Electron paramagnetic resonance study of centres involved in storage processes in K2YF5: RE thermoluminescent phosphors

Ce3+- and Th3+-activated K2YF5 crystals, which have been shown to exhibit thermoluminescence (TL) response to alpha, beta, gamma and X-rays, are investigated using electron paramagnetic resonance (EPR). X-ray irradiation at room temperature of Ce-doped crystals produces at least four distinct paramagnetic centres in the g approximate to 2 range. Irradiation at 77 K leads to the predominant production of only one of these centres, which appears to be a V- or H-type intrinsic trapped hole centre. Most of these centres can also be created in the Tb-doped crystals. In addition, the appearance of strongly angular dependent EPR signals in a wide magnetic field range, suggest that the Th activator ions may act as electron or hole traps. We show that both Ce- and Th-doped crystals exhibit, next to their well-studied TL above room temperature, very strong TL in the 100-300 K range. The possible involvement of the centres observed with EPR in the TL processes is discussed.