Physics of low energy tail in the CCD response function

A response of the frontside illuminated CCDs to monochromatic X-rays shows a noticeable low energy tail in addition to the well understood main peak and escape and fluorescence features. It is generally assumed for silicon detectors that the tail's origin is due to dead layer on the surface of the device  [Scholze and Ulm1994,Lechner and Struder1995]. We have studied the response of the CCD to the monochromatic synchrotron X-rays at BESSY in a wide range of energies and have discovered an unknown feature in the very low energy part of the response function [Prigozhin et al.1998a,Jones et al.1997]. Examining changes in its behavior as a function of energy of incoming radiation we came to the conclusion that it originates from the photons interacting in the gate insulator, which is not a dead layer as was previously thought. We have developed a model that explains the shape of this newly discovered feature as well as the shape of entire low energy tail in the CCD response function.