Physical model of photostimulated luminescence of x-ray irradiated BaFBr:Eu2+

Abstract

A dynamical model of the temporal dependence of photostimulated luminescence of the storage phosphor BaFBr:Eu2+ as a result of preceeding x-ray irradiation is presented. The model is based on a monomolecular recombination mechanism. The commonly used bimolecular mechanism leads to contradictions with experimental observations. The monomolecular recombination is explained by the existence of a photostimulable luminescence complex with a recombination center and an electron trap in close proximity. Charge transfer after optical excitation occurs through tunneling. The simulation of the transient charge carrier dynamics is performed through rate equations. Good agreement wih experimentally determined temporal responses for different x-ray doses applied and stimulating light intensities is obtained.

Physics and Current Understanding of X-Ray Storage Phosphors

“…The EPR signal of Eu 2+ did not change upon prolonged X-irradiation [34], nor could a change be observed in the Eu 2+ luminescence [47]. Furthermore, the observation of an almost temperature independent PSL effect by stimulating F(Br --) centres from 4.2 K to RT [25] contradicts the model that electrons move through the conduction band upon photostimulation of F centres. Note that a repeated and extended study of the PSL temperature dependence revealed a strong temperature dependence [26].…”

“…It was found that the decay of the photostimulated luminescence under continued stimulation is temperature independent [48] and that the increase in the PSL intensity is proportional to the X-ray dose [25]. This latter result implies that retrapping of electrons after photostimulation does not occur, which makes electron-hole recombination via tunnelling more likely than via the conduction band.…”

“…Although the principle of information storage and read-out mechanism is simple, it is not clearly known how the recombination energy is transferred to the activator Eu 2+ which emits at 3.18 eV (390 nm). The PSL decay time is 750 ns [1,25]. In many years of thorough research and optimisation BaFBr : Eu 2+ has already reached a very high level of performance.…”

“…The initial exponential decrease indicates a tunnelling recombination of pairs of electron (F centre) and hole trap centres (Pr 4+ ) which are separated by definite distances having only a small variation. In BaFBr doped with the activator Eu 2+ the PSL process occurs also via a tunnelling recombination between the created electron and hole trap centres [25]. A subsequent stimulation at RT destroys all the remaining Pr 4+ centres.…”

Two-dimensional measurement of natural radioactivity of granitic rocks by photostimulated luminescence technique.

“…Eu-doped alkali halides have been introduced to such phosphors. In particular, barium fluorobromide doped with Eu 2+ (BaFBr:Eu 2+ ) has been investigated extensively as a phosphor which is capable to store X-ray information (von Seggern et al, 1988;Koschnick et al, 1991;Thomas et al, 1991). Takahashi et al (1984) and Iwabuchi et al (1994) Kawano and Ueda, 1966).…”