Emission characteristics of Eu$$^{2+}$$ and Eu$$^{3+}$$ under x-ray and alpha irradiation in Eu-doped CaF$$_2$$ crystals

Calcium fluoride crystals doped with europium (CaF[Formula: see text]:Eu) have long been used as conventional inorganic scintillators. Their luminescence is primarily attributed to emission from Eu[Formula: see text] centers, typically around 420 nm. However, it has been reported that increasing the Eu concentration leads to enhanced Eu[Formula: see text] emission in the wavelength range of 590-700 nm. In this study, CaF[Formula: see text]:Eu crystals with various Eu concentrations (0, 0.3, 1, 2, 3, and 5 mol%) were grown and irradiated with both X-rays and α-particles from a ²⁴¹Am source. Radioluminescence spectra were measured for each excitation condition and compared across samples. The results clearly show that the relative contribution of Eu[Formula: see text] emission is approximately twice as high under α-particle irradiation compared to X-ray excitation. This enhancement was found to be consistent for Eu concentrations above 1%, and independent of the crystal thickness or geometry. To the best of our knowledge, this is the first report demonstrating that the emission intensity ratio of Eu[Formula: see text]/Eu[Formula: see text] centers in CaF[Formula: see text]:Eu scintillators depends on the type of incident radiation.