X-ray excited optical luminescence, photoluminescence, photostimulated luminescence and x-ray photoemission spectroscopy studies on BaFBr:Eu

The results of x-ray excited optical luminescence (XEOL), photoluminescence (PL), photostimulated luminescence (PSL) and x-ray photoemission spectroscopy (XPS) studies on the x-ray storage phosphor BaFBr:Eu are presented in this paper. Analyses of XEOL, PL and PSL spectra reveal features corresponding to the transitions from 4f(6) 5d(1) to 4f(7) configurations in different site symmetries of Eu2+. Increasing x-ray dose is seen to lead to a red shift in the maximum of the PL excitation spectrum for the 391 nm emission. The XEOL and XPS spectra do not show any signature of Eu3+ in the samples studied by us, directly raising doubts about the model of Takahashi et al in which Eu2+ is expected to ionize to Eu3+ upon x-ray irradiation and remain stable until photostimulation. XEOL and PSL experiments with simultaneous x-ray irradiation and He-Ne laser excitation as well as those with sequential x-ray irradiation and laser stimulation bring out the competition between the F centre population and depopulation rates. A time scale similar to 1-2 s, possibly related to the process of production of the electron/hole traps themselves, during x-ray irradiation, is observed. Relaxation of these traps upon photostimulation is also seen.