Transcript Larson_SST_stat

SST Status
SST Status
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All SSTs are functioning
No electronics failures (12 ADC channels x 5 spacecraft)
All attenuators are operating nominally.
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automatically closed at high count rate (>20 kHz) since April 2008
Automatically close inside of 8 Re (regardless of count rate)
All ion detectors on all spacecraft are showing noticeable signs of degradation.
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Caused by total accumulated ion implantation over mission lifetime.
Results in increase in effective dead layer of incident side of detector.
(Each energy channel is shifted up in energy)
Degradation rate is not uniform among detectors:
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Some detectors are MUCH worse than others
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Degradation rate is not based on detector production batch.
Not dependent on look direction.
Appears random.
Not (primarily) due to (penetrating) radiation belt particles.
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Spacecraft B&C are the worst
Spacecraft E is best
Commanding error resulted in 4 perigee passes with attenuators left open (summer 2007) but this
resulting in only minimal damage.
Primary damage from low energy ions (30-100 keV) that produce recombination centers near
detector surface.
Effect was largely mitigated by increase in bias voltage (Dec 2007) on all spacecraft.
Electron detectors do not show obvious detector degradation.
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Foils “protect” detectors from intense flux of low energy ions.
Have found an energy dependent reduction in geometric factor. Attributed to “defocusing
effect” of low energy electrons scattering in foil away from detectors.
SST Status
• Calibration Efforts:
– In-flight:
• Search for Steady periods of moderately high flux with single energy
distribution (as Maxwellian as possible).
• Calibration is based on comparison with ESA particle spectrum and
extrapolation to higher energy
• For Ions:
– Use model of dead layer to determine energy shift for each detector
(elevation angle)
• For Electrons:
– Determine relative geometric factor based on comparison with ESA
electron spectrum.
– Ground Calibration:
• Will begin ion implantation experiment with Jeff Beeman at LBL
using ion beam implantation machine starting next year.
• Modeling electron scattering (defocusing) using GEANT4
• Modeling instrument response to penetrating particles with GEANT4
– Code updates are eminent. Can expect some
SST Pitfalls
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SST measures energetic particles (>30 keV)
Cross contamination
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SST is sensitive to light
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Sunpulse (once per spin)
Moonlight to lesser extent.
There is a sunpulse blockout command that mitigates effect (but does not remove).
Pulse “moves around” with season.
Loss of sensitivity in Some detectors (bad in 3 of 40)
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Very Energetic electrons (>350 keV) can contaminate “open” detectors.
Very Energetic electrons (>350 keV) can contaminate “foil” detectors.
Does not distinguish composition.
Possible premature radiation damage.
Ions only (Electrons ok)
2007-May-October
Spacecraft B & C primarily
Fixed by increase in Bias Voltage.
Attenuator state
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Changes geometric factor by ~128
All spacecraft in “auto” mode as of 2008-04-02 when outside of 8 Re.
Be wary of
Attenuators
Flipping on and
off