We describe the effect of nanoscale spatially coupled trapping centre (TC)–luminescent centre (LC) pairs on the thermoluminescence (TL) properties of LiF : Mg,Ti. It is shown that glow peak 5a (a low-temperature satellite of the major glow peak 5) arises from localized electron–hole (e–h) recombination in a TC–LC pair believed to be based on Mg2+–Livac trimers (the TCs) coupled to Ti(OH)n molecules (the LCs). Due to the localized nature of the e–h pair, two important properties are affected: (i) heavy charged particle (HCP) TL efficiency: the intensity of peak 5a relative to peak 5 following HCP high-ionization density irradiation is greater than that following low ionization density irradiation in a manner somewhat similar to the ionization density dependence of the yield of double-strand breaks (DSBs) induced in DNA. Our experimental measurements in a variety of HCP and fast neutron radiation fields have demonstrated that the ratio of glow peaks 5a/5 is nearly independent of particle species for the protons, deuterons and He ions investigated, is somewhat dependent on HCP energy, and is roughly 2–3 times greater than the peak 5a/5 ratio in low ionization density electron and photon fields. The intensity ratio of peak 5a/5 thus has the potential of estimating the ratio of dose deposited via high ionization density interactions compared to low ionization density interactions in a nanoscale volume without any prior knowledge of the characteristics of the radiation field, (ii) non-linear TL dose response: the relative lack of competitive processes in the localized recombination transitions leading to the TL of composite peak 5 versus the dose-dependent competitive processes in conduction band-mediated delocalized luminescence recombination leads to non-linear dose response (supralinearity) for composite peak 5 and a dependence of the supralinearity on ionization density. This behaviour is modelled in the framework of the unified interaction model (UNIM).
The hypothesis that glow peak 5a arises from localised e-h capture is confirmed by the following experimental observations: (i) The high conversion efficiency (CE) (CE5a-->4 = 3 +/- 0.5) of peak 5a to peak 4 (a hole-only trap) deduced from detailed Im-Tstop optical bleaching studies at 310 nm compared to the much lower CE of peak 5 (an electron-only trap) (CE5-->4 = 0.0026+/-0.012). (ii) The lack of an increase in the sensitivity of glow peak 5a following 2.6 MeV and 6.8 MeV He ion irradiation in 'sensitised' material compared to the factor two increase in the sensitivity of peak 5; (S/S0)5a = 0.86+/-0.12, compared to (S/S0)5 = 2.0+/-0.2. (iii) The late entry into saturation of the 2.6 MeV and 6.8 MeV He ion TL-fluence response curves for peak 5a compared to peak 5 in sensitised and normal material resulting in the following values for the track radial saturation parameter: (r50)5a = 100+/-20) Angstroms compared to (r50)5 = 380+/-30 Angstroms. (iv) The low value of 0.1 for the 'track-escape' parameter of peak 5a deduced from the Extended Track Interaction Model analysis of He ion TL fluence response compared to order of magnitude greater values for peaks 5 and 5b.
The relative intensity of glow peak 5a in the composite glow peak 5 of LiF:Mg,Ti (TLD-100) is very weak following gamma irradiation, and has been estimated at approximately 0.1 of the intensity of peak 5. Typical glow curve analysis using computerised glow curve deconvolution with unconstrained variation of the peak shape parameters, yields values of the relative intensity of glow peak 5a varying from 0 to 15%. Due to the potential of peak 5a to fulfil the criteria of a quasi-tissue-equivalent nanodosemeter which estimates quality factor, considerable efforts have been invested in ancilliary techniques to improve the reliability of the estimation of the intensity of peak 5a. Optical bleaching and thermal annealing techniques were used to obtain single-peak glow curves consisting of peak 4 only and peak 5 only. A multi-stage CGCD protocol was then constructed using these peak shape parameters for peaks 4 and 5, which allows more accurate estimation of the relative intensity of peak 5a. Following 60Co irradiation of ten chips to a dose level of 1 Gy, the technique yields a relative intensity of 0.08 +/- 0.008 (1 SD).
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 glow peak conversion efficiency of peak 5a (T-m = 187degreesC) to peak 4 traps is very high at a value of 3 +/- 0.5 (1 SD) whereas the glow peak conversion efficiency of peak 5 (T-m = 205degreesC) to peak 4 traps is 0.0026 +/- 0.0012 (1 SD). 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 mechanisms. Optical ionisation of the 'singly-trapped' electron (peak 5), however, can lead to peak 4 only via multi-stage mechanisms involving charge carrier transport in the valence and conduction bands, a mechanism subject to competitive processes. The conduction/valence band competitive processes lead to the factor of one thousand decrease in the conversion efficiency of peak 5 compared to peak 5a.
A technique was developed to detect the crystallization of the upper-central surface of sapphire crystals grown in crucibles. This technique takes advantage of the large difference in optical absorption coefficients of molten and solid sapphire at the melting point, and is based on the recording of pyrometer readings throughout the course of growth. At the point of solidification of the observed sapphire surface, a knee occurs on the recorded graph. The accumulated data enable the optimization of growth rates at the end of the growth as well as the duration of the growth by proper timing of the beginning of crystal cool-down. It is also possible to detect and terminate unsuccessful growth runs at an early stage.
We describe the growth of sapphire single crystals, in dome geometry, by the gradient solidification method (GSM). Double walled molybdenum crucibles are utilized to grow the near-net-shape geometry. This significantly shortens the fabrication process of the dome to final dimensions, thus reducing the costs. The process is used to grow domes with different shapes (hemispherical and others), sizes (up to 6 inches in diameter) and crystallographic orientations (including the optical axis). Control of the thermal fields and growth rates is very important for the prevention of grain boundaries and scattering centers, as well as for the prevention of geometric distortions arising from surface tension effects.
This comment points out an omission in the above review which failed to mention the development of near-net-shaped sapphire dome growth at the NRCN. Domes of up to 6 inches in diameter and with different crystallographic orientations are routinely grown.
This comment points out an omission in the above review which failed to mention the development of near-net-shaped sapphire dome growth at the NRCN. Domes of up to 6 inches in diameter and with different crystallographic orientations are routinely grown.
Large sapphire single crystals with the geometry of near net shape domes are grown by the gradient solidification method (GSM). The growth is carried out in double-wall molybdenum crucibles, and perfect crystals, free of grain boundaries and scattering centers, are produced. The process is used to grow sapphire domes with different crystallographic orientations, including the optical axis. This process significantly shortens the fabrication process of the dome, thus leading to a cheaper, high-quality, product.
Large, high quality, single crystals of sapphire and calcium fluoride are routinely grown at the NRCN laboratories via the Gradient Solidification Method (GSM). This method was initially developed for the growth of dome-shaped sapphire crystals and then also applied to the production of bulk cylindrical crystalline boules. The diameters of the currently grown crystals are 150 mm and 85 mm for the sapphire and calcium fluoride respectively. The development of growth procedures for larger diameter sapphire crystals (up to 12") is underway. The growth processes were investigated and crystal imperfections (i.e. grain boundaries, twins, scattering centers, etc.) were correlated with various growth conditions. The optical and mechanical properties of the grown crystals were measured on fabricated components.
