CXC EPHIN Status and Alternatives Michael Juda

CXC EPHIN StatusPage 2 Outline 1.EPHIN description 2.Thermal issues 3.+27V rail anomaly and impacts 4.Operations constraints 5.Contingencies 6.Future plans

CXC EPHIN StatusPage 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 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

CXC EPHIN StatusPage 4 EPHIN Description

CXC EPHIN StatusPage 5 EPHIN Location

CXC EPHIN StatusPage 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

CXC EPHIN StatusPage 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

CXC EPHIN StatusPage 8 +27V Rail Anomaly

CXC EPHIN StatusPage 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)

CXC EPHIN StatusPage 10 Impact of Reduced HV on E1300

CXC EPHIN StatusPage 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 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

CXC EPHIN StatusPage 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

CXC EPHIN StatusPage 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

CXC EPHIN StatusPage 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

CXC EPHIN StatusPage 15 Reference Links General EPHIN Information EPHIN Leakage Currents EPHIN +27V-rail Supply Current-Limit Episodes HRC Use in RADMON Process