Which detectors can I use to analyze this flare?

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Revision as of 22:28, 5 September 2014

Introduction

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.

Example 1 in the A0 Attenuator State

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~ Failed to parse (PNG conversion failed; check for correct installation of latex, dvips, gs, and convert): $s^{-1}~cm^{-2}~keV^{-1}$ , 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 [2] applicable for that time of the flare. This has traditionally been called a "semi-calibrated photon spectrum." Since no background was subtracted, it 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 background and the off-diagonal elements are small. 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, at the time of the plot, the instrument was in attenuator state 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 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. The solar signal is the peak at around 7 keV that falls off at lower energies down to 3 keV and extends up to about 25 keV before merging with the background spectrum at higher energies. Here is a breakdown of what you can determine about each of the nine 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 (see 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 energies unless an updated response matrix is available.

Example 2 in the A1 Attenuator State

Another recent example after the latest anneal of a flare in the A1 attenuator state is shown in Figure 2.

Figure 2: Similar to Figure 1 during a C-class flare on 03 September 2014 at 02:52 UT.

This example shows the condition of the different detectors after the latest anneal when only five were segmented. Consequently, the spectrum of solar photons can only be seen for four detectors - 1F, 3F, 6F and 8F, and 9F. As usual in this attenuator state, the solar count flux spectrum peaks at about 11 keV and falls dramatically below 6 keV due to the attenuation of the overlying material now including the thin attenuator. The Fe-line complex peak at 6.7 keV can be seen in the spectra from the detectors with the best energy resolution. The background spectrum dominates at energies above ~25 keV in this example. The information about the different detectors that can be gleaned from these plots is as follows:

• Detectors 1F, 3F, 6F and 8F, and 9F are the "good detectors in this case and can be used between ~6 and 25 keV.

• Detectors 2F, 4F, 5F, and 7F were not segmented at this time and cannot be used for this event. The background level is higher for these unsegmented detectors since counts from interactions anywhere in the full detector volume are now recorded in the electronics channel normally reserved for front-segment events. This necessitates using a higher electronic threshold of up to 20 keV to limit the total rate. Consequently, these detectors cannot be used at energies below ~20 keV. Flares that extend to higher energies should give count rates above the enhanced background rates but it remains to be seen if the response matrices will need to be modified to account for modified sensitivities since the anneal.

Example 3 in the A3 Attenuator State

Another example from earlier in the mission on 20 January 2005 is shown in Figure 3.

Figure 3: Similar to Figure 1 during an X-class flare on 20 January 2005 at 06:50:40 UT.

This is for a much more intense flare in the A3 attenuator state, i.e. with both the thick and thin attenuators in place above the detectors. Thus, we see the solar count flux spectrum peaking, as is usual in this attenuator state, at ~18 keV and extending above background to >100 keV. In the A3 state the strong attenuation at low energies is such that almost all counts below 5 or 6 keV are from K-escape events (i.e. photons above the germanium K-edge at 11 keV that have photoelectric interactions that result in the escape of the K-shell photon from the detector). Hence, counts below this energy should not be used as indicated by the hatched areas on the plots. The close alignment of the photon spectra for all detectors indicates that they can all be used above ~6 keV although there are still small differences between detectors as indicated below. As with the example in attenuator state A1, counts below ~6 keV are almost all K-escape events and should not be used.

• Detector 1F has a higher count rate above about 60 keV than the other detectors. This is because the grids above this detector are made of molybdenum instead of tungsten, and they start to become transparent to X-rays above this energy. The photon spectrum shows that this is taken into account in the response matrix for this detector.

• Detector 2F has poorer energy resolution than the other detectors as indicated by the enhanced count rate below about 4 keV compared to the other detectors.

• Detectors 3F, 4F, 5F, 6F, 8F, and 9F all have similar count flux spectra and can be used from 6 to 100 keV for all applications.

• Detector 7F shows reduced count flux below about 12 keV and enhanced rates above about 25 keV. The reduced rates at low energies are due to the higher threshold for this detector since it has poorer energy resolution. Even though the photon spectrum looks OK down to 6 keV, it should not be used below about 10 keV since the statistics would be very low. The excess above 25 keV is not taken into account in the response matrix since it also shows up as excess flux in the photon spectrum. It is most probably caused by pulse pile-up because the electronics is less able to reject pile-up events when the threshold is high. It can be corrected for in spectral analysis by including the pileup function in OSPEX when analyzing data for this detector alone.