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RHESSI Detectors Successfully Annealed
January 16, 2008
The performance of the nine RHESSI germanium detectors has been gradually degrading as expected since launch in 2002 because of radiation damage inflicted by the charged particles of the Earths radiation belts. To correct this problem, the detectors were annealed by heating them to over 90 degrees C for a week during November. Upon cooling them back down to their operating temperature of ~90 K and turning on the high voltages, all but one of the detectors showed considerable improvement in performance with approximately half the accumulated radiation damage being removed. The detector performance has been returned to what it was in mid to late 2005. All front segments have nearly their full effective area again, and most rear segments are now capable of observing nuclear lines effectively. Neither was the case before the anneal. This will allow high- resolution X-ray and gamma-ray observations of solar flares to be continued. One detector that had been anomalous since soon after launch is currently not operational. Although the cryocooler has lost some efficiency, it is keeping the detectors at ~94 K, about 4 K above their pre-anneal temperature with only a slight increase in power. Detector performance will be fully evaluated in the coming weeks as the occasional flare occurs during this period of very low solar activity near the minimum in the 11- year cycle.
Brian Dennis
