SIMULATION OF POINT-DEFECTS IN HIGH-DENSITY LUMINESCENT CRYSTALS - OXYGEN IN BARIUM FLUORIDE

Barium fluoride is an example of a high-density scintillator for detec ting high-energy radiation. In use, its luminescent transmission is se riously degraded by radiation damage, This effect is associated with o xygen, among other impurities. At one time it was suspected that oxyge n O-, having been dissociated from a defect complex by radiation damag e, absorbed some of the luminescent energy of the crystal. This explan ation has now been abandoned, and the present work shows quantitativel y that it is not correct. Specifically, a detailed study of the optica l absorption of O- shows that its excitation energy, split by spin pol arization, is similar to 50% higher than the luminescent frequencies o f the crystal. Instead, color centers, such as F centers, have come to be suspected. One origin of the color center is shown here to be the dissociation of a defect complex made up of an O2- ion bound to a fluo ride vacancy, accompanied by electron transfer from oxygen to vacancy, forming an F center. The study of the optical excitation of O- is use d to assess the qualitative and quantitative importance of the main el ements of the physical model and computational method in such a simula tion. These elements include the ion-size effect of Ba2+ ions, spin-po larization effects in ground and excited states. electric quadrupole m oment consistency between the O- ion and the embedding BaF2 crystal, b asis set augmentation and optimization in the treatment of a quantum m olecular cluster that includes the impurity for both ground and excite d states, correlation correction, and projection of excited states ont o spin eigenstates.