EPHIN Status and Alternatives

1. EPHIN Status and Alternatives Michael Juda

2. Outline • EPHIN description • Thermal issues • +27V rail anomaly and impacts • Operations constraints • Contingencies • Future plans EPHIN Status

3. EPHIN Description • 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 • 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 EPHIN Status

4. EPHIN Description EPHIN Status

5. EPHIN Location EPHIN Status

6. EPHIN in RADMON • 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 EPHIN Status

7. Thermal Issues • 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

8. +27V Rail Anomaly EPHIN Status

9. Impact of HV reduction • 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

10. Impact of Reduced HV on E1300 EPHIN Status

11. Operations Constraints • 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

12. Contingencies • 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

13. HRC vs EPHIN • 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 high-energy monitoring capability EPHIN Status

14. Future Plans • 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

15. Reference Links 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