Thermal annealing of radiation-induced optical absorption of BaF2 and CaF2 crystals irradiated with fast xenon ions

Abstract

BaF₂ and CaF₂ crystals possessing high radiation resistance and as high-time resolution scintillators are used in high energy physics. In the present work, radiation defects in BaF₂ and CaF₂ crystals are investigated upon irradiation with 220 and 230 MeV ¹³²Xe ions to fluences of 10¹¹-10¹⁴ ions/cm². Optical absorption spectra were measured in the range from 10 to 1.5 eV. To study thermal annealing of the coloration centers created in BaF₂ and CaF₂ single crystals, step-by-step annealing of radiation defects was carried out. Multiple heating-cooling-measurement cycles were performed under identical conditions with increasing annealing temperature (T_ann) by 20-30 K. All spectra were measured at RT. It is assumed that the radiation-induced absorption band at 9.7 eV in CaF2 is made up of a combined contribution of anion clusters created in the ion track region and complex hole aggregates.  The center responsible for the 8.15 eV band also has a hole nature and a structure close to the 6.5 eV center.  In the UV region of the BaF₂ spectrum, a slight increase of the 9.18 eV band is observed in the temperature range 450-575K, which is accompanied by a decrease of the bands with maxima of 6.5 and 8.2 eV, i.e., the latter is also of hole nature. The bands 7.4 eV, 2.24 eV and 3.04 eV practically do not change in the whole annealing temperature range. By analogy with CaF2, in Xe-irradiated BaF2 single crystals, the high-energy bands below the fundamental absorption can also be of anion cluster nature.