Naturally occuring radioactive material

Apart from a few singular cases, there are three families of naturally occuring radioactive isotopes, namely Thorium 232, Uranium 235 and Uranium 238 with their respective daughter products. All three nuclei have have-lives of the order of 1-10 billion years, which is why a substantial amount of them is still left since the formation of the earth. The daughter products have much shorter half-lives, down to just microseconds. All three families end in various stable isotopes of lead. The best-known daughter product is probably Radium 226 (the radium extracted from pitchblend by the Curies).

Daughter products accumulate over time until they are in equilibrium with their mother isotope, i.e. until they are produced at the same rate (by decay of the mother isotope) as they decay themselves. Their radiation adds to the radiation of the mother isotope. As a result of the many short-lived decay products in the natural families, samples containing Uranium, Thorium or Radium are usually sources of mixed alpha, beta and gamma radiation.

The singular cases are basically Kalium 40 and Rubidium 87 which are also nearly stable (half-lives of 1 resp. 50 billion years). Of course the border between stable and radioactive is quantitative, but with much longer half-lives (e.g. Ge 76 with 1.5e21 years) it becomes difficult to observe any decay at all in a (human) lifetime.

Natural Kalium contains about 0.01% radiaoactive K 40, the rest is made up of the stable isotopes K 39 (93.3%) and K 42 (6.7%). This small fraction is responsible for a natural activity of about 26Bq per gram of natural Kalium. Rubidium contains a much larger fraction, namely 28%, of radioactive Rb 87, but the half-live is also longer and thus leads to about 870Bq per gram of natural Rubidium. Rb 87 is a pure beta(-) emitter, while K 40 emits both beta(-/+) and gamma radiation.

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