There are two important sources of radiation: they are natural and man-made.
Natural background radiation
The radiation that exits all around us is called natural background radiation. All living organisms including man have been continuously exposed to ionizing radiations emitted from different sources, which always existed around us. The sources of natural radiation are cosmic rays and naturally occurring primordial radionuclides such as 238U, 232Th, 235U, and their decay products as well as the singly occurring natural radionuclides like 40K and 87Rb, which are present in the earth crust, soil, rocks, building materials, ore, and water in the environment. Background radiation is a constant source of ionizing radiation present in the environment and emitted from a variety of sources. Natural radiations originated from three major sources: terrestrial, extraterrestrial, and internal (intake of natural radionuclides and their daughter product) sources of radiation.
Terrestrial sources of radiations
Terra means earth; the radiation originated from the earth crust is called terrestrial radiation. The primordial radionuclides (238U, 232Th, and 40K) present in varying amounts in soil, rocks, water, and atmosphere are the sources of terrestrial radiation. The bulk of the natural radiation is mainly due to 40K and 238U, 232Th, and their decay products. Natural uranium consists of three isotopes 234U, 235U, and 238U. 238U is present in an abundance of 99.28% with a half-life of 4.5 × 109 years and 235U in abundance 0.72% with a half-life of 0.7 × 109 years. Thorium is one of the important natural primordial radionuclides with a half-life of 1.4 × 109 years. It is about four times more abundant in nature than uranium. Average crustal abundance of 232Th is 7.2 ppm. All substances found in the terrestrial system contain variable amounts of 238U and 232Th; they undergo radioactive decay until they become stable isotopes. The two main important radioactive series are given in Tables 2 and 3.
The bulk of natural radiation comes from the primordial radionuclides such as 238U, 235U and 235Th. They decay into other radioactive isotope as a part of radioactive series. These series are naturally occurring radioactive series, which have existed since the earth was formed. The nuclei in each series decay by emitting α, β and γ particles until stable (lead). These radioisotopes are chemically bound to minerals in rocks and soils and pose no biological hazards except radon, thoron and its progeny. Radon and thoron are noble radioactive gases, the higher concentrations of these gases and progenies are inhaled to produce lung cancer. According WHO and UNSCEAR, radon and their progeny are the second leading lung cancer after tobacco smoking.
Parent nuclide | Half-life T1/2 | Decay mode (% branch) | Decay energy (MeV) | % Intensity | Daughter nuclide | γ-emission energy (keV) | % γ-emission intensity |
---|---|---|---|---|---|---|---|
238U | 4.5 × 109 years | α (100) | 4.198 | 79.0 | 234Th | 49.55 | 0.063 |
4.151 | 20.9 | 113.50 | 0.0102 | ||||
234Th | 24.10 days | Β (100) | 0.199 | 70.3 | 234Pa | 63.28 | 4.1 |
0.104 | 19.2 | 92.37 | 2.4 | ||||
0.103 | 7.6 | 92.79 | 2.39 | ||||
234Pa | 1.17 m | β (99.84) | 2.269 | 98.2 | 234U | 1001.03 | 0.837 |
