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Glossary of Nuclear Science Terms

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z

– A –


Any material that stops ionizing radiation. Lead, concrete, and steel attenuate gamma rays. A thin sheet of paper or metal will stop or absorb alpha particles and most beta particles.


The number of disintegrations per unit time.

Alpha decay

The emission of an alpha particle by the nucleus. The atomic number of the parent nucleus is decreased by two and the atomic weight is decreased by four.

Alpha particle (alpha radiation, alpha ray)

A positively charged particle consisting two neutrons and two protons (essentially a Helium-4 nucleus). It is the least penetrating of the three common forms of radiation, easily being stopped by a sheet of paper.


A particle of matter indivisible by chemical means. It is the fundamental building block of elements.

Atomic number

The number assigned to each element on the basis of the number of protons found in the element’s nucleus.

Atomic weight (atomic mass)

Approximately, the sum of the number of protons and neutrons found in the nucleus of an atom.

– B –

Background radiation

The radiation of man’s natural environment originating primarily from the naturally radioactive elements of the earth (including radon) and from cosmic rays. The term may also mean radiation extraneous to an experiment.


This is the SI unit for measuring radioactivity. It is defined as 1 disintegration per second.

Beta decay

The decay of a parent nucleus into a daughter nucleus by the emission of a beta particle when one of the neutrons in the nucleus transforms into a proton. The atomic weight of the nucleus remains the same, but the atomic number increases by one.

Beta particle (beta radiation, beta ray)

An electron (negatively charged -) or positron (positively charged +), which has been emitted from an atomic nucleus or neutron in the process of radioactive decay. Beta particles are more penetrating than alpha particles but less than gamma rays or x-rays.

– C –


Radioactive material deposited or dispersed in materials or places where it is not desired.

Curie (Ci)

Another unit to describe the activity of a sample of material. One curie equals thirty-seven billion disintegrations per second, or approximately the radioactivity of one gram of radium. It has been taken over by the Becquerel in the SI scheme.

– D –


One of the resultant decay products formed by a nucleus undergoing a nuclear disintegration, eg
Parent nucleus   –decays–>   Daughter nuclei

Decay (radioactive)

The change in a radioactive nuclide by the spontaneous emission of alphabeta, or gamma rays, or by electron capture. The end product is a less energetic, more stable nucleus.DecontaminationThe removal of radioactive contaminants.DoseA general term denoting the quantity of radiation or energy absorbed in a specific mass.

– E –

Electromagnetic radiation

Radiation consisting of electric and magnetic waves that travel at the speed of light. Examples: light, radio waves, gamma rays, x-rays.


An elementary particle of unit electrical charge and a mass 1/1837 that of the proton.

Electron capture

A radioactive decay process in which an orbital electron is captured by and merges with the nucleus, transforming a proton into a neutron. The mass number is unchanged, but the atomic number is decreased by one.

Excited state

The state of an atom or nucleus when it possesses more than its rest (ground state) energy. The excess energy is released eventually as electromagnetic radiation.


The quantity of radiation to which an object is exposed.

– F –


The splitting of a heavy nucleus into two daughter nuclei (which are nuclei of lighter elements), accompanied by the release of a relatively large amount of energy in the form of kinetic energy of the two parts and in the form of emission of neutrons and gamma rays.

Fission products

Nuclei formed by the fission of heavy elements. They are of medium atomic weight and almost all are radioactive. Examples: strontium-90, caesium-137.

– G –

Gamma ray

Electromagnetic radiation characterised by its high energy and short wavelength. It is a highly penetrating type of radiation. Gamma rays are associated with energy level transitions within the nucleus.

Geiger counter

A Geiger-Müller detector and measuring instrument. It contains a gas-filled tube which discharges electrically when ionising radiation passes through it and a device that records these events.


This is the SI unit for absorbed dose of ionizing radiation. 1Gy = 1 J/kg.

– H –


A characteristic property of each radioactive isotope. The half-life is the time in which half of a given source would be expected to have decayed.

– I –

Induced radioactivity

Radioactivity that is created by bombarding a substance with neutrons or with charged particles.


An atom that is electrically charged due to the loss (becomes positively charged) or gain (becomes negatively charged) of an electron (or electrons).

Ionising radiation

Radiation capable of exciting the bound atomic electrons in a medium.


To expose to some form of radiation.


One of several nuclides with the same number of neutrons and protons capable of existing for a measurable time in different nuclear energy states.


Isotopes of a given element have the same atomic number (same number of protons in their nuclei), but different atomic weights (a differing number of neutrons in their nuclei). For example, uranium-238 and uranium-235 are isotopes of uranium.

– J –

– K –


The capture by an atom’s nucleus of an orbital electron from the first K-shell surrounding the nucleus.


One thousand electron volts.

– L –

– M –


One million electron volts.

– N –


An electrically neutral particle with negligible mass. It is produced in many nuclear reactions such as in beta decay.


One of the basic particles which make up an atom. A neutron and a proton have about the same mass, but the neutron has no electrical charge. Whilst relatively stable within the nucleus, isolated neutrons deacy with a half life of about 1000 seconds


A constituent of the nucleus; that is, a proton or a neutron.


The core of the atom, where most of its mass and all of its positive charge is concentrated. Except for hydrogen, it consists of protons and neutrons.


Any species of atom that exists for a measurable length of time. A nuclide can be distinguished by its atomic weightatomic number, and energy state.

– O –

– P –


A radionuclide that decays to another nuclide which may be either radioactive or stable. The resulting decay product is known as the daughter product.


A quanta of electromagnetic energy. Photons have momentum but no mass or electrical charge.


One of the basic particles which makes up an atom. The proton is found in the nucleus and has a positive electrical charge equivalent to the negative charge of an electron and a mass similar to that of a neutron.

– Q –

– R –


Radiation Absorbed Dose. A unit of measuring the absorbed dose of ionizing radiation. One rad is equal to the absorption of 100 ergs of radiation energy per gram of matter.


The spontaneous decay or disintegration of an unstable atomic nucleus accompanied by the emission of radiation.


The emission and propagation of energy by means of electromagnetic waves or particles.


A radioactive isotope. A common term for a radionuclide.


A radioactive nuclide. An unstable isotope of an element that decays or disintegrates spontaneously, emitting radiation.

Rate meter

An electronic instrument that indicates, on a meter, the number of radiation induced pulses per minute from radiation detectors such as a Geiger-Muller tube.

– S –


An electronic instrument for counting radiation induced pulses from radiation detectors such as a Geiger-Muller tube.

Scintillation counter

An instrument that detects and measures ionising radiation by counting the light flashes (scintillations) induced by the radiation.

The Sievert

Unit indicating the biological damage induced by radiation. 1 Sv = 1 J/kg. The Sievert is equal to the absorbed dose in tissue (Gy) multiplied by the ‘quality factor’ for the particular type of ionising radiation. The quality factor is a dimensionless number representing the relative effect produced by the same absorbed doses of different types of radiation. In older terminology the quality factor was referred to as the Relative Biological Effectiveness


A protective barrier, usually a dense material, which reduces the passage of radiation from radioactive materials to the surroundings.


A radioactive material that produces radiation for experimental or industrial use.


Incapable of spontaneous radioactive decay.


A series of mathematical theorems to aid in the analysis of data by attaching significances to results.

– T –


Movement from one state to another.

– U –

– V –

– W –

– X –


Electromagnetic radiation characterised by its high energy and short wavelength. It is a penetrating type of radiation, similar to, but lower in energy, than a gamma ray. X-rays are associated with transitions of electrons in the inner atomic orbitals.

– Y –

– Z –


The atomic number of a nucleus.