Sources of natural and artificial radiation

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)