Theoretical models of thermoluminescence (TL) and some physical properties of zircons are presented. Descriptions of the TL equipment and the experimental methods used, including the etching techniques, are given. Several methods of analysing the TL glow curves are described and some values of kinetics parameters are obtained. Radiation damage, caused by the decay of uranium (and thorium) present as impurities in the crystals, is estimated by measuring the spontaneous-fission track density in them (rather than a-tracks, which are not revealed by etching the zircons). The distribution of fission tracks in a crystal also gives a representation of the uniformity of radiation damage in it. It is found that TL sensitivity as well as the amount of natural TL are adversely affected by the radiation damage. By annealing the crystals, the damage is "healed-up" as can be seen from the reduction in fission-track density, and the TL sensitivity, recovers, reaching a maximum when all the tracks vanished. Induced radiation damage, caused by bombarding the crystals with heavy ions (e.g. U-23β,α , etc.) is also found to adversely affect the TL properties. Annealing can, once again, heal the induced damage. The etching time (to fully develop the tracks) and the x-ray diffraction angle of the (112) plane are show~ to decrease with increasing radiation damage. The TL spectra (between 400-600 nm) of less damaged and synthetic zircons have two peaks at _480 nm and ",580 nm , However in highly damaged zircons (natural track density is greater than ",1 X 108 cm-2), broad TL spectra with a possible peak at -600 nm are obtained. TL in zircons is found to fade anomalously and the rate of fading increases with radiation damage. In highly damaged zircons, the long wavelengths (yellow-orange) fade faster than the shorter wavelengths (blue-green). A possible mechanism for the anomalous TL fading is also discussed. Phototransfer-thenmolwninescence (PTTL) is used to study the high temperature peak ( '"500oC). Optical bleaching using UV and white light on some of the TL peaks is investigated. It is concluded that zircon is not suitable for conventional TL dating because of anomalous fading of the "normal" (-500oC) TL peaks. However, it is possible,. by means of PTTL, to use the "high" temperature peak (between _600-650oC) for TL dating.
|Amin, Y. Mohd
|University of Birmingham
|Electronic Thesis or Dissertation
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