Space radiation environment effects on X-ray CCD background

Abstract

Charge coupled devices (CCDs) are often employed as the detector of choice for observing X-rays in space. The instrument background experienced in orbit has a major impact on the overall sensitivity of the camera system. The instrument background for the European Space Agency X-ray Multi Mirror (XMM-Newton) mission was found to be much greater in orbit than that originally predicted pre-launch, the reasons for which still being up for discussion. The Geant4 toolkit provides a framework for Monte Carlo simulations in a variety of areas in science and is used here to simulate the instrument background for several CCD based detectors in-orbit in order to gain a further insight into the formation of the instrument background continuum. The missions discussed in this paper include the ESA XMM-Newton mission, the NASA Swift mission and the Japanese Space Agency Suzaku mission. These three missions allow a comparison between the effects of both the mission orbit and the detector construction on the instrument background count-rate. Analysis of the results from the simulation lead to the conclusion that knock-on electrons, produced when protons pass through the shielding, dominate the instrument background continuum at the XMM-Newton Highly Elliptical Orbit (HEO) with a perigee and apogee of approximately 7000 and 120 000 km respectively, forming an additional background component not considered in the pre-launch study. The surface properties of the detector and shielding have the greatest impact on the level of the instrument background due to the interaction length of the knock-on electrons produced. At the Low Earth Orbit (LEO) of Swift and Suzaku at approximately 600 km, the impact of the knock-on electrons is reduced due to the differing in-flux of protons, and the form of the instrument background can vary greatly with the detector construction. The inconsistencies between this study and the pre-launch simulations are discussed. Sensitivity considerations regarding the instrument background are deliberated with a view towards future missions.