Radioactivity of elements

Where the number of both protons and neutrons in an atom is known we are able to identify a specific isotope of a specific element and this is termed a nuclide. Some naturally occurring elements are radioactive and specific isotopes of these elements are called radionuclides. This term implies that their nuclei are unstable and spontaneously decay, transforming the nucleus into that of a different element. Radioactive decay is written in equations that look a little like those for chemical reactions, but they need to express the atomic mass of the elements involved and the type of rotation emitted. A number of modes of radioactive decay are possible, and here we outline some of the common ones. The decay of potassium (40K) can be written:

In this transformation an electron of the potassium is captured by the nucleus, a proton within it is converted to a neutron and excess energy is lost as a g particle. Thus the Z number of the nucleus is decreased by 1 from K (Z = 19) to Ar (Z = 18), but the mass number is unchanged. This so-called gamma radiation (g) is essentially a photon that carries a large amount of electromagnetic energy. This transformation is very important for the atmosphere as it produces the stable form (isotope) of argon which emanates from the potassium-containing rocks of the earth and accumulates in the atmosphere.

Unstable heavy elements with an excess of protons in the nucleus decay to produce radiation as an a particle (alpha decay), which is in fact a helium (He) nucleus, for example:

in the atmosphere. As the helium nucleus contains two protons and two neutrons the nucleus Z number changes from that of U (Z = 92) to Th (Z = 90), while the mass number decreases by 4. Another source of helium is the alpha decay of radium (Ra):

which also produces the inert, but radioactive, gas radon (Rn) discussed in Box

Other heavy elements with an excess of neutrons in the nucleus decay by transforming the neutron into a proton by ejecting an energized electron known as a negative beta particle (b-). An example of beta decay is:

As one neutron has been transformed into a proton the Z number in the nucleus is increased by 1 from Rb (Z = 37) to Sr (Z = 38), but the mass number is unaffected.

While many radionuclides are natural, human activities have produced either artificial radionuclides, or have greatly increased levels of otherwise natural ones. These anthropogenic radionuclides are produced by nuclear power generation (e.g. power stations, satellites and submarines), by reprocessing of nuclear waste or from nuclear weapons. For example, the atmospheric testing of nuclear weapons in the 1950s and 1960s vastly increased the concentrations of tritium (3H), 14C and 137Cs (caesium) and dispersed them worldwide. Consequently, the fallout of these isotopes from the atmosphere also increased, producing a characteristic 'spike' increase in their flux to the surfaces of the oceans and the land. This sudden arrival of radionuclide has been used to trace movements of water masses and mixing rates in the oceans (see Box 7.1), while its burial in sediments (e.g. saltmarshes) can be used as a time marker.

Nuclear weapons testing was deliberate; however many other releases of radionuclides are accidental. These have included fires and spillages at nuclear reprocessing plants resulting in releases of an assortment of nuclides to the atmosphere and the oceans, including super-heavy elements from the actinide group of the Periodic Table (Fig. 2.2) such as plutonium (Pu). Similarly, accidental sinking of nuclear submarines has released radionuclides to the bottom waters of the in the atmosphere. As the helium nucleus contains two protons and two neutrons the nucleus Z number changes from that of U (Z = 92) to Th (Z = 90), while the mass number decreases by 4. Another source of helium is the alpha decay of radium (Ra):

oceans. Perhaps most infamous was the fire and explosion at the Ukranian Chernobyl power plant in 1986 which released a cocktail of radionuclides (e.g. 131I (iodine), 134Cs and 137Cs) into the local area and the atmosphere. Most of the fallout at distance from the source was mediated by rainfall over parts of Europe, resulting in the contamination of upland pasture, where rainfall was heaviest, and ultimately of livestock and milk.

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