Background

The HEPC & LEPC detectors will encounter cosmic background from the diffuse x-ray background and scattered solar x-rays, as well as non-X-ray background from charged particles, cosmic-ray-induced secondary radiation from the spacecraft structure, and high energy x-ray background photons which penetrate the spacecraft and detector structures. The total non-X-ray background is estimated to be $\sim 5\times10^{-4}$ counts cm^-2 s^-1 keV^-1 for either HEPC or LEPC. In a 2$^{\prime}$ diameter circle ($\sim 4.7$ mm for an 8 m focal length), this corresponds to a rate of $\sim 2\times10^{-3}$ counts s^-1 for HEPC (2 - 25 keV) and $\sim 7\times10^{-4}$ counts s^-1 for LEPC (0.2 - 8 keV).

The diffuse x-ray background in HEPC & LEPC may be estimated using the effective area shown in Figure 22 and a model xrb spectrum. We use the ROSAT spectrum of Wang and McCray (1993) below 3 keV, and the HEAO-1 spectrum of Boldt (1987) above 3 keV, and find a value of $\sim 2$ c s^-1 deg^-2 for HEPC and $\sim 12$ c s^-1 deg^-2 for LEPC. In a 2$^{\prime}$ diameter circle, these correspond to rates of $\sim 1.8\times10^{-3}$ counts s^-1for HEPC and $\sim 1.1\times10^{-2}$ counts s^-1 for LEPC. Readers should be aware, however, that the soft component of the xrb is dominated by galactic contributions and can vary with viewing direction by as much as a factor of 5. The total xrb in HEPC & LEPC may therefore vary by a large factor.