RHESSI's Anneal Adventure

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(continuing the draft)
(Draft finished modulo images)
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dead (passive) portions, most notably around the outside of the rear
dead (passive) portions, most notably around the outside of the rear
detector segments.
detector segments.
 +
 +
[[Image:127fig1.jpg|600px|center|thumb|Figure 1: Results of the 2nd RHESSI anneal, for detector G9 only.
 +
The two panels show before/after comparison background spectra for front and rear segments (see [http://sprg.ssl.berkeley.edu/~tohban/nuggets/?page=article&article_id=27 earlier Nugget]).
 +
Note the dramatic improvement in resolution of the line features.]]
For reasons that are not fully understood, heating the detectors to
For reasons that are not fully understood, heating the detectors to
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RHESSI's detectors were first annealed in November 2007, and the adventure
RHESSI's detectors were first annealed in November 2007, and the adventure
reported here is the second time.
reported here is the second time.
-
Figure ?? shows the spectrum of
+
Figure 1 shows the spectrum of
background in a typical front and rear segment.   
background in a typical front and rear segment.   
Note the improvement
Note the improvement
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==RHESSI images==
==RHESSI images==
-
The bottom line in our analysis of the glitch and the anneal is of course the
+
The bottom line in our analysis of the glitch and the anneal is of course the creation of images.
 +
The anneal (see Figure 1)
 +
 
 +
==Conclusions==
 +
 
 +
RHESSI has brilliantly survived its second anneal, even though it occurred somewhat anomalously.
 +
The best results will come from the flare observations to be obtained soon, but we know enough already to be very pleased indeed.
 +
We look forward to stalwart service by RHESSI, the only instrument capable of imaging the fundamentally important hard X-ray
 +
and γ-ray emissions of solar flares.

Revision as of 03:00, 13 May 2010


Nugget
Number: 127
1st Author: David Smith
2nd Author: Mark Lewis
Published: 2010 May 14
Next Nugget: TBD
Previous Nugget: A History of Solar Oblateness
List all



Contents

Introduction

RHESSI's germanium detectors periodically require maintenance in the form of annealing. This just involves warming the detectors up so that their natural damage due to high-energy particles essentially fades away. The procedure is time-consuming and somewhat difficult, not to mention adventuresome, but the RHESSI team had planned to do its second anneal operation just about now in preparation for the major flares now beginning to happen as a part of Solar Cycle 24.

By an amazing coincidence, just as our planning cycle for the anneal operation was beginning, a totally unexpected glitch actually turned RHESSI off completely, including the cryocooler itself - an operational no-no, but of course the first step in an anneal operation since the detectors need to be heated, not cooled. RHESSI's cryocooler is a simplified and ruggedized version of a thermodynamic Stirling engine and has the task of cooling the detectors - at the cost of some tens of watts of power - to below 100 K. The fact that this was totally unplanned made the operations a bit of an adventure, but the results seem to have been very satisfactory. This Nugget is just to keep RHESSI fans up to date on its status.

Radiation Damage and Annealing

First, some background information. Energetic protons and neutrons encountered in Earth orbit penetrate RHESSI's germanium detectors and create small disordered regions in the germanium crystal lattice. When a solar X-ray or gamma ray interacts in a detector, it liberates a cloud of electron/hole pairs in the semiconductor that are swept to the detector's electrodes by the applied electric field, producing the current signal we detect with our electronics. But the disordered regions tend to become negatively charged and are effective hole traps. By removing some of the charge moving through the crystal, they degrade the response of the detector; a monochromatic gamma-ray source, instead of looking like a nice, narrow Gaussian line in the spectrum, will grow a tail on the low-energy side that gets more and more severe as radiation damage increases. In about a minute, the trap will release the hole (too late to count, of course) and be ready to do further mischief. At the most extreme levels of radiation damage, the space density of negative charges from unfilled traps is so great that the resulting field can cancel the externally applied field, leaving parts of the detector volume with no field or a field pointing in the wrong direction. At this point, the detector has dead (passive) portions, most notably around the outside of the rear detector segments.

Figure 1: Results of the 2nd RHESSI anneal, for detector G9 only. The two panels show before/after comparison background spectra for front and rear segments (see earlier Nugget). Note the dramatic improvement in resolution of the line features.

For reasons that are not fully understood, heating the detectors to approximately 100 C will remove the tendency for most of the disordered regions to become negatively charged and function as hole traps. This temperature is not nearly high enough to re-crystallize the lattice, but this process is still referred to as "annealing". RHESSI's detectors were first annealed in November 2007, and the adventure reported here is the second time. Figure 1 shows the spectrum of background in a typical front and rear segment. Note the improvement both in the ability to resolve narrow background lines and in the total count rate (related to the active detector volume).

The Operations Adventure

On March 17 something happened on board RHESSI. We still do not know the exact details, but basically an unexpected noise spike appeared on the main spacecraft power bus. This tripped an alarm that shut all systems down. The cryocoolers should have continued functioning, but the automatic shutdown (a safety feature) also turned the battery heaters off. They froze, and no power was available during orbit night (this can be very, very cold in space). It did not help matters that this glitch appeared right after a series of Berkeley ground station contact passes, when command control is normally available, so that the problem did not show up for half a day.

However there was spacecraft power during the sunlit portions of the orbit, so eventually there was no problem about getting things sorted out. But in a case such as this, one is flying blind - the spacecraft is in an unplanned configuration, and so one must be very careful in recovering it. Of course it turned out that the "trip" protection (either software and hardware) that turned RHESSI off was not fully documented, and so considerable discussion of almost-forgotten spacecraft engineering ensued. Everything recovered well enough so that the anneal could begin, and the silver lining of this story is that the RHESSI team had not had to spend a lot of time planning this delicate operation: it was forced on us. The "trips" have now been disabled, so that this cannot happen again, and the RHESSI team is working on replacing the positive features of this capability. The spacecraft bus had always been noisy, which was a nuisance before this glitch, and so probably there is no fundamental problem.

The adventure here illustrates how tricky the operation of a delicate, remote spacecraft can be. For example, Figure X shows how RHESSI pointing changed as a result of the glitch. Such a change might be associated with something mechanical, such as a micrometeorite impact on one of the solar panels. Instead we think that it is just the re-initialization of the sequences of on-board magnetic torque commands required to force RHESSI to follow the apparent motion of the Sun.

RHESSI images

The bottom line in our analysis of the glitch and the anneal is of course the creation of images. The anneal (see Figure 1)

Conclusions

RHESSI has brilliantly survived its second anneal, even though it occurred somewhat anomalously. The best results will come from the flare observations to be obtained soon, but we know enough already to be very pleased indeed. We look forward to stalwart service by RHESSI, the only instrument capable of imaging the fundamentally important hard X-ray and γ-ray emissions of solar flares.

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