Which detectors can I use to analyze this flare?

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Revision as of 16:37, 10 September 2014

Introduction

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.

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

The issue is now particularly important because some of the detectors did not separate electronically into a front and rear segment after the two most recent anneals. Following the 2012 anneal, detectors 2 and 4 failed to segment, although detector 4 did segment about a year later. Following the 2014 anneal, detectors 2, 4, 5, 7, and 8 did not segment. Unsegmented detectors still operate as single large-volume detectors and have similar sensitivity to that of segmented detectors, however their background rates are higher and their energy resolutions are worse. Nevertheless they are still perfectly adequate for spectroscopy and imaging above ~20 keV.

We have generated quicklook front-segment spectral plots for times throughout the mission that can help determine which detectors are OK to use and over what energy range, for light curves, spectroscopy, and imaging of a specific flare. The plots show the count spectra and the corresponding 'semi-calibrated' photon spectra for the nine detector front segments for a one minute interval centered on the peak time of every contiguous event in the RHESSI flare catalog. An example is shown in Figure 1 for a flare on 02 May 2014 just prior to the 2014 anneal.

The plots are available in Browser via the 'Detector Spectra' check-box under the 'Flare Quicklook' option (click the magnifying glass to expand). Use the flare-seeking button on Browser to step through the data by flare. The PNG files displayed in Browser can be directly accessed here.

Description of Plots

As the example in Figure 1 shows, the plot for each flare has two sections - an upper panel with the count-flux spectra between 1 and 100 keV 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 subtracted. 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 on-line 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 (This is typically between ~10 and 100 keV for large flares). These photon plots serve primarily to show the relative sensitivity of the front segments and how well the on-line 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 response matrix for each detector requires the use of OSPEX. OSPEX also allows for the inclusion of albedo, and corrections for such instrumental effects as pulse pile-up and changes in the energy resolution and energy calibration for individual detectors.

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 RHESSI Spectrometer.) The crosshatched area on each plot indicates the attenuator-state dependent lower energy range that should not be used for 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.

Figure 1 and the two examples given below show spectra 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.) The interpretation of these quick-look spectral plots in the three attenuator states is discussed below.

A0 Attenuator State (no attenuators)

The plot in Figure 1 is for a small flare on 02 May 2014 recorded in the A0 attenuator state just prior to the start of the most recent anneal. As is usual in this attenuator state, the count-flux spectrum from the flare peaks at ~7 keV. The plots for the different detectors show which ones have recorded an increase in count rate from the flare. In this case, all detectors except 6F show the the solar signal peaking at this energy. The plotted photon spectra are a useful representation of the solar spectrum from ~3 keV to ~15 keV. The lower energy is set by both the electronic lower threshold level for each detector and the 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. Above ~15 keV in this case, the calculated photon spectrum is an indication of the background spectrum under the false assumption that all the photons came from the solar direction.

Here is a breakdown of what you can determine about each of the nine detectors from the count-flux and photon flux spectra shown in Figure 1.

• Detectors 1, 3, 4, 5, and 8 all show similar photon spectra above 3 keV so can all be used for spectroscopy and imaging. 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 much below ~10 keV.

• Detector 6F shows no flare peak at 7 keV 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 below ~12 keV indicating that there is a problem with the response matrix for this detector at low energies. You could try remaking these plots in IDL to see if an improved response matrix is available for this detector since the quick-look plots were made (see date and time in lower right-hand corner of the plot).

In general, weak events like this require careful background subtraction and considerations of different attenuation at low energies possibly from degrading thermal blankets over the detectors.

• Detector 9F shows lower count fluxes than the 'good' detectors below ~8 keV, and that discrepancy is not removed in the photon spectrum. Consequently, you might not want to use detector 9F at lower energies.

Figure 2: Total count and photon flux spectra for individual detector front segments during the M-class flare on 03 September 2014 at 13:40 UT.

A1 Attenuator State (thin attenuators)

An example of the M-class flare on 03 September 2014 is shown in Figure 2. This flare was recorded in the A1 attenuator state after the most recent anneal completed in August 2014. As is usual in this attenuator state, the solar count flux spectrum peaks at ~11 keV. The solar flux can be seen above background at energies up to ~30 keV. The count-flux spectrum falls at energies below ~6 keV due to the absorption of the overlying material, which now includes the thin attenuator. This strong attenuation at low energies is such that almost all counts below ~6 keV are from K-escape events, i.e. incident photons with energies 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>The RHESSI Spectrometer, Smith et al. 2002, Sol. Phys. 210: 33-60)</ref> The RHESSI Spectrometer p. 56 <ref>Sampaio et al., Phys. Rev. A 89,012512 (2014)</ref>K-shell decay rates and fluorescence yields in Ge). As a result, counts much below ~6 keV should not be used.

• Detectors 1F, 3F, 6F and 8F, and 9F are the 'good' detectors in this case. The calculated 'semi-calibrated' photon spectra closely follow the same curve for these five detectors showing that they can all be used between ~6 and ~30 keV. The Fe-line complex peak at 6.7 keV can be seen in the spectra from the detectors with the best energy resolution (1F, 3F, and 6F).

• Detectors 2F, 4F, 5F, and 7F were not segmented at this time and their calculated photon spectra show that the current response matrices does no correctly allow for their modified sensitivities in this state. Their background levels are higher 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 new response matrices will need to be generated to account for modified sensitivities in the unsegmented state.

Figure 3: 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 3. As is usual in this attenuator state, the solar count-flux spectrum peaks at ~18 keV. The solar flux can be seen above background at energies up to at least 100 keV. As in the A1 attenuator state, the count-flux spectrum falls at energies below ~6 keV due to the absorption of the overlying material, which now includes both the thick and the thin attenuators. Consequently, almost all counts below ~6 keV are from K-escape events, and counts much below ~6 keV should not be used.

The close alignment of the photon spectra for all detectors between 6 and 100 keV indicates that they can all be used over this full energy range although there are still small differences between detectors as indicated below.

• 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 for this detector matches the spectrum for the other detectors indicating that this difference is incorporated in the response matrix.

• 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. Thus, this detector should not be used for spectroscopy when high energy resolution is important.

• 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 fluxes below ~12 keV and enhanced rates above ~25 keV. The reduced rates at low energies are due to the higher electronic threshold needed because of the high background rate from this detector most probably caused by caused by its poorer energy resolution. Thus, even though the photon spectrum matches the other detectors down to 6 keV, it should not be used below about 10 keV since the count rates 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 should be possible to take this into account in OSPEX spectral analysis by including the pileup function when analyzing data for this detector alone.

Conclusion

Please check out these quicklook detector spectral plots in Browser or here. Use them to decide which detectors can be used for any particular flare of interest, and over what energy range. Keep in mind that you can always generate them yourself for any time interval by running the IDL program hsi_spectrum_check_dets.pro. For example, the command to generate the plot in Figure 1 is:

hsi_spectrum_check_dets,time='2-may-2014 00:55:06', /no_gui

This is useful to look at different time intervals during a flare and to determine if the detector response matrices has been improved since the archived quicklook plots were created (creation date/time shown in the lower right corner of each plot). Changes to the response matrices and energy calibrations have been infrequent in the past but continue to be necessary as we learn more about variations in detector response with time.