is dissolved in the liquid, the counter is a "liquid scintillation counter" and may detect beta and gamma rays.

Sealed source

means any radioactive material that is encased in a capsule designed to prevent leakage or escape of the contents; the NRC defines sealed sources in terms of encapsulated byproduct material.

Sievert

(Sv) is the SI unit for dose equivalent. It is equal to 100 rem.

Stable isotopes

are those isotopes of an element that do not disintegrate and are thus not radioactive. Stable isotopes may be used in magnetic resonance imaging or in tracer methods that use mass spectrometry for detection.

Stochastic—

see Nondeterministic.

Teletherapy

is radiation therapy by means of an external beam of radiation. The source is commonly a cobalt-60 irradiator or a linear or other accelerator.

Transmutation

means changing one element into another. This occurs in radioactive decay whenever a charged particle such as an alpha or beta particle is emitted.

Weighting factor

is a way of taking into account the particular anatomic area irradiated in a calculation involving the equivalent dose to the whole body. The weighting factor for an organ or tissue is the proportion of the risk of stochastic effects resulting from irradiation of that area to the total risk of stochastic effects when the whole body is irradiated. (See Committed effective dose equivalent.)

Worker or employee in the radiation industry

is an individual who may be exposed as a result of his or her occupation.

X-rays

are short-wavelength electromagnetic radiations (photons) that emanate from energy changes in electronic shells.

X-ray machines

are used in medicine for both diagnostic and therapeutic purposes. In an x-ray machine, a beam of x-rays is generated when a stream of electrons hits an anode target; the beam is directed by collimation toward the field to be irradiated. After being attenuated by passing through an object such as a patient, x-rays are detected by a screen–film combination inside a cassette or by a fluorescent screen in an image-intensifier tube. The screen–film combination yields a film for diagnosis by a physician; the image-intensifier tube signal is fed into a video system for immediate viewing, as in fluoroscopy. X-rays may be used therapeutically to treat cancer.