Frequently, the ejected electrons possess sufficient energy to cause other atoms to ionize. The magnitude of the radioactivity in a sample can be expressed as an activity, exposure, or absorbed dose. The SI unit of activity is 1 disintegration per second or becquerel. Exposure is the amount of ionization caused by radioactive material. One roentgen is the amount of radiation that produces ions with a total charge of 1 electrostatic unit in 1 cm 3 of dry air.
Absorbed dose is the amount of energy absorbed by a substance exposed to ionizing radiation.
Different kinds of radiation cause different biological effects for the same amount of energy absorbed. For this reason, roentgen equivalent in man or rem was introduced. One rem is equal to one rad multiplied by a factor, Q, that accounts for the relative biological effect of radiation on humans. The ionizing power of radiation depends on the type of radiation. However, alpha particles lose most of their energy after traveling only a few centimeters in air or less than 0.005 mm in aluminum.