Natural radiation, also defined background radiation, has always existed in nature, and life has developed, and keeps on proliferating, in a naturally radioactive environment. There are different sources of background radiation and they can be responsible for either internal or external exposure. Doses from natural sources are summarized in Figure A.10. The worldwide annual effective dose is 2.4 mSv and, considered a world population of 5.3 billion people, the collective dose is 13 x 106man Sv [UNSCEAR, 2000].
Cosmic rays are a source of external exposure. They can be divided into primary and secondary radiation. Primary radiation can be further divided, depending on its origin, into galactic and solar, the second being less significant. Outside the Earth atmosphere the main component of cosmic radiation is positively charged particles, mostly protons, of energy between 102 and 105 MeV; they constitute the so-called primary radiation (galactic and solar). When these particles approach Earth they are deflected by the terrestrial magnetic field according to their momentum. In their travel toward the ground, primary radiation particles interact with the atmosphere, producing many particles such as electrons, photons, mesons, protons and neutrons: these are called the secondary radiation.
Secondary radiation particles themselves can interact with the atmosphere or decay, producing so-called avalanche ionization: from a single starting event up to 10 8 particles can be generated. At about 20 km from sea level cosmic radiation is constituted almost exclusively of its secondary component [Galli and Mancini, 1996]. The typical range of effective dose per person per year is 0.3-1.0 mSv, with average effective dose ~ 0.4mSv [UNSCEAR, 2000]. For locations high above the sea level very large doses are received, i.e., in La Paz, Bolivia (3,600 m), the average dose due to cosmic rays is 2.02 mSv per year. A flight at an altitude of 8 km causes a dose rate of 2.8 mSvh-1 [Galli and Mancini, 1996].
Inside the Earth there are radionuclides whose half life (T =2) is comparable with the Earth's age. In fact the Earth's core is still hot thanks to the energy released by radionuclides in their decay processes. The most significant for dose computation
are K40 (T1/2 = 1.28 x 109yr), Th232 (T1/2 = 1.41 x 1010yr), U238 (T1/2 = 4.47x
109 yr); of secondary importance are Rb (T1/2 = 4.7 x 10 yr) and U235 (Tj=2 = 7.04 x 108yr). Most radionuclides belong to one of the three families of uranium, thorium and actinium (see Figures A.7, A.8 and A.9) [Galli and Mancini, 1996]. In all three families radon (Rn) appears. Radon appearance is the clearest evidence that the Earth's crust is radioactive. Terrestrial radiation can be responsible for internal or external exposure.
A.22.214.171.124 External exposure from terrestrial radiation
External exposure to gamma-rays from natural radionuclides can occur both outdoors, since radionuclides are present in the Earth's crust, and indoor, as they may be present in construction material. Combining outdoor and indoor exposure, for a person spending 80% of time indoors, a range of 0.3-0.6mSv per person per year is typical. Worldwide-averaged annual effective exposure is estimated at -0.5 mSv [UNSCEAR, 2000].
A.126.96.36.199 Internal exposure from terrestrial radiation
Potassium isotopes are present in the human body with a weight percentage 0.18%; the isotope K40 has an isotopic abundance 1.18 x 10~4, and its main decay
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