Answer to Question #3408 Submitted to "Ask the Experts"

The promethium used in the watch, assuming it was promethium, would have been Pm-147. Although the activities varied from one timepiece to another, it would be unlikely that your watch contained more than 100 microcuries (μCi). Compare this to 4,000 microcuries, the annual limit of intake (ALI) for promethium-147. If the ALI were ingested, the resulting effective dose equivalent would be approximately 5,000 millirem (5 rem). If you had consumed the entire 100 μCi, the upper limit estimated for the watch, this would equate to 125 mrem, approximately one third to one half of the average American's yearly exposure to natural radiation. With regard to what might be left today, consider that almost ten half-lives of promethium-147 have passed since the late 1970s. There would be so little left (a fraction of a microcurie perhaps) that it would be difficult to detect. In other words, don't worry. But don't do it again.

The following might be of some interest even though it doesn't apply directly to your situation. It is taken from a discussion of radioluminescent materials at the Oak Ridge Associated Universities Web site: "Promethium-147 (Pm-147) is a low energy beta emitter with a 2.6 year half-life. It emits a gamma ray at 121 keV in a small fraction of its decays. The Pm-147, mixed together with ZnS in a polymer paint, is either an insoluble oxide or is incorporated into ceramic microspheres. For a watch or clock containing Pm-147, the activity is approximately 0.05 mCi (50 μCi). The dose to an individual from a Pm-147 containing timepiece is due to the emission of Bremmstrahlung and gamma rays. The low energy beta particles cannot penetrate the case. According to NUREG-1717, a person continuously wearing a watch containing the maximum allowable level of Pm-147 would receive less than 400 mrem to a small area of skin on the wrist and less than 1 mrem to the gonads. NUREG-1717 also calculated the effective dose equivalent to the wearer of a Pm-147 containing watch using two scenarios. In one, the effective dose equivalent was estimated to be 0.2 mrem per year. In the other scenario, the estimated effective dose equivalent was 0.4 mrem per year. The estimated effective dose equivalent to a homeowner due to a clock containing Pm-147 was 0.002 mrem per year. For the purpose of these calculations, the activity was assumed to be 100 μCi per timepiece (watch or clock). The National Radiological Protection Board (NRPB) of Great Britain estimated that the use of a Pm-147 containing pocket watch would result a dose to the gonads of up to 0.2 mrem and a maximum dose of 35 mrem to the skin."