Spectral evidence for defect clustering: Relevance to radiation dosimetry materials

Highlights: • Detailed analysis of emission spectra can reveal evidence of defect clustering. • Defect clustering important in many or most TL and OSL materials. • Localized and delocalized recombination processes prominent, including tunnelling. • Defect clustering can cause high TL/OSL efficiency, fading, and anomalous E and s values. The accumulation of experimental evidence available in the literature, especially from optical emission and excitation spectra, demonstrates that defect sites in luminescent materials are primarily complexes extending over multiple lattice sites. Included in such luminescent materials are those used in luminescence dosimetry (using thermoluminescence and/or optically stimulated luminescence). When defect complexes form, an understanding of recombination processes in these materials must include charge transfer via both delocalized and localized mechanisms, including both excited state and ground state tunnelling among nearest-neighbour sites. The various clustering mechanisms and recombination pathways that result from defect cluster formation are complex and described by a multitude of relevant parameters. There is no obvious route to optimising so many parameters, except by trial and error through massive amounts of experimentation, as everything from composition, growth, powder formation and subsequent thermal treatments can make dramatic changes to final luminescence and dosimetric properties. Reliance on idealised historic models is inappropriate in terms of improving practical dosimeters.