In this memo, I review the available MCP total rate data from times just prior to shut-down by runs of SCS-107 due to radiation (both autonomous and via ground command). Given the limited performance data available during times of high radiation I combine it with data from the observed total counts (mostly due to the cosmic-ray background) to derive a proposal for a threshold that could be used within the RADMON process. Finally, I discuss the merits of implementing the monitoring of this channel within the RADMON process.
 |
| Figure 1: Flux in the three EPHIN channels monitored within the RADMON process and the HRC MCP total and antico shield rates prior to the SCS-107 trigger of 2001:360. The red horizontal lines on the EPHIN flux plots are the current RADMON thresholds; for most RADMON triggers (including this one) the E1300 threshold was half its current value. |
The following is a tabulation of the radiation-safing events for which the MCP HV was at the operational level at the time of the SCS-107 activation. The links go to plots of the fluxes and rates similar to figure 1.
| Event Date | Cause |
|---|---|
| 2001:228 | Autonomous activation due to EPHIN E1300 |
| 2001:324 | Ground activation due to ACE P3 fluence |
| 2001:360 | Autonomous activation due to EPHIN E1300 (shown in Figure 1) |
| 2003:297 | Ground activation due to ACE P3 fluence |
| 2003:336 | Autonomous activation due to EPHIN P4GM |
| 2004:210 | Autonomous activation due to EPHIN E1300 at the fringe of the radiation zone |
| 2004:257 | Autonomous activation due to EPHIN P4GM and E1300 |
| 2004:312 | Ground activation due to ACE P3 fluence |
| 2005:134 | Autonomous activation due to P4GM |
| 2006:080 | Autonomous activation due to EPHIN E1300 at the fringe of the radiation zone |
For these events the MCP total rate displays evidence for an increase for only the E1300 events and one of the ACE P3 events which had the HRC at the aim-point of the telescope.
 |
| Figure 2: HRC-I MCP total rate vs time since launch. |
The data in figure 2 was extracted from the HRC secondary science telemetry using continuous time intervals over which the HRC-I MCP HV was observed to be at the operational level and attempting to filter out "glitches" created during telemetry format changes by iteratively clipping the rate. The total counts dose can be obtained from the data by summing the products of the mean rate over the interval and the interval duration; the resulting total accumulated counts in the HRC-I as a function of time is shown in figure 3.
 |
| Figure 3: HRC-I accumulated counts history. |
In a little over six years of operation the HRC-I MCP has recorded about 2.2×109 counts. The data for the HRC-S (rate and accumulated counts) are very similar with the total counts being slightly less due to a slightly shorter time spent with its MCP HV at the operational level. Most of the time with the MCP HV at the operational level has been spent with ACIS observing and the HRC at the stowed location and is dominated by the cosmic-ray background. The cosmic-ray background is essentially uniformly spread over the detector area so this counts dose corresponds to 30-35 counts per MCP pore (for reference note that laboratory tests showed a 10% reduction in MCP gain at a dose of ~500 counts per MCP pore). While the background rate typically remains below 300 counts/s, during HRC observations X-rays from the target source will add to this rate as will the contribution from the flaring, low-energy-proton background component. The rate due to X-rays (or out-of-band photons) can be as high as a few hundreds of counts/s (e.g. the Vega observations on the MCP wing segments or the Crab pulsar observations) and the added rate persists throughout the observation. The flaring background can reach rates of a few thousand but this only happens for short bursts relative to the 10 minutes over which the RADMON process samples the rates.
The RADMON process uses data for the MCP total rate that is telemetered as MSID 2DETART (or 2DETBRT if we switch to the HRC B-side CEA). This MSID only reflects the high-order byte of the rate and so a rate of 5000 counts/s would correspond to a 2DETART value of 19. There have been four instances during the entire mission to date when the telemetry shows 2DETART exceeding a value of 19. One of these instances was the 2001:360 radiation-safing event, during which the value of 19 was exceeded for roughly ten minutes before the MCP HV was reduced. Two of the instances were brief (~10 and ~40 seconds) busts during periods of elevated rates prior to radiation zone entry (on days 2003:142 and 2003:324); the antico shield rates rose at these times as well. The final event was a single high sample on day 2004:362 for which the secondary-science scalar rates showed a corresponding "blip". There have been no cases of MSID 2DETART with a value of 30 (corresponding to ~7900 counts/s) or higher. I propose using a "2DETART" threshold value 29.5; in other words, the OBC would run SCS-107 once we have ten consecutive samples (one sample every 65.6 s) of 2DETART within the RADMON process with a value of 30 or higher).
To the extent that the E1300 channel in the RADMON process is not needed for the protection of ACIS, the MCP total rate could be used to replace it. This would have the benefit of eliminating the possibility of some unwanted RADMON triggers on the fringes of the radiation zones. The following table lists the past RADMON triggers that would have had a difference relative to the current response if the E1300 channel were not used for safing.
| Event Date | Difference |
|---|---|
| 2000:313 | 590.4 second delay to P4GM |
| 2001:228 | 30241.6 second delay to P41GM |
| 2001:308 | 1049.6 second delay to P41GM |
| 2004:210 | 1049.6 second delay to P4GM |
| 2004:257 | 1049.6 second delay to P4GM |
| 2005:234 | 10824.0 second delay to P4GM |
| 2005:251 | No trigger |
| 2005:257 | No trigger |
| 2006:080 | No trigger |
The "Event Date" column links go to plots of the time history of the fluxes in the three monitored EPHIN channels (E1300, P4, and P41) around the RADMON trip time. The current threshold levels are indicated by the horizontal red lines. The RADMON trigger time is given by the solid red vertical line, while the trigger time without the E1300 channel (if there is one) is given by the dashed red vertical line. For the day 2006:080 event the RADMON disable time is given by the solid black vertical line. Four of the nine triggers do not show a significant delay without the E1300 channel. The 2006:080 event was an unwanted trigger, due to the fringe of the radiation zone.
The day 2001:228 event, which has a long delay to trigger without the E1300 channel, was one where the MCP HV was at the operational level the the time that RADMON activated SCS-107. Given the observed response in the MCP rate it is possible that without the E1300 channel the event could have been triggered by the MCP rate being exceeded earlier than by the P41GM channel.
The remaining three events 2005:234, 2005:251, and 2005:257 all exhibit a slow rise in radiation levels, with the latter two requiring the E1300 for a RADMON trip. Since the MCP HV was not at the operational level for these events, it is not clear what result monitoring the MCP total rate in RADMON would have produced. Ground intervention would likely be necessary to provide earlier (or any) safing for radiation environments similar to these if it were desired; the slow rate of increase in the radiation level presumably minimizes any possible damage to the science instruments.
Last modified: Mon Jul 24 15:48:08 EDT 2006
Dr. Michael Juda
Harvard-Smithsonian Center for Astrophysics
60 Garden Street, Mail Stop 70
Cambridge, MA 02138, USA
Ph.: (617) 495-7062
Fax: (617) 495-7356
E-mail: mjuda@cfa.harvard.edu
