V-type Centers Research Articles

Properties of lithium hydride—IV: F-center formation at low temperatures

The growth of the F-band in LiH crystals has been studied in samples exposed to X-rays, thermal neutrons, and β-particles at low temperatures. The F-band growth curves obtained for LiH are compared to growth curves published for alkali halides irradiated under similar conditions. Probable mechanisms for the production of F-centers and H 2 molecules in V-type centers in LiH are discussed. Several LiH crystals containing LiT at various concentrations were prepared so that comparable samples cleaved from these crystals could be used for concurrent measurements of the effect of a regularly rising. F-center concentration on a number of crystal properties. In this way the growths of the F-band, the F-center paramagnetic resonance and the crystal volume were correlated to the steady production of F-centers and interstitial He 3 atoms by tritium decay in the crystals. The optical absorption of the interstitial He 3 atom in LiH was observed at 3.4 eV, and the β-band was found at 4.7 eV. Increasing F-center concentration and thermal annealing reduce the relaxation time of the F-center paramagnetic resonance in LiH. The aggregation of F-centers is associated with more drastic changes in the paramagnetic resonance band.

Optical absorption of cesium halides with excess halogen

X-rayed CsBr crystals show an absorption band at 275 mμ. As this bandcould be attributed to V-type centers, the optical absorption of additively coloured CsBr and CsI crystals with excess halogen was studied. It is not clear whether we are dealing with real V-type centers, or with a solution of the halogen in the crystal.

Paramagnetic resonance study of radiation damage in lithium fluoride

The low temperature annealing processes in LiF crystals X-rayed at 77°K have been studied by means of paramagnetic resonance. Four types of hole centers ( V-type centers), the structures of which are known from previous work, play an important part; namely, the self-trapped holes, the H-centers, the V F -centers and the V t -centers. The relative concentrations of these defects have been measured after X-irradiation at 77°K and after subsequent pulse annealing at successively higher temperatures. Four temperature intervals were found in which the paramagnetic resonance spectra undergo significant changes in intensity. There seems to be a one-to-one correspondence between the annealing processes observed by means of paramagnetic resonance and those detected by other investigators by means of thermoluminescence, conductivity, changes of the optical absorption and of the crystal volume.