Equivalent dose

The equivalent dose is a measure of the biological effect of radiation due to several factors. The factors to take into account are as follows:

  • The type of radiation
  • The absorbed dose
  • The body organs or tissue that have been exposed

Type of radiation and tissue type

The effect of different types of radiation on tissue varies. Some radiations (alpha particles) are more ionising than others (beta and gamma).

This is quantified by using a radiation weighting factor ( W_{R}).

Radiation weighting factor
RadiationRadiation weighting factor ( W_{R})
alpha particles20
beta particles1
gamma rays1
slow neutrons3

Equivalent\,dose = Absorbed\,dose \times Radiation\,weighting\,factor

H = D \times {W_R}

Equivalent Dose ( H) is measured in Sieverts (Sv)

Absorbed Dose ( D) is measured in Grays ( Gy)

Radiation weighting factor does not have units.

The equivalent dose may be due to several types of radiation. Each type will have its own absorbed dose and radiation weighting factor. This means that the equivalent dose from each type of radiation can be added to give a total equivalent dose.

Question

A worker in a nuclear power station receives the following radiations while working in 1 year:

10 mGy of slow neutrons.

25 mGy of gamma rays.

What is the total equivalent dose that the worker has absorbed?

Looking at the previous table, the radiation weighting factor for slow neutrons is 3 and for gamma rays is 1.

For slow neutrons:

Radiation\,weighting\,factor = {W_R} = 3

Absorbed\,dose = D = 10mGy = 0.01Gy

H = D \times {W_R}

H = 3 \times 0.01

H = 0.03Sv

For gamma rays:

Radiation\,weighting\,factor = {W_R} = 1

Absorbed\,dose = D = 25mGy = 0.025Gy

H = D \times {W_R}

H = 1 \times 0.025

H = 0.025Sv

\begin{array}{l}
                Total\,equivalent\,dose = Slow\,neutrons + Gamma\,rays\\
                = 0.03 + 0.025
                \end{array}

= 0.03 + 0.025

= 0.055Sv

The average annual effective dose equivalent received by a person in the UK due to natural sources is about 2 mSv.

This value increases significantly for those working with radioactive sources.