Induced Ultraviolet Fluorescence and Its Release by Visible Light*
In the course of investigations on the detection of induced radioactivity and of extremely weak ultraviolet radiation we have recently observed that the ultraviolet fluorescence of certain substances following their exposure to and their removal from rœntgen or gamma-irradiation persists for unexpectedly long periods of time. This ultraviolet fluorescence, furthermore, is conspicuously increased if the irradiated compounds be exposed to visible light. This is in agreement with an observation made by W. Kudrjawzewa.1
Of twenty-five compounds we have thus far examined sodium chloride, potassium chloride, rocksalt and fluorite crystals exhibit these effects 'most clearly. Impurities inhibit, but previously repeated recrystallization facilitates the subsequent induction of ultraviolet fluorescence. Heating the crystals impairs their fluorescence which, indeed, is completely stopped by dissolving the compounds in distilled water.
A photoelectric Geiger-Muller-counter tube equipped with a cadmium electrode and a quartz window has been used for measuring the ultraviolet fluorescence. This counter is very sensitive to ultraviolet radiation and hardly responds to visible light. The ultraviolet light has also been registered by means of the darkening produced on Eastman hypersensitive panchromatic dry plates.
The fluorescent effects described have been elicited in the following way: Samples of crystals were exposed to radon (100 millicuries, 1½ mm. brass filter, close distance) or to rœntgen rays (200 kv, cardboard filter, 40 cm focal distance, 25 r-min.) for periods of time ranging from a few seconds to ten hours. A few particles thus irradiated cause discharges in the Geiger counter; in general, the number of counts is proportional. . .