**Bethe-Bloch formula** The linear stopping power for a charged particle given by the formua: .

**Bragg curve:** 1) A curve showing the average number of ions per unit distance along a beam of initially monoenergetic ionizing particles, usually alpha particles, passing through a gas. Also known as a Bragg ionization curve. 2) A curve showing specific ionization of an ionizing particle of a particular kind as a function of its kinetic energy, velocity, or residual range.

**Bragg-Kleeman rule:** (also known as the Bragg rule) An empirical rule according to which the mass stopping power of an element for alpha particles is inversely proportional to the square root of the atomic weight.
**bremsstrahlung** Radiation that is emitted in the form of x-ray photons (light quanta) by an electron accelerated in its collision with the nucleus of an atom.

**delta-ray**: energetic electrons with sufficient kinetic energy to produce secondary ionization following initial interaction with a passing charged particle. The majority of the energy loss of a heavy charged particle such as the alpha occurs via these delta-rays. (Knoll, 1989).

**linear energy transfer (LET): **See also stopping power, linear.

**positronium**: The bound state of a positron and an electron.

**range** (of charged particles): for heavy charged particles (i.e., not electrons), the range is the average distance travelled before a particle has lost all of its original kinetic energy. Electrons have winding, tortuous paths in material and therefore range is usually defined as the minimum thickness which will stop all electrons.

**straggling, energy:**The broadening of the distribution of kinetic energies in a beam of initially monoenergetic charged particles due to the statistical nature of the energy deposition process as the particles pass through the material.

**straggling, range:** The statistical distribution of the range in which a charge particle travels while losing its kinetic energy in an absorbing medium.
**stopping number**An expression appearing in the Bethe-Bloch formula for linear stopping power of a material. It is proportional to atomic number Z of the absorber.

**stopping power, linear** : the energy lost by a charged particle passing through a substance per unit length of path; related concepts are mass, atomic, molecular, and relative stopping power. See also <linear energy transfer.

**stopping power, mass**: the stopping power of a material normalized for the density of the material. Given in units of energy per thickness in gm/cm**2.
________________________________________________________

Douglas J. Wagenaar, Ph.D., wagenaar@nucmed.bih.harvard.edu