Degradation of back-illuminated CCD´s due to low energy protons

To investigate the behavior of a back illuminated CCD as an X-ray detector in a space environment some devices have been irradiated by protons. These CCD´s have a depleted layer of about 30 micron. The proton irradiations have been carried out with unshielded CCD´s in a calibrated monoenergetic proton beam with energy in the range between 0.75 to 1.6 MeV from an R.J. Van de Graaff Generator of the Utrecht University. These protons are stopped in the depleted part of the CCD so it can be expected that they will have a strong influence on the Charge Transfer Efficiency (CTE). Calculations with TRIM showed that protons of about 1.4 MeV are stopped just in the buried channel in which the charge transfer takes place. The proton irradiations were performed in bunches of about 10⁸ protons/cm². Before and after each irradiation the CTE was determined with X-rays from a MgF₂ anode producing 0.667 and 1.25 keV radiation. Afterwards the CTE was determined with AlK radiation (1.486 keV) as function of operating temperature between -70 and -130°C. It is shown clearly that protons with energies in the range from 1.25 to 1.45 MeV produce the largest degradation. For higher energies the effects diminish again.