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.

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 H_T,R) is calculated by the formula

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

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

To obtain the average equivalent dose H_R for a given combination of tissues (such as a whole organism), one may compute a weighted average of the doses H_T,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" W_f 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

• International Commission on Radiological Protection. ICRP Publication 60: 1990 Recommendations of the International Commission on Radiological Protection. Elsevier Science Pub Co (April 1, 1991). ISBN 0-08-041144-4.

1. ^ "Radiation weighting factors". European Nuclear Society. http://www.euronuclear.org/info/encyclopedia/r/radiation-weight-factor.htm. Retrieved 9 June 2011. 
2. ^ Nuclear Regulatory Commission. "NRC Regulations: §34.3 Definitions". United States Government. http://www.nrc.gov/reading-rm/doc-collections/cfr/part034/part034-0003.html. Retrieved 2007-03-14. 

See also

• Ionizing radiation units
• Rad (unit)
• Gray (unit)
• Banana equivalent dose
• Counts per minute
• Curie
• Becquerel
• Roentgen (unit)
• Roentgen equivalent man
• Sievert

External links

• Dose equivalent - glossary of the European Nuclear Society