NUCLEAR FISSION AND FUSION
Otto Hahn and Fritz Strassman (1902-80) discovered nuclear fission in 1938. When an isotope of uranium-235 was bombarded with neutrons, it split into two lighter nuclei along with, on average, three neutrons. These neutrons were capable of bombarding and splitting other nuclei, causing more fission to take place. If the mass of uranium-235 was above a certain level (the critical mass) this produced a chain reaction. It was the production of this chain reaction which, in turn, led to the development of the first nuclear bomb. Fission is used in both nuclear reactors and atomic weapons. Nuclear fusion occurs when two small nuclei collide and combine, breaking the weak nuclear force and releasing energy. This type of reaction releases considerably more energy than a fission, process for a given mass of material. However, unlike nuclear fission, humankind has not yet found a way to properly contain or control the process. Many scientists today are searching for the key to controlled room-temperature fusion referred to as ‘cold fusion’. An example of uncontrolled fusion reaction is the hydrogen (thermonuclear) bomb, which relies on the fusion of light atoms to give heavier atoms, with the destruction of matter releasing the observed energy.
NUCLEAR PARTICLES
The proton and neutron, which were once thought to be the basic blocks of matter, are now known to be made up of over 200 elementary particles. Elementary particles can be divided into two types: hadrons which are heavy particles subject to the strong force, and leptons which are small particles not subject to the strong force. Elementary particles have a further distinction between fermions, which are permanently existing particles, and bosons, which can be produced and destroyed freely.
Every type of particle is thought to have a companion antiparticle, which is opposite to it in some characteristic way. For instance, the positron, with positive charge, is the antiparticle of the electron, with a negative charge. Some particles, such as the photon, serve as their own antiparticles. Protons and neutrons are composed of simpler particles named quarks. The six types (‘flavours’) of quark are: up, down, charmed, strange, top and bottom. The proton is considered to consist of two up quarks and a down quark, whilst the neutron consists of two down quarks and an up quark. Mesons are short-lived subatomic particles composed of two quarks each. Mesons jump between protons and neutrons, thus holding them together. Neutrinos are particles which can carry much energy away from nuclear reactions, such as those involved in radioactivity, but they are difficult to detect, as they only interact very weakly with ordinary matter. They are capable of passing right through the Earth undetected.
