# Nuclear Physics - Equations and Definitions

Subatomic particles - those particles which make up the atom, i.e. proton, neutron and electron.

Electron - a subatomic particle with a charge of -1 and mass in the order of 1/1800 a single proton.  (In standard units its mass is 9.1110-31kg and its charge is -1.610-16C.)

Proton - a subatomic particle with a charge of +1 and mass 1u.  (A u is the unified atomic mass unit and is equal to 1.710-27kg, its charge is 1.610-16C.)  It is a nucleon, in that it is found within the nucleus of atoms.

Neutron - a subatomic particle with a charge of 0 and mass 1u.  It is a nucleon, in that it is found within the nucleus of atoms.

Proton number (atomic number) - the number of protons in an atom, this defines the element.

Nucleon number (mass number) - the total number of protons and neutrons.  Given as a multiple of u, the unified mass number.

Isotope - the various mass numbers of an element, i.e. each isotope of the same element has the same proton number but different mass number.  Isotopes have the same chemical behaviour but different nuclear behaviour.

Ion - a charged particle, atoms are ionised negatively by gaining electrons or positively by losing electrons.

Radioactive decay - a change in the nucleus which emits some nuclear energy in the form of nuclear radiation.

Nuclear stability - a measure of how likely a nucleus is to decay.  I.e. a stable isotope is not likely to decay.  All nuclei are tending towards stability.

Nuclear radiation - the high energy emissions from the nucleus of atoms.

Alpha - a nuclear radiation consisting of two protons and two neutrons.  It is also described as a Helium nucleus.

Beta - a high energy (high speed) electron emitted from a nucleus when a neutron becomes a proton.

Beta plus - a high energy (high speed) positron emitted from a nucleus when a proton becomes a neutron.  (This is the antimatter partner of an electron!)

Gamma - very high frequency electromagnetic radiation from the nucleus.

Penetrating power - a measure of the ability of the various radiations to penetrate matter, usually expressed as a range in air and a material which will stop it.

Ionisation power - a measure of how likely each radioactive particle is to cause an ionisation.

Activity - number of decays per second of a sample of a radioactive material.  Has units Becquerel (Bq) which is equivalent to per second.

Half-life - The average time taken for half of a sample to decay, or for the activity to half.

Quantum - a discrete amount of energy.  Referring to the changes of energy level in the Bohr model of the atom.

Energy levels - a discrete amount of energy which it is possible for an electron to exist at within the atom.

Photon - a wave-particle of light.  The mechanism by which electromagnetic interractions are carried.  Higher energy photons have higher frequencies.

Absorption - a photon of an equal energy to a discrete energy level change is taken in by an electron an atom.  The electron is raised to a higher energy level, i.e. it is excited.

Emission - an electron de-excites, i.e. it falls from a higher energy level to a lower one, it emits, (gives out) a photon of the corresponding energy.

Absorption spectrum - a continuous spectrum of light where the absorbed frequencies appear as dark lines.

Emission spectrum - a pattern showing only the emitted frequencies of light against a dark background.

Background radiation - radiation which is constantly randomly occurring at all times.  This is different at different places on earth, most of this is entirely natural.

Contamination - a radioactive material getting into a normally non-radioactive material, for example the human body.

Radiotherapy -  use of ionising radiation to kill cells in a tumor.  Rays are focused on cancerous tissue and beam is rotated ensuring healthy cells have only a minimal dose of radiation.

Fission - large unstable nuclei absorb neutrons and split into daughter nuclei, nuclear energy is released along with two or three neutrons.

Chain reaction - the neutrons released by fissions go on to cause other fissions.  As each fission releases more than one neutron the number of fissions per second increases rapidly.

Fusion - the joining of two smaller nuclei to form one larger nuclei.  Nuclear energy is released in this process.