Fig. 6

(Left) Dependence of the Eu\(^{3+}\)/Eu\(^{2+}\) emission ratio on Eu concentration, obtained from radioluminescence measurements. Red and blue points represent data under \(\alpha\)-particle and X-ray irradiation, respectively. (Right) Ratio of the Eu\(^{3+}\)/Eu\(^{2+}\) emission intensity under \(\alpha\)-irradiation to that under X-ray irradiation, plotted as a function of Eu concentration. The ratio remains nearly constant for Eu concentrations above 1%.