2.2.4. External gamma-ray dose rate
The distribution of external exposure due to terrestrial radiation (
226
Ra,
232
Th and
40
K) in a given place depends on the
geographical characteristics of the place. Anthropomorphic alteration to the natural environment (as in for instance as a result of
operation of a coal-fired power plant) can also increase radiation
exposure in the natural environment, which also increases radiation exposure of the public. The resultant dose can be calculated
according to the formula given inUNSCEAR (1993):
D¼0:461CRaþ0:623CThþ0:0414CK ð4Þ
where,Dis the absorbed dose rate in air (nGy/h), andCRa, CThand
CKare the activity concentrations of
226
Ra,
232
Th and
40
K in soil
(Bq/kg), respectively. The processing of materials containing
NORM via mechanical, chemical or thermal treatment methods
can disrupt radioactive equilibrium due to the separation of
different radionuclides into various process streams. However,
about 98% of the external gamma dose rate from the
238
U series
is delivered by the
226
Ra sub series. Thus disequilibrium, if any,
between
226
Ra and
238
U will not affect the dose estimation from
the measurement of
226
Ra (Gusain et al., 2012).