Detector Technologies for WSO

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


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


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

4


CCD Quantum Efficiency

5

WFC-3 E2v CCD

GOES E2V CCD64 deviceEVE - SDO


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


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


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 <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


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


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


J-PEX MCP Detector


J-PEX Detector Performance


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


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


Multilayer PCB TWA


Image Charge Performance

Position errorCentral 23 x 36:

X - 13.2 µm rms

Y - 12.4 µm rms


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


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


The end goal is a 32 x 32 array, effectively 1024 PMTs


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


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

Detector Technologies for WSO

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