3-D electron tracking ? crossed strip with TDCs

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X-ray Imaging
Imaging achieved with crossed-strip electrodes

• Well X-ray imagers are proposed for LOBSTER-ISS
  focal plane detector
• Requires large area (20 cm), low cost, low power
• ESA Space Station accommodation study under way
• Two-year development track to demonstrate flight
  capabilities

5-cm detector in test fixture with
charge-division readout.
Well Array
X-ray window/ Drift Electrode
Bias Electronics
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X-ray Imager Performance

• Measured Position Resolution
• 285 mm fwhm (400 micron pitch)
• Energy Resolution
• DE/E (fwhm) 50 /

Fe55 spectrum
X-ray image (right) of shadow mask (left) with
750 micron slits on a 2mm pitch. Note that the
image resolution is equal in each dimension.
Stable operation at high gas gain -- at moderate
voltage
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Electron trackers 2-D pixelized readout

• Multiplexing with active matrix backplane
• Very low quiescent power?very large area
• Images 2-D track projection
• 3rd dimension from orthogonal detectors
  downstream (stereo imaging) and/or sparse TPC
• NGHEG, long-range GRAPWG mission

Lithographically fabricated well on top of
thin-film transistor (TFT)
Multiplexing concept
2-D tracking with TFT readout
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Electron trackers 3-D readout

• Time Expansion Chambers (TEC)
• z coordinate from timing
• In well detectors, drift field and gain are
  independently adjustable drift speed can be 3
  mm/?s in Xe/CO2
• Modest time resolution requirements 100 ns time
  buckets for 100 ?s rms spatial resolution
• x-y imaging done with crossed-strip readout,
  like our X-ray imagers

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ACT Tracker Idea

• 20 1m x 1m TEC panels, 2.5 atm-m Xe
• 3º electron direction for 1 MeV ?-ray
• 100 ?m (rms) vertex determination
• 2 energy resolution for 0.5 MeV e
• 10 interaction probablility
• Effective range of 2 MeV e 35 cm

We welcome guidance, suggestions, criticism, help.
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X-ray Imager Performance (cont.)
X-ray image of shadow mask with 100 micron slits
on a 2mm pitch.
Stable operation at gas gain 30,000 -- at
moderate voltage
Pulse-height spectrum of Fe55 X-rays.
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TEC Power
Case study Silicon Vertex Tracker (SVT) front
end on STAR
SVT Well Tracker Idea MIP input (mean)
24,000e 150,000e (at gas gain5000) Peaking
time 30 ns 100 ns (leading edge) - 1000 ns
(zero-crossing) Analog acquisition rate 40
MHz 10 MHz time buckets 256 256 Power
per channel 15 mW ?
Add 2 mW for Switched Capacitor Array (SCA)
Total 17 mW worst case Total power for 20 panels,
400 ?m pitch 1700 W

• Power reduction strategies
• Slow the acquisition rate from 40 MHz to 10 MHz
• Power strobing (use calorimeter for trigger)
• Optimize design -- zero-crossing or constant
  fraction timing
• Dont implement fast timing on every channel

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Detector in test fixture without drift electrode.
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Imaging Well Detector Electrodes
Copper sputtered platinum
Sputtered platinum
Well cathodes
Well anodes
Laser-cut trace in sputtered platinum
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Alignment is computer adjusted to match template
image (in red).
Machine-vision control software continually
adjusts alignment during machining.
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Continuing Well Developmentfor Lobster

• Window development and testing
• Life-time tests
• Lobster-sized detector with flight-like readout
  system
• Continued work on understanding and optimizing
  performance
• New funding opportunities SRT, Explorer
  Technology