Glossary Q - R

Radiation (Ionizing)

Energy given off as either particles or rays from the unstable nucleus of an atom.

Radiation Sickness (Syndrome)

The set of symptoms that results when the whole body (or a large part of it) has received an exposure of greater than 50 rads of ionizing radiation. The earliest symptoms are nausea, fatigue, vomiting, and diarrhea. Hair loss, hemorrhaging, inflammation of the mouth and throat, and general loss of energy may follow. If the exposure has been approximately 1,000 rad or more, death may occur within two to four weeks.

Radiation Warning Symbol

An officially prescribed symbol (a magenta or black trefoil) on a yellow background. It must be displayed where certain quantities of radioactive materials are present or where certain doses of radiation could be received.

Radioactive Contamination

A deposit of radioactive material in any place where it may harm persons, equipment, or the environment.

Radioactive Decay

The process in which an unstable (radioactive) nucleus emits radiation and changes to a more stable isotope or element. A number of different particles can be emitted by decay. The most typical are alpha or beta particles.

Radioactivity

Spontaneous transformation of the nucleus of an atom; this resulting in a new element, generally with the emission of alpha or beta particles often accompanied by gamma rays.

Radioisotope

Isotopes of an element that have an unstable nucleus. Radioactive isotopes are commonly used in science, industry, and medicine. The nucleus eventually reaches a more stable number of protons and neutrons through one or more radioactive decays. Approximately 3,700 natural and artificial radioisotopes have been identified.

Radionuclide

An unstable form of a nuclide.

Radium

Radium is a naturally-occurring radioactive metal. Radium is a radionuclide formed by the decay of uranium and thorium in the environment. It occurs at low levels in virtually all rock, soil, water, plants, and animals. Radon is a decay product of radium.

Risk

The probability of injury, disease, or death under specific circumstances. Risk can be expressed as a value that ranges from zero (no injury or harm will occur) to one hundred percent (harm or injury will definitely occur).Risk-based standards limit the risk that releasing a contaminant to the environment may pose rather than limiting the quantity that may be released.

• Absolute Risk, the excess risk attributed to irradiation and usually expressed as the numeric difference between irradiated and non-irradiated populations (e.g., 1 case of cancer per million people irradiated annually for each rad). Absolute risk may be given on an annual basis or lifetime basis.
• Relative Risk, the ratio between the number of cancer cases in the irradiated population to the number of cases expected in the unexposed population. A relative risk of 1.1 indicates a 10 percent increase in cancer due to radiation, compared to the "normal" incidence.

Roentgen Equivalent Man (Rem)

A unit of equivalent dose. Rem relates the absorbed dose in human tissue to the effective biological damage of the radiation. Not all radiation has the same biological effect, even for the same amount of absorbed dose.