Alpha particle - The nucleus of the helium atom containing two protons and two neutrons. It is a highly stable nucleus and is emitted in one piece from many heavier nuclei in a process known as alpha-decay. Alpha particles were known before it was understood that they comprised of protons and neutrons. They are also doubly ionized helium atoms, i.e. helium atoms with both electrons removed.

Anti-matter - matter made up entirely of anti-particles.

Anti-particle - a type of subatomic particle that is like a mirror image of the original particle in that many of its key properties are reversed. For instance, the anti-proton has the same mass as the proton but is negatively charged. When particles and their corresponding anti- particles meet, the mutually annihilate each other in a burst of energy. Similarly, a particle and anti-particle pair can be created out of pure energy. The anti-electron is known as a positron, and has a positive charge with the same magnitude as the negative charge on an electron.

Atomic mass unit (amu) - the conventional unit for expressing the mass of nuclei. It is one twelfth of the mass of a neutral carbon atom, 12C. The mass of a nucleus in amu is approximately equal to the mass number of a nucleus.

Atomic number - symbol Z, the number of protons in a nucleus. Also the total number of electrons in a neutral atom, since, if the charges are to balance the number of electrons outside the nucleus must equal the number of protons in the nucleus.

Baryons - Hadrons that are composed of three quarks. The lightest baryons are the protons and neutrons. Heavier hadrons, such as the delta particle, are unstable. Baryons and mesons (which contain just a quark and an anti-quark) make up the two types of hadrons.

Beta decay - the process, governed by the weak nuclear force, whereby protons and neutrons can transform into each other. When a neutron beta decays, an electron plus an anti-neutrino are released. A free neutron can beta decay because its mass exceeds the sum of the proton and electron masses. A proton can only beta decay within a nucleus with excess energy and becomes a positron plus a neutrino; this happens within nuclei with an excess of protons..

Beta ray - the old name given to the electrons, or positrons, emitted from nuclei in the process of beta decay.

Borromean nucleus - A certain type of unstable nucleus that acts as though it is made up of three distinct parts; The bulk of the nucleus makes up the core, while two nucleons (usually neutrons) 'float' around outside it. These three constituents (core plus two nucleons) are held together very weakly by the strong nuclear force in such a way that if any one of them is removed then the force between the remaining two is too weak to hold them together and they too fall apart. This behaviour is unique in nature. The term Borromean comes from the field of mathematics known as knot theory in which the borromean rings are interlocked such that each one holds the other two together. Examples of Borromean nuclei are 6He, 11Li and 14Be, all of which are also halo nuclei.

Bubble chamber - a device for revealing the paths of charged (highly ionizing) particles as they pass through a `supersaturated' liquid. Sometimes a liquid, such as liquid hydrogen, can be heated above its boiling point if it is very pure. However, a charged particle passing through such a `super-saturated' liquid will leave a series of bubbles at the points where the particle ionized the liquid.

Chain reaction - when a uranium nucleus is induced to undergo fission by the absorption of a neutron, it will, itself release a few neutrons. These can then, in turn, induce other nuclei to undergo fission, and so on, throughout a significant volume of uranium. This is a neutron chain reaction.

Cloud chamber - a device for revealing the paths of charged (highly ionizing) particles as they pass through `saturated' water vapour. There is a limit to the amount of water vapour air can hold, but that limit can sometimes be exceeded. However, a charged particle passing through such a `saturated' vapour will leave a series of droplets at the points where the particle ionized the vapour.

Cyclotron - an accelerator for charged particles in which the particles are held in a spiral orbit in a vacuum chamber by a magnetic field, and given a series of pushes by an electric field.

Delta particle - a relation of the nucleon, this particle can be thought of as an excited state of the nucleon with a slightly greater mass.

Diffraction - The property of all waves whereby they spread out on encountering a obstacle. The amount of spreading depends on the frequency of the wave and the size of the object.

Driplines - these are the limits on a Segre chart beyond which nuclei do not exist. So-called because it is as if nucleons inserted into such a nucleus drip straight out again.

Electromagnetic radiation - any radiation consisting of self-sustaining electric and magnetic fields. All electromagnetic radiation travels in a vacuum with the same speed; the speed of light. Light, radio waves, ultraviolet, gamma and infrared radiation are fundamentally the same, differing only in frequency and wavelength. All their differing effects on matter result from the difference in frequency and hence the difference in energy of the photons.

Electron - The first elementary particle to be discovered. It belongs to the class of particles known as leptons. Electrons are very light negatively charged particles and are the constituents of atoms outside the nucleus. Electrons have a mass of 9x10-31kg, about one two- thousandth of the mass of the lightest atom, the hydrogen atom. They have zero size and so are considered to be `point particles'. They are the carrier of electricity in metals.

