Natural radioactive isotopes have been present since the formation of the Earth and are produced continuously by nuclear reactions of cosmic rays with atoms in the atmosphere.

As seen in Section 6.2.5, the elements in the universe are produced by nuclear reactions. Of course, these nuclear reactions produce both stable and radioactive isotopes. The half-lives of some radioactive isotopes are several billion years, com­parable to the age of the Earth and the universe. These radioactive isotopes cannot be formed under natural conditions characteristic to the Earth; thus, as a result of the radioactive decay, their quantity and radioactivity have been decreasing contin­uously since the Earth was formed. However, because of these long half-lives, their radioactivity have been significant until now. These radioactive isotopes are called “nucleogenesis” or “primordial” isotopes and can be classified into two groups. The first group contains of the isotopes in the natural radioactive decay series (235U, 238U, and 232Th; see Figures 4.4—4.6). The most important members of these decay series are the parent nuclides (235U, 238U, and 232Th) and the daughter nuclides with relatively long half-lives and the daughter elements of these daughter nuclides, for example, 226Ra, 210Pb, 210Bi, and 210Po. Gaseous radon isotopes (222Rn, 220Rn) are especially important because they enter the lungs through breath­ing, and their solid daughter elements (the lead, bismuth, and polonium isotopes produced from the radon isotopes in the 238U and 232Th series) are incorporated in the lung tissues, causing internal irradiation. Many of these isotopes emit alpha par­ticles with a short range. The alpha particles transfer their high-energy radiation within a short range inside the lungs. Since the radioactive isotopes of the decay series are always present in the building material, radon gas accumulates in closed

Nuclear and Radiochemistry. DOI: http://dx. doi. org/10.1016/B978-0-12-391430-9.00013-5

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spaces (such as houses and caves). Therefore, the activity of radon is an important part of the background irradiation affecting living organisms.

In the second group of the primordial isotopes, there are the long-life nuclei pro­duced during nucleogenesis, which transform into stable daughter nuclides in one step. For example, 40K, 50V, 87Rb, 113Cd, 115In, 123Te, 138La, 144Nd, 147,148Sm, 152Gd, 156Dy, 174Hf, 176Lu, 186Os, 187Re, and 190Pt isotopes can be mentioned in this context. The most important radionuclide in this group is the radioactive iso­tope of potassium, 40K. The potassium ion is an essential ion in living organisms; its quantity is significant and plays an important biological role. Of course, the abundance of 40K in living organisms is the same as in every other potassium com­pound. This means that the radioactivity of 40K that is present in the body of adult people is about 3500—4000 Bq, depending on the mass of the body. The 40K iso­tope emits gamma radiation with high energy (1.46 MeV) and the range of these gamma photons is long. Thus, gamma photons leave the human body, and so the living organisms irradiate each other.

Many natural radioactive isotopes are produced continuously via nuclear reac­tions of the nuclei of atmosphere (nitrogen, oxygen, and argon) with cosmic radia­tion. As seen in Section 4.3.6, the basic isotope of the radiocarbon dating, 14C, is produced from the 14N in the air in an (n, p) nuclear reaction (see Section 6.2.1). Beside radiocarbon, many radioactive isotopes are produced in this way, e. g., 3H, 7,10Be, 22Na, 26Al, 32,33P, 35S, 36Cl, and 39Ar. These nuclides form from the 40Ar isotope of air under the effect of the cosmic radiation by spallation (see Section 5.5.2).