Thermoluminescence sensitivity of ulexite after UV irradiation

The effects of UV radiation on the thermoluminescence (TL) properties of natural materials, in contrast to synthetic materials, have been scarcely studied. We report on the UV-induced thermoluminescence emission of a Turkish ulexite (NaCaB5O6(OH)(6)center dot 5H(2)O) that displays very complex TL glow curves, with at least three groups of components peaked at 130-140 degrees C, 240 degrees C and, 340 degrees C, wherein the last group is weaker. Such emission could be associated with structural changes in the lattice as well as alkali self-diffusion processes. The UV exposure performed at controlled temperatures (at room temperature (RT), 50 degrees C and 100 degrees C) produced a (i) different evolutions of the intensities of each maximum, which are directly related to the controlled thermal treatment; (ii) different intensity ratios among the groups of components; (iii) different activation energies (E-a) (1.13 eV for RT, 0.99 eV for 50 degrees C and 0.49 eV for 100 degrees C) calculated using the initial rise method; and (iv) similar scattering values (12.4%, 8.2% and 12.8%), which were not a function of the controlled temperature. The thermal stability tests conducted on this borate at different temperatures, based on the T-stop protocol, confirm the presence of a continuum in the distribution of the trap system with progressively increasing E-a (from 0.60 to 0.90 eV). (C) 2015 Elsevier B.V. All rights reserved.