ARTICLE
Natural radioactivity measurements and external dose estimation
by car-borne survey in Douala city, Cameroon
S.D. Takoukam Soh
1,2
, Saïdou
1,2,*
, M. Hosoda
3
, J.E. Ndjana Nkoulou II
2
, N. Akata
4
,
O. Bouba
1
and S. Tokonami
5
1
Nuclear Physics Laboratory, Faculty of Science, University of Yaoundé I, P.O. Box 812 Yaoundé, Cameroon.
2
Nuclear Technology Section, Institute of Geological and Mining Research, P.O. Box 4110 Yaoundé, Cameroon.
3
Hirosaki University Graduate School of Health Sciences, Hirosaki City, Aoromi, Japan.
4
National Institute for Fusion Science, 322-6 Oroshi, Toki, Gifu 509-5292, Japan.
5
Department of Radiation Physics, Institute of Radiation Emergency Medicine, Hirosaki University, Hirosaki City, Aomori 036-8564,
Japan.
Received: 23 January 2018 / Accepted: 27 July 2018
Abstract – A car-borne survey was carried out in Douala, the largest city in Cameroon to make a
detailed distribution map of the absorbed dose rate in the city, to locate the high natural radiation areas
useful later to carry out indoor radon, thoron, and thoron progeny measurements. Gamma-ray dose rates
were measured using 3-in 3-in NaI(Tl) detector. Activity concentrations of
238
U,
232
Th and
40
K in soil
from Douala city were determined by two methods: the first, using in situ gamma spectrometry and the
second, at the laboratory using a NaI(Tl) detector. A heterogeneous distribution of absorbed dose rates in
air was observed on the dose rate distribution map, and varies from 29 to 86 nGy h
1
with an average of
50 nGy h
1
, lower than the world average value of 59 nGy h
1
. The activity concentrations with NaI(Tl)
detector varied from 18 to 47 Bq kg
1
for
238
U, 21 to 54 Bq kg
1
for
232
Th, and 10 to 410 Bq kg
1
for
40
K
with averages of 29, 38, and 202 Bq kg
1
respectively, for in situ measurements. They vary between
29–98 Bq kg
1
for
238
U, 29–92 Bq kg
1
for
232
Th, and 40 to 79 Bq kg
1
for
40
K, with averages of 60, 57,
and 56 Bq kg
1
respectively for soil samples collected at Douala III subdivision. The highest value of the
annual effective dose for in situ measurements by car was observed at Ndogbong and was found to be
0.7 mSv y
1
, higher than the world average value of 0.5 mSv y
1
.
Keywords: car-borne survey / NaI(Tl) detector / natural radioactivity / air absorbed dose rate / external effective dose
1 Introduction
Exposure to natural radiation sources varies substantially
from one area to another and even locally (UNSCEAR, 1982).
Gamma radiation from natural radionuclides such as
238
U,
232
Th
and
40
K is the main source of external exposure. There are three
sources of environmental radioactivity: terrestrial, manmade and
cosmic. The most significant terrestrial radionuclides include the
uranium and thorium decay series, potassium and rubidium
(EPA, 2009). The terrestrial component is due to the radioactive
nuclides that are present in air, soil, rocks, water and building
materials whose amounts vary significantly depending on the
geological and geographical features of the regions. Cosmic
radiation from space contributes to the background changes
chiefly through elevation and latitude (UNSCEAR, 2000).
Although background radiation is present everywhere, radionu-
clide concentrations and distributions are not constant (EPA,
2009). For ages, humans have been exposed to radionuclides that
occur naturally in the environment. It is therefore important to
measure the activity concentrations of radionuclides in the living
environment.
There have been many surveys to measure natural
radioactivity and to estimate corresponding radiation dose
to the public in Cameroon. According to Guembou et al.
(2017), absorbed dose rates and annual effective dose due to
radioactivity in sand used as building material in Douala,
were normal and within the recommended limits. Also,
Saïdou et al. (2015a, 2015b) reported no significant radiological
risk to population living in the oil-bearing Bakassi peninsula,
in the uranium-bearing regions of Poli and Lolodorf. The
average total radiation dose and external radiation dose
were respectively 5.9 and 0.6 mSv y
1
for Poli, 7.6 and
0.7 mSv y
1
for Lolodorf, and 22.3 and 0.3 mSv y
1
for Bakassi.
*Corresponding author: saidous2002@yahoo.fr
Radioprotection 2018, 53(4), 255–263
© EDP Sciences 2018
https://doi.org/10.1051/radiopro/2018032
Available online at:
www.radioprotection.org