Terrestrial, Atmospheric, and Aquatic Natural Radioactivity

Publisher Summary This chapter discusses terrestrial, atmospheric, and aquatic natural radioactivity. There are about 340 nuclides in nature of which about 70 are radioactive. The latter are called the naturally occurring radionuclides and are found mostly as isotopes of heavy elements. The radioactivity of the natural radionuclides of the uranium ( 235 U and 238 U) and thorium ( 232 Th) series is essentially because of alpha and beta decay modes plus the gamma de-excitation of nuclei. The mechanisms that determined uranium and thorium distributions through geological times in the Earth’s asthenosphere, mantle, core–mantle boundary, and outer core have been discussed by Bao. The concentrations of 40 K, 232 Th, and 238 U in the Earth’s crust are assumed to have increased throughout geological times. However, taking into account that during the last 2 gigaannum (Ga) these three radionuclides have kept decaying, the annual external radiation dose from the terrestrial nuclides present in the Earth’s crust is believed to have been maintained constant at about 1.6 mGy y –1 during this same period. The nuclides of the 238 U, 235 U, and 232 Th radioactive decay series are illustrated in the chapter with their historical names, half-lives, and main radiation.