Experimental evidence for the aggregation of photostimulable centers in BaFBr:Eu2+single crystals by cross relaxation spectroscopy

Spatial correlation between the activator ion ${\mathrm{Eu}}^{2+}$ and electron and hole traps generated by x irradiation is thought to play a decisive role in the photostimulated emission process of the storage phosphor BaFBr:${\mathrm{Eu}}^{2+}$. We report the first direct experimental evidence for correlations among F centers, hole centers, and ${\mathrm{Eu}}^{2+}$ obtained by measuring cross relaxation effects in optically detected paramagnetic resonance. BaFBr treated with the known procedures for removing oxygen still retains 50 ppm or more ${\mathrm{O}}^{2\mathrm{-}}$. This ion is critically involved in the formation of F and hole centers.