Radioactive Substances in Nature

Radioactive substances are widely distributed on the earth. Some are found in the atmosphere, but the major part is present in the lithosphere. The most important ones are the ores of uranium and thorium, and potassium salts, including the radioactive decay products of uranium and thorium. Uranium and thorium are common elements in nature. Their concentrations in granite are about 4 and 13mg/kg, respectively, and the concentration of uranium in seawater is about 3 pg/l. Some uranium and thorium minerals are listed in Table 1.1. The most unportant uranium mineral is pitchblende (UsOs). Uranium is also found in mica. The most important thorium mineral is monazite, which contains between about 0.1 and 15% Th. 

The measurement of natural radioactivity is an important tool for dating, e.g. for the determination of the age of minerals (see section 16.1). 

Mineral Composition Cone of U [%] Cone ofTh [%] Deposits 

Monazite Phosphates of Th and Rare Earths 0.1-15 Brazil, India, Russia, Norway, Madagascar 

The radioactive atoms with half-lives 1 d that are found in nature are listed in Table 1.2. The table shows that radioactivity is mainly observed with heavier elements and seldom with light ones (e.g. and Rb). Be, Be, and (tri- 

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