Optical absorption and luminescence in lithium fluoride TLD-100

Abstract

The isothermal decay at 25 °C of the optical absorption bands in irradiated LiF TLD-100 single crystals has been compared with the decay, under the same conditions, of thermoluminescence (TL) and dielectric loss. The decay of the 380-nm band can be described by a third-order process suggesting that this band is directly related to free Mg-vacancy dipoles, which cluster during aging to form trimers. When compared with recent measurements which indicate that the TL peaks 2 and 3 are not related to the Mg dipoles [G. C. Taylor and E. Lilley, J. Phys. D 15, 1243, 1253 (1982)] the present results infer that the 380-nm band is not directly related to these TL peaks either, in contrast to earlier conclusions [J. H. Jackson and A. M. Harris, Phys. Lett. 29A, 423 (1969)]. This assertion receives some support from pulse annealing measurements and from x-ray-induced luminescence spectra. In particular, the latter results reveal that the spectrum consists of several overlapping bands, the relative intensities of which depend upon the aggregation state of the Mg within the LiF lattice. An emission peaking at ∼460 nm is increased if the concentration of Mg-vacancy dipoles is increased, and an emission at ∼420 nm is related to the number of dipole aggregates (Mg trimers) present. As Mg is precipitated from solid solution the main emission approaches ∼410 nm, which is that seen in Mg-free, Ti-doped LiF. A comparison of these data with recent thermoluminescence spectra measurements leads to the suggestion that TL peak 2 is caused by Mg dipole/Ti complexes, and that TL peak 5 is caused by Mg trimer/Ti complexes.