The Why Files The Why Files --

Radiation reassessed

The measurement morass
A casual observer might figure there are just too many ways to measure ionizing radiation, and that casual observer would be right.

SI (International Standard) units:

Sievert (Sv): used to derive equivalent dose; 1 Sievert = 100 rem.

Equivalent dose in Sv = absorbed dose in Gy x (Q) (see rem, below).

Gray (Gy): measures the absorbed dose. One gray is equivalent to 100 rads; one rad = 0.01 gray, also called "one centi-gray."

Q: a factor reflecting the true harm of a particular type of radiation. The same dose of radiation is more damaging if it has a higher Q.

U.S. units:

Rad (radiation absorbed dose): measures the absorbed dose; can be used for any type of radiation or material. One rad = 10 mGy.

Rem (roentgen equivalent man): used to derive "equivalent dose" and compare the effective biological damage of radiation. The dose is often expressed in terms of thousandths of a rem (millirem, or mrem). Equivalent dose in rem = absorbed dose (rad) x Q (see above). 1 rem = 10 millisievert.

side views of paper, wood, concrete, with different types of radiation passing through some but not others
Different types of radiation have different abilities to pass through materials. Diagram: DOE

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Megan Anderson, project assistant; Terry Devitt, editor; S.V. Medaris, designer/illustrator; David Tenenbaum, feature writer; Amy Toburen, content development executive

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