This portion of the site is just getting underway. Bits and pieces will be added on a regular basis. 


Introduction to Radiation and Radioactive Decay



Atoms with a specific number of protons and neutrons in the nucleus (e.g., atoms with 6 protons and 6 neutrons are one nuclide while atoms with 7 protons and 8 neutrons are a different nuclide). 

Atomic Number (Z)

The number of protons in the nucleus of an atom. It determines which element the atom belongs to. Hydrogen has an atomic number of 1, helium has an atomic number of 2, lithium has an atomic number of three . . . carbon has an atomic number of six . . . uranium has an atomic number of 92, etc.

Atomic Mass Number (A)

The total number of protons and neutrons in the nucleus of an atom. A nuclide is typically identified by specifying the element and the atomic mass number. For example, carbon-12 (C-12) has an atomic mass number of 12 and uranium-238 (U-238) has an atomic mass number of 238.


Different nuclides of the same element. In other words, atoms that have the same number of protons but different numbers of neutrons.  For example, carbon-12 (C-12) which has 6 protons and 6 neutrons and carbon-14 (C-14) which has 6 protons and 8 neutrons are isotopes.  As a rule, people use the term incorrectly. They use “isotope” when they should use “nuclide”.

Stable nuclides

Atoms with a stable combination of neutrons and protons in the nucleus  (e.g., 6 protons and 6 neutrons). In other words, a nuclide that is not radioactive.  


Atoms with an unstable combination of neutrons and protons in the nucleus (e.g., 6 protons and 8 neutrons). In other words, a nuclide that is not stable. It is radioactive and will undergo decay.

Radionuclides that have too many neutrons to be stable typically undergo beta decay (e.g., Cs-137). Radionuclides that have too many protons to be stable typically undergo positron decay (e.g., F-18) or electron capture (e.g., I-125). Radionuclides that have too many neurons and too many protons to be stable typically undergo alpha decay (U-238) or beta decay (Bi-214).

Chart of the Nuclides

A chart, analogous to the periodic table of the elements, that provides information about the radioactive properties of the various nuclides. It also provides information about the neutron absorption properties of the nuclides and the yields of nuclear fission.

Decay (radioactive decay)

A process by which a radionuclide changes its number of neutrons and protons to a more stable combination. Some radionuclides decay to a stable decay product (e.g., P-32 decays to stable S-32) while other radionuclides decay to a radioactive decay product (e.g., Ra-226 decays to radioactive Rn-222). Some authors prefer the term "transformation" to decay. 


A general term referring to particles (e.g., electrons) and electromagnetic waves (e.g., radio waves) that carry energy. Electromagnetic radiation can be thought of as “photons” or packets of pure energy. 

Radiation is released during the decay of radionuclides. It is also emitted by some electronic devices when they are operating, e.g., x-ray tubes. During the 1960s, the x-ray emissions from some color television sets were found to be unacceptably high and design changes had to be made.

It is common for people to confuse radiation with radioactive material. Radiation is emitted by radioactive material just as light is emitted by a light bulb. For example, it would be more appropriate to say that the accident at Three Mile Island released radioactive material rather than radiation. 

Ionizing radiation 

Radiation that possesses enough energy to ionize atoms, i.e., strip an electron away from an atom. Alpha particles, beta particles, positrons and neutrons are examples of particulate ionizing radiation. X-rays and gamma rays are examples of electromagnetic ionizing radiation. Ionizing radiation can be produced in a variety of ways, e.g., electronically via an x-ray tube, or as a result of the decay of a radionuclide.

Non-ionizing radiation 

Radiation that does not possess enough energy to ionize atoms. Radio waves, microwaves, infrared, visible light and ultraviolet are examples of non-ionizing radiation.

Alpha particles

Particles consisting of two protons and two neutrons that are ejected from the nucleus of a radionuclide when the latter decays. An alpha particle is a helium-4 nucleus and it has a charge of +2. It carries kinetic energy, usually in the 4 to 8 MeV range. An alpha particle only travels 2-6 cm in air. In the body, it might traverse one to two cells. A type of ionizing radiation.  

