Radiation Units

•A curie is a very large amount of radioactivity. Contamination of individuals usually involve µCi to mCi quantities. Nuclear medicine patients are injected with µCi to mCi quantities of radioactive material for routine diagnostic exams.

•The basic unit of radiation dose is the rad. The rad is defined as the deposition of 0.01 joule of energy (a small amount) per kilogram (kg) of tissue.

•A rad of x-rays, a rad of gamma rays, and a rad of beta particles are about equally damaging to tissue. However, a rad of another type of ionizing radiation, such as alpha particles or neutrons, is much more damaging to tissue than a rad of gamma rays.

•The rem was introduced to take into account this variation in tissue damage. This is important because a person may be exposed to more than one type of radiation. For example, it was found that 100 rad of gamma and beta radiation produced the same effect as 100 rad of x-rays. However, only 20 rad of neutrons and 5 rad of alpha particles produced the same effect as 100 rad of x-rays. Therefore, neutron and alpha radiations were more potent and required fewer rad to produce the same effect.

•The number of rem is calculated by multiplying the number of rad by a radiation weighting factor that accounts for the relative amount of biological damage produced by a specific type of radiation. The radiation weighting factor for x-rays, gamma rays, and beta particles is 1. Thus, a rad of one of these radiations is equal to one rem. For other types of radiation (that are less likely to be present in accidents), the quality factor may be higher.

•The International Scientific System (SI) assigns different units to the quantities:

1 R = 2.58 X 10-4 C kg-1 1 gray (Gy) = 100 rad

1 sievert (Sv) = 100 rem 1 becquerel (Bq) = 1 disintegration per second