Equivalent dose


Equivalent dose

The equivalent absorbed radiation dose, usually shortened to equivalent dose, is a computed average measure of the radiation absorbed by a fixed mass of biological tissue, that attempts to account for the different biological damage potential of different types of ionizing radiation. It is therefore a less fundamental quantity than the total radiation energy absorbed per mass (the absorbed dose), but is a more significant quantity for assessing the health risk of radiation exposure. For a further estimate of damage on different types of tissue (which vary in sensitivity to radiation) a further quantity called effective dose can be calculated (see article for more). However, the latter requires additional input information about various organs, and also fractionation of radiation to different parts of the body, if any.

Equivalent dose is dimensionally a quantity of energy per unit of mass, and is usually measured in sieverts or rems.

Contents

Computation

Conventional quality factors (relative biological effectiveness) to calculate equivalent doses[1]
Radiation Energy Q or RBE
x-rays, gamma rays, electrons,
positrons, muons
  1
neutrons < 10 keV 5
10 keV - 100 keV 10
100 keV - 2 MeV 20
2 MeV - 20 MeV 10
> 20 MeV 5
protons > 2 MeV 2
alpha particles, Nuclear fission products,
heavy nuclei
  20

The equivalent dose cannot be measured directly. The dose for each tissue T and each type of radiation R (often denoted by HT,R) is calculated by the formula

H_{T,R} = Q \times D_{T,R} \,

where DT,R is the total energy of radiation R absorbed in a unit mass of tissue T, and Q is a radiation quality factor that depends on the type and energy of that radiation. The quality factor is also known as the relative biological effectiveness of the radiation. It connects the simple deposited energy of the radiation (a physics quantity easy to measure with a sensor) to the equivalent dose, which is —to the extent that the estimation of the Q or RBE is accurate— the biological reaction or damage from this particular amount and type of radiation.

The value of Q or RBE is 1 for x-rays, gamma rays and beta particles, but higher for protons, neutrons, alpha particles etc.

To obtain the equivalent dose HT for a given mix of radiation types and energies, one simply adds the individual equivalent doses HT,R over all types of radiation energy dose R in the mix.

To obtain the average equivalent dose HR for a given combination of tissues (such as a whole organism), one may compute a weighted average of the doses HT,R over all T, with the weight of each dose being proportional to the mass of tissue T present in the combination.

In order to obtain an effective dose (also known as effective dose equivalent) for radiation given to only part of the body, the equivalent doses of radiation must be further weighted by a "weighting fraction" Wf specific to each tissue, which attempts to quantify the fraction of health risk for a human (or other mammal) attributable to this type of tissue. The weighting fractions for all the tissues add to 1.0. See the article on effective dose for this calculation.

Units

The SI unit of measure for equivalent dose is the sievert, defined as the dose of absorbed radiation that has the same biological effect as a dose of one joule of gamma rays absorbed in one kilogram of tissue. In the United States the roentgen equivalent man (rem), equal to 0.01 sievert, is still in common use, although regulatory and advisory bodies are encouraging transition to sieverts.[2]

Related quantities

To quantify the radiation risk of a particular environment or activity, one may use the equivalent dose received per unit of time spent in it, usually measured in sievert per year or sievert per hour.

References

See also

External links


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