Radioluminescent Clock Face - Radium (ca. 1950s)

I suspect that this radioluminescent clock face was made by the Western Clock Company (Westclox) of Illinois. There is nothing particularly unusual about it (ca. 4"  x  3 1/2"). 

The exposure rate at one foot is approximately 5 uR/hr above background. This would suggest that it contains 0.5 uCi, or a little less, of radium-226, a fairly typical value for clocks like this.

Radium-containing clocks were last sold in the United States in 1978.  In fact, it has been estimated that as many as 2.5 million clocks containing Ra-226 were sold by a Florida firm from 1976 to 1978. The total number of such clocks that have been sold in the US is probably over 100 million. 

 

Based on the results of one survey conducted in Tennessee, the average whole body exposure due to a radium-containing radioluminescent clock might be on the order of 7 to 9 mrem per year. Exposure rates to the head would be somewhat higher, possibly 5-10 times higher, because these are usually alarm clocks that would be positioned on a night stand near the head of the bed.

The photo to the right shows an autoradiograph of the clock face. It was produced by placing the clock face against Polaroid film for approximately 20 hours. The image was created by the action of radium's beta particles and gamma rays  on the film. The image is not due to any emitted light.

References

National Council on Radiation Protection and Measurements. Radiation Exposure from Consumer Products and Miscellaneous Sources. NCRP Report No. 56. 1977.

National Council on Radiation Protection and Measurements. Radiation Exposure of the U.S. Population from Consumer Products and Miscellaneous Sources. NCRP Report No. 95. 1987.

 

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Last updated: 05/10/11
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