Emission characteristics of Eu²⁺ and Eu³⁺ under x-ray and alpha irradiation in Eu-doped CaF₂ crystals

Calcium fluoride crystals doped with europium (CaF ₂ :Eu) have long been used as conventional inorganic scintillators. Their luminescence is primarily attributed to emission from Eu ²⁺ centers, typically around 420 nm. However, it has been reported that increasing the Eu concentration leads to enhanced Eu ³⁺ emission in the wavelength range of 590–700 nm. In this study, CaF ₂ :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 ³⁺ 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 ²⁺ /Eu ³⁺ centers in CaF ₂ :Eu scintillators depends on the type of incident radiation.