2.9 Activity and dose quantities and units
Physical quantities in atomic and nuclear physics are defined and expressed in units that have been adopted by the International Organization for Standardization (ISO), and are described in ISO (1992a) and ISO (1992b). Further references for quantities and units are recent publications of the International Commission on Radiological Protection (ICRP, 1991, 1993) and the United Nations Scientific Committee on Effects of Atomic Radiation (UNSCEAR, 1993). Apart from the International System (SI) units of nuclear physics, other units in common use in the field of radioactivity and the environment can be found in IAEA Safety Glossary (2007). This section describes a selection of SI and conventional units in common use in these fields. The symbols of the basic parameters used in the published ISO standards and literature are as follows:
| Z | atomic number (number of protons/electrons) |
| A | mass number (number of nucleons) |
| N | number of entities (eg. particles, pulses, counts) |
| n | frequency of events (eg. count rate), (s-1) |
| λ | decay constant (s-1) |
| T½ | half-life (s) |
| σ | effective cross section (m2) |
| σ | standard deviation (eg. σ(n) - standard deviation of a count rate) |
| E | energy (eV), 1 eV = 1.602×10-19 J |
| μ | linear attenuation coefficient (cm-1) |
| μ/ρ | mass attenuation coefficient (cm2/g); |
| ρ | density (g/cm3) |