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International Journal of Marine Science 2014, Vol.4, No.35
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10
Table 3 Efficiency, background and MDA for energies under
investigation
NUCLIDES @ KeV EFFICIENCY BKGD
CPM
MDA*
(dpm/g)
Pb-214
Ac-228
Pb-214
Tl-208
Bi-214
Cs-137
Ac-228
Bi-214
K-40
295.0
338.4
351.9
583.1
609.3
661.6
911.2
1120.2
1460.1
0.205
0.133
0.170
0.049
0.053
0.085
0.043
0.026
0.043
0.18±0.01
0.14±0.01
0.30±0.01
0.28±0.01
0.31±0.01
0.04±0.01
0.22±0.01
0.09±0.00
0.37±0.01
0.37
0.79
0.30
0.43
0.78
0.09
1.29
2.63
4.67
Note: * The counting time of MDA was 70 hours.
3.4Theoretical calculations
The gamma-ray radiation hazards due to the specified
radionuclides were assessed by different indices.
Radium equivalent activity (Ra
eq
), a widely used
hazard index, is an index that has been introduced to
represent the specific activities of
226
Ra,
232
Th and
40
K
in sediments by a single quantity. Its calculations
based on the assumption that 10 Bq
226
Ra/kg, 7
Bq
232
Th/kg and 130 Bq
40
K/kg produce the same
gamma dose rate, which takes into account the
radiation hazards associated with them. It calculated
using the following equation (UNSCEAR, 2000; El
Mamoney and Khater, 2004; Derin et al., 2012):
K
Th
Ra
eq
C
C C Ra
130
10
7
10
Where C
Ra
, C
Th
and C
K
are the activity concentrations
(Bq/kg dry weight) of
226
Ra,
232
Th and
40
K,
respectively.
The external hazard index is obtained from Ra
eq
expression through the supposition that its maximum
allowed value (equal to unity) corresponds to the
upper limit of Ra
eq
(370 Bq/kg). This index value
must be less than unity in order to keep the radiation
hazard insignificant. The external hazard index can be
defined as following equation (Beretka and Mathew,
1985; Singh, et al., 2005; El-Taher, 2010):
1
4810
259
370
K
Th
Ra
ex
A A A
H
Where A
Ra
, A
Th
and A
K
are the specific activities of
226
Ra,
232
Th and
40
K in Bq/kg, respectively.
The absorbed dose rates (D) due to gamma radiation
from natural radionuclides at 1 meter above the
ground surface, assuming uniform distribution of the
naturally occurring radionuclides (
226
Ra,
232
Th and
40
K), were calculated by Monte Carlo method based
on guidelines provided by UNSCEAR (2000). This
calculation was performed after assuming that the
contributions from other naturally occurring
radionuclides are insignificant. The value of D was
calculated by (UNSCEAR, 1988 and 2000; Derin et
al., 2012):
D (nGy/h) = 0.462 C
Ra
+ 0.604 C
Th
+ 0.042 C
K
Where C
Ra
, C
Th
and C
K
are the specific activities
(Bq/kg dry weight) of
226
Ra,
232
Th and
40
K,
respectively.
To estimate the annual external effective dose rates,
the conversion coefficient from absorbed dose in air to
effective dose (0.7 Sv/Gy) and an outdoor occupancy
factor (0.2) proposed by UNSCEAR (2000), were
used. Accordingly, the annual effective dose rate
(mSv/yr) (AEED) was calculated by:
Acknowledgment
The authors would like to express deep gratitude to Prof.
William Burnett, Department of Earth, Ocean and Atmospheric
Sciences, Florida State University, Tallahassee, USA, for his
contribution that the radioactivity measurements and
calculations were carried out in his laboratory. The authors also
thank the maritime staff of Coastal Research Institute for the
help in samples collection.
References
Badr A.A., El-Fishawi N.M., and Khafagy A.A., 1993, Grain
size characteristics of coastal sediments along
Rosetta-Burullus stretch, Egypt, Water Science J., 13,
43-48
Beretka I., and Mathew P. I., 1985, Natural radioactivity of
Australian building materials, waste and by-products,
Health Phys., 48, 87–95
Cochran J.K., Feng H., Amiel D., Beck A., 2006, Natural
radionuclides as tracers of coastal biogeochemical
processes, J. Geochem. Explor., 88(1-3), 376-379
Dean,R.G., and Dalrymple R.A., 2002, Coastal processes with
engineering applications, 04