1.224 | 1.007 | 766.38 | 0.294 | ||||
IT (0.16) | * | 234Pa | 73.92 | * | |||
234Pa | 6.70 h | β (100) | 0.642 | 19.4 | 234U | 131.30 | 0.029 |
0.472 | 33.0 | 946.00 | 0.021 | ||||
234U | 2.5 × 105 years | α (100) | 4.7746 | 71.38 | 230Th | 53.20 | 0.123 |
4.7224 | 28.42 | 120.90 | 0.0342 | ||||
230Th | 7.5 × 104 years | α (100) | 4.6870 | 76.3 | 226Ra | 67.672 | 0.373 |
4.6205 | 23.4 | 143.872 | 0.0483 | ||||
226Ra | 1600 years | α (100) | 4.7843 | 94.45 | 222Rn | 186.21 | 3.59 |
4.601 | 5.55 | 262.27 | 0.0050 | ||||
222Rn | 3.8235 days | α (100) | 5.4894 | 99.92 | 218Po | 511.00 | 0.076 |
218Po | 3.10 m | α (99.98) | 6.0024 | 100.0 | 214Pb | ** | |
β (0.02) | * | 218At | |||||
218At | 1.60 s | α (100) | 6.0024 | 100.0 | 214Bi | * | |
214Pb | 26.8 m | β (100) | 0.671 | 48.9 | 214Bi | 351.93 | 35.1 |
0.728 | 42.2 | 295.22 | 18.2 | ||||
1.023 | 6.3 | 241.99 | 7.12 | ||||
214Bi | 19.9 m | β (99.98) | 3.272 | 18.2 | 214Po | 609.31 | 44.6 |
1.542 | 17.8 | 1764.50 | 15.1 | ||||
1.507 | 17.02 | 1120.29 | 14.7 | ||||
α (0.02) | 5.452 | 53.9 | 210Tl | 1238.11 | 5.78 | ||
5.516 | 39.2 | 2204.21 | 4.98 | ||||
214Po | 164.30 μs | α (100) | 7.6868 | 99.99 | 210Pb | 799.7 | 0.0104 |
210Tl | 1.30 m | β (100) | 4.209 | 30.0 | 210Pb | * | |
1.863 | 24.0 | ||||||
210Pb | 22.3 years | β (100) | 0.0166 | 0.0631 | 210Bi | 46.54 | 4.25 |
0.0631 | 16.0 | ||||||
210Bi | 5.013 days | β (100) | 1.1615 | 100 | 210Po | ** | |
210Po | 138.376 days | α (100) | 5.3043 | 99.99 | 206Pb | 803.10 | 0.00122 |
206Pb | Stable end product |
Table 2.
Decay series of uranium (238U)
No gamma rays observed.
Parent nuclide | Half-life T1/2 | Decay mode (% branch) | Decay energy (MeV) | % Intensity | Daughter nuclide | γ-emission energy (keV) | % γ-emission intensity |
---|---|---|---|---|---|---|---|
232Th | 1.4 × 1010 years | α (100) | 4.0123 | 78.2 | 228Ra | 63.81 | 0.263 |
3.9472 | 21.7 | 140.88 | 0.021 | ||||
228Ra | 5.75 years | β (100) | 0.0392 | 40.0 | 228Ac | 13.52 | 1.6 |
0.0128 | 30.0 | 16.24 | 0.72 | ||||
0.0257 | 20.0 | 12.75 | 0.30 | ||||
228Ac | 6.15 h | β (100) | 1.158 | 29.9 | 228Th | 911.20 | 25.8 |
1.731 | 11.66 | 968.97 | 15.8 | ||||
2.069 | 8.0 | 338.32 | 11.27 | ||||
228Th | 1.9116 years | α (100) | 5.4232 | 72.2 | 224Ra | 84.373 | 1.22 |
5.3404 | 27.2 | 215.98 | 0.254 | ||||
224Ra | 3.66 days | α (100) | 5.6854 | 94.92 | 220Rn | 240.99 | 4.10 |
5.4486 | 5.06 | 292.70 | 0.0062 | ||||
220Rn | 55.6 s | α (100) | 6.2881 | 99.87 | 216Po | 549.76 | 0.114 |
216Po | 0.145 s | α (100) | 6.7783 | 100 | 212Pb | 804.9 | 0.0019 |
212Pb | 10.64 h | β (100) | 0.335 | 82.5 | 212Bi | 238.63 | 43.3 |
0.574 | 12.3 | 300.09 | 3.28 | ||||
0.159 | 5.17 | 115.18 | 0.592 | ||||
212Bi | 60.55 m | β (64.06) | 2.248 | 86.57 | 212Po | 727.33 | 6.58 |
1.521 | 6.81 | 1620.50 | 1.49 | ||||
0.627 | 2.92 | 785.37 | 1.102 | ||||
α (35.94) | 6.0508 | 69.19 | 208Tl | 39.86 | 1.06 | ||
6.0899 | 27.12 | 288.20 | 0.337 | ||||
5.7675 | 1.78 | 452.98 | 0.363 | ||||
212Po | 0.299 μs | α (100) | 8.7849 | 100.0 | 208Pb | ** | |
208Tl | 3.053 m | β (100) | 1.796 | 48.7 | 208Pb | 2614.53 | 35.64 |
1.286 | 24.5 | 583.19 | 30.4 | ||||
1.519 | 21.8 | 510.77 | 8.13 | ||||
208Pb | Stable end product |
Table 3.
Decay series of thorium (232Th)