Cosmic radionuclides

In the marine environment, the numerous radionuclides can be classified into three broad categories based on their production or origin (1) those derived from the weathering of continental rocks, the primordial radionuclides, (2) those formed from cosmic radiation, the cosmogenic radionuclides, and (3) those artificially introduced into nature, the anthropogenic or transient radionuclides and tracers. The primordial radionuclides (e.g. Th, and U) were... 

Cosmogenic radionuclides are formed in the upper atmosphere by the interaction of cosmic rays, primarily from the sun, with elements present in the atmosphere (e.g. 0, and " Ar). Their half-lives range from months to... 

Cosmogenic radionuclides are produced in the upper atmosphere by spallation reactions of cosmic rays with atmospheric elements. The most common... 

Background Radiation—The amount of radiation to which a member of the general population is exposed from natural sources, such as terrestrial radiation from naturally occurring radionuclides in the soil, cosmic radiation originating from outer space, and naturally occurring radionuclides deposited in the human body. 

A large number of radionuclides are produced continuously in the upper atmosphere through various interactions between gases and cosmic radiation [9,10]. Some of these radionuclides are produced also in the soil and bodies of surface water by cosmic radiation that penetrates the earth s atmosphere to interact with materials at the surface of the earth. Radionuclides which are of hydrologic interest and which are also produced primarily in the atmosphere (prior to 1945) are listed in Table 1. 

A fundamental assumption made for most dating with atmospheric radionuclides is that the cosmic radiation flux and hence, the natural production of the radionuclides has been constant with time. Various studies of this problem using 14C and tree-ring calibration have been made. Isotopic studies of meteorites have also been useful [17]. Considering the probable lack of basic accuracy of dating water, the problem of changes in cosmic ray flux is not serious. 

In this article we plan to focus on two aspects (i) the transport of radionuclides to the ocean floor and the processes which govern their distribution in deep-sea sediments and (ii) the application of deep-sea sediments to retrieve historical records of large scale phenomena, e.g. long term changes in the rate of production of nuclides by cosmic rays. Even while discussing these aspects, our emphasis will be mainly on the processes rather than on the details of the chronometric method. 

Exposure to natural sources of radiation is unavoidable. Externally, individuals receive cosmic rays, terrestrial X-rays, and gamma radiation. Internally, naturally occurring radionuclides of Pb, Po, Bi, Ra, Rn, K, C, H, U, and Th contribute to the natural radiation dose from inhalation and ingestion. Potassium-40 is the most abundant radionuclide in foods and in all tissues. The mean effective human dose equivalent from natural radiations is 2.4 milliSieverts (mSv). This value includes the lung dose from radon daughter products and is about 20% higher than a 1982 estimate that did not take lung dose into account (Table 32.4). 

Numerous radionuclides have been applied to marine sedimentary problems. These are generally grouped into cosmic-ray produced (cos-mogenic) nuclides ( C, °Be, Be, "Al), nuclear bomb-produced (fallout) nuclides ( Cs, plutonium isotopes, 24iAm), and naturally occurring nuclides ultimately derived from the decay of 38u, and 232xh parents. 

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