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Characteristics of the on-orbit background of the Chandra x-ray observatory high-resolution camera

The Chandra X-ray Observatory (CXO) High Resolution Camera (HRC) is a microchannel plate (MCP) based X-ray detector with heritage from similar detectors flown on the Einstein and ROSAT missions. The HRC consists of two detectors in a common housing. Improvements from the previous instruments include: fabricating the MCP from 'low-noise' glass (glass that contains a reduced level of radioactive isotopes) and surrounding the detector housing on five sides with an active coincidence detector. Both of these improvements help to maximize the X-ray signal to background noise ratio. The on-orbit background is dominated by cosmic ray and solar-wind particles. The temporal behavior of the background has two parts: a quiescent level and a flaring component. The quiescent level slowly changes with time and is correlated with the high-energy particle flux as measured by the Electron Proton Helium Instrument (EPHIN), the CXO radiation detector. The flaring component is associated with times of elevated low-energy particle flux, primarily from the Sun. A combination of on-board vetoing and filtering during ground processing provides a substantial rejection of the non-X-ray background. This work was supported by NASA contract NAS8-39073 to the Chandra X-ray Center.