MEASURED AND MODELED CCD RESPONSE TO PROTONS IN THE ENERGY-RANGE 50 TO 300 MEV

A deep depletion X-ray sensitive CCD was exposed to monoenergetic, collimated protons of energies 50, 100, 200 and 300 MeV at the proton irradiation facility (PIF) at the Paul Scherrer Institute. Measurements were made at angles-of-incidence 0 degrees, 20 degrees and 45 degrees to the detector normal. The energy-loss spectra and spatial distribution of the collected charge were derived and compared to theoretical predictions which take into account the CCD geometry and the generation and diffusion of charge within the active device. These data were then used to determine the efficiency with which protons could be discriminated from 0.3-10 keV X-rays. By applying energy deposition and size requirements (in terms of the number of reporting pixels) on each event, it was found that X-rays, single proton and multiple proton events could all be separated unambiguously although with varying efficiencies. For energy depositions in the range 0.3 to 7 keV, 100% particle discrimination can be achieved for all sizes of X-ray event. Above 7 keV, the rejection efficiency can still be maintained at 100%, if one is prepared to reject between 0 and 39% of the X-rays depending on incident proton energy, by the selective use of size and energy cuts. It was found that for this CCD the dominant form of masquerading events (i.e., events which cannot be discriminated against) arise from proton interactions in the readout register. Since the register constitutes only 1% of the active area, the number of contaminating events is a small fraction of the total incident flux. Approximately 99.9% of charged particles can still be rejected on size and energy grounds whilst still accepting 100% of 7 to 10 keV X-rays.