CXC
Q uickTim e™ and a
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EPHIN Status and Alternatives
Michael Juda
Outline
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1.
2.
3.
4.
5.
6.
CXC
EPHIN description
Thermal issues
+27V rail anomaly and impacts
Operations constraints
Contingencies
Future plans
EPHIN Status
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EPHIN Description
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• EPHIN (Electron, Proton, and Helium Instrument)
provides on-board particle radiation sensor for
safing function
– Flight-spare of EPHIN unit in COSTEP on SOHO
– Contains 7 detectors
• Passivated ion-implanted Si (detectors A, B, and F)
• Lithium-drifted Si (detectors C, D, and E)
• Scintillator with PMT readout (detector G)
– Signals combined to provide 13 particle “coincidence”
channels
•
•
•
•
EPHIN Status
4 electron channels covering 0.25-10.4 MeV
4 proton channels covering 5-53 MeV
4 alpha-particle (He) channels covering 5-53 MeV/nucleon
1 “Integral” channel for particles with energies higher than the
above ranges
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EPHIN Description
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EPHIN Status
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EPHIN Location
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EPHIN Status
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EPHIN in RADMON
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• EPHIN data is provided to the on-board computer for
potential use in radiation monitoring (RADMON)
– Rate data from the 13 coincidence channels
– Rate data from the individual detectors (not in RADMON now)
– “Aliveness” data
• The RADMON process currently monitors three of the
coincidence channels to identify a high-radiation
environment
– In high-radiation an on-board sequence is run to safe the science
instruments and stop the observing program
Channel Particle Energy Range (MeV)
P4GM
Protons
5.0 - 8.3
P41GM
Protons
41.0 - 53.0
E1300
Electrons
2.64 - 6.18
Xn indicates a detector threshold level
Strike-thru indicates a "NOT" condition
EPHIN Status
Coincidence Condition
A1 A4 B0 C0 D0 E0 F0 G0
A1 A2 B0 C0 D0 E0 F0 G0
A0 A1 B0 C0 D0 E0 F0 G0
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Thermal Issues
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• EPHIN is mounted on the sun-ward side of the spacecraft
• Degradation of passive thermal control surfaces (e.g. MLI)
has led to temperatures increasing faster than pre-launch
expectations
• High temperatures have caused anomalous EPHIN
performance
– High detector leakage currents at high temperature exceed design
capability of +27V supply leading to a current-limit condition
– Drop in +27V supply output that leads to a drop in detector HV
– Hysteresis in temperature to recover from anomaly
• High temperatures could lead to permanent degradation or
failure of EPHIN
– Drop in HV may lead to loss of compensation in Si(Li) detectors
– Component/workmanship-related failure
EPHIN Status
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+27V Rail Anomaly
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EPHIN Status
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Impact of HV reduction
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• Reduced HV on detector G reduces its
anticoincidence efficiency
– Higher E1300 rate observed which could lead to
unnecessary radiation safing and lost science time
• No evidence in past events of lowered sensitivity to radiation
• Reduced HV on detectors C, D, and E could lead
to permanent degradation in their performance
– Si(Li) detectors require sufficient HV bias to maintain
compensation
• HV level unknown (not available in telemetry)
– Increased noise in detectors is expected to lower the
sensitivity in the EPHIN coincidence channels
– No degradation observed to-date that can be attributed
to the anomaly events (16 episodes)
EPHIN Status
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Impact of Reduced HV on E1300
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EPHIN Status
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Operations Constraints
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• Avoid episodes of +27V rail anomaly as much as possible
– Plan observations such that the attitude profile keeps the predicted
EPHIN temperature below the onset temperature with a margin
• Margin selected to limit episodes to ~5/year
• Limit on duration of observations in the 60-130 deg pitch range
– Pitch range of concern grows with time as the degradation of thermal
control surfaces continues
• Requires extensive (re)work in the long-term schedule
• Constrained science targets are occasionally expected to
trigger the anomaly
– Schedule a long-duration, cold attitude to follow the science target
to speed recovery from the anomalous condition
– Adjust safing time before radiation zone entry to minimize
possibility of safing trigger from higher E1300 level
EPHIN Status
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Contingencies
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• Change thresholds of monitored EPHIN channels
or which EPHIN channels are monitored in
response to degraded EPHIN performance
• RADMON process has been modified to read
HRC anticoincidence and MCP total rate data
– HRC antico shield and MCP trigger rates replaced He
coincidence channel rates
– HRC rates only reflect the high-energy end of the
EPHIN measurements
• OK match to P41GM but dynamic range is more limited
• Less good match to E1300 and none to P4GM
EPHIN Status
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HRC vs EPHIN
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• Ceiling on HRC rate is
lower than where we
would normally safe
for high-radiation
• Using HRC for safing
could lead to
unnecessary safing
and lost science time
• Use it when EPHIN
cannot deliver highenergy monitoring
capability
EPHIN Status
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Future Plans
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• Raise E1300 threshold to minimize possibility of
unnecessary safing during +27V rail anomaly
episodes
• Investigate the gains from turning off the detector
G HV
– Less current draw should raise the temperature of the
onset of the +27V rail anomaly
– Thresholds in the RADMON process may require
modification
• Investigate modifications to the temperature
margin used in scheduling observations
– Budget to allow for more anomaly occurrences
EPHIN Status
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Reference Links
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General EPHIN Information
http://hea-www.harvard.edu/~juda/memos/ephin/index.html
EPHIN Leakage Currents
http://hea-www.harvard.edu/~juda/memos/ephin/leakage_current/index.html
EPHIN +27V-rail Supply Current-Limit Episodes
http://hea-www.harvard.edu/~juda/memos/ephin/current_limit/index.html
HRC Use in RADMON Process
http://hea-www.harvard.edu/~juda/memos/FN443_HRC_in_RADMON.pdf
EPHIN Status
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