Performance characteristics of a doped alkali halide (KBr:In)-based imaging plate (IP) and dosemeter (D) both the devices utilizing optically stimulated luminescence (OSL) and suitable for UV-light and x-ray energy recording are described. By exploiting (silicon) photodiode array as a photodetector, the detective quantum efficiency of 0.1 for IP (KBr:In) can be achieved. A very wide dynamic range, 1010:1 for UV (6.35 eV) and 1011:1 for x-ray (44 kV tube voltage) OSL recording at the spatial resolution of 1 mm2, is favorable for digital imaging and considerably improves image quality. These possibilities are illustrated by presenting available information capacities, energy consumption per bit as a function of the spatial resolution, and signal contrast ratio as a function of the input energy if using the KBr:In material with optimum characteristics (the activator concentration of 1017 cm-3, the 245 nm UV-light sensitization of UV detecting, minimum concentration of Ca+2 ions, etc.). Both the radiation dose storage is stable only up to 18 degrees Celsius and 200 s after an x-bremsstrahlung write at this temperature. When used for dose measuring, KBr-In monocrystals can be exploited for low-intensity x-ray detection and provide a possibility of the space background measuring in a second time interval.
By calibrating the photostimulated luminescence (PSL) response with the known intensity of a standard light source, efficiency of the KBr:In powder material to the x- ray bremsstrahlung exposure at room temperature is estimated. At 44 and 85 kV voltages on the x-ray tube, the values of (8 plus or minus 2) multiplied by 105 and (18 plus or minus 4) multiplied by 105 photons from 1 mm2 area of the 1 mm thick material are obtained as PSL responses on a 1 mR x-ray exposure, respectively. Under an assumption of the uniform x-ray excitation within the material, the maximum values for the x-ray irradiation dose and the PSL response both corresponding to the upper limit of linearity range are tentatively appraised, using the data for PSL response saturation under UV-irradiation.
Radiation-induced effects in doped alkali halides, mainly in KBr:In, are studied by the luminescence technique. The activator luminescence during a 10 s under UV-light or electron irradiation and, after it, the pulsed photostimulated luminescence on a phosphorescence background were investigated. The obtained results allow us to conclude that the main host lattice excitation relevant to both the luminescence processes mentioned above is a very mobile excitonic excitation including a photon phase and the self- trapped exciton in its composition. The photon phase, as we suppose, represents a free exciton luminescence at room temperature. In this phase, via multiple reabsorption in the low-energy exciton absorption band, the excitonic excitation can overcome large distances. In the phase of the self- trapped exciton, the induced decay within a perturbed region around the activator hole center takes place. As a result, the pairs of F-type electron and activator hole centers, contributing to the photostimulated luminescence under stimulation with light in the F absorption band, are formed. The above-mentioned defect formation process is dominant over the excitation of activator ion at low activator concentration. At high activator concentration, when strong competition between reabsorption in the exciton absorption band and absorption in the local center band connected with the activator takes place, the activator luminescence prevails. The activator luminescence spectrum is found to be sensitive to the F or F-aggregate centers formation in the region near the activator hole center.
The photostimulated luminescence is studied for CsI-Tl crystal after irradiation with ultraviolet light in the 80 - 300 K temperature range. The PSL creation spectrum coincides with the D absorption band at 80 K. Three bands are observed in the stimulation spectra at 80 K: 1400, 950, and 600 nm. The 1400 and 950 nm stimulation bands are presumably explained as optical transitions in the Tl0 and Vk centers situated in the spatial correlated pairs. The stimulation at 600 nm band is ascribed to the unperturbed Tl0 centers.
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