What Is Natural Radioactivity?

Natural Radioactivity was discovered by Henri Becquerel in march 1896. It is one of the most well-known accidental discoveries in the history of physics. In addition to physical and chemical changes, matter can undergo a third type of change known as nuclear change. This occurs when nuclei of certain isotopes spontaneously change or are made to change into one or more different isotopes. Three types of nuclear change are natural radioactive decay, nuclear fission, and nuclear fusion. The law of conservation of matter does not apply to nuclear changes because they convert a small but measurable amount of the mass in a nucleus into energy. This type of change is governed by the law of conservation of matter and energy: The total amount of matter and energy involved in any nuclear change remains the same.

Natural radioactive decay is a nuclear change in which unstable isotopes spontaneously emit fast-moving particles, high-energy radiation, or both at a fixed rate. The unstable isotopes are called radioactive isotopes or radioisotopes. Radioactive decay into various isotopes continues until the original isotope is changed into a new stable isotope that is not radioactive. Radiation emitted by radioisotopes is damaging ionizing radiation. The most common form of ionizing energy released from radioisotopes is gamma rays, which form high-energy electromagnetic radiation. High-speed ionizing particles emitted from the nuclei of radioactive isotopes are most commonly of two types: alpha particles (fast-moving, positively charged chunks of matter that consist of two protons and two neutrons) and beta particles (high-speed electrons). The relative penetrating power of alpha, beta, and gamma ionizing radiation.

All of us are exposed to small amounts of harmful ionizing radiation from both natural and human sources. Each type of radioisotope spontaneously decays at a characteristic rate into a different isotope. This decay rate can be expressed in terms of half-life-the time needed for one-half of the nuclei in a radioisotope to decay and emit their radiation to form a different iso- tope. The decay continues, often producing a series of different radioisotopes until a nonradioactive isotope is formed. Each radioisotope has a characteristic half-life, ranging from a few million of a second to several billion years. An isotope's half-life cannot be changed by temperature, pressure, chemical reactions, or other factors. Half-life can be used to estimate how long a sam- Pie of a radioisotope must be stored in a state container before it decays to what is considered a safe level. A general rule of thumb is that such decay takes about 10 half-lives. Thus people must be protected from radioactive waste containing iodine-131 (which concentrates in the thyroid gland) for 80 days (10 x 8 days).

In contrast, plutonium-239 (which is produced in nuclear re- actors and used as the explosive in some nuclear weapons) can cause lung cancer when its particles are inhaled in minute amounts; it must be stored safely for 240,000 years (10 x 24,000 years)-four times longer than the latest version of our species has existed.