Decomposition by micro-organisms also has an impact on the industrial economy. Food products, such as cheese and yoghurt, require the activity of specific micro-organisms, but if these cultures are contaminated by other microbes the process rapidly deteriorates. Similarly, the microbial contamination of food can result in changes of consistency, smell, or taste—changes which reduce its acceptability. The growth of particular groups of organisms during the preparation or storage of food can be responsible for outbreaks of food poisoning. Microbial decomposition of food can be delayed by storage in high salt or sugar concentrations or in weak acids (pickling); drying (desiccation) or cooling (refrigeration); and killing the micro-organisms by heat (canning and pasteurization) or radiation.
Radioactive elements such as uranium (U) and thorium (Th) decay naturally to form different elements or isotopes of the same element. (Isotopes are atoms of any elements that differ in mass from that element, but possess the same general chemical and optical properties.) This decay is accompanied by the emission of radiation or particles (alpha, beta, or gamma rays) from the nucleus, by nuclear capture, or by ejection of orbital electrons. A number of isotopes decay to a stable product, a so-called daughter isotope, in a single step (for example, carbon-14), whereas others involve many steps before a stable isotope is formed. Multistep radioactive decay series include, for example, the uranium-235, uranium-238, and thorium-232 families. If a daughter isotope is stable, it accumulates until the parent isotope has completely decayed. If a daughter isotope is also radioactive, however, equilibrium is reached when the daughter decays as fast as it is formed.
Radioactive decay may take different routes. Thus, if the isotope decays by alpha emission, it loses the two protons and two neutrons that make up an alpha particle; the atomic number (number of protons) is reduced by two and the atomic mass (number of nuclear particles, or nucleons) by four. In beta decay, or electron loss, a radioactive nucleus can gain or lose one unit of electric charge without changing the number of nucleons. More radioactive substances are beta-ray emitters than alpha-ray emitters.
A third important mode of decay involves electron capture; the nucleus of an atom absorbs an electron, which unites with a proton of the nucleus to form a neutron. Thus, the atomic number is reduced by one, but the mass of the nucleus remains unchanged. The fourth mode of decay, gamma radiation, consists of the emission of waves of electromagnetic energy. Scientists describe the radioactivity of an element in terms of half-life—the time the element takes to lose half of its activity through decay. This covers an extraordinary range of time, from a few microseconds to billions of years.
At the end of the period constituting one half-life, half of the original quantity of the radioactive element has decayed; after another half-life, half of what was left is halved again, leaving one quarter of the original quantity, and so on. Every radioactive element has its own half-life; for example, that of carbon-14 is 5,730 years and that of uranium-238 is 4.5 billion years. Radiometric dating techniques are based on radio-decay series with constant rates of isotope decay. Once a quantity of a radioactive element becomes part of a growing mineral crystal, that quantity will begin to decay at a steady rate, with a definite percentage of daughter products in each time interval. These “clocks in rocks” are the geologists’ timekeepers.
Radiocarbon dating techniques, first developed by the American chemist Willard F. Libby and his associates at the University of Chicago in 1947, are frequently useful in deciphering time-related problems in archaeology, anthropology, oceanography, pedology, climatology, and recent geology. Through metabolic activity, the level of carbon-14 in a living organism remains in constant balance with the level in the atmosphere or some other portion of the Earth’s dynamic reservoir, such as the ocean. Upon the organism’s death, carbon-14 begins to disintegrate at a known rate, and no further replacement of carbon from atmospheric carbon dioxide can take place. The rapid disintegration of carbon-14 generally limits the dating period to approximately 50,000 years, although the method is sometimes extended to 70,000 years. Uncertainty in measurement increases with the age of the sample.
Although the method is suited to a variety of organic materials, accuracy depends on the half-life to be used, variations in levels of atmospheric carbon-14, and contamination. (The half-life of radiocarbon was redefined from 5,570 ± 30 years to 5,730 ± 40 years in 1962, so some dates determined earlier required adjustment; and due to radioactivity more recently introduced into the atmosphere, radiocarbon dates are calculated from AD 1950.) The radiocarbon timescale also contains other uncertainties, and errors as great as 2,000 to 5,000 years may occur. Postdepositional contamination, which is the most serious problem, may be caused by percolating groundwater, incorporation of older or younger carbon, and contamination in the field or laboratory.
