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

Abstract

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

The concept of quasi-tissue-equivalent nanodosimeter based on the Glow Peak 5a/5 in LiF:Mg, Ti (TLD-100)

“…The successful fit of both the decrease of peak 5 and the increase of peak 4 with the same meanlives and a constant conversion efficiency supports the hypothesis that the increase of peak 4 arises directly from ionisation of electrons from the composite trapping structure of peak 5 13 . It seems highly unlikely, however, that transfer of electrons from peak 5 to peak 4 via ionisation and diffusion in the conduction band could result in such a high conversion efficiency.…”

“…(i) The high conversion efficiency (CE) (CE 5aĺ4 = 3 ± 0.5) of peak 5a to peak 4 (a hole-only trap) deduced from detailed I m -T stop optical bleaching studies at 310 nm (4eV) compared to the much lower CE of peak 5 (an electron-only trap) (CE 5ĺ4 = 0.0026 ± 0.012) 13,14 . Using computerized glow curve deconvolution it is possible to quantitatively study the conversion of peaks 5a and 5 to peak 4.…”

Thermoluminescence solid-state nanodosimetry--the peak 5A/5 dosemeter

“…In the geminate recombination mechanism responsible for peak 5a, delocalised charge carrier traffic in the conduction band/valence band is not involved, whereas the opposite is true for peaks 4 and 5 in which the recombination mechanism is completely delocalised. The TC giving rise to composite peak 5 is associated with a 4 eV optical absorption band and very strong conversion of peak 5a to peak 4 following 4 eV optical excitation and stepped (I m 2T stop ) annealing is the primary experimental evidence supporting the above hypothesis for the TL absorption mechanisms giving rise to composite peak 5 (11) . Owing to the 'two-energy transfer event' nature of peak 5a, it is expected on purely statistical grounds that its relative intensity following HID irradiation would be increased relative to the same ratio following LID radiation because of the very high levels of dose (ionisation density) in the HCP track (12) .…”

Localized and delocalized transitions and optical excitation of LiF:Mg,Ti following alpha and beta irradiation

“…Intensity of composite peak 5 (T m ¼ 205 1C) rapidly decreases but the intensity of peak 4 increases [1][2][3]. The conversion efficiency is defined as CEðtÞ ¼ I 4 ðtÞ À I 4 ð0Þ I 5 ð0Þ À I 5 ðtÞ ,…”

“…In the coupled trapping center (TC)-luminescent center (LC) model of composite peak 5 ( Fig. 1) peak 5a is believed to arise from geminate recombination between a locally trapped electrode-hole (e-h) in the TC/LC pair, peak 5 arises from capture of an electron only and peak 4 arises from the capture of a hole only [1][2][3]. Very high CE of peak 5a to peak 4 is easily understood in the context of this model.…”