4. Charged-Particle Stopping Powers and Related Quantities

Abstract

Charged-particle stopping powers have been treated in detail in ICRU Report 37 (ICRU, 1984a), Report 49 (ICRU, 1993), and Report 73 (ICRU, 2005; Sigmund et al., 2009). In the present Report, consideration has been confined to electrons (and positrons), protons, and a particles for three key materials: graphite, air, and liquid water. Additionally, information on C ions in these materials has been included in recognition of their increasing use in radiation therapy. The stopping power quantifies the average energy loss per pathlength of charged particles in matter. In principle, the stopping power consists of three contributions, namely the electronic (or collision), the radiative, and the nuclear stopping power. The nuclear stopping power is negligible for electrons; it can be significant for light and heavy ions (ICRU, 1993; 2005), but only at rather low kinetic energies. A recommendation issued jointly by the ICRU and IAEA (Wambersie et al., 2004) calls “light ions” those nuclei with an atomic number equal to, or smaller than, that of neon nuclei (Z 1⁄4 10), leaving the name of “heavy ions” to all heavier nuclei. Results for the nuclear stopping power from ICRU Report 49 (1993) will be included in the tables presented in this Report for completeness. The radiative stopping power for electrons and positrons can be quite significant; for light and heavy ions, it is smaller to first approximation by a factor of (me/M) , where me is the rest mass of the electron and M the rest mass of the incident ion. Thus, the radiative stopping power can be ignored for protons and heavier ions at the kinetic energies considered in this Report.