60. Radioactivity and Counters

Small "license free" sources of activity < 0.1 microcurie can be taken into the class to activate a small hand held counter. Cloud chamber alpha sources of lead 210, beta source of strontium 90, thoriated tungsten welding rod, uranium glass (alpha beta gamma), Americium (alpha gamma) from a smoke detector and a hot fiesta ware cup and saucer with uranium glaze are available.

Americium-241, with a half-life of 432 years, is used in most domestic smoke detectors. Am-241 decays by emitting alpha particles and 60 keV gamma radiation to become neptunium-237.

Uranium glass was used to make a yellow tinted dinnerware from Victorian times.  Sometimes called Vasoline glass, or depression glass, our bowl measures about 5 µSv/hr.  You can tell uranium glass because it fluoresces green under blue or UV light.

Fiesta ware was the largest selling dish line in American history – 200 million dishes were shipped since 1936.  The red/orange color glaze contains uranium.  The government seized the company's uranium supply in 1943 out of fear it could be used to make a bomb.  A single plate contains about 4.5 grams of uranium, mostly U-238.  Production resumed in 1959 with depleted uranium (depleted of U-235) and continued until 1972 when it was discontinued out of concerns about uranium and lead leaching out of the glaze.  Fiesta ware is the hottest source we have and measures over 100 µSv/hour at the plate surface, but this is not considered dangerous for display purposes.  U-238 has a half-life of 4.5 billion years.

Radiation safety standards limit public exposure to 1 mSv/year and occupational exposure to 50 mSv/year.  Natural background radiation is about 3 mSv/year.

Contrast that with a typical medical dose as shown below.  This reading was taken about two hours after injection with Technetium-99m, a radioisotope used for heart imaging with a 6 hour half life. 

A handheld Gieger-Mueller tube is available.