413 Page 4 of 15 Arab J Geosci (2023) 16:413

Ci
Ai (Bq∕kg) = (1)
𝜀(E).P.m.t

√
( )2 ( )2 ( 𝜇𝜀 )2 ( 𝜇m )2
𝜇Ci
(2)
𝜇P
𝜇A = A + + +
Ci P 𝜀 m

where Ai is the activity concentration of the radionuclide

of interest; Ci is the net area of the energy peak E (keV); P
is the probability of gamma-ray emission at a given energy
E; Ɛ(E) is the detection efficiency of the detector as a func-
tion of the energy E; m is the mass of the sample in kg; t is
the counting time in seconds; and μ the standard deviation.
Fig. 2  Sample collection technique: a view of a sample point as per-
formed in this study
Radiological parameters

Gamma spectrometry system Radium equivalent activity

The detection system Due to the non-uniformity of the distributions of 226Ra,

232
Th, and 40K in the studied samples, the equivalent radium
The thalium-doped sodium iodide detector (NaI (Tl)), Canberra activity is calculated to assess the actual activities of 226Ra,
232
brand model 802, was used for the measurement of radioactivity Th, and 40K in the soil samples. This calculation is made
by gamma spectrometry. This detector is protected by a cylindrical assuming that 370 Bq ­kg−1 of 226Ra, 259 Bq ­kg−1 of 232Th,
lead castle (50 mm) to reduce background noise from cosmic and 4810 Bq ­kg−1 of 40K produce the same gamma equiva-
radiation and from the laboratory environment; inside are lent dose (Bangrota et al. 2017).
mounted two layers of 1 mm cadmium and 1 mm copper to Raeq = ARa + 1.43 × ATh + 0.077 × AK (3)
protect against X-rays from the interaction of background radiation
with the outer layer of lead and those from cadmium respectively With ARa, ATh, and AK the activity concentrations of
(Senthilkumar et al. 2016). This detector is equipped with a crystal 226
Ra, 232Th, and 40K in Bq ­kg−1, respectively.
measuring 7.6 × 7.6 c­ m2, with a resolution of 7.5% at 662 keV
and a multi-channel analyzer of 1024 channels. The detector is Dose rate
also associated by an optical coupling with a photomultiplier tube
that provides an output signal, which is then amplified and shaped In radiation protection, the dose rate is the dose absorbed
by a preamplifier and an amplifier (Blanchard et al. 2017). The per unit of time by the exposed material. The dose rate
spectra are acquired using Genie 2000 software version 3.3. quantifies the level of gamma radiation to which an
individual has been exposed. This parameter takes into
Calibration and calculation of the activity concentration account the concentration of radionuclides (226Ra, 232Th,
40
K) and the dose conversion factor, which is 0.462 for
For radiometric measurement of the radioactivity of the sam- 238
U (226Ra), 0.604 for 232Th, and 0.0417 for 40K, respec-
ples, the counting time for the detection of the emitted pho- tively. These conversion factors are used to convert the
tons was set to be 1­ 05 s (28 h). The multi-gamma-ray stand- activity concentrations of the radionuclides into the
ard sources of 137Cs, 60Co, 54Mn, 152Eu, and 22Na, traceable dose rate. Thus, the dose rate in the air at 1 m above the
to international standards and emitting gamma radiation in ground is given by Eq. (4) (Akkurt and Günoğlu 2014;
the 344.3–1407.5 keV range, were used for energy calibra- El Bahia et al. 2017):
tion. The standard sources of 208Tl (2614.4 keV), 40K (1460.8
keV), 228Ac (940.1 keV), and 137Cs (661.6 keV) contained in Dose_rate (nGy∕h) = 0.462 × ARa + 0.604 × ATh + 0.0417 × AK
(4)
a customer-supplied 500-ml Marinelli Beaker were used to
perform the efficiency calibration. The activities of 226Ra and
232
Th were determined using the energy lines of 214Bi (lines Annual effective dose (AED)
1120.3 keV and 1764.5 keV) and 228Ac (lines 911.6 keV and
969.1 keV) respectively. The line 1460.8 keV was used for According to the International Committee for Radiological
40
K. Activity concentration and uncertainties (IAEA 1989) Protection (ICRP), the effective annual dose coming from
were calculated according to the following equations: natural radioactive sources is limited at 1 mSv ­year−1. Our