Electron capture - The process in which an electron is absorbed by a nucleus wherein it combines with a proton to make a neutron and a neutrino.

Energy levels - The allowed discrete (quantised) values of energy that a nucleus can have. Each kind of nucleus has a unique pattern of energy levels. Atoms and molecules also have unique patterns of energy levels.

Energy valley (nuclear) - Not all nuclei have the same amount of energy per nucleon. If all nuclei are arranged according to N and Z, as in a Segre chart, and a line is raised from the position of each nucleus, proportional to the energy per nucleon, then the tops of those lines will form a surface taking the appearance of a valley. The stable nuclei will be those near the bottom of the valley. The nuclei higher up the valley will undergo radioactive transformations, losing energy and sliding down the sides of the valley.

Excited state - Any energy level of a nucleus above its ground state.

Fission - the process whereby a heavy nucleus splits into two roughly equal smaller nuclei, releasing energy locked up inside it. Usually, fission takes place after a nucleus has been stimulated by the absorption of a neutron, but spontaneous fission does exist. Fission is the process that produces nuclear power through controlled chain reactions.

Frequency - The number of vibrations of an oscillating system that occur in one second. For the case of waves, it is the number of wave crests that pass by a fixed point in one second. It is measured in Hertz ( = cycles per second).

Fusion - the nuclear process whereby two light nuclei can overcome the mutual electric (Coulomb) repulsion to fuse together. This is accompanied by the release of a large amount of energy and is the source of energy in the Sun and other stars. It is hoped that fusion will one day be harnessed as an energy source on Earth for mankind.

Gamma ray - a high-energy photon (particle of light) released from within atomic nuclei when they find themselves in an unstable excited state. Gamma rays can also be absorbed by nuclei which then become excited.

Gluon - massless particle, never seen isolated outside hadrons, which generate the attraction between the quarks within the hadrons, holding them together.

Ground state - The lowest energy level of a nucleus (or atom).

Hadrons - All particles that interact via the strong nuclear force. Hadrons are composed of quarks; protons and neutrons are hadrons, as are the mesons.

Half life - the time after which half of a large collection of identical radioactive nuclei will have decayed.

Halo nucleus - A certain type of exotic nucleus discovered in the mid 1980s that has many more neutrons than the stable isotope of that element. This sometimes results in the outer one or two neutrons being very weakly linked to the rest of the nucleons and thus spend much of their time far beyond the range of the strong nuclear force that binds them top the rest of the nucleus in the first place. Such nuclei are highly unstable and only exist due to strange rules of quantum mechanics. Examples of one neutron halo nuclei are 11Be and 19C, while two-neutron halo nuclei tend to be Borromean. Nuclei with proton halos also exist (such as 8B) but in that case the repulsion between the proton's positive charge and the rest of the nucleus means that it cannot stray very far out or it falls off. Thus proton halos tend to be smaller than neutron halos.

Interference - A property of waves whereby two waves overlap each other to produce a pattern of peaks (when two wave crests meet and combine) and troughs (when a wave crest meets a wave trough and the cancel out). The pattern of dark and light that results allows as to learn about the waves and about the system in which interference takes place.

Ion - an atom, or molecule, which is no longer electrically neutral, usually because one or more electrons have been knocked out, but the term can apply to an atom with an extra electron (negative ion).

Ionize - the process of removing electrons from atoms or molecules so that they are no longer electrically neutral. Alpha, beta and gamma rays ionize the atoms of matter with which they interact.

Isobar - Nuclides with different numbers of protons and neutrons but the same total number of protons plus neutrons, i.e. the same atomic mass number.

Isomer - A nucleus that is in a long-lived excited state (called an isomeric state or a metastable state). Certain nuclei can remain in such excited states due to certain quantum properties they have which forbid them from dropping down to a lower energy level by emitting gamma rays.

Isotones - Nuclides with the same number of neutrons but different numbers of protons.

Isotope - All nuclides of a particular element (having the same number of protons) but with different numbers of neutrons are known as isotopes of that element. Thus 12C and 14C are different isotopes of carbon.

Isotope shift - the optical spectrum of an atom depends almost entirely on the electrons outside the nucleus, but the size of the nucleus does have a tiny but measurable effect. This means that careful measurements of the optical spectra of atoms allow us to measure the size of the nucleus. This is very useful for nuclei that live for too short a time for measurements based on electron scattering to be made.

Leptons - One of the two classes of elementary particles in nature. They include the electron, muon and tau particle, along with their corresponding neutrinos.

Linear accelerator - an accelerator for charged particles in which the particles are given a series of pushes down a long straight vacuum chamber by oscillating electric fields.

