Radiation Health and Safety

Several units of measurement are used to describe levels and doses of radioactivity.

As is the case in everyday life, the units used in the United States are not the same

as the SI units of measurement.

In the United States, the degree of radioactivity is often measured in curies

(Ci). Less commonly used in the United States is the SI unit, the becquerel (Bq).

Both units measure the number of disintegrations per second; 1 Ci is 3.7 1010

dps (disintegrations per second), while 1 Bq represents 1 dps. The curie and the

becquerel are used to report the amount of radioactivity when multiple kinds

of unstable nuclei are decaying and to report amounts necessary for medical

purposes.

By itself, the degree of radioactivity does not provide a good measure of the

amount of energy in the radiation or the amount of damage that the radiation can

cause to living tissue. Two additional kinds of information are necessary. The first

is the amount of energy absorbed; the second is the effectiveness of the particular

kind of radiation in causing tissue damage. The amount of energy absorbed by

living tissue is measured in rads. Rad is an acronym for “radiation absorbed dose.”

One rad represents 0.01 J of energy absorbed per kilogram of tissue. Its SI equivalent

is the gray (Gy); 1 Gy denotes the absorption of 1 J per kilogram of tissue.

Different forms of radiation cause different amounts of biological damage. The

amount of damage depends on how strongly a form of radiation interacts with

matter. Alpha particles cannot penetrate the body any farther than the outer layer

of skin. If particles are emitted within the body, however, they will do between

10 and 20 times the amount of damage done by -rays, which can go entirely

through a human body without being stopped. In determining the amount of biological

damage to living tissue, differences in damaging power are accounted for

using a “quality factor.” This quality factor has been set at 1 for and radiation,

5 for low-energy protons and neutrons, and 20 for particles or high-energy protons

and neutrons.

Biological damage is quantified in a unit called the rem (an acronym for “roentgen

equivalent man”). A dose of radiation in rem is determined by multiplying the

energy absorbed in rads by the quality factor for that kind of radiation. The rad

and the rem are very large in comparison to normal exposures to radiation, so it

is more common to express exposures in millirems (mrem). The SI equivalent of

the rem is the sievert (Sv), determined by multiplying the dose in grays by the

quality factor.