Introduction+to+Naturally+Occurring+Radioactivity

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Our world is radioactive and has been since it was created. Over 60 radionuclides (radioactive elements) can be found in nature, and they can be placed in three general categories:


 * 1) Primordial - from before the creation of the Earth
 * 2) Cosmogenic - formed as a result of cosmic ray interactions
 * 3) Human produced - enhanced or formed due to human actions (minor amounts compared to natural)

Radionuclides are found naturally in air, water and soil. They are even found in us, being that we are products of our environment. Every day, we ingest and inhale radionuclides in our air and food and the water. Natural radioactivity is common in the rocks and soil that makes up our planet, in water and oceans, and in our building materials and homes. There is nowhere on Earth that you cannot find Natural Radioactivity. Radioactive elements are often called radionuclides, just nuclides or incorrectly radioactive isotopes. There are over 2000 different radioactive nuclides. Often, radionuclides are symbolized based on the element and on the atomic weight, as in the case of radioactive hydrogen or tritium with an atomic weight of 3 is shown as H-3 or 3 H where the latter is preferred in the present text. As another example, Uranium with the atomic weight of 235 would be shortened to U-235 or 235 U.

=Primordial Radionuclides=

Primordial radionuclides are left-over from when the world and the universe were created. They are typically long-lived, with half-lives often on the order of hundreds of millions of years. Radionuclides that exist for more than 30 half-lives are not measurable. The progeny or decay products of the long-lived radionuclides are also under this heading. Here is some basic information on some common primordial radionuclides:


 * ~ Table 1: Primordial nuclides ||
 * Nuclide || Symbol || Half-life || Natural Activity ||
 * Uranium 235 || 235 U || 7.04 •10 8 a || 0.72% of all natural uranium ||
 * Uranium 238 || 238 U || 4.47 •10 9 a || 99.2745% of all natural uranium; 0.5 to 4.7 ppm total uranium in the common rock types ||
 * Thorium 232 || 232 Th || 1.41 •10 10 a || 1.6 to 20 ppm in the common rock types with a crustal average of 10.7 ppm ||
 * Radium 226 || 226 Ra || 1.60 •10 3 a || 0.42 pCi/g (16 Bq/kg) in limestone and 1.3 pCi/g (48 Bq/kg) in igneous rock ||
 * Radon 222 || 222 Rn || 3.82 days || Noble Gas; annual average outdoor air concentrations range in the US from 0.016 pCi/L (0.6 Bq/m 3 ) to 0.75 pCi/L (28 Bq/m 3 ) ||
 * Potassium 40 || 40 K || 1.28 •10 9 a || Soil - 1-30 pCi/g (0.037-1.1 Bq/g) ||

Some nuclides like 232 Th have several members in its decay chain. You can roughly follow the chain starting with 232 Th:

232 Th⟶ 228 Ra⟶ 228 Ac⟶ 228 Th⟶ 224 Ra⟶ 220 Rn⟶ 216 Po⟶ 212 Pb⟶ 212 Bi⟶ 212 Po⟶ 208 Pb(stable)

or starting with 238 U: 238 U⟶ 234 Th⟶ 234m Pa⟶ 234 U⟶ 230 Th⟶ 226 Ra⟶ 222 Rn⟶ 218 Po⟶ 214 Pb⟶ 214 Bi⟶ 214 Po⟶ 210 Pb⟶ 210 Bi⟶ 210 Po⟶ 206 Pb(stable)

Some other primordial radionuclides are 50 V, 87 Rb, 113 Cd, 115 In, 123 Te, 138 La, 142 Ce, 144 Nd, 147 Sm, 152 Gd, 174 Hf, 176 Lu, 187 Re, 190 Pt, 192 Pt, 209 Bi.

Natural Radioactivity in Soil
How much natural radioactivity is found in a volume of soil that is 1 square kilometer and 50 centimeter deep? The following table is calculated for this volume (total volume is 5 • 10 5 m 3 ) and the listed activities. It should be noted that activity levels vary greatly depending on soil type, mineral make-up and density (~1.58 g/cm 3 used in this calculation). This table represents calculations using typical numbers.
 * ~ table 2: Average Natural Radioactivity in an upper crust volume corresponding to one square kilometer by 50 cm deep, data taken from the table next page. We suppose equilibrium between 238 U and 226 Ra and an emanation factor of 222 Rn of 0.5. ||
 * Uranium 238 || 36 Bq/kg (1 pCi/kg) || 1500 kg || 28 GBq (0.78 Ci) ||
 * Thorium 232 || 44 Bq/kg (1.2 pCi/kg) || 7900 kg || 34 GBq (0.93 Ci) ||
 * Potassium 40 || 850 Bq/kg (23 pCi/kg) || 2528 kg || 670 GBq (18 Ci) ||
 * Radium 226 || 36 Bq/kg (1 pCi/kg) || 0.78 g || 28 GBq (0.78 Ci) ||
 * Radon 222 || 28 kBq/m 3 (0.76 nCi/m 3 ) || 2.4 µg || 14 GBq (0.38 Ci) ||
 * ||  || Total: || >774 GBq (>21 Ci) ||

=Human Produced Radionuclides=

Humans have used radioactivity for one hundred years, and through its use, added to the natural inventories. The amounts are small compared to the natural amounts discussed above, and due to the shorter half-lives of many of the nuclides, have seen a marked decrease since the halting of above-ground testing of nuclear weapons. Here are a few human produced or enhanced nuclides:

( 238 U + n 239 U 239 Np +β -  239 Pu+β - ) || = =
 * ~ Table 3: Human Produced Nuclides ||
 * Nuclide || Symbol || Half-life || Source ||
 * Tritium || 3 H || 12.3 a || Produced from weapons testing and fission reactors; reprocessing facilities, nuclear weapons manufacturing ||
 * Iodine 131 || 131 I || 8.04 days || Fission product produced from weapons testing and fission reactors, used in medical treatment of thyroid problems ||
 * Iodine 129 || 129 I || 1.57•10 7 a || Fission product produced from weapons testing and fission reactors ||
 * Cesium 137 || 137 Cs || 30.17 a || Fission product produced from weapons testing and fission reactors ||
 * Strontium 90 || 90 Sr || 28.78 a || Fission product produced from weapons testing and fission reactors ||
 * Technetium 99 || 99 Tc || 2.11•10 5 a || Decay product of 99 Mo, used in medical diagnosis ||
 * Plutonium 239 || 239 Pu || 2.41•10 4 a || Produced by neutron bombardment of 238 U