Photostimulated luminescence

Fig. 6. The response of photostimulated luminescence to X-ray exposure level. O, A, for MoKa, , A, for CuKp. IPs were read under three different sensitivities (E) (O. E = 4000, A, A E = 400, , E = 40) and 3.6 preset dynamic range...

Fig. 2. Schematic representation of the mechanism of photostimulated luminescence in BaBrF Eu. (A) Formation of a colour centre (F) under X-ray irradiation by trapping of an electron in a bromine vacancy with trapping of the hole formed in the valence band by a hole trapping centre (HT) in the vicinity (B) Release of the trapped electron by laser irradiation and transfer of the electron-hole recombination energy to Eu2+. Relaxations after electron transfers have not been represented.

Halides can form colour centres with absorption bands in the visible range. Doped with europium whose decay time of the allowed 5d —< 4f transition is of the order of 1 fis, they are the most appropriate materials. Because of the presence of fluorine, only BaCIF Eu and BaBrF Eu have a high enough chemical stability for screens fabrication. The highest efficiencies of photostimulated luminescence have been obtained with the fluorobromide (Amax = 390 nm). 

The mechanisms of the photostimulated luminescence of Eu2+-doped fluorohalides have been extensively studied. In BaBrF Eu an excess of fluorine favours the formation of F centres in bromine sites. This results in a red shift of the absorption induced by X-rays which increases the luminescence yield for stimulation by a He—Ne laser at 633 nm [69], Various interpretations have been proposed both about the nature of the hole-trapping centre and the electron-hole recombination mechanism [70], It was initially assumed that holes are trapped by Eu2+, leading to the formation of Eu,+ [71]. However after long X-ray irradiation no change in the EPR signal of Eu2+ was observed and the luminescence of Eu3+... 

Two new fluorine-rich barium fluororochlorides, BaI2Cl5FI9 and Ba7Cl2FI2, have been recently obtained [76,77]. While in BaF2 the Eu2+ 5d - 4f emission is quenched by ionization processes (Sec. 10.5.1), in these fluorochlorides Eu2+ shows an intense emission. Only part of chlorine can be substituted by bromine. The efficiency of the photostimulated luminescence after X-ray irradiation increases with the bromine content. The new fluorohalides have higher chemical stability and are denser than BaCIF Eu and BaBrF Eu. 

A photostimulated luminescence emission (2max = 358 nm) has been recently obtained with Ce3+ incorporated in the elpasolite Cs2NaYF6 [70]. The decay time, 42 ns, is identical to that obtained by direct excitation. 

A typical image plate phosphor is doped BaFBr. When exposed to x-rays, oxidizes to Eu. Thus produced electrons may either recombine with Eu or they become trapped by F-vacancies in the crystal lattice of BaFBr. The trapped electrons may exist in this metastable state for a long time. They are released when exposed to a visible light and emit blue photons during recombination with Eu ions, e.g. see K. Takahashi, K. Khoda, J. Miyahara, Y. Kanemitsu, K. Amitani, and S. Shionoya, Mechanism of photostimulated luminescence in BaFX Eu (X = Cl, Br) phosphors, J. Luminesc. 31-32,266 (1984). 

EN 13751 2002 DIF using photostimulated luminescence herbs, spices, seasonings, shellfish... 

He-Nc laso. The red laser light (A = 633 nm) stimulates recombination, resulting in photostimulated luminescence. The intensity of the photostimulated luminescence is proportional to the X-ray dose. For each spot of the laser beam on the screen the intensity of the photo.stimulated luminescence is measured by a photomultiplier tube and stored in a computer. The X-ray photograph in the computer can be visualized on a monitor or by making a hard copy. 

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. 

---