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