ENERGY DEPOSITION IN X-RAY CCDS AND CHARGED-PARTICLE DISCRIMINATION

We report the results of investigation into ionization energy loss in X-ray CCDs and the ability of fast protons to masquerade as X-ray events by their energy and spatial signatures. The study was made to explore background contamination in X-ray detectors intended to operate in the mixed radiation environment of space. It was carried out using the ionization energy loss code of Hall [1] which includes atomic electron binding effects. Three devices were considered in our study; a front illuminated 35 mu m deep depletion device developed for the Joint European X-ray Telescope, and two CCDs baselined for ESA's XMM mission - a PN device of active depth 280 mu m and a back-illuminated MOS device of active depth 90 mu m It was found that for the JET-X device, 4.9% of minimally ionizing protons deposit less than 10 keV in the active volume of the CCD. Whilst the frequency distribution of such events versus energy deposited increases up to a peak energy of similar to 9 keV, the number having a morphology consistent with X-ray events simultaneously decreases. Thus, when coupled with the requirement that events should extend no more than 2 pixels spatially, the fraction of masquerading events drops to 0.016%. The analysis was also carried out for the two XMM CCDs. For the PN device the fraction of events depositing less than 15 keV is miniscule whereas almost 4% of events were recorded for the MOS device. When coupled with an event size requirement of less than or equal to 2 pixels, the fraction of events falls to 1 X 10(-9). A surprising result of the present work is that the ability of a CCD to discriminate against minimally ionizing particles is dependent on pixel size. For a JET-X like device this varies by a factor 10 for pixel sizes ranging from 13 to 35 mu m on a side. In fact, the optimum pixel size for both the JET-X and XMM MOS devices was found to be near 40 mu m. We conclude that given the energy deposition and morphological distinctions between photon and particle events, CCDs used in X-ray astronomy can be optimally designed to provide simultaneously the highest signal-to-noise ratios for X-rays and rejection ratios for charged particles.