absorbed dose

Absorbed dose is the quantity of ionizing radiation received per unit mass within a given period of time. The SI unit of absorbed dose is the gray (Gy). One gray is the absorbed dose when the energy per unit mass imparted to matter is 1 joule per kilogram. A former unit, the rad, = 0.01 gray.


Different materials that receive the same exposure may not absorb the same amount of energy. The absorbed dose (D) due to an amount of energy (E) being absorbed by a unit mass (m) of absorbing material is D = E/m.


The absorbed dose rate is the absorbed dose per unit of time. Its unit is the Gy/h.


Equivalent dose

Equivalent dose is a measure of the local radiation damage caused to biological tissue through exposure to intense ionizing radiation, such as alpha particles. Local damage to DNA is harder for the body to repair than the more diffuse damage from gamma rays and beta particles. This relative biological effectiveness gives rise to the use of equivalent dose, which is the absorbed dose multiplied by an appropriate radiation weighting factor (WR). The value of WR is 20 for alpha particles, and 1 for both beta particles and gamma rays. The resulting unit of equivalent dose is the sievert (Sv). Submultiples are the millisievert and microsievert. Prior to 1990, the weighting factor used was referred to as the quality factor (QF).


Another quantity, known as the effective dose, which also has units of Sv, is the equivalent dose weighted for the differing radiosensitivities of individual tissues. Associated with this is a factor called detriment, which gives the probability of stochastic effects occurring.


Equivalent dose is often referred to simply as "dose" in every day use of radiation terminology. The old unit of "dose equivalent" or "dose" was rem.


Units conversion

Dose in Sv = absorbed dose (in grays) × WR


Dose in rem = dose in rad × QF


1 Sv = 100 rem


1 rem = 10 mSv (millisievert = one thousandth of a sievert)


1 Gy air dose equivalent to 0.7 Sv tissue dose


1 R (roentgen) exposure is approximately equivalent to 10 mSv tissue dose