Introduction

There are several radionuclides present in the environment and are classified into two categories based on their origin, naturally and anthropogenically occurring. The naturally occurring radionuclides can be divided into three different types: the primordial radionuclides that are present in soils and rocks since the formation of the earth such as 238U-series, 232Th-series, and 40K [1]; the radiogenic radionuclides formed from the radioactive decay of 238U-series and 232Th-series such as 222Rn, 220Rn and their subsequent decay products 218Po, 214Pb, 214Bi, 214Po, 210Pb and 210Po [2]; and the cosmogenic radionuclides such as 3H, 7Be, 10Be, 14C, 26Al, 32Si or 36Cl produced from the spallation reactions in the upper atmosphere by the interaction of cosmic rays with the atmospheric molecules [3]. On the other hand, the anthropogenically occurring radionuclides also present in the environment are those related to human nuclear activities (atomic bomb testing, nuclear reactor accidents, release from spent nuclear fuel reprocessing facilities, and so on), such as 137Cs, 131I, 3H, 90Sr or 236U [4]. Uranium and thorium are present in the soil in the form of different isotopes, and they play an important role in the nuclear industry and present radiological hazards for biological life. Furthermore, after the release from spent nuclear fuel reprocessing facilities, atomic bomb testing and nuclear power plant accidents occurred in the past decades, uranium 236U and several other anthropogenic radionuclides have been released into the environment [5]. Although Cameroon is a country where nuclear facilities such as power plants or research reactors do not exist, the country uses nuclear technology in various activities, such as in medicine, industry and research. The use of nuclear technology has generated some 200 radioactive sources in use alongside 50 disused sources (those sources considered as nuclear waste, and containing enough radioactive material harmful to humans and the environment). Managing these radioactive sources by keeping them under regulatory control, and assuring the security of the public and environment has been a major focus of the country’s National Agency for Radiation Protection (NRPA). This regulatory body is in charge of both radiation safety and nuclear security [6].

To date, it is a great challenge to measure the anthropogenic sources of uranium isotopes, e.g., 236U [7,8,9]. Although there are standard techniques to measure the naturally occurring uranium isotopes such as 234U, 235U, 238U directly by alpha spectrometry or mass spectrometry, most laboratories in developing countries still find it challenging afford [10, 11]. To overcome this handicap, researchers from such laboratories often assumed a radioactive equilibrium between 226Ra and 238U. The 226Ra activity concentration can be accurately determined by gamma-ray spectrometry if the sample is adequately prepared to allow 226Ra and its decay products 214Bi and 214Pb to reach secular equilibrium after 30 days. Thereafter, 238U activity concentration is deducted from 226Ra activity concentration by gamma-ray spectrometry. In Cameroon, several research publications dealing with 238U measurements by gamma-ray spectrometry with the hypothesis of radioactive equilibrium between 238U and 226Ra have been reported [12,13,14,15]. However, the relationship between 238U and 226Ra in these areas should be studied to assess the uncertainties in the measurement of 238U from 226Ra.

In the present study, surface soil samples from the seaside, gold mining sites, volcanic terrains, areas of uranium and thorium deposit in Cameroon were collected. The objective was to evaluate their radionuclide contents using two different techniques, i.e. inductively coupled mass spectrometry (ICP-MS) and gamma-ray spectrometry. Meanwhile, the measurement of 236U to demonstrate its potential sources is new in Cameroon. In addition, the correlation and determination of radioactive or secular equilibrium that exists between 238U and 226Ra were done to shed light on the uncertainties of the method used in determining 238U activity concentration through 226Ra by gamma-ray spectrometry taking into account the radioactive equilibrium between them from the previous report.

Materials and methods

Sampling areas

As shown in Fig. 1, a total of fifteen soil samples were collected in four different regions of Cameroon. Two of them came from the Kitongo area (North Region), a village located in the vicinity of the Kitongo granite burden that hosts a uranium-rich rock [12, 16, 17]. Eight soil samples were collected from the City of Douala which is an industrial and business hub of the Country where previous laboratory and in-situ radioactivity measurements in soil were conducted using the gamma-spectrometry method [18,19,20]. Three other soil samples were collected from areas of uranium and thorium deposit within Lolodorf and Eseka localities in the southwestern region of the Country. In these localities, abnormally high air dose rates have been reported [21,22,23,24,25,26]. The two remaining soil samples were each collected from the Tiko and Owe in the South West Region of the country. These two villages are found at the foot of Mount Cameroon, which is an active volcano along the western coast of Africa with the highest peak of 4100 m high [27]. This region has a huge and fertile agricultural land due to its rich volcanic soil.

