Identification of defect centers responsible for TL in fluorapophyllite crystal using EPR and optical absorption techniques

Abstract

In this work, the natural fluorapophyllite crystal (KCa4Si8O20(F,OH)‧8(H2O)) has been studied by techniques such as X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), thermoluminescence (TL), optical absorption (OA), and electron paramagnetic resonance (EPR). This crystal, sensitized by heat treatment at 700 ◦C, has four TL peaks which, when superimposed, give rise to a broad peak centered at 225 ◦C, and an emission band between 300 and 600 nm centered at 475 nm. The kinetic parameters of the TL peaks were determined using the Tm-Tstop and deconvolution methods. In addition, a statistical analysis of the deconvolution residue was performed to evaluate the accuracy of the deconvolution. Electron paramagnetic resonance (EPR) analysis identified the defect centers induced in the crystal by gamma radiation: a center due to V4+, and a center associated with oxygen (O􀀀 ion). The V4+ center is interpreted as a (VO2+) radical (g ≈ 1.967). The O􀀀 center is characterized by axial symmetry, and the center relates to the TL peaks from 230 to 306 ◦C.