Physics and Engineering of Radiation Detection

606 Chapter 11. Dosimetry and Radiation Protection

particular type of radiation and the location of its source, that is

HT,R=wR·DT,R, (11.2.1)

whereHT,Ris the equivalent dose due to radiation typeR,DT,Ris the mean ab-
sorbed dose delivered by radiationR,andwRis the radiation weighting factor. The
radiation weighting factor is given by

wR=QR·NR, (11.2.2)

whereQRandNRare thequalityandmodifiedfactors for the radiation typeR
respectively. For external sources of radiation,NRis taken to be unity. For internal
sources,NRis assigned a value by some relevantauthority. In most cases a value of
NR= 1 can be taken and the knowledge of the quality factor is sufficient to calculate
the weighting factor. The quality factors for different radiation types are listed in
Table 11.2.1^1.
Note that the above formula is valid for only one type of radiation. In case
of mixed field, the total equivalent dose can be obtained by simply summing the
contribution due to individual types of radiation, that is

HT=

∑

R

wR·DT,R. (11.2.3)

Since the weighting factor is a dimensionless quantity, the equivalent dose is also
measured in units ofJ/kg. However now the specific name given to the unit is sievert
represented by the symbolSv. There is also an older unit calledrad equivalent for
manorrem, which is still in limited use. The conversion fromSvtormis fairly
simple with

1 Sv≡ 100 rem.

Example:

In a mixed radiation environment, a person receives 20mGyofγ-ray dose

and 2mGyof slow neutron dose. Calculate the total equivalent dose received

by the person.

Solution:

As the source is external, we can takeNR= 1 and the weighting factors for

the two radiation types as given in Table 11.2.1 are

wγ=1 andwn=5.

(^1) In table-11.2.1 as well as in earlier sections the particle energies have been given inelectron voltunits
(keV,MeV) instead of the conventional SI units ofJoules. This convenient unit of energy is defined as the
energy attained by an electron when it is made to accelerate by an electric potential difference of 1 volt.
E=qV=(1. 6 × 10 −^19 C)(1JC)
This gives
1 eV=1. 6 × 10 −^19 J.