Radiation effects on the proportional counter X-ray detectors on board theNEAR spacecraft

The X-ray proportional counters on board the Near Earth Asteroid Rendezvous (NEAR) spacecraft have exhibited a resolution degradation and recovery phe nomenon several times during the long cruise phase of the mission. The reso lution is checked periodically by commanding an Fe-55 source into the windo w area. The degradation is seen as a low energy tailing of the 5.9 keV phot opeak. Two events have occurred which provided good spectral data for bette r understanding the degradation phenomenon In November 1997 a large solar p article event occurred that degraded the resolution and excited copper in t he collimator. Eventually the detectors returned to normal. In January 1998 the spacecraft performed an Earth swingby gravity assist maneuver. The nea r Earth environment excited the magnesium and aluminum in the filter elemen ts. The copper line was also produced. The NEAR spacecraft was launched in February 1996 and will rendezvous and orbit the asteroid 433 Eros in early 1999. (C) 1994 Elsevier Science B.V. All rights reserved.