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Radioactive food!  Potassium chloride (KCl) is the main
ingredient in Nu-Salt, NoSalt, and other substitutes, and
0.0117% of potassium atoms are beta-emitting K-40.
The radiation is easily detected with a Geiger counter.

When starting a radioactive material collection, one of the first places to stop is the grocery store.  Potassium, in the form of its chloride, is sold as there as salt substitute for old folks who just love the great flavor of salt, but have to limit their sodium intake.  Naturally-occurring potassium is radioactive.  117 out of every million potassium nuclei are of the isotope K-40, which not only emit beta particles and energetic gamma rays, but also spit out high-velocity positrons (anti-electrons) on rare occasions.  Finally, in a few percent of decays, the K-40 nucleus will not emit a particle at all, but will actually kidnap one of the atom's own inner electrons in a process called electron capture (EC).  The 3 oz. shaker of Nu-Salt above contains approximately 2600 Bq (0.07 microcuries) of activity.  A 2" pancake Geiger tube will register 400 CPM, about eight times local background, if the contents of one 3 oz. Nu-Salt shaker are placed in a paper cup against the GM tube.

Potassium is also found (depending on locality and season) in road de-icing crystals (KCl), and stump remover (KNO3).  Besides being a substitute for table salt and a road de-icer, potassium chloride is the favored heart-stopping drug administered in lethal injections at your local state penitentiary.  If you have trouble getting potassium from any of these sources, note that it is also concentrated in certain feldspars, wood ash, fertilizers, and bananas.

Brazil nuts are another source of consumable radioactivity.  Due to the plant's chemistry, it tends to retain natural radium such that the nut may harbor levels of radium and its daughters that are several times in excess of soil concentrations.  Unlike salt substitute, however, Brazil nuts will not cause a noticeably higher reading on a Geiger counter.  Specialized instruments, like gas-flow proportional counters, must be used to measure the activity in the nut.