13
413 Page 2 of 15 Arab J Geosci (2023) 16:413

according to the European Commission, the segment of the concentrations of 226Ra, 232Th, and 40K. The obtained results
chain starting from the fifth descendant, i.e., 226Ra, is radio- could be used as a national database to contribute to the
logically the most important (EC 1999). Thus, in this work, development of a reference map of the level of environmen-
we will refer to 226Ra rather than 238U. tal radioactivity in Cameroon and to complete the work done
Human exposure to natural ionizing radiation can occur by Ngachin (Ngachin et al. 2008) in some localities of the
externally (irradiation) or internally (contamination). Irra- South-West region of Cameroon.
diation is due to the above-mentioned radionuclides, which
continuously emit gamma radiation. Contamination occurs
through the ingestion of food contaminated with radioac- Materials and methods
tive particles or the inhalation of radioactive particles in the
air, in particular radon, which is an alpha emitter result- Presentation of the study area
ing from the alpha decay of 226Ra. The degree of human
exposure depends on the geological structure of the soil, The study area is the inhabited surrounding valley of Mount
which affect the distribution of radionuclide in the Earth Cameroon, located in the South-West region of the country.
crust (Nguelem et al. 2016). Therefore, soil radioactivity It is an almost elliptical shaped area, of which the major
analysis plays a very important role in assessing the level of axis is about 60 km and the minor axis is about 35 km.
exposure of human beings because long-term exposure to The summit of Mount Cameroon, called Fako (coordinates
even low-dose radiation, such as naturally occurring ionizing 4°13′N–9°10′E), found at 4095 m above sea level, making
radiation, can increase the risk of long-term effects such as it the highest mountain in Cameroon, is located at the center
cancer incidence. of the elliptical shaped area. The southwest part of the study
238
U (226Ra), 232Th, and 40K are present in soils, espe- area plunges directly into the Gulf of Guinea, and the Moun-
cially more so in volcanic area. Among the soils likely to tain presents, from sea level to the peak, a regular slope.
present a potential radiological risk in Cameroon are those Only the small cone of Mont Etinde, an adjacent crater of
of the slopes of Mount Cameroon. These soils derive from 1713 m, somewhat breaks the uniformity of the slope. The
volcanic rocks (Ngachin et al. 2008) emitted during the vol- climate of the study area is equatorial, with two seasons: a
cano’s various eruptions. Mount Cameroon is still active, dry season that lasts four months, from December to March,
and its last eruption dates back to 2000 (Tchinda 2010). and a long rainy season that runs from April to November.
During its various eruptions, several products (fumaroles, The average temperature hovers around 26 °C. The study
ashes, and magma) are often emitted toward the surface and area is dominated by soils formed from the weathering of
are deposited around the volcanic chimney. Under the effect volcanic rocks. These rocks have basaltic facies and belong
of rainfall, erosion, or human activities, these products flow to the upper quaternary black series (Siefferman 1969).
down the slopes of the mountain to the inhabited areas. Among the volcanic rocks encountered in this area, we have
For environmental studies, it is crucial to know exactly alkaline basalts, basanitoids, hawaits, mugeorites, tephritis,
which radionuclide contributes the most to the radiological and picrites (Nicolas 2010). The eruptions of 1982 and 1999
exposure of populations in a given region of the country in mainly enriched the soil with alkaline basalts and hawaits
order to understand the decisive factor for human exposure (Deruelle 2000). The geochemical study of these basalts and
to ionizing radiation in that region. This usually facilitates Hawaiian rocks reveals the presence of compounds such as
regulatory decision-making process. It is also important to ­SiO2, ­TiO2, ­Al2O3 ­Fe2O3, MnO, MgO, CaO, N ­ a2O, and K
­ 2O,
know the similarities and correlations between the radio- whose proportions are given in Table 1.
nuclides and the calculated radiological parameters, thus These elements enrich the soil with minerals and favor
allowing the regulatory authorities to take decisions and agricultural development and applications.
follow-up with appropriate measures. For this reason, a sta- The study site included 10 localities as shown in Fig. 1:
tistical analysis was performed on the obtained results and Limbé, Buéa, Ekona Muyuka, Ikata, Bafia, Likoko, Muny-
this allows for the detection of the factor that mainly influ- engue, Idenao, and Bakingili. Samples were randomly col-
ences the calculated radiological parameters. lected in these localities.
This work is part of a radiation protection monitoring
and assessment program, which is a set of measures imple- Sampling and samples’ preparation
mented to protect humans, foods, infrastructures, and the
environment against the harmful effects of ionizing radia- A total of 31 soil samples were collected to assess the
tion. Thus, our work focuses on evaluation of the radioactiv- natural radioactivity levels. The sampling method con-
ity of the soils of the slopes of Mount Cameroon and their sisted of delimiting an area of 1 ­m2 (Guembou et al. 2019;
radiological consequences on the health of the local popu- Penabei et al. 2018); the surface was swept superficially
lations. It is achieved through the measurement of activity to remove any residue therein. Then five holes of 5 to 15

