The Effectiveness of the Proportional Counter With a Specially Designed Filter as a Solar X-Ray Monitor on the NEAR Mission

The X-ray instrument package on the Near Earth Asteroid Rendezvous (NEAR) spacecraft is designed to provide direct measurements of the major elemental abundances for the asteroid 433 Eros. These will be the first such measurements ever made for a 'primitive' body. For this task to be performed, simultaneous measurements must be provided of both the X-ray lines characteristic of major elements at the asteroid's surface as well as the solar flux, which is the source of these lines (1,2). The solar flux consists of a continuum which is generated by thermal bremstrahlung and radiative recombination, with superimposed major lines which result from emission due to transitions in H and He-like ions. The relative contribution of each component of the solar flux varies as the level of solar output changes, and thus solar output must be monitored to correct for varations in surface data caused by the changing levels of solar output.