Charge diffusion effects in CCD X-ray detectors. I. Theory.

Abstract An analysis is presented of the effect of charge diffusion on the performance of CCD X-ray detectors when used for single photon counting. It is found that the energy resolution of such detectors can be severely degraded if the thickness of the field-free region below the depletion layer is comparable to the minority carrier diffusion length (typically 50–100 μm) and that satisfactory performance can only be achieved if this thickness is substantially less than the depth of the depletion region (normally ∼ 7 μm in presently available devices). The formulae for epitaxial devices are particularly simple, making prediction of pulse height spectra straightforward. The analysis is also relevant to the calculation of soft error rates (SERs) in random access memories, to the use of CCDs as detectors of ionising particles and to the discrimination of cosmic ray events in CCDs used for visible astronomy.

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