Fig. 1
figure 1

Locations of the sampling areas

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Gamma-ray spectrometry analysis

Before the gamma-ray spectrometric analysis, the soil samples were oven-dried at 110 °C for 24 h, then disaggregated and separated using a 2 mm standard stainless sieve. The recovered soil fractions for each sample weighing from 50 to 100 g were placed in the cylindrical plastic container (U-8 type: diameter = 48 mm; high = 58 mm) and sealed tightly using an adhesive glue to avoid the escaping of radon or thoron gases from the sealed containers. The sealed containers were stored for at least 45 days to reach secular equilibrium among 226Ra, 232Th, and their respective progeny gamma emitters. Activity concentrations of natural radionuclides were measured using a high purity germanium (HPGe) detector (GEM40190, AMETEK ORTEC, USA) with a relative efficiency of 30% and 1.85 keV energy resolution of Full Width at Half Maximum (FWHM) at 1.33 MeV of 60Co. The detector was coated inside a 10 cm thick lead shielding, covered with 5 mm of copper and 5 mm of plexiglass layers to ensure a low background configuration. For the gamma-ray spectral data analysis, the detector was coupled with a multi-channel analyzer (MCA-7, Seiko EG&G, Japan), and the computer software gamma vision was used (Seiko EG&G, Japan). Energy and efficiency calibrations of the detector were carried out using the multi-nuclides radioactive standard gamma volume source set that contains 109Cd, 57Co, 139Ce, 51Cr, 85Sr, 137Cs, 54Mn, 88Y, and 60Co radionuclides supplied by the Japan Radioisotope Association with gamma energies ranging from 88 to 1,836 keV and having an overall uncertainty less than 10%. The counting time for every soil sample was set for 260,000 s to lower the counting uncertainty. Assuming secular equilibrium with their progenies in uranium and thorium decay chains, the activity concentrations of 226Ra and 232Th were determined. The activity concentration of 226Ra was determined by weighing the sum of counts in the photoelectric peak channel of the gamma-ray energy line of 609.3 keV from 214Bi and 352 keV from 214Pb. To determine 232Th activity concentration, the counts in the photoelectric peak channel of the single gamma-ray energy line of 911.2 keV from 228Ac were considered. Activity concentrations of 40K and 137Cs were derived directly from the counts in the photoelectric peak channels of the single energy gamma-ray lines 1460.8 and 662 keV, respectively [28, 29].

Triple-quadrupole inductively coupled plasma-mass spectrometry (ICP-QQQ) MS/MS analysis

Detailed sample digestion and purification were done following procedures described in [16, 30]. 1 g of sieved soil samples were ashed in a muffle furnace at 450 °C for at least 2 h to decompose organic matter. The ashed soil samples were digested with 20 mL concentrated HNO3 for 24 h on a hot plate followed by filtration. After filtration, the eluate was diluted with Milli-Q water to get a 6 M HNO3 solution for DGA resin purification. Following resin preconditioning and sample loading, interference elements were eluted with 20 mL of 6 M HNO3 and 30 mL of 8 M HNO3 solutions. Finally, uranium was eluted with 15 mL of 0.1 M HNO3. Subsequently, the uranium fraction was evaporated to dryness and re-dissolved in 1.5 mL 4% HNO3 The 238U was measured with an Agilent 8800 ICP-QQQ operated in single MS mode (Agilent Technologies, Santa Clara, CA, USA) after proper dilution of an aliquot of 20 µL sample solution. Uranium atom ratios were obtained using the residual sample solution with the same Agilent 8800 ICP-QQQ operated in tandem MS mode. Under tandem MS (MS/MS) mode measurement conditions, O2 was used as a reaction gas to form UO+ with a flow rate of 0.1 mL min− 1. The method detection limits for 234U, 235U, 236U and 238U were 3.04 × 10− 4, 2.49 × 10− 6, 3.50 × 10− 6 and 5.08 × 10− 5 Bq kg− 1, respectively, in 1 g soil sample. The detection limit of 236U/238U atom ratio monitored as 236U16O/238U16O could reach 10− 9.

