Which detectors can I use to analyze this flare?

From RHESSI Wiki

This is the most common question we get asked by many people trying to analyze RHESSI imaging and/or spectroscopy data. The answer is not always easy and it is becoming more and more difficult as the individual detectors respond differently to radiation damage and to the four anneals that have now been completed. However, we now have new front-segment spectral plots available in Browser for all identified contiguous flares that can help answer this question. These plots show both the count flux spectrum and the corresponding "semi-calibrated" photon spectrum for each of the nine detector front segments. The plots cover a one minute interval centered on the peak times of all identified contiguous events in the RHESSI flare catalog. They can be obtained for any given flare by selecting "Detector spectra" under "Flare Quicklook" in Browser.

Figure 1: Total count and photon flux spectra for individual detector front segments during a flare on 02 May 2014 at 00:54:36 UT.

An example of these plots is shown in Figure 1 for a one minute interval centered on the peak of a small flare in the A0 attenuator state. The nine total count-flux spectra in units of counts s^-1 cm^-2 keV^-1, one for each front segment, are each fully corrected for decimation and live time and use 0.3 keV energy bins below 10 keV increasing logarithmically up to 100 keV. The photon flux spectra are computed from the corresponding count flux spectra by multiplying by the diagonal elements of the detector response matrix applicable for that time of the flare. This has traditionally been called "semi-calibrated." Since no background was subtracted, this is equivalent to assuming that all the counts were from solar photons and that the off-diagonal elements are negligible. Photon spectra determined in this way approximate the actual solar spectrum only in the energy range where the solar counts are significantly above the background levels. For the case shown in Figure 1, this would be below ~20 keV. Even in that case, they serve only to show the relative sensitivity of the nine detector front segments and how well the detector response matrices correct for the known sensitivities of the different detectors. Since the attenuator state atthis time was A0, i.e. no attenuators were in place above the detectors, an estimate of the photon spectrum can be made down to ~3 keV, the approximate energy level of the electronic lower threshold level for each detector and the effective cutoff from the absorption of material in front of the detectors including the thermal blankets, both inside the cryostat and above the top grids, and the beryllium windows on the cryostat. Counts recorded below ~3 keV are electronic noise and cannot be used in the determination of the photon spectrum, hence the hatched areas in the plots.

From this plot you can already begin to decide which detectors are OK to use for specific applications such as light curves, spectroscopy, and imaging. The count flux plots show immediately which detectors have recorded an increase in count rate from the flare shown by the peak at around 7 keV that falls off down to 3 keV and up to about 25 keV before merging with the background spectrum at higher energies. All other detectors show the peak from the flare photons but with increased scatter below 10 keV. Here is a breakdown of what you can determine the other detectors from these two plots:

Detectors 1,3, 4, 5, and 8 all show similar photon spectra above 3 keV and so can all be used. These i call the "good" detectors for this event. Note that detector 3F has lower count rates than the others but its response matrix takes care of this in computing the photon spectrum. It does mean that this detector suffers less from pulse pile-up although that is not a factor for this weak flare.

Detector 2F has a similar count flux spectrum to the "good" detectors but shows a much broader peak as a result of its poorer energy resolution. You may not want to use it for spectroscopy much below 10 keV.

Detector 6F shows no flare peak in the count flux spectrum and so should not be used at all for this event.

Detector 7F shows a similar count flux spectrum to the "good" detectors but the calculated photon spectrum is much higher than for the other detectors indicating that there is a problem with the response matrix for this detector below ~10 keV. The detector response matrices may be updated as we learn more about the variation of the detector responses with time so that you could remake these plot yourself in IDL to see if an improved response matrix has been generated for this detector since these quicklook plots were generated as indicated by the date and time in the bottom right-hand corner of each plot.

Detector 9F shows lower count fluxes than the "good" detectors below about 9 keV and that discrepancy is not removed in the photon spectrum. Consequently, you might not want to use data from this detector at lower nergies unless an updated response matrix is available.

Another example from earlier in the mission is shown in Figure 2.

Figure 1: Similar to Figure 1 during an X-class flare on 20 January 2005 at 06:50:40 UT.

This is a much more intense flare with both the thick and thin attenuators in place above the detectors during this one minute interval centered on the peak time.