Detector Technologies for WSO

1
Detector Technologiesfor WSO

• Jon Lapington
• Space Research Centre
• University of Leicester

2
Outline

• Choice of detector MCPs or CCDs?
• MCP detectors
• Photocathodes
• Microchannel plates
• Image readout devices
• The Vernier Anode
• Image Charge technique
• Readout developments

3
CCD Option

• Detectors of choice in optical and X-ray
  applications
• High QEs 80 achievable
• High performance down to 200nm e.g. WFC3
• QE 60 _at_ 250nm
• read noise 3 e-
• Dark current 1 e-/hr _at_ -80C

4
CCDs a possibility?

• Pros
• Ubiquitous
• Monolithic
• No HV required
• Fixed pixel imaging
• High Spatial resolution
• High local/global count rate

• Cons
• Low QE 100-200nm
• Not photon counting
• Dark noise limits SNR
• Cooling
• Long integrations
• Accurate pointing
• Format limitations
• Radiation damage

5
CCD Quantum Efficiency
WFC-3 E2v CCD
GOES E2V CCD64 device
EVE - SDO
6
MCPs preferred

• Pros
• True photon counting
• Flexible format
• Mature technology
• High spatial resolution
• High temporal resolution
• QE 30 - 40 for LSS ?
• Low background
• No cooling
• Radiation hard

• Cons
• HV required
• Vacuum/hermetically sealed pre-launch
• Contamination sensitive
• Ageing gain depression
• Over-bright shutdown
• Local count rate limitation

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MCP detector overview

• Detection
• Bare MCP ions, electrons neutrons
• Photocathode photons
• Window 1200 to 120 nm
• Windowless 200 nm to 10 keV
• Amplification
• 1/2/3 MCP stack
• Gain up to 108 e-
• MCP pore ø down to 2µm
• Pulse risetime down to 80 ps
• Image readout
• Electronic
• Resistive anode
• Wedge and strip, TWA, Vernier anode
• CODACON, MAMA
• Delay line
• Parallel strip readout (cross strip, etc.)
• Hybrid electronic
• EBCCD, MediPix2, Timepix

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Photocathodes

• Event detection via photoelectron released from a
  photocathode
• Windowed - above 120 nm
• Semi-transparent photocathode
• Alkali halide, bi-alkali, multi-alkali S20, GaAs
  (NEA)
• QE up to 25-30
• Windowless - below 250 nm
• opaque photocathode deposited directly on MCP
• CsI, KBr, CsTe, (GaN), (Diamond) etc
• Alkali halides up to 50 in XUV
• GaN 71 reported
• Response up to 10 keV
• Poor energy resolution in X-rays

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FUV photocathodes

• All window cut off below 120 nm
• Windowless detector necessary
• Typically 15000Å CsI, KBr deposited on MCP
• Hermetic/vacuum enclosure pre-launch
• Mechanical, on orbit, one-shot door
• Web photoelectrons - resolution/QE trade-off
• Optimal QE not always achieved historically
• MCP manufacturing variability

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MCP characteristics

• Gain
• Typically 1-5 pC for high resolution electronic
  readouts
• Format
• Chevron or Z stack
• Double or triple thickness
• Noise
• Low noise lt0.1 cm-2 s-1
• Lifetime
• Gain plateau
• 0.1C cm-2 to 1C cm-2 1012ct cm-2

• Spatial resolution
• Fundamentally limited by MCP pore geometry
• Pore diameters 2 µm
• LSS format 6µm pore Ø
• Count rate
• Global rate limited by MCP strip current
• Point source rate lt 1000 ct s-1

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Advantages of MCPs for LSS

• Curved focal plane detector
• Slumped manufacture
• Ground and etched
• Large, flexible format
• Proven technology
• QE of 40 possible at FUV
• Curved image readouts possible

12
Image readout design

• Performance conflicts
• Higher resolution requires higher gain
• Higher count rate requires lower gain
• Extended lifetime requires lower gain
• Conflict resolution
• Develop high resolution readouts requiring lower
  gain
• Design choices
• Improve existing readout techniques
• Maximise dynamic range (WSA ? TWA)
• Utilize dynamic range more efficiently (Vernier
  anode)
• Increase electrode/channel number
• Potential conflict with mass/vol./pwr resources
• Resolve by use of miniaturization - multichannel
  ASICs

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Readout comparison
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Vernier Anodegeometric charge division

