Optically induced conversion of glow peaks 4 and 5 in LiF:Mg,Ti due to localized and de-localized recombination mechanisms

The composite structure of glow peak 5 in LiF:Mg,Ti (TLD-100) has been investigated using optical bleaching by 310 nm (4 eV) light. The decay of composite peak 5 and the simultaneous growth of peak 4 are described by two exponential components of the same mean life for both peaks. The conversion of peak 5 to peak 4, as a function of the temperature of post-irradiation annealing, reveals that the conversion efficiency of charge from peak 5a traps to peak 4 traps is very high (30%) whereas the conversion efficiency from peak 5 traps to peak 4 traps is no greater than a few percent. Glow peak 5a is believed to arise from geminate recombination of a locally trapped electron-hole pair in a spatially correlated trapping center (TC) and luminescent center (LC). Glow peak 5 results from recombination via electron diffusion in the conduction band following thermal release of a singly trapped electron in the TC/LC structure. Glow peak 4 arises from a singly trapped hole in the TC/LC structure. The high conversion efficiency of peak 5a to peak 4 arises from direct optical ionisation of the electron in the electron-hole pair, leaving behind a singly trapped hole (peak 4), a direct mechanism relatively free of competitive processes. Optical ionization of the 'singly trapped' electron (peak 5), however, can lead to peak 4 only via multi-stage mechanisms involving charge carrier transport in the conduction band, a mechanism subject to competitive processes, which leads to an order of magnitude decrease in the observed conversion efficiency.