Beta particles

Electrons ejected from the nucleus of a radionuclide when the latter decays. A beta particle possesses kinetic energy, usually in the 0 to 2 MeV range. Depending on the energy, a beta particle can travel up to one meter in air (although not in a straight line) and one or so centimeters in human tissue. It has a negative charge (often expressed as -1, but in reality it is 1.6 x 10-19 coulombs). A type of ionizing radiation. 


Antimatter electrons ejected from the nucleus of a radionuclide when the latter decays. It has the same mass as a beta particle (0.00549 atomic mass units) but it has a positive charge whereas a beta particle has a negative charge. Immediately after it is produced (during radioactive decay or as a result of a high energy photon interacting via pair production) the positron will destroy itself and an electron. When this happens the positron and electron are converted into pure energy: two 511 keV photons that head off in opposite directions. 

Gamma Rays

Electromagnetic ionizing radiation emitted from the nucleus of an atom when it de-excites. Gamma rays are best thought of as "photons," i.e., packets of pure energy. The energy of a gamma ray is usually between 50 and 2000 keV (0.05 to 2 MeV). Gamma rays can be emitted during radioactive decay. Some radionuclides (e.g., H-3) never emit gamma rays during decay. Some radionuclides always emit a gamma ray during decay (e.g., Co-60).  Some radionuclides emit a gamma ray in a specific fraction of their decays. For example, 85% of the decays of Cs-137 result in the emission of a gamma ray. During the decay of a single atom, gamma rays at different energies can be emitted. For example, every time an atom of Co-60 decays, two gamma rays are emitted, one at 1173 keV and the other at 1332 keV.  If excited, for example by being struck by neutrons, a stable nuclide can emit gamma rays.

X- Rays

Although we usually think of x-rays as being produced by x-ray machines, x-rays can also be emitted during the decay of a radionuclide (radioactive material).  They can even be produced by a non-radioactive material when it is exposed to radiation, e.g., the devices used to measure the lead content in paint do so by measuring the intensity of the characteristic lead x-rays emitted when the paint is exposed to a low level radiation source.

There are two types of radiation that are lumped together under the name x-rays: characteristic x-rays and bremsstrahlung. 

Characteristic x-rays have unique energies that increase with the atomic number of the element (e.g., iron has a characteristic x-ray at 6.4 keV while uranium has an x-ray at 111 keV).  They are emitted when a electron in an atom falls from one energy level (a shell)  to fill a vacancy in a lower energy level.  There are different types of characteristic x-rays, e.g., K, and L x-rays.

The other type of "x-ray" is known as bremsstrahlung. In fact, most of the radiation emitted by an x-ray tube is bremsstrahlung  (some characteristic x-rays are also produced). Bremsstrahlung is emitted when electrons change direction. The higher the energy of the electrons and the higher the atomic number of the material through which they are traveling, the greater the intensity of the bremsstrahlung. Unlike characteristic x-rays, bremsstrahlung photons have a continuous range of energies. The highest energy that bremsstrahlung photons can have is the maximum energy of the electrons producing them. 


The rate of decay of a radioactive material, i.e., the number of decays per unit time. 

The curie (Ci), an old unit of activity, is defined as 3.7 x 1010 decays per second (37,000,000,000 dps).  Smaller quantities include the millicurie (mCi), the microcurie (uCi) and the picocurie (pCi).

The becquerel (Bq), the new unit of activity, is defined as one decay per second. Larger quantities include the kilobecquerel (kBq), the megabecquerel (MBq), and the gigabecquerel (GBq).


The time over which the half the atoms of a particular radionuclide decay. Iodine-131 (I-131) has a half-life of 8 days while uranium-238 (U-238) has a half-life of four and a half billion years.


Introduction to Dosimetric Quantities and Units


A quantity that describes the intensity of gamma rays and x-rays in air. More specifically, it is a measure of the ionization of the air that results from the gamma rays and x-rays.  It is an unnecessary quantity. Some workers use the quantity absorbed dose in air or air kerma instead. 

The units of the quantity exposure are roentgen (R) and coulombs per kilogram (C/kg).

Museum Directory

Last updated: 07/25/07
Copyright 1999, Oak Ridge Associated Universities