Spectral changes and wavelength dependent thermoluminescence of rare earth ions after X-ray irradiation

The thermoluminescence spectra of rare earth doped materials after X-ray irradiation typically vary with the glow peak temperature. Additionally, there are many examples where, for the same dopant ion, the expected component emission lines peak, but at different temperatures. This unusual behaviour is discussed in terms of changes in proximity of coupling between trapping and recombination sites. Changes in the energy barriers for recombination influence alternative routes for charge transfer to rare earth sites which can involve different higher energy states of the rare earth dopants. Proposed mechanisms include selective tunnelling, or barrier crossing, in addition to normal charge transfer from remote trapping sites. The model successfully describes numerous examples in terms of the energy scheme for the rare earth ions. Whilst the standard emission lines are recorded in the glow curve spectra they do not always occur at the same temperature, and, even for the same rare earth dopant, they can differ by as much as 30 degrees C. These wavelength dependent variations in peak temperature not only offer information on the proximity of trap and recombination sites, but also introduce issues in conventional activation energy analysis when recording is with polychromatic light. The concepts are relevant for related types of measurement, such as optically stimulated thermoluminescence.