Fluorescence, Phosphorescence, and Photostimulation of NaCl(AgCl) with High-Energy Irradiation

Quantitative results are presented on the light-emitting and storage properties of silver chloride activated sodium chloride crystals excited with high-energy radiation. The characteristic of the strong light emission, mostly in the ultraviolet range, upon stimulation by longer wavelength light after pre-excitation with high-energy radiation, is extensively investigated, as well as the fluorescence and phosphorescence. A practically linear stimulated light response as a function of the roentgen dose over a range from about 10^-2 to 10⁴ roentgens is found. Such a measurement of stimulated light can be performed a considerable time after the exposure to the high-energy radiation and still provide a determination of the dose. Both the ordinary phosphorescence and the stimulated light phosphorescence under suitable conditions may decay very slowly and show many other similarities. The measurements show that the amount of energy stored by the crystal which can later be released as emitted light is greater than 3 percent of the energy totally absorbed. After a sufficiently long stimulation by visible light the rate of de-excitation becomes very small; further stimulation with light of shorter wavelengths produces a rise in the amount of stimulated light and an increase in the de-excitation rate. The amount of stored energy which can be stimulated as light decays slowly with storage time. This effect is closely correlated with the coloration well known with pure NaCl crystals; bleaching a crystal results in a reduced stimulation. Most of the experiments were carried out with a radium gamma-ray source, but measurements with x-rays and fast electrons show very similar results. Experiments with fast electrons were performed with doses up to 500 000 rep; saturation then occurs, but after de-excitation (bleaching) the crystal could be re-excited and be used again with no appreciable change in properties. The counting properties of these crystals are rather poor because of small peak heights. A rather complete explanation of these properties can be given by a model assuming a continuous distribution of traps over a small range. The energy released from these traps goes to the activator, from which the energy is emitted as light.