• Geometric charge division using 9 electrodes
• 3 groups of 3 sinusoidal electrodes
• 3 cyclic phase coordinates
• Cyclically varying electrodes allow
• Determination of a coarse position using a
  Vernier type technique
• Spatial resolution greater than charge
  measurement accuracy
• The full unique range of the pattern can be
  utilized
• JPEX 3000 x 3000 FWHM pixel format
• Easy to reformat e.g. 6000 x 1500, etc.
• Up to 200 kHz max. global count rate

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J-PEX MCP Detector
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J-PEX Detector Performance
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Imaging spectral lines
Line width s

• Line width s
• Line profile top hat
• Assuming MCP pore delta response
• FWHM s
• Extent s pore ø
• Convolve with noise gaussians
• Centroid error from pore
• Readout noise

FWHM s
Extent s ø
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Image Charge Technique

• Pros
• Stable charge distribution
• No secondary e- effects
• No partition noise
• Readout
• Mechanically separate
• Electrically isolated
• ltlt100 electrode area
• ?Low capacitance
• Cons
• Infinite charge distribution

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Tetra Wedge Anode
PCB Layer 1
PCB Layer 2
Y axis
X axis
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Multilayer PCB TWA
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Image Charge Performance
Position error Central 23 x 36 X - 13.2 µm
rms Y - 12.4 µm rms
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Image Charge Optimizations

• Image Charge uses capacitive coupling
• No direct charge collection
• Electrode area can be ltlt 100
• Low inter-electrode capacitance
• Beneficial for MCP gain/rate/lifetime trade-off
• Vernier redesigned as 3 sets of parallel strips
• Readout constructed as 3 layer flexi PCB
• Improved peformance due to lowered capacitance
• Can be simply curved to match curved focal
  plane/MCPs

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TWA detectorfor a UV spectrometer

• Detector
• Conservative performance requirements
• Low risk MCP detector
• One design for all spectrographs
• KBr and CsI photocathodes
• Redesigned Wedge and Strip (TWA)
• Readout using Image Charge technique
• Compact, low mass design
• 40 µm FWHM resolution
• Maximum event rate 10,000 ct/s
• Electronics
• One electronics board per spectrograph
• Hybrid analog electronics
• Digital processing using FPGA
• No processor or software
• Radiation hardened to suit HEO
• Standard control and data i/f
• Engineering unit already built

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Charge division readout limitations

• Requires accurate charge measurement
• longer shaping times for adequate SNR
• high MCP gain required 107 electrons
• High gain MCP suffers from
• Lower local and global count rate
• Shorter lifetime
• Higher power requirements
• Serial event processing
• Readout electrodes have global scope
• Detector is paralysed while each event is
  processed

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Prototype detector for life science applications
Window
Photon
Photocathode
Photoelectron
MCP stack
MCP electron gain
Resistive anode
Charge localization
Current induced on readout electrode
Electrode array
ASIC preamp and discriminator timesphoton event
Readout electronics PCB with ASIC electronics
underside
LVDS logic out
TDC FPGA processing
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The end goal is a 32 x 32 array, effectively 1024
PMTs
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NINO ASIC (CERN)
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2D Parallel Strip Readout (Lapington - Leicester)
NINO ASICs

• 2D parallel strip readout 128 electrodes 200 µm
  pitch (25mm x 25mm, scaleable)
• Charge spread over 3 strips per axis
• Capacitively coupled signal via Image Charge
• Stable charge distribution, no degradations due
  to secondary electrons, no feed-throughs
• Threefold charge comparison ? fixed 100 µm
  pixel
• Discriminator timing (amplitude walk) ? sub-pixel
  centroiding (MCP limited resolution)
• Excellent counting statistics - comparison does
  not allow multiple event counting
• No explicit charge measurement, no ADCs required
• Matched to fast (6 ns dead-time) multi-channel
  preamp/discriminator ASIC (developed at CERN)

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Conclusions

• Detector choice
• CCDs would require substantial development
  programme
• MCP detector technology is mature
• High spatial resolution, large format UV
  detectors available
• UK has specific relevant unique expertise
• ROSAT WFC/HRI, Chandra HRC/HRCS, JPEX, SOHO,
  XMM-OM, TAUVEX, etc.

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Single pores picked out with a 14 micron wide
slit mask - resolution is 10 microns FWHM
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Uniform illumination of JPEX detector showing
filter support mesh
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2-D Vernier Schematic
Pitch 3
Pitch 2
Pitch 1
Line of constant phase - triplet A
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Variation of the 3 phases with position
Phase A
Phase B
Phase C
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Generation of fine and coarse x, y
X A B
Y B C
A C - X/16 - Y/4