Which detectors can I use to analyze this flare?

From RHESSI Wiki

Revision as of 22:50, 8 September 2014

Introduction

This is the most common question we are asked by people trying to analyze RHESSI imaging and/or spectroscopy data. The answer is not always straightforward and it is becoming more difficult as the individual detectors respond differently to radiation damage and the four detector anneals completed so far (Nov. 2007, Mar. 2010, Jan. 2012, and 26 June to 13 August 2014).

Figure 1: Total count and photon flux spectra for individual detector front segments during the M-class flare on 03 September 2014 at 13:40 UT.

We now have quicklook front-segment spectral plots for times throughout the mission that can help answer this question. An example for the recent M-class flare on 03 September is shown in Figure 1. These plots can help you decide which detectors are OK to use for the analysis of data a specific flare for various applications such as light curves, spectroscopy, and imaging. The plots are available in Browser via the 'Detector Spectra' check-box under the 'Flare Quicklook' option (click the magnifying glass to expand). They show the count-flux spectrum and the corresponding 'semi-calibrated' photon spectrum for the nine detector front segments for a one minute interval centered on the peak time of each contiguous event in the RHESSI flare catalog. Use the flare-seeking buttons on Browser to step through the data by flare. These plots should help you decide which detectors are OK to use for specific applications such as light curves, spectroscopy, and imaging for a specific time/flare.

Description of Plots

Each plot for a given flare is divided into two sections - an upper panel showing the count-flux spectrum for each of the nine detector front segments, and a lower panel with the corresponding 'semi-calibrated' photon flux spectra.

The count flux spectra are in units of counts cm^-2 s^-1 keV^-1 corrected for decimation and live time but with no background subtraction. The energy bin widths are 0.3 keV below 10 keV and increase logarithmically to 3 keV at 100 keV to give a total of 138 bins.

The 'semi-calibrated' photon flux spectra are in units of photons cm^-2 s^-1 keV^-1. They are computed by multiplying the corresponding count flux spectrum by the diagonal elements of the detector response matrix for the attenuator state at that time [2]. 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 background and the off-diagonal elements are small ((typically ~10 to 100 keV for large flares). These photon plots serve primarily to show the relative sensitivity of the front segments and how well the detector response matrices correct for the known sensitivities of the different detectors. More accurate spectral analysis with background subtraction and using all elements of the detector response matrix for each detector requires the use of OSPEX OSPEX.

Interpreting these plots depends on the attenuator state at the time the spectrum was obtained. (Attenuators are aluminum disks that can be automatically brought into the field of view of the detectors to reduce the effects of pulse pile-up during the largest flares <ref>The RHESSI Spectrometer, Smith et al. 2002, Sol. Phys. 210: 33-60)</ref>.) The attenuator-state dependent cross-hatched energy range on each plot indicates the lower energy range that generally should not be used for further data analysis. The upper energy limit (not shown on these plots) depends on the flare intensity and the energy at which the solar flux dips below the background level.

Three examples are given below to illustrate how to interpret the plots in the three possible attenuator states - A0 when no attenuators are in place above the detectors, A1 when the thin attenuators are above the detectors, and A3 when both the thick and thin attenuators are above the detectors. (Note that the A2 state with just the thick attenuators above the detectors is not used.)

Note the date and time of plot creation in the lower right corner. The detector response matrices may be updated as we learn more about the variation of the detector responses with time. In cases where the photon spectrum is incorrect for a particular detector, you could check whether an improved response matrix has been generated since these quicklook plots were created by remaking these plots yourself in IDL.

Figure 2: Total count and photon flux spectra for individual detector front segments during a flare on 02 May 2014 at 00:54:36 UT.

A0 Attenuator State (no attenuators)

An example of these plots is shown in Figure 2 for a small flare in the A0 attenuator state.

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 and the beryllium windows on the cryostat. Counts recorded below ~3 keV are electronic noise and should not be used in the determination of the photon spectrum.

The count flux plot shows 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 ~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 so can all be used. We'll call these 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. (This detector will therefore suffer less from pulse pile-up, but 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 below ~10 keV.

• Detector 6F shows no flare peak in the count flux spectrum so it 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.

• 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 detector 9F at lower energies.

Figure 3: Similar to Figure 1 during a C-class flare on 03 September 2014 at 02:52 UT.

A1 Attenuator State (thin attenuators)

The solar count flux spectrum peaks at ~11 keV and falls dramatically below 6 keV due to the overlying material, which now includes the thin attenuator. Counts below ~6 keV should not be used.

An example of a flare in the A1 attenuator state after the June 2014 anneal is shown in Figure 3.

After this anneal, only five detectors were segmented. Consequently, the spectrum of solar photons can only be seen for those five detectors - 1F, 3F, 6F and 8F, and 9F. 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.

• 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.

Figure 4: Similar to Figure 1 during an X-class flare on 20 January 2005 at 06:50:40 UT.

A3 Attenuator State (thin and thick attenuators)

An example of an intense flare in the A3 attenuator state on 20 January 2005 is shown in Figure 4.

The solar count flux spectrum peaks at ~18 keV and extends above background to >100 keV. The strong attenuation at low energies is such that almost all counts below ~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 - see <ref>Sampaio et al., Phys. Rev. A 89,012512 (2014)</ref>K-shell decay rates and fluorescence yields in Ge).). Counts below ~6 keV should not be used.

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 ~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 ~12 keV and enhanced rates above ~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 are 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.