Depolarization of a photoelectret under depth-nonuniform excitation of the sample

A simple theoretical model is given and explaination is made of the experimental observations of the recently carried out destroying the photoelectret state (PES) in Bi12SiO20 (BSO) by X-ray irradiation. It is assumed that during the irradiatoin two regions are formed divided by a sharp boundary - an excited region I with mobile non-equilibrium carriers and non-excited region II without mobile carriers. According to the experimental conditions, the isolating layers are between the sample and the electrodes, the total photoelectret charge is zero and the PE charge before the irradiation have a barrier distribution. For the determination of a PE charge the method of photodepolarization is used. When the photoelectret is irradiated in region I, mobile carriers are generated which move under the influence of the electrical field in this region and begin to accumulate on the boundary plane between the excited and non-excited regions, thus forming a 'shifted' charge layer. There is no movement of charges in region II. The distribution of the charges and the electric field in such a multilayer system is described by a system of equations. It is established that during the X-ray irradiation the PE charge gradually decreases. However, the maximum charge which can be destroyed is found to be a function of the thickness of the excited region and becomes equal to the initial charge when an excitation of the whole sample takes place. The consideration done explains the experimentally observed seeming loss of sensitivity of the BSO to the X radiation