The field in which the subatomic fragments emitted in radioactive decay (alpha-, beta-, gamma-rays) or produced by high-voltage accelerators (electrons, protons, x-rays) are applied to the problems of science, engineering, industry, and medicine. The techniques are extraordinarily versatile and sensitive and are basically inexpensive. A disadvantage that limits the range and extent of these applications is the health hazard that may be involved. See also Radioactivity; Radioisotope.
Tracer applications are based on two principles. First is the chemical similarity of radioactive atoms and other atoms of the same element. Periodically a few of the radioactive atoms decay, emitting some penetrating subatomic fragments that can be detected one by one, usually through their ability to cause ionization. Thus the movement of a particular element can be followed through various chemical, physical, and biological steps. The second principle involves the characteristic half-life and nature of the emitted fragments. This makes a radioactive species unique and thereby detectable above a background of radioactive emitters associated with elements. For discussions of radioisotope techniques relating to tracer methodology See also Activation analysis; Isotope dilution techniques; Radioactive tracer; Radioecology.
Penetration and scattering applications arise from the fact that subatomic fragments can penetrate a thick section of a material, and yet a small fraction of the incident particles can be backscattered by a relatively thin section. The oldest application of the penetrating properties of energetic ionizing photons is radiography. An extension of this technique is autoradiography. Since World War II the penetration and scattering properties of beta- and gamma-rays have been applied in industry in the form of thickness gages. See also Autoradiography; Radiography.
The absorption of small amounts of energy from ionizing particles and ionizing photons has chemical effects that have been the basis of several practical applications. The oldest application of this principle is radiation therapy. For example, in cancer therapy the local affected areas are irradiated by external beams of gammas from cobalt-60 or of radiation from accelerators. Radioactive sources have also been administered internally to induce beneficial biochemical reactions in patients afflicted with various ailments. See also Isotopic irradiation; Radiology.
A related area is the radiation sterilization of biomedical supplies. The advantages to this method of biochemical destruction of microscopic life are that (1) unlike steam sterilization, it can be performed at low temperatures on plastics and other thermally unstable materials, and (2) unlike germicidal gases, ionizing radiation can reach every point in the treated product. Radiation-sterilized objects are not radioactive.
The radiation preservation of food is an area of considerable promise. Small doses can inhibit sprouting in potatoes, kill insects in wheat, and sterilize pork products but practical applications have been sharply limited due to a cautious role by regulatory authorities in approving such procedures.
Kinetic energy of emissions in radioactive decay can be converted to useful forms of light, heat, and electricity. See also Luminous paint; Nuclear battery.
