RHESSI's Anneal Adventure

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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
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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.

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 ?? 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).

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