Atomic or nuclear fission is the process of splitting a heavy nucleus into two lighter nuclei. Some nuclides split up spontaneous as a type of radioactive decay. In other elements, fission is induced through the reaction of an incident radiation with the nucleus. High energy rates are released during the fission reaction (difference between the binding energies); this energy is used to produce heat and electricity e.g. in nuclear power plants.

Gamma rays are a form of nuclear radiation that consists of photons emitted by radioactive elements from the nucleus. This high energetic light emission is also produced from subatomic particle interaction, such as electron positron annihilation. Gamma radiation, similar to x-radiation can injure and destroy tissue, especially cell nuclei.
Gamma rays have in general very high frequencies, short wavelengths, are electrically neutral and penetrate matter. The interaction of gamma rays with matter depends on the nature of the absorber as well as the energy of the gamma rays; these interactions determine also the type and amount of shielding needed for radiation protection.

See also Radiation Safety, Lead Equivalence, Lead Apron, Leaded Glove, Glove-Box, Radioactive Decay Law and Radiation Worker.

(H) The Hounsfield scale displays radiodensity in a linear scale of gray shades expressed in Hounsfield units (HU). The Hounsfield scale is a quantitative transformation of the attenuation coefficient.
The Hounsfield value -1000 is defined as the radiodensity of air, 0 H that of distilled water at standard pressure and temperature, and denser tissues like for example cranial bone can reach 2000 H. The radiation attenuation of dental fillings or artificial implants depends on atomic number of the elements used. Titanium usually has an amount of +1000 HU. Iron steel can have a density greater than the highest range (traditional 3095 H) covered by the standard Hounsfield scale of a CT scanner. Areas with attenuation coefficients that exceed the scale's maximum are white areas in which no detail is visible.
Some CT machines are relatively tolerant, precise representing regions with very high densities. Sometimes, an option is available to select an extended CT number scale.

Isomers are nuclides with the same number of neutrons and protons of a given element in different states of excitation. Most of these unstable isomers decay very quickly (~ 10^-12 seconds).
They are characterized in the chemical formula by an m (e.g. Tc-99m).

See also Isomeric Transition.

Mesons are elementary particle which are strongly interacting bosons belonging to the hadron family. More than 140 different kinds of mesons are discovered by now. Mesons can be negatively or positively charged or neutral.