[en] We studied the response of high-resistivity MOS CCDs to monochromatic X-ray illumination at energies ranging from 1.47 to 5.9 keV and discovered a new feature in the low-energy tail of the response function. We attribute it to the photons interacting in the gate insulator and explain the shape of the entire low-energy tail assuming that it is formed by electron charge clouds partially formed in the gate oxide. The size of the charge clouds derived from this model is much smaller than previously reported. This can be explained by a different field distribution in a buried channel CCD