The effect of proton damage on the X-ray spectral response of MOS CCDs for the Swift X-ray telescope

The effect of non-ionising energy loss of protons in charge-coupled devices is to displace silicon atoms and any dopant materials present from their lattice positions to form lattice defects which in turn can trap electrons (IEEE Trans. Nucl. Sci. NS-40 (1993) 1628). A CCD operating as a photon counter for X-ray spectroscopy relies on the efficient transfer of charge from one region to another. The number of defects produced will reduce the charge transfer efficiency and hence degrade the spectral resolution of the energy distribution of interest (Jet-X Project Document: JET-X(94) UL-230 WP2220 (1994)). The Swift X-ray telescope will be equipped with a single EPIC MOS CCD22 as developed for the XMM project SPIE 3445 (1998) 13. It is the aim of this study to determine the effect of the radiation environment on the performance of the CCD and its impact on the scientific objective of the X-ray telescope, to probe the X-ray afterglow of gamma-ray bursts.