Luminescence mechanisms in Tb- and Dy-doped magnesium tetraborate samples

The luminescent properties of thermoluminescent detectors based on magnesium tetraborate doped with terbium or dysprosium and sodium were studied. Analysis of the obtained results allows verifying the charge state of the dopant ions. Magnesium tetraborate was prepared by a solid-state reaction between boric acid and magnesium carbonate or magnesium oxide. After grinding, the reaction mixture was dried at 333–353 K, dehydrated at 473 K, and annealed in two stages at 1023 K and 1143 K, respectively. Dysprosium, terbium, and sodium impurities were introduced before the final annealing step. The photoluminescence signal was recorded using a spectrometer based on a double monochromator under monochromatically selected excitation from a xenon lamp. Cathodoluminescence was recorded with a CCD spectrometer under electron-beam excitation. Photoluminescence decay kinetics were measured with a photomultiplier tube under laser excitation, whereas cathodoluminescence decay kinetics were recorded using a photomultiplier tube and an oscilloscope. Photoluminescence, photoluminescence excitation, and cathodoluminescence spectra, as well as photo- and cathodoluminescence decay curves, were obtained for magnesium tetraborate samples doped with terbium or dysprosium and sodium. Dysprosium photoluminescence was absent in the non-irradiated sample and appeared only after electron-beam excitation. This indicates a change in the charge state of the dysprosium dopant. Thermally stimulated luminescence of the samples is associated with radiative transitions of terbium and dysprosium in the 3+ charge state. The dopant charge state determines the excitation pathway of the luminescence center during heating. In the studied samples, dysprosium predominantly occurs in the 2+ charge state, whereas terbium is initially present in the 3+ charge state. The obtained results can be used when selecting rare-earth dopants for the development of magnesium-tetraborate-based thermoluminescent materials. The established relationship between the dopant charge state and the thermoluminescence mechanism provides a basis for targeted control of the practical properties of these materials.