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UV detectors in Space. John Vallerga Space Sciences Laboratory University of California, Berkeley. 200+ Detector Years in Orbit!. Mission# of DetectorsYears EUVE78.3 ALEXIS612 SUMER (SOHO)24 UVCS (SOHO)212.0+ FUSE27.9 IMAGE25.7 - PowerPoint PPT Presentation
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WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
UV detectors in Space
John Vallerga
Space Sciences Laboratory
University of California, Berkeley
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
200+ Detector Years in Orbit!Mission # of Detectors Years
EUVE 7 8.3
ALEXIS 6 12
SUMER (SOHO) 2 4
UVCS (SOHO) 2 12.0+
FUSE 2 7.9
IMAGE 2 5.7
ALICE (Rosetta) 1 3.7+
GALEX 2 4.7+
ALICE (New Horizons to Pluto) 1 1.8+
COS (Hubble) 1 0 (Aug 08)
+ still operating
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
Anode Types (at SSL)
Cross Delayline (XDL)
Cross Strip (XS)
Medipix ASICIntensified CCD
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
Design/Performance Flexibility
Input aperture Window/door
Photocathode KBr - CsI - CsTe - GaN(QE, bandpass) (xray to optical)
MCPs d < 160mm, r > 7cm,(format, resolution) pore spacing > 3µm
Anode Delayline (4 amps)(resolution, gain, rate) Cross Strip (128 amps)
Medipix ASIC (65k amps)
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
Case study of two UV spectrometer detectors
Cosmic Origins Spectrograph
“Hubble class” instrument
Large, reliable, stable, well calibrated and tested.
Extreme QA
ALICE on New Horizons
Pluto mission
Low power, mass, telemetry
“Moderate” resolution, calibration, testing, QA
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
COS
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
Case study of two UV spectrometer detectors (cont.)
Parameter COS ALICE
Size (mm) 178 x 10 38 x 20
Format (pxls) 32768 x 1024 1024 x 32
Resolution (µm) 25 80
Mass (kg) 33.4 0.66
Power (W) 37 1.1
Cost $$$$$$$$$$ $
Both have: curved MCPs, vacuum doors, CsI
photocathodes, 20 kHz ct rates (10% deadtime)
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
COS resolution
Resolution mask image (12 x 10 mm subsection )
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
COS spatial linearity
Residual to linear fit
Pixel size
After correction
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
COS local gain sag
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
COS QE
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
COS flat field
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
Improvements in MCP Detectors since COS
• MCP fixed pattern noise improved
• Electronics (Time to Digital Converters)– Faster (250 kHz at 10% deadtime)– FPGA logic– Lower power
• Readout technologies– Crossed Strip– Medipix and Timepix ASICs
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
MCP Fixed pattern noise
COS flat field 16 x 10 mm
Optical tube flat field 25 mm
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
Cross Strip Anode
Resolution mask, 5 µm pores
Anode strips
Y amps
X amps (ASIC)Charge cloud from MCP
Resolution < 8 µm FWHM at gain of 500000
Count rates > MHz
8µm
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
Medipix/Timepix ASIC readout
• 256 x 256 array of 55 µm pixels
• Integrates counts, not charge
• 100 kHz/pxl
• Frame rate: 1 kHz
• Low noise (100e-) = low gain operation (10 ke-)
• GHz global count rate
• ~1 W watt/chip, abuttable
• Developed at CERN
Input
Preamp
Disc.
Disc. logic Mux. 13 bit
counter –ShiftRegister
Clock out
Shutter
Lower Thresh.
Disc.
Mux.
Previous Pixel
Mask bit
Analog Digital
Upper Thresh.
Next Pixel
Mask bit
Polarity
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
Original Medipix mode readout
Zoom 256 x 256
(14 mm)
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
Timepix version of Medipix
Amplitude rather than counts using “time over threshold’ technique
If charge clouds are large, can determine centroid to sub-pixel accuracy
Tradeoff is count rate as event collisions in frame destroy centroid information
Single UV photon events
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
Factor of 8 improved resolution!
256 x 256 converted to 4096x4096 pixels (3.4µm pixels)
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
Factor of 8 improved resolution!
