TNAG-1508-FCO40-2066-Guangdong-nuclear-power-station-project-at-Daya-Bay-safety-c-1987 — Page 112

Rays

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An unstable isotope spontaneously emits rays from its nucleus,

and the directions of rays emitted by radioisotopes are completely different in the electric field. Judging from their directions, some carry a positive charge, some a negative charge and others are neutral. The positively charged rays are called a rays, the negatively charged rays rays and the neutral rays y rays.

device.

Rays are invisible but can be detected with the aid of special They have the power of penetrating substances and different types of rays have different levels of penetrating powers.

The phenomenon of radioisotopes emitting rays is called decay. Different types of isotopes have different rates of decay and the term, half-life, can be used to indicate such a rate. A half-life is the time taken for the mass of a radioisotope to decrease to half of its original value. Half-lives of radioisotopes vary from thousands of millions of years

to less then a second.

In the natural world there exist a few types of natural radio- isotopes such as radium, uranium, thorium, protactinium, polonium etc. However, most radioisotopes (about 2,000) have to be produced artificially. Artificial radioistopes are mainly produced with the aid of nuclear reactors or electrically charged particle booster. For example, when ordinary cobalt- 59 is put into a reactor for irradiation by neutrons, it will absorb one neutron and become cobalt-60 and at the same time give out y rays. Cobalt-60 is a radioisotope with a longer half-life and its y rays have greater energy. It is often used to produce sources of irradiation for application in r irradiaton equipment, isotopic meters and therapeutical machines.

Apart from the fact that isotopes can be used to produce sources of irradiation, reactors and boosters are themselves large radioactivity- producing installations and are of great use in scientific research and production.

Application of isotopes and rays

The special features of isotopes and rays have been widely applied in various areas such as industry, agriculture, medicine, and scientific research.

1. Tracer atoms (isotopic tracing technology)

Rays irradiated from radioisotopes can be conveniently detected and therefore can be used to trace the movements of substance. Tracing technology in this respect are playing an important role in biology, chemical engineering, hydrology and water conservancy, metallurgy, petroleum industry, environmental protection, agriculture, medicine and biotechnology.

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