Performance of the XMM EPIC MOS CCD detectors

Measurements made during the selection and evaluation of flight CCD detectors for the XMM EPIC MOS cameras have demonstrated near Fano limited resolution at X-ray energies above approximately 3keV. At lower energies some devices exhibit a fractional charge loss which is believed to be due to recombination at the epitaxy/oxide interface. This has been modelled through a Monte-Carlo simulation by assuming that the pinning implant in the etched electrode structure can cause electrons to flow to the front surface, rather than to the buried channel. In spite of this charge loss, spectral response may be characterised using a double Gaussian with residuals of <5%. Quantum efficiency has been measured using a lithium drifted silicon (Si(Li)) reference detector and these measurements combined with analytical and Monte Carlo simulation, event size ratios and cosmic ray detection, all give a value for the effective depletion depth of 30 to 35 mu m.