Particle-dependent radioluminescence color response of Eu-doped CaF$_2$ associated with energy-loss density
Europium-doped calcium fluoride (Eu:CaF$_2$) exhibits characteristic blue Eu$^{2+}$ and red Eu$^{3+}$ emissions. In our previous study, the Eu$^{3+}$/Eu$^{2+}$ radioluminescence intensity ratio under $α$-particle irradiation was approximately twice that under X-ray irradiation. Here, we investigate the relationship between this emission ratio and energy-loss density using H, Li, C, and Ni ion beams produced by a tandem accelerator. Radioluminescence spectra were measured for each ion species, and the effective range-averaged energy-loss density was evaluated using SRIM simulations, spanning approximately 18 to 5900~keV/$μ$m. The Eu$^{3+}$/Eu$^{2+}$ emission-intensity ratio increased systematically from H to C ions by approximately a factor of two, whereas no further systematic increase was observed for Ni ions at substantially higher energy-loss densities. Because the energy-loss density was varied mainly by changing the ion species, its effect cannot be fully separated from that of ion-specific track structure. The X-ray result, included for qualitative comparison, showed a lower emission ratio than the ion-beam measurements, although its effective energy-loss density was estimated only approximately. These results demonstrate a systematic variation in the emission color of Eu:CaF$_2$ across different ion-beam irradiation conditions and provide baseline data for future studies of wavelength-resolved particle discrimination.