16-Jan-2008

<pre>RHESSI Detectors Successfully Annealed

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 R. Dennis
        Solar Physics Laboratory - Code 671
        Heliophysics Science Division
        Sciences and Exploration Directorate
        NASA Goddard Space Flight Center
        Greenbelt, MD 20771
        Telephone: 301-286-7983
        FAX: 301-286-1617
        Email:      Brian.R.Dennis(at)nasa.gov

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