RADMON via HRC Shield Rates

In anticipation of the possible need to use the HRC anitcoincidence shield as a replacement for EPHIN in the RADMON process, I have examined its response during the time around a single RADMON trigger. The chosen RADMON trigger, 2001-Dec-26, was selected only because the HRC HV was at the operational level at the time of the trigger and the HRC was used shortly after the resumption of the science mission. Figure 1 shows the time history of the HRC antico shield rate and the EPHIN Integral and P41 channel fluxes starting about two days before the RADMON trip and ending about a week after the resumption of the science mission.

Comparison of HRC Antico-shield rates to
EPHIN fluxes

Figure 1 HRC antico shield rate (top) and EPHIN Integral (middle) and P41 (bottom) channel fluxes as a function of time from 2001-Dec-24 to 2002-Jan-05. This time period covers the HRC observation before the 2001-Dec-26 RADMON shut-down and includes the HRC observations following the recovery.

Previous work had shown a correlation between the HRC antico shield rate and the EPHIN Integral channel flux during "quiescent" periods:

HRC Antico Rate = (EPHIN Integral Flux)×18800 + 200 In figure 2 the HRC antico rate is plotted for the same time period. The EPHIN Integral channel flux, scaled by this relationship, is over-plotted in red. Clearly, during this interval of increased particle activity the quiescent scaling no longer holds. The break-down of the quiescent relationship can be attributed to a change in the incident particle spectrum. If during times of high particle activity the HRC antico shield is sensitive to the spectral change, it may be advantageous for RADMON purposes. The RADMON process currently uses the EPHIN P41 channel, the highest energy proton channel, for one of its monitors. In figure 2 the blue points show the results of adding a scaled EPHIN P41 channel flux to the scaled Integral channel flux; the scale for the P41 flux, 7×10⁴, was selected to get a rough "eye-balled" match to the HRC antico shield rate over the time interval.

Comparison of HRC Antico-shield rates to
scaled and combined EPHIN fluxes

Figure 2 HRC antico shield rate for the same period as in figure 1. The EPHIN Integral channel flux, scaled by the "quiescent" relationship is over-plotted in red. The over-plotted blue points are the scaled EPHIN Integral channel flux plus the EPHIN P41 channel flux multiplied by 7.0E4.

This single example is somewhat encouraging for the possibility of being able to use the HRC antico shield to substitute for EPHIN (with reduced capability) in the RADMON process. A more complete comparison of the HRC antico shield rate and EPHIN P41 channel flux is required. Figure 3 shows samples of the HRC antico shield rate that are coincident with the EPHIN flux samples from the start of the mission through the end of 2002. Only times when the HRC antico shield HV is at the operating level (i.e. not during radiation events) are plotted. Scatter plots are shown for the EPHIN Integral and P41 channels. The "quiescent" scaling relation between the HRC antico shield rate and EPHIN Integral channel flux is over-plotted on its scatter plot. The departures from the quiescent relation follow many different tracks, suggesting different particle spectra for various departures. The rough scaling used to add the EPHIN P41 channel flux on in figure 2 is over-plotted in the HRC antico shield rate to EPHIN P41 channel flux scatter plot. In this plot we also see at least two different tracks. It should be noted that our current EPHIN P41 channel flux safing threshold is at 8.47 cts s^-1 cm^-2 sr^-1 and the tracks intersect the maximum HRC antico shield rate of 65535 cts s^-1 before they reach this threshold.

Figure 3 HRC antico shield rate versus EPHIN Integral (left) and P41 (right) channel fluxes. The "quiescent" relationship between the antico shield and Integral flux is plotted in the left panel. A ratio of 7×10⁴ is plotted in the right panel.

A better view of a correlation between the HRC antico shield rate and the EPHIN P41 channel flux during times of high particle activity can be obtained by subtracting the quiescently scaled EPHIN Integral channel flux from the HRC antico rate and plotting versus the EPHIN P41 channel flux; this is shown in figure 4. The various tracks appear to follow constant ratios between the "excess" shield rate and P41 flux but the range on the constant spans a factor of five.

Figure 4 HRC antico shield rate minus the scaled "quiescent" EPHIN Integral channel flux versus the EPHIN P41 channel flux. Straight lines are at ratios of 2×10⁴ and 1×10⁵.

Thresholding Example

If we were to adopt a HRC antico shield rate trigger threshold of 6×10⁴ cts s^-1 there would have been seven RADMON trips since the start of the mission. The date are given in the following table along with links to plots of the HRC antico shield rate and EPHIN P4, P41, and E1300 channel fluxes versus time.

Date	Comment
1999-10-13	Fringe of rad zone Prior to increased rad zone padding?	plot
1999-11-11	Fringe of rad zone Prior to increased rad zone padding?	plot
2000-06-10	"Hard" event - Integral Channel response No RADMON trigger	plot
2001-08-16	RADMON trip - E1300	plot
2001-12-26	RADMON trip - P41, E1300	plot
2002-07-15	X3 flare spike RADMON trip day and a half later - P4	plot
2002-11-09	RADMON trip - P4	plot

Last modified: Tue Jan 28 10:28:45 EST 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