Magic number - particular numbers of protons or neutrons lead to nuclei with increased stability compared to the neighbouring nuclei. For neutrons these numbers are 2, 8, 20, 28, 50, 82 and 126. They are the same for protons except that no nucleus with 126 protons is known.

Mass defect - the difference between the mass of a nucleus and the sum of the masses of all its nucleons when the nucleons are not bounded. Note that the total rest mass of decay products is smaller than the rest mass of decaying nucleus; this is called sometimes the mass decrement.

Mass number - symbol A, the total number of protons and neutrons in a nucleus. A=N+Z, where N is the number of neutrons and Z is the number of protons.

Meson - subatomic particle that can be considered as the carrier of the strong nuclear force between nucleons in nuclei. There are various types of mesons and they can be neutral, positively or negatively charged. It is now known that mesons are, like the nucleons, themselves composed of quarks. But unlike the nucleons, which are made up of three quarks, mesons contain a quark and an anti-quark.

Neutrino - a very light particle emitted during beta decay. Its name means 'little neutral one' and until recently was thought to have no mass at all (like the photon). It is now known that there are three types of neutrinos, along with their corresponding anti-particles, but it is the lightest of these that is emitted by nuclei.

Neutron - The other constituent, along with protons, of nuclei; it has a mass very slightly greater than the proton but is electrically neutral. Neutrons cannot survive for long outside the confines of the nucleus and beta-decay, i.e. turn into protons and anti-neutrinos, after about ten minutes.

Neutron star - the compact corpse left behind when a giant star dies in a supernova explosion. It has much the same density as a nucleus. Many experiments are aimed at understanding enough of the properties of nuclear matter at high pressures to enable us to understand neutron stars.

Nuclear matter - broadly speaking, the `stuff ' of nuclei. It is because nuclear matter is incompressible that the density of protons and neutrons at the centre of nuclei is much the same for all nuclei except the very lightest. For the same reason, neutron stars have much the same density as we find at the centre of nuclei.

Nucleon - general term for either a proton or a neutron.

Nuclide - A nucleus of a given number of protons and neutrons. There are about 7000 different possible nuclides, only several hundred of which are stable.

Oblate deformation - The deformation of a sphere that is achieved by squeezing two sides together. The Earth is slightly oblate since it is a little squashed at the poles making the equator slightly longer than it would be if the Earth were a perfect sphere.

Photomultiplier - a very sensitive photon detector, capable of measuring the energy in a very weak pulse of light. It forms part of a scintillation detector for measuring gamma rays. The pulse of light is generated as a scintillation.

Photon - a single particle of light. Einstein proposed in 1905 that light came in packets or light quanta, as an explanation for the photoelectric effect, whereby light can knock out electrons from the surface of a metal. Together with the work of Max Planck, this marked the beginning of the old quantum theory in which light is considered to be composed of discrete lumps. Later, it was realized that these packets of light energy had the same characteristics as particles, such as appearing at a single point in detectors, and they became known as photons.

Pion - The lightest meson, with mass of just over one eighth of the nucleon mass.

Positron - The anti-particle of the electron. It has the same mass as the electron but with opposite (positive) electric charge. The beta radiation from nuclei consists of electrons or positrons together with the (very nearly) undetectable neutrinos.

Prolate deformation - The deformation of a sphere into a 'rugby ball' or `American football' shape. This can be achieved with a balloon, say, by pulling apart two points on either side of the balloon's surface.

Proton - One of the constituents of nuclei and the only constituent of the lightest nucleus, Hydrogen. It has a positive charge of the same magnitude as the negative charge on an electron, and a mass almost two thousand times heavier than the electron. Every neutral atom contains as many protons in its nucleus as electrons orbiting outside it.

Quantum mechanics - The set of physical laws that govern the behaviour of the subatomic world. It began with the ideas of Max Planck and Albert Einstein at the beginning of the twentieth century and was developed into a complete mathematical theory by the mid-1920s by Neils Bohr, Erwin Schrodinger [Editor, o-umlaut] and Werner Heisenberg. Other physicists such as Paul Dirac, Max Born and Wolfgang Pauli made major contributions. While quantum mechanics is the most successful theory in science, underpinning much of modern physics, chemistry, electronics and material science, its predictions remain strange and counter-intuitive, especially when met for the first time.

Quark - components particle of proton, neutron, mesons and other hadrons. They cannot exist isolated, outside hadrons. The proton consists of two `up' quarks, each with a positive charge of magnitude 2/3 the charge of an electron, and one `down' quark, with a negative charge 1/3 the charge of an electron; the neutron has two down quarks and one up quark, making it a neutral particle.

