Investigation of the composite structure of peak 5 in the thermoluminescent glow curve of LiF:Mg,Ti (TLD-100) using optical bleaching

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 growth of peak 4 are described by two exponential components of the same mean lives for both peaks. Using computerized glow curve deconvolution and contrary to previous investigations, it is determined that peak 4 does not decrease in intensity at long bleaching times and is therefore not associated with the 4 eV absorption band. The fast and slow components of the exponential decay are associated with the bleaching of peaks 5a and 5, respectively. Optical bleaching as a function of the temperature of post-irradiation annealing reveals that the conversion efficiency of peak 5a to peak 4 is very high, at a constant value of approximately 30%, whereas the conversion efficiency of peak 5 is no greater than a few per cent. The behaviour of the conversion efficiency as a function of the temperature of the post-irradiation anneal is explained in the framework of the spatially correlated trapping centre/luminescent (TC/LC) centre model. In this model, the TL of peak 5a results from the geminate recombination of a locally trapped electron-hole pair: 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. 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 ionization of the electron in the electron-hole pair, leaving behind a singly-trapped hole (glow peak 4), a direct mechanism not subject to competitive processes. Optical ionization of the 'singly-trapped' electron (peak 5), however, can lead to peak 4 only via a multi-stage mechanism 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 efficiency.