Reply #19: K-40 gives off a beta and a gamma ray. Alpha emitters like Plutonium....

[Junkdrawer]

are a whole different story.

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Potassium is also in this category. There are actually three potassium isotopes: K39, a stable isotope, is the most abundant, at 93.26 % of the total; K41 is next in abundance at 6.73 % and is also a stable isotope. The potassium isotope of interest is a radioactive isotope, K40. It is present in all potassium at a very low concentration, 0.0118 %. It has a very long half-life, 1,260,000,000 years. When it decays 89 % of the events give rise to the emission of a beta ray with maximum energy of 1.33 Mev. The other 11 % of the decays produce a gamma ray with an energy of 1.46 Mev.

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http://rerowland.com/K40.html