The Phosphorescence of Calcium Tungstate Induced by X-rays

The rate of decay of a very phosphorescent sample of calcium tungstate was determined photographically and found to fit the empirical equation of Becquerel, <i>I</i>^-x = <i>a</i>+<i>bt</i>, within the experimental error, from thirty seconds to five minutes after the exposure to x-rays. The phosphorescence of this sample could not be detected photographically more than 50 hours after the exposure of 300 milliampere-seconds at a distance of 30 cm from the target. Under the same conditions the phosphorescence of normal calcium tungstate disappeared in one minute. The intensity of the phosphorescence was found to be closely proportional to the product of the intensity of the x-rays and the time of exposure up to a limiting value, beyond which increased exposure caused a negligible increase in the phosphorescence. In general, these properties are quite similar to those of the “phosphors” of Lenard and others.

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