1 Electron paramagnetic resonance study of rare-earth related centres in K 2 YF 5 :Tb 3+ thermoluminescence phosphors D. G. Zverev, 1† H. Vrielinck, 1‡ * E. Goovaerts 2 and F. Callens 1§ 1 Department of Solid State Sciences, Ghent University, Krijgslaan 281-S1, B-9000 Gent, Belgium 2 Experimental Condensed Matter Physics, Department of Physics, University of Antwerp, Universiteitsplein 1, B-2610 Antwerpen, Belgium (Etienne.Goovaerts@ua.ac.be ) † Dmitry.Zverev@ugent.be ‡ Henk.Vrielinck@ugent.be § Freddy.Callens@ugent.be (corresponding author) *Postdoctoral Fellow of the Research Foundation Flanders (FWO-Vlaanderen) ABSTRACT Rare-earth related centres have been investigated in K 2 YF 5 :Tb 3+ crystals, exhibiting thermoluminescence (TL) below and above room temperature (RT), using electron paramagnetic resonance (EPR) spectroscopy at Q (34 GHz) and W-band (94 GHz). The spectra have been studied prior to irradiation, after exposure in the kGy range to X-rays at 77 K and subsequent pulse annealing up to 570 K. In addition to Gd 3+ , previously studied in detail, we identified Er 3+ and Yb 3+ centres as accidental impurities in as-grown crystals and determined their effective g tensors. The EPR spectra of irradiated and annealed crystals provide evidence for the production of at least three distinct Tb-related trapped hole centres, two of which could definitely be identified as Tb 4+ . Hence, we prove that the Tb 3+ activator ions also act as hole traps in K 2 YF 5 . Pulse annealing experiments indicate that the TL above RT results from thermal release of electrons, recombining at these Tb 4+ ions. KEYWORDS Electron paramagnetic resonance; EPR; K 2 YF 5 ; thermoluminescence, TL; Tb