The same absorbed dose delivered by different types of radiation may result in different degrees of biological damage to body tissues. The total energy deposited is not the only factor which determines the extent of the damage. The equivalent dose was introduced to take into account the dependence of the harmful biological effects on the type of radiation being absorbed. The equivalent dose is therefore a measure of the risk associated with an exposure to ionising radiation . Risks due to exposures to different radiation types can be directly compared when in terms of equivalent dose.
The unit of equivalent dose is the sievert (Sv) and is defined for a given type of radiation by the relationship:
The radiation weighting factor is a dimensionless number which depends on the way in which the energy of the radiation is distributed along its path through the tissue.
The rate of deposition of energy along the track is known as the Linear
Energy Transfer (LET)
of the radiation and has units of 
measured in water (water is considered equivalent to tissue).
Radiation with a high LET (such as heavy charged sub-atomic particles) is more
likely than radiation with a low LET (such as x-rays or beta particles) to
damage the small structures in tissue such as DNA molecules
. This is because
the energy from high LET radiation is absorbed in a
small volume surrounding
the trail of dense ionisation produced by this radiation. The radiation
weighting factor is directly related to the LET of the radiation as is shown
in Fig. 3.1 
.
The radiation weighting factors are used to correct for
differences in the biological damage to tissue caused by chronic exposure to
different radiations. The radiation weighting factors for various radiations
are shown in the Table 3.1 .
Figure: 3.1 Correspondence between LET and radiation
weighting factor
Table 3.1: Radiation weighting factors
for various radiations
The use of the radiation weighting factors to calculate the equivalent dose is illustrated in the following examples:
has absorbed an equivalent dose of:
In other words the dose of
of fast neutrons is
`equivalent' to a dose of
of x-rays. Both can be expressed as an
equivalent dose of 
.
) has absorbed an equivalent dose of:
For example if a person were to receive
of slow neutron ,
of gamma, and 
of fast
neutron radiation the total dose would be calculated as follows:
