When Does an Atom Split?
An atom splits when it is struck by a neutron. The nucleus of the atom then breaks into two roughly equal parts and, at the same time, shoots out several high-speed neutrons. Atoms are so small that they cannot be seen under the most powerful microscope. They are the building bricks of which each element is composed. The Greek word “atom” means “cannot be cut.” But we know now that atoms can be cut, or split. Each one contains minute particles carrying two sorts of electricity: first, the electrons which are negatively charged; and secondly, the central core or nucleus which is made up of protons (positively charged) and neutrons (no charge).
In the 19th century it was discovered that all elements with atomic weights greater than 83 are radioactive and that the nucleus could be divided into several parts. Albert Einstein (1879-1955) calculated in 1905 that splitting an atom would destroy mass and release heat. By thus converting matter into heat energy, vast amount of heat would be obtained by destroying only a very small amount of matter.
Between 1934 and 1938 the Italian Enrico Fermi and the German Otto Hahn discovered that atoms of uranium (atomic weight 92) split when struck by a neutron. In 1939 Jean Frédéric Joliot–Curie found that this splitting, or fission, released two or three more neutrons which in turn produced fission in more uranium nuclei, and so on. It is this chain reaction that makes possible not only the benefits of nuclear power but also the horrors of nuclear warfare.
Fission is a form of nuclear transmutation because the resulting fragments are not the same element as the original atom. The two nuclei produced are most often of comparable but slightly different sizes, typically with a mass ratio of products of about 3 to 2, for common fissileisotopes. Most fissions are binary fissions (producing two charged fragments), but occasionally (2 to 4 times per 1000 events), threepositively charged fragments are produced, in a ternary fission. The smallest of these fragments in ternary processes ranges in size from a proton to an argon nucleus.
Apart from fission induced by a neutron, harnessed and exploited by humans, a natural form of spontaneous radioactive decay (not requiring a neutron) is also referred to as fission, and occurs especially in very high-mass-number isotopes. Spontaneous fission was discovered in 1940 by Flyorov, Petrzhak and Kurchatov in Moscow, when they decided to confirm that, without bombardment by neutrons, the fission rate of uranium was indeed negligible, as predicted by Niels Bohr; it was not.
The unpredictable composition of the products (which vary in a broad probabilistic and somewhat chaotic manner) distinguishes fission from purely quantum-tunneling processes such as proton emission, alpha decay, and cluster decay, which give the same products each time. Nuclear fission produces energy for nuclear power and drives the explosion of nuclear weapons. Both uses are possible because certain substances called nuclear fuels undergo fission when struck by fission neutrons, and in turn emit neutrons when they break apart. This makes possible a self-sustaining nuclear chain reaction that releases energy at a controlled rate in a nuclear reactor or at a very rapid uncontrolled rate in a nuclear weapon.
The amount of free energy contained in nuclear fuel is millions of times the amount of free energy contained in a similar mass of chemical fuel such as gasoline, making nuclear fission a very dense source of energy. The products of nuclear fission, however, are on average far more radioactive than the heavy elements which are normally fissioned as fuel, and remain so for significant amounts of time, giving rise to a nuclear waste problem. Concerns over nuclear waste accumulation and over the destructive potential of nuclear weapons are a counterbalance to the peaceful desire to use fission as an energy source, and give rise to ongoing political debate over nuclear power.