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:
- Primordial - from before the creation of the Earth
- Cosmogenic - formed as a result of cosmic ray interactions
- 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 3H 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 235U.
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 |
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|---|---|---|---|
| Nuclide |
Symbol |
Half-life |
Natural Activity |
| Uranium 235 |
235U |
7.04 •108 a |
0.72% of all natural uranium |
| Uranium 238 |
238U |
4.47 •109 a |
99.2745% of all natural uranium; 0.5 to 4.7 ppm total uranium in the common rock types |
| Thorium 232 |
232Th |
1.41 •1010 a |
1.6 to 20 ppm in the common rock types with a crustal average of 10.7 ppm |
| Radium 226 |
226Ra |
1.60 •103 a |
0.42 pCi/g (16 Bq/kg) in limestone and 1.3 pCi/g (48 Bq/kg) in igneous rock |
| Radon 222 |
222Rn |
3.82 days |
Noble Gas; annual average outdoor air concentrations range in the US from 0.016 pCi/L (0.6 Bq/m3) to 0.75 pCi/L (28 Bq/m3) |
| Potassium 40 |
40K |
1.28 •109 a |
Soil - 1-30 pCi/g (0.037-1.1 Bq/g) |
Some nuclides like 232Th have several members in its decay chain. You can roughly follow the chain starting with 232Th:
232Th⟶228Ra⟶228Ac⟶228Th⟶224Ra⟶220Rn⟶216Po⟶212Pb⟶212Bi⟶212Po⟶208Pb(stable)
or starting with 238U:
238U⟶234Th⟶234mPa⟶234U⟶230Th⟶226Ra⟶222Rn⟶218Po⟶214Pb⟶214Bi⟶214Po⟶210Pb⟶210Bi⟶210Po⟶206Pb(stable)
Some other primordial radionuclides are 50V, 87Rb, 113Cd, 115In, 123Te, 138La, 142Ce, 144Nd, 147Sm, 152Gd, 174Hf, 176Lu, 187Re, 190Pt, 192Pt, 209Bi.
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 • 105 m3) 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/cm3 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 238U and 226Ra and an emanation factor of 222Rn of 0.5. |
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|---|---|---|---|
| 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/m3 (0.76 nCi/m3) |
2.4 µg |
14 GBq (0.38 Ci) |
| Total: |
>774 GBq (>21 Ci) |
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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:
| Table 3: Human Produced Nuclides |
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|---|---|---|---|
| Nuclide |
Symbol |
Half-life |
Source |
| Tritium |
3H |
12.3 a |
Produced from weapons testing and fission reactors; reprocessing facilities, nuclear weapons manufacturing |
| Iodine 131 |
131I |
8.04 days |
Fission product produced from weapons testing and fission reactors, used in medical treatment of thyroid problems |
| Iodine 129 |
129I |
1.57•107 a |
Fission product produced from weapons testing and fission reactors |
| Cesium 137 |
137Cs |
30.17 a |
Fission product produced from weapons testing and fission reactors |
| Strontium 90 |
90Sr |
28.78 a |
Fission product produced from weapons testing and fission reactors |
| Technetium 99 |
99Tc |
2.11•105 a |
Decay product of 99Mo, used in medical diagnosis |
| Plutonium 239 |
239Pu |
2.41•104 a |
Produced by neutron bombardment of 238U ( 238U + n  239U 239Np +β- 239Pu+β-) |