13
Arab J Geosci (2023) 16:413 Page 3 of 15 413

Table 1  Elemental composition (wt%) of representative lavas of the at the four corners and the center of the square as can be
eruption of Mont Cameroon (Deruelle, 2000) seen in Fig. 2. The soil from the five holes made up the
Year of eruption Basalts — 1999 Hawaits — 1999 Basalts — 1982 final sample of 2-kg weight. Appropriate distance between
different sampling points was considered to ensure statisti-
SiO2 45.44 47.13 44.33
cal distribution of samples on the investigated area.
TiO2 3.06 2.97 3.44
In the laboratory, the samples were processed through
Al2O3 15.65 17.23 15.64
a conditioning step, which consists of:
Fe2O3 12.58 11.02 13.47
MnO 0.19 0.20 0.19
– Drying the samples in a UK Gallenkamp electric oven at
MgO 6.49 4.66 5.90
a temperature of 70 °C for 48 h to remove moisture.
CaO 10.56 9.43 11.50
– Grinding the dried samples with a German rotary pulver-
Na2O 4.06 4.95 3.90
izer (Fritish) rotating at a speed of about 60 rpm.
K 2O 1.59 2.02 1.37
– Sieve in order to obtain homogeneous samples with par-
P2O5 0.61 0.76 0.65
ticles not exceeding 2-mm diameter.

The samples were then weighed and introduced into

cm each were dug and samples were collected at this depth hermetically sealed 500-ml marinelli geometries for a
to make sure that they are not contaminated by possible period of 30 days for secular equilibrium to be established
chemicals in the surface layer. These holes are distributed between radium and its decay products.

Fig. 1  Map of the study area and sampling point location in the South-West Region of Cameroon

13
 JOURNAL OF RADIATION RESEARCH AND APPLIED SCIENCES 487

Figure 1. The map of Duhok province and the sample’s locations.