Quark-gluon plasma - when nuclear matter is raised to a high temperature and subjected to huge pressure, as happens in very high energy collisions between heavy nuclei, the boundaries between the nucleons disappear and the quarks and gluons within them form a sort of soup. The Universe very briefly passed through a quark-gluon phase before protons and neutrons emerged after the Big Bang.

Scintillation - the flash of light when a nuclear particle or gamma ray strikes certain substances.

Scintillation counter - a particle detector in which the light from scintillations is detected and measure with photomultipliers.

Segre chart - a chart of the nuclei arranged (usually) with the proton number on the vertical axis and the neutron number on the horizontal axis. Generally, the square on this chart which correspond to particular values of N and Z will indicate key properties of that particular nucleus, such as its radioactivity.

Silicon detector - modern detectors of charged particles are based on the fact that such particles cause electrical signals to be generated when they pass through or are absorbed. Such detectors allow the energies and directions of particles produced in nuclear reactions to be accurately measured.

Spallation - when a very high energy proton or other projectile strikes a nucleus, it is likely to shatter, producing an array of lighter nuclei. Nuclei such as 6Li are believed to have been produced in spallation reactions in interstellar space.

Spectrometer - an instrument that separates radiation into its different wavelengths (or frequencies). Since every nucleus or every different atom radiates different sets of wavelengths, a spectrometer allows us to identify what atoms or nuclei are present in a sample. In addition, the particular pattern of wavelengths provides vital information about the atom or nucleus from which it has been radiated.

Spectrum - A representation showing how the strength (or brightness) of electromagnetic radiation from a given source depends on its wavelength. Also refers to the band of colours we see when light or other radiation is separated according to frequency (or wavelength); the most familiar example of this is the rainbow spectrum of visible light.

Spontaneous fission - The process whereby a heavy nucleus with an excess of neutrons splits roughly in half into two lighter nuclei. Such a process is a type of radioactive decay and takes place without the nucleus needing to absorb any surplus neutrons to prompt it to fission as would be needed in a nuclear reactor.

Super-heavy nuclei - nuclei with Z in the region of 110 or above.

Supernova, type Ia and type II - A supernova is the catastrophic explosion of a star in which it briefly emits as much radiation as all the stars in its galaxy. A type Ia supernova takes place when matter from the companion to a white dwarf star sheds enough matter onto the white dwarf until it exceeds the maximum mass such a star can have. A type II supernova takes place when the nuclear fuel in a giant star is exhausted. Radiation pressure from nuclear reactions can no longer hold the star up, so it collapses inwards. It then bounces out as a result of the incompressibility of nuclear matter, releasing vast amounts of energy, a flood of neutrinos and much matter into space. Many elements of which we and the solar system are made were produced in supernova explosions billions of years ago.

Synchrotron - developed from the cyclotron for much higher energies and involving a circular rather than spiral path. Synchrotrons can accelerate charged particles to very nearly the speed of light.

Theory of relativity - Einstein's special theory is based on two ideas; (I) that the speed of light in a vacuum is always the same, no matter how fast you are moving relative to the source of light, and, (II) the laws of physics are the same no matter how fast your lab is moving at a steady speed. One consequence is that mass m and energy E are equivalent, and that E=mc2.

Tokamak - an apparatus for producing nuclear fusion on Earth. It consists of a vacuum chamber in the form of a torus together with large magnets for keeping the interacting ions on closed paths through the vacuum.

Tunneling (quantum) - quantum mechanics permits particles to appear on the far side of barriers which, according to physics before quantum mechanics, they would not have the energy to surmount. This `quantum tunneling' controls such things as alpha decay, and permits the fusion of light nuclei in stars, allowing stars like the Sun to shine for billions of years.

Uncertainty principle - One of the fundamental ideas in quantum theory, first expressed by the German physicist Werner Heisenberg, which states that for any object certain pairs of properties, such as position and momentum, cannot be known precisely at the same time. However, this feature is not a result of the inevitable 'clumsiness' of our measuring apparatus when it comes to subatomic objects, but rather is a inherent in the objects themselves.

Wave-particle duality - The quantum concept whereby both matter and radiation must be regarded at their most fundamental level as sometimes having wave properties and sometimes particle properties. For example, electrons and photons each behave sometimes like particles and sometimes like waves.

Wavelength - The distance between two consecutive crests (or troughs) of a wave and is inversely proportional to the frequency of the wave. For electromagnetic radiation, the wavelength is equal to the speed of light divided by the frequency.

X-ray - A form of electromagnetic radiation with wavelengths shorter than ultraviolet but longer than gamma rays. Since the wavelength of the radiation is related to energy this means that x-ray photons are less energetic than gamma ray photons. However, there is no sharp dividing line between the two. X-rays have wavelengths ranging between about 10 nanometers down to 10 picometers.