BaFBr is usually contaminated with oxygen which influences the performance of this well known X-ray storage phosphor material. Oxygen is incorporated as a diamagnetic O2- ion on F- sites with a nearby charge-compensating vacancy in the Br- sublattice, as is known from the paramagnetic O-F and F(Br-) centres created by room-temperature X-irradiation and studied in detail with electron paramagnetic resonance (EPR) techniques. It is shown that BaEBr:O2-F has a luminescence band excited at 4.95 eV, peaking at 2.43 eV with two radiative lifetimes of 0.2 ms and 1 ms, respectively. With luminescence-detected EPR (i.e. optically detected magnetic resonance (ODMR)), it is shown that the luminescence is due to an excited triplet state of an O2-F-Br- vacancy pair. Time-resolved ODMR measurements reveal the same radiative lifetimes as seen in luminescence. No singlet emission was observed. The zero-field splitting can be explained assuming a F(Br-)-O-F pair defect as the excited triplet state.

A detailed investigation of the optical absorption and emission bands from the two types of F centre in BaFBr is reported, using optical, magneto-optical, and optically detected electron paramagnetic resonance and electron nuclear double resonance (ENDOR) techniques. With conventional ENDOR, the superhyperfine interactions of the F(Br-) centre are investigated in detail. It was found from the luminescence experiments that the two types of F centre are spatially correlated when they are created by X-irradiation in qualitative agreement with the observation of cross relaxation effects, as reported elsewhere.

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The temperature dependence of the luminescence from activators in BaFBr : EuZt photostimulated between 10 and 300 Kin the optical absorptions of F centres produced by room temperature x-irradiation, revealed two thermally-activated processes. The photostimulated luminescence (PSL) increases sharply inintensity when excited between 60 and 100 K. After the low-temperature PSL is exhausted by continuous F centre excitation, it can be almost entirely regained by warming the crystal to 300 K. This thermal cycle can be repeated several times. It is shown by EPR that no Eu3+ is formed upon x-irradiation at 300 K. These results are explained with a model for the PSL mechanism which involves a loose aggregate of E d t , F, and hole centres.

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An 0centre formed upon hole trapping by 0'in X-irradiated BaFBr is long-lived at room temperature and has been detected by EPR below 30 K. It can also be produced in low wncentrations by expos-of BaFBr to subbandgap (200400 nm) radiation. The formation of F(Br-) centres at chargecompensating bromide ion vacancies is a corollary of the photoionivltion of Oz-. High eoneentrations of oxide have been successfully introduced into this material by doping with anhydrous BaO. including Ba"O for unambiguous identification of the 0defect.Oxide addition and the formation of 0-produces a series of optical bands that have been assigned using a wmbination of optical and magneboptical techniques. The inBwnce of the material preparation conditions on the concentration of 0'is dircussed and possible mechanisms for the formation of 0are proposed

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Holetrapping sites and the mechanism of the photostimulated luminescence of the xray storage phosphor RbI:Tl+The oxygen-free x-ray storage phosphor BaFBr, doped with Sr 2ϩ and having Br Ϫ vacancies due to fluorine excess, is shown to have a high photostimulability shifted appreciably to lower photon energy compared to nondoped BaFBr because of the predominant generation of F A ͑Sr 2ϩ ͒ centers on the Br Ϫ sublattice upon room-temperature x irradiation. The F A ͑Br Ϫ ,Sr 2ϩ ͒ centers are less stable than unperturbed F͑Br Ϫ ͒ centers. Therefore, the redshift of the photostimulability disappears upon thermal activation above room temperature. From the failure to observe an F center infrared emission, it is concluded that F and F A centers are spatially correlated to hole centers of yet unknown nature.