256 x 256 converted to 4096x4096 pixels (3.4µm pixels)
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
Zoomed
5-6 pattern resolved = 57 lp/mmLinewidth = 8.8 µm
5-6
The MCP pore spacing of 8µm limits further improvement
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
Standard specifications
• Format size
• Spatial resolution
• Quantum Efficiency
• Linearity (integral and differential)
• Deadtime per event
• Maximum rate (global and local)
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
Additional specifications
• Pixel size (bits per event, X,Y,T,P etc)• Lifetime• Thermal stability• Fixed pattern distortions• Response uniformity• Livetime characteristics• Calibration accuracy
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
Lifetime (MCP gain loss)
• High-fluence missions concern
• Gain loss can affect response due to threshold
• Can also affect imaging performance
• Somewhat ameliorated by HV increase
– Localized loss a problem
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
FUSE MCP gain loss(Sahnow, SPIE 5488, 2004)
Total fluence map (through 2004)
Gain vs. fluence
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
Scrub to stabilize gain loss
MCP gain initially decreases rapidly with extracted charge as adsorbed gasses are removed
Rate of gain loss vs. extracted charge eventually decreases
Scrub “end point” dependent upon expected fluence of misison
Exposure to gas at atmospheric pressures resets gain to initial higher levels - hence the need for sealed tubes or closable doors
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
0
0.5
1
1.5
2
2.5
0 50 100 150
Scrub Current vs. Time
COS test plates (10-10-10µm)
Current (µA)
Time (hours)
0.0001
0.001
0.01
0.1
1
10
100
1000
10
4
10
5
0 0.05 0.1 0.15 0.2 0.25
Scrub of COS Test plates (10-10-10µm)
Scrub gain of ~ 50,000 . Nov. 16-23, 1999
Relative GainGain Sag (%/C cm
-2
)
Extracted Charge (C cm
-2
)
Relative Gain
Gain Sag
0.0001
0.001
0.01
0.1
1
10
100
1000
10
4
10
5
0 0.1 0.2 0.3 0.4 0.5
Post-Scrub Lifetest on COS sample plates (10-10-10µm)
Full gain. Nov. 24-28, 1999
Relative Gain
Gain sag (%/C cm
-2
)
Extracted Charge (C cm
-2
)
Relative Gain
Gain Sag
COS Test Scrub
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
COS Flight Scrub
Initial Scrub Post Rework
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
Detector walk due to channel bias
Three different rear fields,fixed gain
BiasBias
Three different gains,fixed rear field
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
COS flat field
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
Deadtime
0
20000
40000
60000
80000
100000
0 50000 100000 150000 200000
Digital Event Counter (cps)
Fast Event Counter (cps)
Non-paralyzable fit:
DEC = R / (1 + R*t)
FEC= R * exp(-R*330ns)
Best fit: t = 7.4µs
FUV-01 Segment A
FUSE COS
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
Detector “Oddities” in Space
EUVE
Filter pinholesDeadspotThe “smudge”
FUSEThe “Worm”Current spikes and event burstsY bloomingThermal driftWalk correction
GALEX (sealed tube)Current transientsFUV “Blob”Current “anomaly”FUV “flash”
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
EUVE
Smudge
Cosmic rays saturating charge amps
DeadspotBest focus gain sag
Pinholes in Al/B filterAfter launch, vibration?
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
FUSE (Sahnow, SPIE 4854, 610,2003)
The “Worm” - focus near QE grid
Current spikes and event burstsassociated with Space Weather
Y bloomingfunction of input count rate - anode charging?
Thermal driftpoorly monitored
Walk distortion in Xset pre-launch for nominal gaincorrectable in list (X,Y,P) mode
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
GALEX (Morrissey, SPIE 6266, 2006)
Current transientsassociated with space weathersealed tube concern
FUV “Blob”window charging
Current “anomaly”MCP short that was annealed
FUV “flash”significant breakdown event that cured itself
WSO Detector Workshop Leicester University, Dec. 2007, John Vallerga
Lessons Learned
• Start early• Build Prototypes• Stress Prototypes• Build Flight Spares• Include lots of “knobs” to adjust in orbit
– Hardware– Firmware– Software