Photostimulable storage phosphor

Energy storage phosphors are materials in which an irradiation induces ionization of some ions followed by capture of the extracted electrons by vacancies, forming colour centres, or oxidizing cations. By irradiation in the absorption bands of the centres so created (photostimulation) or temperature rise, the trapped electrons can be released and the recombination energy with holes transferred to a luminescent ion. 

The stoted energy can be released by thermal or optical stimulation. In the case of thermal stimulation the irradiated phosphor is heated to a temperature at which the energy barrier AE can be overcome thermally. The trapped electron (or hole) can escape irom the trap and recombine with the trapped hole (or electron). In the case of radiative recombination, luminescence is observed which is called thermally stimulated luminescence (TSL) (compare Sect. 3.5). Under optical stimulation the energy of an incident photon is used to overcome AE. The luminescence due to optical stimulation is called photostimulated lumine.scence (PSL). The phenomenon of stimulated luminescence from storage phosphors has been known since 1663 (Boyle). Storage phosphors have found a wide range of applications, e.g. as infrared detectors and in the field of dosimetry [3J. 

It has been found that only the bromine F centers contribute to the photostimu-lability, although the X-ray irradiation creates both fluorine and bromine F centres [18]. These authors have also derived estimates of the concentrations of defect centers in a particular BaFBr Eu- sample. Even if these values arc not very reliable, they illustrate how complicated the physical mechanisms in a storage phosphor may be 82% of the centers created by irradiation are fluorine F centers or variants therebf these do not contribute to the photostimulable luminescence. The remaining 18% of the created centers are bromine F centers. Of these about one quarter arc spatially correlated to the hole center and the Eu ion, i.e. they yield PSL via a tunelling mechanism the others are not correlated and need thermal activation via the conduction band in order to yield PSL. These estimated concentrations depend strongly on the history of the sample and on the Eu concentration. 

Another X-ray storage phosphor is RbBr TI+. The luminescent center is the TP (6s ) ion which emits by a 6s6p - 6 transition (Sect. 3.3.7). The electron is trapped at a bromine vacancy, the hole is assumed to be trapped at a Tl ion. The storage state can, therefore, be characterized by F -f Tl . The PSL center consists of these two centers optical stimulation excites the F center, and the electron recombines with the hole on thallium yielding TP in the excited state [19]. The efficiency of the photostimulated luminescence of RbBr TI+ decreases above 230 K due to a thermal instability of one of the trapped charge carriers. 

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