Relative and (maybe) Absolute RHESSI Detector Efficiency: 2002-2008

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Revision as of 05:45, 22 December 2009

Introduction

RHESSI has now been observing for nearly eight years. How has detector sensitivity changed since it was launched? To test the relative sensitivity for detectors we use a method and IDL routine devised by Brian Dennis and Kim Tolbert. For a sample of flares, we fit a Temperature, T and Emission Measure, EM near the flare peak for detector 1. Next, thermal components are fit to the spectra for the other eight detectors, with T fixed at the value found for detector 1, T₁. This gives us a measure of the relative sensitivity of the detectors.

The Flare Sample

We want to do this calculation for as many flares as possible. Here we chose every flare on the RHESSI Flare List which was observed with no attenuators, no particle events, no data gaps, with the spacecraft at low geomagnetic latitude, and at least 5 minutes from the SAA. In addition to the flare interval being "clean" in this manner, we also require that the 12 second time intervals before the flare start time and after the flare end time be "clean", so that we can use those times to calculate the background level. This process resulted in a sample of 10661 flares. Spectra were fit for each flare for the same time range that was used to find the flare position. This time interval is usually an interval of 2 minutes or less (depending on flare duration) at the flare peak in the 6 to 12 keV range. The spectra were assumed to be isothermal, and fit for the energy range from 6 to 20 keV.

Not all of the flares were fit successfully for all detectors; flares which did not have count rates that were more than 3 sigma above the background level in more than 2 channels were discarded; flares for which the calculated EM was less than 10⁴³cm ^{− 3} were discarded; flares with background-subtracted count rates totaled over the 6 to 20 keV range which were less than zero were discarded. These tests were applied for each of the detectors 3, 4, 5, 6, 8, and 9. Spectra from detectors 2 and 7 have been ignored, since those detectors have reduced energy resolution in the 6 to 20 keV range.

The final sample contains 7740 flares, from the start of the mission until 1 November 2009.

Results for Relative Sensitivity

Figure 1:This is the relative detector efficiency for front detectors 3, 4, 5, 6, 8, and 9

Figure 1 shows 60 day averages of the ratio between the EM of each of detectors 3, 4, 5, 6, 8, and 9 and detector 1 for the full time range. From the plot we see that relative sensitivity was the same for all detectors until 2006. Early in 2006, the relative sensitivity of detector 3 began to drop. Later in 2006 the detector 5 sensitivity rolls over. Detectors 4, 6, 8, and 9 lose sensitivity during 2007. The first data gap on the plot is for the detector anneal which took place in November of 2007. Post-anneal, the detectors recover sensitivity. The second data gap on the plot is just a quiet time...

Note that the relative sensitivity of all detectors seems to be greater than 1 for the early part of the mission. This may be a systematic effect resulting from holding the temperature fixed at the level found using detector 1 for each of the detectors. This possibility is being investigated.

What about absolute sensitivity?

Figure 2:60 day averages of the ratio of RHESSI detector 1 EM to GOES fluxes for the full sample.

For each flare we also measured GOES flux averaged for the spectral interval. We expect (why?) that the GOES detectors are less susceptible to decay then the RHESSI detectors which suffer from radiation damage. For most of the flares GOES 10 data were used, but there are gaps in the GOES 10 coverage, so there are flares with GOES 11 and 12 data included. (It turns out that even if non-GOES 10 flares are discarded, we find the same results. Figure 2 shows 60 day averages of the ratio RHESSI detector 1 Emission Measure to GOES fluxes for the full sample, which should show the variation of the relative sensitivity of detector 1 to GOES. Since we see that the other detectors lose sensitivity, we expect that detector 1 should lose some sensitivity, but it seems to gain sensitivity relative to the GOES detectors.

Figure 3:GOES Hi channel flux versus RHESSI detector 1 Emission Measure for the full sample.

Looks like a blob for small flares, do the same plot for 6 to 20 keV RHESSI detector 1 count rate.

Figure 4:GOES Hi channel flux versus RHESSI detector 1 6 to 20 keV count rate for the full sample.

Restrict to the flares with greater than 1.0e45 emission measure. This leaves 2580 flares.

Figure 5:GOES Hi channel flux versus RHESSI detector 1 6 to 20 keV count rate for the flares with EM greater than 1.0e45 cm^(-3).

Plot 60 day averages as a function of time.

Figure 6:60 day averages of the ratio of RHESSI 6 to 20 keV counts to GOES fluxes for the flares with EM greater than 1.0e45 cm^(-3).

Show samples of flare ratio results for two different time ranges.

Figure 7:The ratio of RHESSI 6 to 20 keV counts to GOES fluxes for flares with EM greater than 1.0e45 cm^(-3) for two different time periods, one early in the mission, one just before anneal,

What does the sample change do to the original plot?

Figure 8:This is the relative detector efficiency for front detectors 3, 4, 5, 6, 8, and 9, but using count rates and only the small sample of 2580 flares