Transformation of defects arising in CsI(Tl) crystals under daylight

Abstract

PACS 78.40.Ha, 78.55.Fv The part played by the activator in delocalization of the excitation and defect formation in a CsI(Tl) crystal has been studied. Basing on the investigation of the long term afterglow kinetics for CsI(Tl) crystals with various activator concentrations, it has been found that the cation substituted Tl + ions participate in the process of the charge carriers delocalization in CsI(Tl) crystals under daylight. Non-radiative decay of electronic excitations with the F center formation is stimulated by oxygen containing anions. The subsequent destruction of the F centers is accompanied by both, luminescence of the excitons localized near Tl + and activator defect formation. The luminescence of Tl + centers is reabsorbed by the stable activator defects arising in CsI(Tl) crystals under irradiation. As a result, a sensitized luminescence of activator defects appears along with Tl + luminescence.1 Introduction CsI(Tl) crystals are very sensitive to the daylight, the short wave edge of which is limited by glass transparency (λ > 320 nm). Under exposure to the daylight, they store energy that is released as an afterglow. Identical activator color centers are formed in a CsI(Tl) crystal doped with carbonate ions under the scattered daylight with a wavelength of not more than 400 nm as well as in CsI(Tl) without any oxygen-containing impurities under an ionizing radiation. Delocalization of the charge carriers under the daylight is necessary for the formation of the color centers. The activator plays the primary role in this process. According to [1], the mobile charge carriers can arise as a result of the light absorption by the precipitates of the activator. The authors [2] suppose that the cause of the activator defect formation is the two-photon absorption by Tl + ion with the subsequent creation of an F-H pair. Cs + and Tl + ions have close radii of 1,68 and 1.49 Å, correspondingly, forming a solid solution of substitution CsI:TlI within the wide interval concentrations, that is limited by the concentration of about 0.3 % TlI [3]. The detectors made on the basis of CsI(Tl) crystals just with this activator concentration can show the highest resolution. According to [3], the deterioration of the resolution of NaI(Tl) and CsI(Tl) detectors is connected mainly with micro-inhomogeneities due to the decay of the solid solution. This experimental fact proves that the delocalization of the excitation in a CsI(Tl) crystal with an optimal activator content cannot be explained by the presence of the precipitates. Under exposure to the daylight, the long-term afterglow appears in every CsI(Tl) crystal while in the presence of carbonate ions in the lattice the stable activator color centers arise too. Like in a KCl (Tl) crystal [4], the appearance of the activator color centers in CsI(Tl) can be connected with F center destruction. According to [5] the number of F centers formed in CsI lattice under irradiation is stimulated by CO 3 2-ions. The purpose of this work is to study the part played by the activat...