Which detectors can I use to analyze this flare?

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Number: 235
1st Author: Brian Dennis
2nd Author: Kim Tolbert
Published: September 15, 2014
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Figure 1: Count- and photon-flux spectra for detector front segments during a small flare on 02 May 2014.

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 because the individual detectors respond differently to radiation damage and to the four anneals completed so far (Nov. 2007, Mar. 2010, Jan. - Feb. 2012, and Jun. - Aug. 2014).

The issue is particularly important now 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 both detectors eventually segmented much later. Following the 2014 anneal, detectors 2, 4, 5, and 7 did not segment. Unsegmented detectors operate as single large-volume detectors and have similar effective area to that of segmented detectors. 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 an archive of quicklook front-segment spectral plots for times throughout the mission to help determine which detectors and energy ranges are OK to use 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 each flare in the RHESSI flare catalog, even if the flare is divided into multiple events in the catalog. An example is shown in Figure 1.

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 spectrum 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 detector response matrix for the attenuator state at that time. 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 count rates are significantly greater than background rates and the off-diagonal elements are small (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 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. This also allows for the inclusion of albedo (X-rays reflected from the Sun) in the analysis, and corrections for instrumental effects such as pulse pile-up (one kind of saturation) and changes in the energy resolution and 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 at high count rates - see Smith et al. (2002) p. 41 & 58.) The hatched 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.

Figures 1, 2, and 3 show spectra in the three possible attenuator states - A0 (no attenuators), A1 (thin attenuators), and A3 (both thin and thick attenuators). (Note that the thick attenuators are never used by themselves.) The interpretation of these quick-look plots in the three attenuator states is discussed below.

A0 Attenuator State (no attenuators)

An example of a small flare in the A0 attenuator state on 2 May 2014, just prior to the 2014 anneal, is shown in Figure 1. As is usual in this attenuator state, the count-flux spectrum from the flare peaks at ~7 keV. The separate spectra show which detectors have recorded an increase in count rate from the flare. In this case, all detectors except 6F show the solar signal peaking at this energy.

The plotted photon spectra are a useful representation of the solar spectrum which in this case extends above background up to ~15 keV. The lower energy is set by both the electronic lower threshold level for each detector and the absorption of material in front of the detectors including the thermal blankets and the beryllium windows on the cryostat. Thus, counts recorded below ~3 keV are electronic noise and should not be used in the determination of the photon spectrum. The calculated photon spectrum above ~15 keV in this case 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 detector from the spectra shown in Figure 1.

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

Figure 2: Count- and photon-flux spectra for detector front segments during an M-class flare on 03 September 2014.

A1 Attenuator State (thin attenuators)

An example of an M-class flare in the A1 attenuator state on 03 September 2014, just after the 2014 anneal, is shown in Figure 2. 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 Smith et al. (2002) p. 56 and Sampaio et al. (2014). As a result, counts below ~6 keV should not be used.

Figure 3: Count- and photon-flux spectra for detector front segments during an X-class flare on 20 January 2005.

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, as in the A1 attenuator state, almost all counts below ~6 keV are from K-escape events, and counts 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.


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, and over what energy range. You can 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

Generating the plots yourself will allow you to examine different time intervals (don't include times of attenuator changes!). You can then determine if the detector response matrices have 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.

If after all of this, you still don't know which detectors to use for any particular flare, feel free to ask any RHESSI team member.


[1] The RHESSI Spectrometer, Smith et al., 2002, Sol. Phys. 210: 33-60.

[2] K-shell decay rates and fluorescence yields in Ge, Sampaio et al., 2014, Phys. Rev. A 89,012512.

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