detector technologies for wso
DESCRIPTION
Detector Technologies for WSO. Jon Lapington Space Research Centre University of Leicester. Outline. Choice of detector: MCPs or CCDs? MCP detectors Photocathodes Microchannel plates Image readout devices The Vernier Anode Image Charge technique Readout developments. CCD Option. - PowerPoint PPT PresentationTRANSCRIPT
3-5 December, 2007 WSO Detector Workshop, Leicester
Detector Technologiesfor WSO
Jon Lapington
Space Research Centre
University of Leicester
3-5 December, 2007 WSO Detector Workshop, 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
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CCD Option
• Detectors of choice in optical and X-ray applications
• High QE’s 80%+ achievable • High performance down to 200nm e.g.
WFC3– QE: 60% @ 250nm– read noise: 3 e- – Dark current: 1 e-/hr @ -80°C
3-5 December, 2007 WSO Detector Workshop, Leicester
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
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3-5 December, 2007 WSO Detector Workshop, Leicester
CCD Quantum Efficiency
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WFC-3 E2v CCD
GOES E2V CCD64 deviceEVE - SDO
3-5 December, 2007 WSO Detector Workshop, Leicester
MCPs –preferred
Pros• True photon counting• Flexible format• Mature technology• High spatial resolution• High temporal resolution• QE 30 - 40% for LSS λ• 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
– Hybrid: optical• Intensified CCD, CID, APS
PHOTOCATHODEe-
104-108 e-
Conductive coating
Microchannel Plate Cross-section
Incident electron
-
+
Output Electrons
HV
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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
3-5 December, 2007 WSO Detector Workshop, Leicester
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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3-5 December, 2007 WSO Detector Workshop, Leicester
MCP characteristics
• Gain– Typically 1-5 pC for high
resolution electronic readouts
• Format– Chevron or Z stack
– Double or triple thickness
• Noise– Low noise <0.1 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 < 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
3-5 December, 2007 WSO Detector Workshop, Leicester
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 Vernier Anode Intensified CCD Intensified APS Delay line Parallel strips –
interpolated position
Discrete pixel array
Medipix2
Image Format 30×20 mm (flexible)
25 mm Ø 25 mm Ø Up to 100×100 mm
Currently 45×45 mm (Cross-Strip)
32×32 256×256
Pixel Format (resolution elements)
3k×2k( JPEX) (up to 4k×4k, 8k×2k, etc.)
2048×2048 >2k×2k 3000×3000 Currently 5k×5k (up to 10k×10k - Cross-Strip)
32×32 256×256
Number of channels
9 256×256 (CCD pixels)
256×256 (APS pixels)
4 128/axis (2D parallel strip) 2/mm/axis (Cross-strip)
1024 64k
Readout Resolution (FWHM)
10 µm <10 µm
MCP limited 30 μm MCP limited 0.5 mm 55 μm
Dynamic range Global 2×105 2×105 400 kHz
>1MHz (goal) > 1MHz >10MHz (2D
parallel strip) MCP limited 266 µs / frame
Local MCP limited CCD frame rate MCP limited kHZ/pixel MCP limited >10 MHz/channel 200 kHz / pixel Time resolution ~ ns CCD frame rate
limited 2 μs <100 ps ~10-20 ps (using
NINO ASIC) < 10 ps 266 µs
Digital resolution
12 bit - - 13 bit 12 bit (Cross-Strip)
n/a 13 bit counter
MCP gain 1.5×107 5×105 5×105 107 ~5×105 – 2D parallel strip 5×106 - Cross-strip
5×105 ~104
Comments High MCP gain 4 µm electronic noise limited. Flexible format
Can suffer from cyclic nonlinearity due to centroiding errors
Can suffer from cyclic nonlinearity due to centroiding errors
Low channel count but requires high gain, limited parallel capability
High channel count for realistic formats, multiple simultaneous event capability
Event rate MCP limited, crosstalk →double counting, overcome with intelligent readout
Single MCP, low unsaturated gain, thresholding inaccuracies
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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 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
Line width = s
3-5 December, 2007 WSO Detector Workshop, Leicester
Image Charge Technique
Pros• Stable charge distribution• No secondary e- effects• No partition noise• Readout
– Mechanically separate– Electrically isolated– <<100% electrode area– ►Low capacitance
Cons• Infinite charge distribution
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Tetra Wedge Anode
X axis
Y a
xis
PCB Layer 1PCB Layer 2
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Multilayer PCB TWA
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Image Charge Performance
Position errorCentral 23 x 36:
X - 13.2 µm rms
Y - 12.4 µm rms
3-5 December, 2007 WSO Detector Workshop, Leicester
Image Charge Optimizations
• Image Charge uses capacitive coupling – No direct charge collection– Electrode area can be << 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/sElectronics• 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
3-5 December, 2007 WSO Detector Workshop, Leicester
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
Photocathode
MCP stack
Resistive anode
Electrode array
Readout electronics:PCB with ASIC electronics underside
Photon
Photoelectron
MCP electron gain
Charge localization
Current induced on readout electrode
ASIC preamp and discriminator timesphoton event
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)
Parameter Value
Peaking time 1ns
Signal range 100fC-2pC
Noise (with detector) < 5000 e- rms
Front edge time jitter < 25ps rms
Power consumption 30 mW/ch
Discriminator threshold
10fC to 100fC
Differential Input impedance
40Ω< Zin < 75Ω
Output interface LVDS
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2D Parallel Strip Readout (Lapington - Leicester)
• 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)
Y axis25 mm
X axis 25 mm
NINO ASICs
Charge footprint
128 sense strips at 200 μm pitch
NINO ASICs