Spontaneous change of the nuclei of atoms accompanied by the emission of radiation. Such atoms are called radioactive. It is the property exhibited by the radioactive isotopes of stable elements and all isotopes of radioactive elements, and can be either natural or induced. See radioactive decay.
A radioactive material decays by releasing radiation, and transforms into a new substance. The energy is released in the form of alpha particles and beta particles or in the form of high-energy electromagnetic waves known as gamma radiation. Natural radioactive elements are those with an atomic number of 83 and higher. Artificial radioactive elements can also be formed.
Devices such as the Geiger-Muller tube, a photographic plate, or an electroscope can detect radioactivity. An electronic counter on the Geiger-Muller instrument displays a digital reading of the amount of radiation detected.
The discovery of radioactivity
Radioactivity was first discovered in 1896, when French physicist Henri Becquerel observed that some photographic plates, although securely wrapped up, became blackened when placed near certain uranium compounds. A closer investigation showed that thin metal coverings were unable to prevent the blackening of the plates. It was clear that the uranium compounds emitted radiation that was able to penetrate the metal coverings. Pierre and Marie Curie soon succeeded in isolating other radioactive elements. One of these was radium, which was found to be over 1 million times more radioactive than uranium.
Further investigation of the nature of radiation by Ernest Rutherford revealed that there are three types of radiation: alpha particles, beta particles, and gamma rays. Alpha particles are positively-charged, high-energy particles emitted from the nucleus of a radioactive atom. They consist of two neutrons and two protons and are thus identical to the nucleus of a helium atom. Because of their large mass, alpha particles have a short range of only a few centimetres in air, and can be stopped by a sheet of paper. Beta particles are more penetrating and can travel through a 3 mm/0.1 -in-thick sheet of aluminium or up to 1 m/3 ft of air. They consist of high-energy electrons emitted at high velocity from a radioactive atom that is undergoing spontaneous disintegration. Gamma rays comprise very high-frequency electromagnetic radiation. Gamma rays are stopped only by direct collision with an atom and are therefore very penetrating; they can, however, be stopped by about 4 cm/1.5 in of lead. When alpha, beta, and gamma radiation pass through matter they tend to knock electrons out of atoms, ionizing them. They are therefore called ionizing radiation. Alpha particles are the most ionizing, being heavy, slow moving, and carrying two positive charges. Gamma rays are weakly ionizing as they carry no charge. Beta particles fall between alpha and gamma radiation in ionizing potential.
Detection of radioactivity
Detectors of ionizing radiation make use of the ionizing properties of radiation to cause changes that can be detected and measured. A Geiger counter detects the momentary current that passes between electrodes in a suitable gas when ionizing radiation causes the ionization of that gas. The device is named after the German physicist Hans Geiger. The activity of a radioactive source describes the rate at which nuclei are disintegrating within it. One becquerel (1 Bq) is defined as a rate of one disintegration per second.
Radioactive decay occurs when an unstable nucleus emits alpha, beta, or gamma radiation in order to become more stable. The energy given out by disintegrating atoms is called atomic radiation. An alpha particle consists of two protons and two neutrons. When alpha decay occurs (the emission of an alpha particle from a nucleus) it results in the formation of a new nucleus. An atom of uranium isotope of mass 238, on emitting an alpha particle, becomes an atom of thorium, mass 234. Beta decay, the loss of an electron from an atom, is accomplished by the transformation of a neutron into a proton, thus resulting in an increase in the atomic number of one. For example, the decay of the carbon-14 isotope results in the formation of an atom of nitrogen (mass 14, atomic number 7) and the emission of an electron. Gamma emission usually occurs as part of alpha or beta emission. High-speed electromagnetic radiation is emitted from the nucleus in order to make it more stable during the loss of an alpha or beta particle. Isotopes of an element have different atomic masses. They have the same number of protons but different numbers of neutrons in the nucleus. For example, uranium-235 and uranium-238 both have 92 protons but the latter has three more neutrons than the former. Some isotopes are naturally radioactive (see radioisotopes) while others are not. Radioactive decay can take place either as a one-step decay, or through a series of steps that transmute one element into another. This is called a decay series or chain, and sometimes produces an element more radioactive than its predecessor. For example, uranium-238 decays by alpha emission to thorium-234; thorium-234 is a beta emitter and decays to give protactinium-234. This emits a beta particle to form uranium-234, which in turn undergoes alpha decay to form thorium-230. A further alpha decay yields the isotope radium-226.
The rate of radioactive decay
The emission of radioactivity by an atom occurs spontaneously and quite unpredictably. However, in a sample containing many radioactive atoms, the overall rate of decay appears to be governed by the number of nuclei left undecayed. The time taken for half the radioactive atoms in a sample to decay remains constant and is called the half-life. Radioactive substances decay exponentially with time, and the value of the half-life for a substance can vary from a fraction of a second to billions of years.
We are surrounded by radioactive substances. Our food contains traces of radioactive isotopes and our own bodies are made of naturally radioactive matter. In addition, we are bombarded by streams of high-energy charged particles from outer space. Radiation present in the environment is known as background radiation and we should take this into account when considering the risk of exposure to other sources. Alpha, beta, and gamma radiation are dangerous to body tissues because of their ionizing properties, especially if a radioactive substance is ingested or inhaled. Illness resulting from exposure to radioactive substances can take various forms, which are collectively known as radiation sickness.
Radioactivity in use
Radioactivity has a number of uses in modern science, but its use should always be carefully controlled and monitored to minimize the risk of harm to living things. A small quantity of a radioactive tracer can be used to follow the path of a chemical reaction or a physical or biological process. Radiocarbon dating is a technique for measuring the age of organic materials. Another application is in determining the age of rocks. This is based on the fact that in many uranium and thorium ores, all of which have been decaying since the formation of the rock, the alpha particles released during decay have been trapped as helium atoms in the rock. The age of the rock can be assessed by calculating the relative amounts of helium, uranium, and thorium in it. This calculation can help to estimate the age of the Earth at about 4.6 billion years. In medicine, radioactive emissions and electromagnetic radiation can be used therapeutically; for example, to treat cancer, when the radiation dose is very carefully controlled (see radiotherapy).
Nuclear fission and fusion
Fission of a nucleus occurs when the nucleus splits into two approximately equal fragments. The fission of the nucleus results in the release of neutrons and a large amount of energy. In a nuclear reactor, the fission of uranium-235 is caused by bombarding it with neutrons. A nuclear chain reaction is caused as neutrons released by the splitting of atomic nuclei themselves go on to split other nuclei, releasing even more neutrons. In a nuclear reactor this process is carefully controlled to release nuclear energy. In nuclear fusion, two light nuclei combine to form a bigger nucleus. As fusion is accompanied by the release of large amounts of energy, the process might one day be harnessed to form the basis of commercial energy production. So far, no sustained fusion reaction has been achieved.
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