Action Item 25: Fluence from Missed SI Safing
At the 2003-Aug-19 RADMON threshold crossing and Rad-zone preparation
telecon, an open issue to address the impact on ACIS in raising
the number of consecutive samples above threshold that are
required for the RADMON process to declare high radiation was:
What is the added fluence due to the proposed change in the event
of a failure to safe ACIS prior to radiation zone entry. This is
my response to an action I took to determine the values.
What I did
Starting from EPHIN level 0, I considered data on an orbit-by-orbit
basis (apogee-to-apogee). For each orbit I found the first times when
each of the monitored EPHIN channels exceeded the RADMON threshold
for four and for ten consecutive samples. The earliest time of the
four sample and ten sample set was selected as the RADMON trigger
time. I also determined a nominal RADMON disable time for each
orbit by subtracting 10 ks from the EE1RADZ orbital event or
from 5 hours before perigee (whichever is earlier). The
expected CRM fluence due to a failure to safe ACIS at the
radiation zone entry can then be calculated by determining the
average CRM flux between the nominal RADMON disable time and the
four-sample trigger time and multiplying by the elapsed time
between these points. The added fluence in going from the
four-sample trigger time to the ten-sample trigger time can be
calculated in a similar manner. The resulting fluences are plotted
below.
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Expected CRM fluence versus orbit number if the SI safing had not
occurred.
Top panel: CRM fluence in going from the
normal SI safing time (RADMON disable used as reference)
until the time when the RADMON process would run SCS-107 due
to a monitored EPHIN channel having four consecutive samples
above the trigger threshold.
Bottom Panel: Added CRM
fluence in going from the time with four samples to
requiring ten consecutive samples above the trigger threshold.
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The Bottom Line
In general, the added fluence in going from four to ten samples is at
least an order of magnitude less than the fluence from missing the
safing in our current configuration. The analysis points out the
potential hazard to ACIS of missing the safing actions prior to
even a single radiation zone entry.
Last modified: Wed Aug 20 09:30:20 EDT 2003
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