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.
Alpha particle (alpha radiation, alpha ray)
A positively charged particle (a Helium-4 nucleus) made up of two neutrons and two protons. It is the least penetrating of the three common forms of radiation, being stopped by a sheet of paper. It is not dangerous to living things unless the alpha-emitting substance is inhaled or ingested or comes into contact with the lens of the eye.
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 and from the cosmic rays. The term may also mean radiation extraneous to an experiment.
Beta particle (beta radiation, beta ray)
An electron of either positive charge (ß+) or negative charge (ß-), which has been emitted by an atomic nucleus or neutron in the process of a transformation. 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 wanted.
A radioisotope generator system.
Curie (Ci)
The basic unit used to describe the intensity of radioactivity in a sample of material. One curie equals thirty-seven billion disintegrations per second, or approximately the radioactivity of one gram of radium.

- D -

A nucleus formed by the radioactive decay of a different (parent) nuclide.
Decay (radioactive)
The change of one radioactive nuclide into a different nuclide by the spontaneous emission of alpha, beta, or gamma rays, or by electron capture. The end product is a less energetic, more stable nucleus. Each decay process has a definite half-life.
The removal of radioactive contaminants by cleaning and washing with chemicals.
That property of a substance which is expressed by the ratio of its mass to its volume.
A 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 with a unit electrical charge and a mass 1/1837 that of the proton. Electrons surround the atom's positively charged nucleus and determine the atom's chemical properties.
Electron capture
A radioactive decay process in which an orbital electron is captured by and merges with the nucleus. The mass number is unchanged, but the atomic number is decreased by one.
Washing solution (The solution that is introduced into the cow).
The washings obtained by elution (the solution that comes out of the cow).
To separate by washing (to milk).
Excited state
The state of an atom or nucleus when it possesses more than its normal energy. The excess energy is usually released eventually as a gamma ray.

- F -

The splitting of a heavy nucleus into two roughly equal parts (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, cesium-137.

- G -

Gamma ray
A highly penetrating type of nuclear radiation, similar to x-radiation, except that it comes from within the nucleus of an atom, and, in general, has a shorter wavelength.
Geiger counter
A Geiger-Müller detector and measuring instrument. It contains a gas-filled tube which discharges electrically when ionizing radiation passes through it and a device that records the events.
A cow-a system containing a parent-daughter set of radioisotopes in which the parent decays through a daughter to a stable isotope. The daughter is a different element from that of the parent, and, hence, can be separated from the parent by elution (milking).

- H -

The time in which half the atoms of a particular radioactive nuclide disintegrate. The half-life is a characteristic property of each radioactive isotope.
Health physics
That science devoted to recognition, evaluation, and control of all health hazards from ionizing radiation.

- I -

Induced radioactivity
Radioactivity that is created by bombarding a substance with neutrons in a reactor or with charged particles produced by particle accelerators.
An atomic particle that is electrically charged, either negative or positive.
Ionizing radiation
Radiation that is capable of producing ions either directly or indirectly.
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.
Isometric transition
A mode of radioactive decay where a nucleus goes from a higher to a lower energy state. The mass number and the atomic number are unchanged.
Isotopes of a given element have the same atomic number (same number of protons in their nuclei) but different atomic weights (different number of neutrons in their nuclei). Uranium-238 and uranium-235 are isotopes of uranium.

- 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.
Microcurie (µCi)
One millionth of a curie (3.7 x 104 disintegrations per second).
To elute a cow.
A trademark of Union Carbide Corporation that is used to identify radioisotope generator systems for educational use.

- 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 weight, but the neutron has no electrical charge.
Nuclear reactor
A device in which a fission chain reaction can be initiated, maintained, and controlled. Its essential components are fissionable fuel, moderator, shielding, control rods, and coolant.
A constituent of the nucleus; that is, a proton or a neutron.
The science, technology, and application of nuclear energy.
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 weight, atomic number, and energy state.

- O -

- P -

A radionuclide that decays to another nuclide which may be either radioactive or stable.
A quantity 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: a hydrogen nucleus.

- Q -

- R -

Radiation Absorbed Dose. The basic unit of an absorbed dose of ionizing radiation. One rad is equal to the absorption of 100 ergs of radiation energy per gram of matter.
Radioactive dating
A technique for estimating the age of an object by measuring the amounts of various radioisotopes in it.
Radioactive waste
Materials which are radioactive and for which there is no further use.
The spontaneous decay of disintegration of an unstable atomic nucleus accompanied by the emission of radiation.
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 gamma radiation by counting the light flashes (scintillations) induced by the radiation.
Secular equilibrium
A state of parent-daughter equilibrium which is achieved when the half-life of the parent is much longer than the half-life of the daughter. In this case, if the two are not separated, the daughter will eventually be decaying at the same rate at which it is being produced. At this point, both parent and daughter will decay at the same rate until the parent is essentially exhausted.
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.
The accidental release of radioactive materials.

- T -

A small amount of radioactive isotope introduced into a system in order to follow the behavior of some component of that system.
The transformation of one element into another by a nuclear reaction.


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