RHESSI is Annealing Now

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Number: 229
1st Author: Albert Shih
2nd Author: Martin Fivian
Published: July 7, 2014
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RHESSI's detectors are high-resolution germanium spectrometers, widely used in many situations that require gamma-ray spectroscopy, for example in the measurement of emission lines from excited nuclear states. Solar flares have these, and they provide the most direct way to learn about the essential process of particle acceleration in these eruptions.

Germanium detectors in space accumulate radiation damage in the hostile environment above Earth's protective atmosphere. This results in reduced resolution and calibration errors, as a result of crystal defects that appear in the germanium. The time scale for this degradation is a few years, but it shortens as time goes on. Luckily, it turns out that simply heating the detectors up to about 100 degrees Centigrade for a week to ten days can reset these defects via an annealing process.

We do not anneal the RHESSI detectors very often, since each anneal requires about six weeks for the very carefully done heating and cooling processes. An earlier Nugget describes the result of the first anneal, in 2008, and also details some of RHESSI's special properties.

The current anneal

The annealing operation now under way is the fourth, and we anticipate that it will restore some of the degradation accumulated since the previous anneal in 2012. Figure 1 shows the time histories of detector ("cold plate") temperatures for the four processes; at the time of writing the process is about halfway through its "hot" (about 100 C) phase and the slow process of returning to a correct operating temperature will commence in mid-July.

Figure 1: Time history of RHESSI temperatures during the three prior annealings, plus the beginning of the present one.

Normal operations will begin in mid-August and in the meanwhile, the RHESSI team was hoping that the Sun would not produce any major flares. But our hopes were dashed; almost as soon as the anneal began, a quite remarkable event occurred, as shown in Figure 2 (see also Sam Freeland's coverage of this event, which was a coronal disturbance originating from a highly occulted flare). The occurrence of this event during RHESSI's down time just emphasizes the importance of our unique instrument. Apart from the Gamma-ray Burst Monitor on the Fermi spacecraft that provides X-ray and gamma-ray spectroscopy above ~10 keV with relatively modest energy resolution but no imaging, RHESSI will be the only source of high-resolution X-ray imaging spectroscopy prior to the launch of the STIX instrument on Solar Orbiter several years from now.

Figure 2: A remarkable "gradual rise and fall" event, overlapping two days of RHESSI Browser standard plots, now annotated to show the annealing operation in progress. This event originated on the invisible hemisphere of the Sun, and would likely have produced interesting coronal hard X-ray sources for RHESSI to study (Ref. [1]).

What do we expect?

When the anneal has been completed, we will write another Nugget on the results. At a minimum we expect that the energy resolution and calibration of most of RHESSI's nine independent detectors will improve drastically. This should enable RHESSI to observe the remainder of the present solar maximum with high-quality hard X-ray imaging spectroscopy, as it has done since launch in February, 2002.


[1] "Hard X-ray emission from the solar corona"

Facts about RHESSI is Annealing NowRDF feed
Has article subject RHESSI  +, and detector anneal  +
RHESSI Nugget Date7 July 2014  +
RHESSI Nugget First AuthorAlbert Shih  +
RHESSI Nugget Index229  +
RHESSI Nugget Second AuthorMartin Fivian  +
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