Introduction

Spacecraft

The XMM-Newton spacecraft consists of three X-ray telescopes and an Optical/UV telescope. The three X-ray mirror modules are formed by 58 nested shells, with an outermost diameter of 70 cm and a focal length of 7.5 m. Each mirror shell is made up of a forward parabolic section followed by a hyperbolic section. In order to focus an X-ray photon two scatters are required: one in each mirror section. There are three types of focal plane instruments, all of which use CCDs as detectors: three imaging cameras placed on the optical axis of each telescope (two EPIC-MOS and one EPIC p-n) and two off-axis spectrometers (RGS).

Orbit

The XMM-Newton observatory was launched in December 1999 into a highly eccentric elliptical orbit, with an apogee of 114000 km, a perigee of 7000 km and an inclination of 39º. On its path, the spacecraft crosses the Earth's radiation belts where there are trapped electrons and protons.

Radiation Effects

As XMM -Newton crosses the radiation belts, it is subjected to a bombardment of energetic electrons and protons, as well as cosmic rays (especially at apogee) all of which may contribute to the detector background. While electron deflectors, placed behind the X-ray mirrors, divert electrons of up to 100 keV energy away from the optical axis, there are no mechanisms that can stop protons from reaching the focal plane detectors. Protons of energies in the range of 100's keV to a few MeV can scatter at low angles of incidence through the mirror shells and reach the focal plane. These protons, because of their low energy, can produce a high non-ionising dose in unshielded CCD detectors leading to a loss in Charge Transfer Efficiency (CTE). This is the most likely cause of the CTE degradation reported to have occurred in the ACIS instrument on NASA’s Chandra X-ray observatory early on in the mission. The launch of XMM-Newton coincided with the start of the current period of increased solar activity, which can enhance significantly the fluence of damaging protons during solar events.

Our Task

The initial task of the Space Environments and Effects Section was to verify that low energy protons were indeed responsible for the damage observed on Chandra’s detectors and analyse the possibility that XMM-Newton would be likely to suffer similar degradation. An accurate assessment of the radiation environment encountered by the spacecraft in orbit was carried out and mitigation techniques that could be adopted to protect the XMM-Newton detectors were explored. A computer model was developed using the Monte-Carlo Geant4 tool-kit to investigate the interaction of low energy protons with the telescope optics and detectors as they propagate from the telescope aperture to the focal plane. This model was applied separately to the two geometries of the  XMM-Newton and Chandra observatories to estimate the level of potentially damaging protons for all XMM-Newton X-ray detectors and for the ACIS camera on Chandra. The results of this study have been published in the ESA document Radiation Environment Induced Degradation on Chandra and Implications for XMM-Newton.

As the XMM-Newton mission progresses successfully, the model predictions can now be compared with spacecraft data and the study is being extended, refining the geometry and physics models of the Geant4 simulation, to investigate further the radiation background induced by protons on XMM-Newton’s CCD instruments.