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UV detectors in Space

UV detectors in Space. John Vallerga Space Sciences Laboratory University of California, Berkeley. 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

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UV detectors in Space

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  1. UV detectors in Space John Vallerga Space Sciences Laboratory University of California, Berkeley

  2. 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

  3. Anode Types (at SSL) Medipix ASICIntensified CCD Cross Delayline (XDL) Cross Strip (XS)

  4. 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)

  5. 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

  6. COS

  7. Case study of two UV spectrometer detectors (cont.)

  8. COS resolution Resolution mask image (12 x 10 mm subsection )

  9. COS spatial linearity Residual to linear fit Pixel size After correction

  10. COS local gain sag

  11. COS QE

  12. COS flat field

  13. 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

  14. MCP Fixed pattern noise COS flat field 16 x 10 mm Optical tube flat field 25 mm

  15. 8µm Cross Strip Anode Anode strips Resolution < 8 µm FWHM at gain of 500000 Count rates > MHz Yamps X amps (ASIC) Charge cloud from MCP Resolution mask, 5 µm pores

  16. 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

  17. Original Medipix mode readout Zoom 256 x 256(14 mm)

  18. 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

  19. Factor of 8 improved resolution! 256 x 256 converted to 4096x4096 pixels (3.4µm pixels)

  20. Factor of 8 improved resolution! 256 x 256 converted to 4096x4096 pixels (3.4µm pixels)

  21. Zoomed 5-6 5-6 pattern resolved = 57 lp/mmLinewidth = 8.8 µm The MCP pore spacing of 8µm limits further improvement

  22. Standard specifications • Format size • Spatial resolution • Quantum Efficiency • Linearity (integral and differential) • Deadtime per event • Maximum rate (global and local)

  23. Additional specifications • Pixel size (bits per event, X,Y,T,P etc) • Lifetime • Thermal stability • Fixed pattern distortions • Response uniformity • Livetime characteristics • Calibration accuracy

  24. 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

  25. FUSE MCP gain loss(Sahnow, SPIE 5488, 2004) Total fluence map (through 2004) Gain vs. fluence

  26. 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

  27. COS Test Scrub

  28. COS Flight Scrub Initial Scrub Post Rework

  29. Detector walk due to channel bias Bias Bias Three different rear fields, fixed gain Three different gains, fixed rear field

  30. COS flat field

  31. Deadtime FUSE COS

  32. Detector “Oddities” in Space EUVE Filter pinholes Deadspot The “smudge” FUSE The “Worm” Current spikes and event bursts Y blooming Thermal drift Walk correction GALEX (sealed tube)Current transients FUV “Blob” Current “anomaly” FUV “flash”

  33. EUVE Smudge Cosmic rays saturating charge amps Deadspot Best focus gain sag Pinholesin Al/B filter After launch, vibration?

  34. FUSE(Sahnow, SPIE 4854, 610,2003) The “Worm” - focus near QE grid Current spikes and event bursts associated with Space Weather Y blooming function of input count rate - anode charging? Thermal drift poorly monitored Walk distortion in X set pre-launch for nominal gain correctable in list (X,Y,P) mode

  35. GALEX(Morrissey, SPIE 6266, 2006) Current transients associated with space weather sealed tube concern FUV “Blob” window charging Current “anomaly” MCP short that was annealed FUV “flash” significant breakdown event that cured itself

  36. Lessons Learned • Start early • Build Prototypes • Stress Prototypes • Build Flight Spares • Include lots of “knobs” to adjust in orbit • Hardware • Firmware • Software jvv@ssl.berkeley.edu

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