The geological composition formed of red beds of any other seasons because of the variation in soil
silt, limestone, and hard clay. Conglomerate and regain moisture, which evidently affect the seasonal variation
is existed within the low folded area. The composition in soil air radon concentration. (Hosoda et al., 2010;
of the soil throughout investigation area is mostly Maeng et al., 2019; O.B. Modibo et al., 2011). Therefore,
characterized by thick sedimentary cover, broad syn­ this research adopts dry soil to eliminate the effect of
clines and well-marked folds of asymmetrical anticli­ moisture content and rainfall. Thus, the collected sam­
nes, very low permeability, and moisture of sediment ples of soil dried at room temperature for four days
of depth from (surface to 10 cm) of 6–7% (Abdulah & then dried in an oven at 110°C for one hour. Then the
Ramadhan, 2011). dried samples were sieved through 250 mm mesh to
obtain a homogeneous powder (Amin et al., 2018).
In preparation to measure 222Rn, a mass of 125 g
2.2. Sample collection and preparation
from each of the soil samples were stored in a cup of
A total of 40 soil samples were carefully gathered in 10 cylindrical plastic tube. Each plastic tube contain of
locations of Duhok governorate. Four measurement two valves, the dimension and the valves of the plastic
points were taken in Duhok district, three in Sumel containers shown in Figure 2. These plastic tubes
and three points in Amadi district. At each sample stored for one month to acquire secular equilibrium
point soil was taken in depths 10 cm, 20 cm, 30 cm, between 222Rn and its progeny (Ismail & Jaafar, 2010).
and 40 cm. Measurements were taken in July (dry While to estimate concentration of 226Ra in soil,
season). In each point an area of about 50 cm×50 cm smaller tubes of about 13 cm3 (Figure 3) were used.
was remarked and accurately cleared of any sediment These tubes were filled with soil and also stored for
or debris depending on a standard methodology one month to acquire secular equilibrium between
226
(Abdulah & Ramadhan, 2011). Ra and its progeny.
It was found that radon exhalation rate has a strong
dependence on the moisture and temperature of the
2.3. Radon activity measurement
soil (Soares et al., 2020). Radon exhalation rate had
a tendency to increase when the soil moisture content In this work the solid-state detector RAD7 was used to
below 8%, While it decrease steadily when the soil measure 222Rn concentration in soil. The benefit of this
moisture content exceeded 8% (Maeng et al., 2019; detector is the measurement of 222Rn and 220Rn con­
Soares et al., 2020). The theory is as follows: Water centration separately and having short measurement
works to promote radon exhalation up to a particular time. The measurement set up is shown in Figure 4; it
moisture level. Water keeps more radon if the water consists of a cylindrical plastic tube, RAD7 Professional
content exceeds a certain level, and consequently, Electronic Radon Detector and a vinyl tube that con­
exhalation is then restrained (Hosoda et al., 2008). tained desiccant (CaSO4). When plastic tube connected
Much research has been conducted concerning the to the closed loop, both valves of the plastic tube are
relationship between the soil radon exhalation rates opened. The accumulated radon gas will pass from the
and the soil moisture content (Maeng et al., 2019) and plastic tube to the desiccant, after that to the inlet filter
effect of rainfall (Masahiro et al., 2007). In many studies of RAD7. The air is leaving from the outlet of RAD7.
radon exhalation rate in summer season is higher than Inside RAD7 air is decayed and detects alpha particle
488 W. A. ALHAMDI AND K. M. S. ABDULLAH

Figure 2. Plastic tube with its cup used for soil radon gas accumulation.

data were transferred from RAD7 to computer. Then,

the obtained spectrum was analyses by Capture
software.
For quality assurance and tracking the background
radiation, radon concentration is measured for a blank
container following the same manner as for the soil
samples in term of starts, stops and time of exposure.

2.4. Estimation of 222Rn exhalation rate

After the determination of radon concentration, sur­
face and mass exhalation rate of 222Rn can be calcu­
lated using equations 1 and 2 (Amin et al., 2018).
Figure 3. Sample tube used for measuring radium content.
λVCRn
Ex ¼ (1)
A½½T þ ð eð λTÞ 1Þ=λ�
that emitted from polonium isotopes. Then alpha tech­
nique is used by RAD7 which convert alpha particle λVCRn
right away to electrical sign. Also this detector capable EM ¼ (2)
M½½T þ ð eð λTÞ 1Þ=λ�
to separate the different between electrical pulses
generated from 218Po and 214Po with energies 6 MeV Where: λis the radon decay constant (0.00756 h−1), T is
and 7.69 MeV, respectively. the exposure time in hours, V is the volume
The experiment is done in a dry condition, with (0.00104 m3) (the difference between the volume of
humidity less than 8% (by purging RAD7 sometimes the container and the volume of the sample), and A is
before starting the test). Then RAD7 is setup onto four the surface area of the sample (0.014 m2), M is the mass
cycle mode, each mode about one hour. Finally, the of the soil.

Figure 4. Experiment and schematic diagram system of radon measurement.