A lithium fluoride (LiF)-based photo-fluorescent film dosimetry system provides a means of determining absorbed dose to materials by the photo-stimulated emission of wavelengths longer than that of the stimulation wavelength. The absorbed dose is obtained from the amount of the light emission. Imperfections within the ionic lattice of alkali-halide compounds such as LiF act as traps for electrons and electron holes (positively charged negative-ion vacancies). These imperfections are known as color centers because of the part they play in the compound's ability to absorb and then release energy in the form of visible-light photons. Like an atom, these color centers have discrete, allowed energy levels, and electrons can be removed from these sites when energy of the appropriate wavelength and intensity is transferred to the material. The resulting fluorescence spectra contain discrete peaks that can cover a range of wavelengths, depending upon the type of alkali-halide (8). An example of fluorescence spectra from a LiF-based dosimeter is provided in Fig. 1. A system of optical filters within a light-detecting instrument (that is, fluorimeter) can be used to block all but a narrow range of wavelengths that are desired for use. Theories on how color centers are formed, how luminescence mechanisms work, and their application in dosimetry are found in Refs (8-13). For characterization studies on specific photo-fluorescent dosimeters see Refs (1-7) and (14-19).

In the application of a specific dosimetry system, absorbed dose is determined by use of an experimentally-derived calibration curve. The calibration curve for the photo-fluorescent dosimeter is the functional relationship between ΔE_f and D, and is determined by measuring the net fluorescence of sets of dosimeters irradiated to known absorbed doses. These absorbed doses span the range of utilization of the system.

Photo-fluorescent dosimetry systems require calibration traceable to national standards. See ISO/ASTM Guide .

The absorbed dose is usually specified relative to water. Absorbed dose in other materials may be determined by applying the conversion factors discussed in ISO/ASTM Guide .

During calibration and use, possible effects of influence quantities such as temperature, light exposure, post-irradiation stabilization of signal, and absorbed-dose rate need to be taken into account.

Photo-fluorescent dosimeters are sensitive to light, especially during irradiation and post-irradiation stabilization (7). Some color centers are sensitive to the UV and blue regions of the spectrum, while other centers are only sensitive to the UV. Therefore, they need to be packaged in appropriate light-tight packaging shortly after manufacture, and during use they need to be packaged or the appropriate filters placed over room lighting. Filtering the light fixtures involved during irradiation may be required for irradiations using low-energy X-rays or electrons where unpackaged dosimeters are used.

The signal from photo-fluorescent dosimeters either increases or decreases with time following irradiation, depending on the color center utilized (19). This stabilization process, which can last from hours to days depending on storage temperature (and dose for some color centers) can be accelerated and stabilized by heat treating the dosimeters after irradiation and before readout (see 9.2).

Note-Also shown are transmission curves for green and red emission filters.

FIG. 1 Excitation Spectrum and Resulting Fluorescence Spectrum from the Sunna LiF-based Film Dosimeter