EPR and optical absorption specta of X‐irradiated NaF: U,Li crystals

Abstract

Investigation of radiation-induced defects in NaF and NaF:Li crystals led to discovering a nuniber of effects, allowing to use these crystals as a three-diniensional recording medium (CAIMI et al.), for suppressive writting for information storing (CASASENT, CAIMI), and to create laser tunable in infrared region (MOLLENAUER; ALYBAKOV et al. 1980). Absorption spectra and EPR of irradiated NaF:U crystals were studied by ALYBAKOV and co-workers (1966) and LUPEI and co-workers. The latest investigations showed that crystal properties may be changed considerably by means of doping with two impurities simultaneously. I n this respect the investigation of earlier unstudied NaF : U,Li crystals was of a certain interest. Some results of such an investigation are presented in this note.Studied crystals were grown by the Kyropoulos method in a platinum crucible from NaF of special purity. The melt was doped with 0.1 molyn of LiF and 0.001 molyn of uranyle nitrate. X-irradiation was performed at room temperature (tungsten target tube, 55 kV, 10 m A ) . The absorption spectra were measured on an Unicam SP8-100 spectrophotonieter using a nitrogen cryostat. EPR spectra were recorded on a Varian E-112 radiospectroiiieter at liquid nitrogen temperature.The luminescence spectrum of NaF: U,Li (the line locations and their intensities) turned out the same as that of NaF: U crystal. As to absorption spectra, doping with lithiuni results in the growth of absorption caused by the presence of U6+ ions in the lattice (Fig. 1). Under the X-irradiation absorption of U6+ gradually becomes weaker (Fig. 2 ) ; new weak absorption lines appear simultaneously at 554 and 581 nm. Intensity of these new lines increases slightly up to 5 hrs of X-irradiation, and under the further irradiation it remains invariable. The weakening of U6+ absorption is connected with the change of uranium valence at the expense of capture of electrons. Uranium in crystals is in a diamagnetic six-valent state, therefore unirradiated crystals show the EYlt signals. X-irradiated NaF: U,Li crystals show complex E P R spectrum, indicative of the presence of Paramagnetic centers of several types in these crystals. Investigation of angular dependences of the most intensive EPR lines showed that they are due to four different centers; two of them (Tg, and Tg,)