Results and discussion

Radionuclides content of the soil samples

Eight different radionuclides 226Ra, 232Th, 40K, 137Cs, 234U, 235U, 236U, and 238U activities concentration were determined in the soil samples sampled from different areas in Cameroon: Douala, Kitongo, Lolodorf, Eseka, Tiko, and Owe. As shown in Tables 1 and 2, the activity concentrations range from 24.2 to 282.9, 21.6–157.0, 19.7–717.0, < 0.1–6.9, 7.0–269.0, 7.5–420.6, 0.318–18.1, 4.34 × 10− 5 – 1.35 × 10− 3, and 7.2–266.1 Bq kg− 1 for 226Ra, 232Th, 40K, 137Cs, 234U, 235U, 236U, and 238U, respectively. Meanwhile, other reports from in-situ measurements in the City of Douala show concentrations that vary from 18 to 47 Bq kg− 1 for 238U, 21 to 54 Bq kg− 1 for 232Th, and 10 to 410 Bq kg− 1 for 40K with averages of 29, 38, and 202 Bq kg− 1, respectively [18]. Accordingly, the concentration obtained from laboratory measurements using NaI(Tl) detector varies from 29 to 98 Bq kg− 1 for 238U, 29–92 Bq kg− 1 for 232Th, and 40–79 Bq kg− 1 for 40K, with averages of 60, 57, and 56 Bq kg− 1, respectively [18]. In comparison, reports on radionuclides measurement in soils within the campuses of the University of Douala show mean values of 226Ra, 232Th, and 40K of 25.5, 66.0, and 39.1 Bq kg− 1 for Campus 1 and 24.5, 66.7, and 28.2 Bq kg− 1for Campus 2, respectively [19]. These values are similar to those obtained in the present study for Douala.

Table 1 Radionuclides measured by HPGe detector
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Table 2 Radionuclides 236U and 238U measured by ICP-MS/MS
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In previous studies conducted at another areas, results obtained using the portable NaI(Tl) scintillation detector for in-situ measurement Poli area near Kitongo show activity concentrations for 238U, 232Th, and 40K ranging respectively from 13 to 52, 10–67, and 242–777 Bq kg− 1 with respective average values of 32, 31, and 510 Bq kg− 1 [31]. In the Lolodorf area, activity concentrations for 226Ra, 232Th, and 40K from the NaI(Tl) detector ranged from 5 to 120, 2–170, and 50–253 Bq kg− 1, respectively, with respective averages values of 22, 37, and 98 Bq kg− 1 [22]. Furthermore, reports on work carried out on soils at Melondo and Gombas localities within the Lolodorf area using CANBERRA alpha spectrometer with ion-implanted silicon detectors show average values of activity concentrations for 238U, 235U, 234U, 232Th, and 226Ra of 126 ± 12, 5 ± 1, 131 ± 10, 400 ± 22, and 154 ± 28 Bq kg− 1, respectively at Melondo; and 170 ± 11, 9 ± 2, 179 ± 12, 200 ± 18, and 416 ± 7 Bq kg− 1, respectively at Ngombas [23]. Meanwhile, in the same area, the HPGe detector recorded activity concentrations as follow: 226Ra values vary from 60 to 270 Bq kg− 1 with a mean value of 130 ± 10 Bq kg− 1; 232Th from 100 to 700 Bq kg− 1 with a mean value of 390 ± 30 Bq kg− 1; whereas, 40K vary from 370 to 1,530 Bq kg− 1 with a mean value of 850 ± 70 Bq kg− 1 [25]. 226Ra and 232Th from the present investigated sites fall within the same range as the values reported in previous studies [12, 18, 19, 31–32]. However, the results for Kitongo and Lolodorf show a contrast between the values of radionuclides measured in soils and rocks [17, 24]. Beyala et al. [24] found activity concentration values from 0.92 to 228 Bq kg− 1 for 235U and from 20 to 4868 Bq kg− 1 for 238U around Lolodorf, which are higher than those recorded in soils in this study. The present study shows average activity concentration values of 226Ra (58.0 ± 1.0 Bq kg− 1) and 232Th (81 ± 1 Bq kg− 1) that are above the global averages of 32 and 45 Bq kg− 1, respectively [1]. Meanwhile,40K activity concentration show lower average values of 175 ± 5 Bq kg− 1 than the global average of 412 Bq kg− 1 [1]. Generally, previous measurements on 40K activity concentration appear higher than the global average [12, 31] but lower in the rainforest and coastal areas [23, 24, 26].

Reports on radioactivity within Cameroon show great variation in different environments within different parts of the country. According to [33], East Cameroon shows average activity concentrations for 226Ra, 232Th, and 40K of 40.1, 29.4, and 216.9 Bq kg− 1 respectively, while in-situ measurements for 40K, 238U and 232Th gave concentrations of 197 ± 21, 37 ± 13, and 32 ± 7 Bq kg–1, respectively [32, 33]. On the other hand, areas of bauxite deposits in the western part of the country gave average values of 671 ± 272, 125 ± 58, 157 ± 67, 6 ± 3 and 99 ± 69 Bq kg− 1 for 40K, 226Ra, 232Th, 235U, and 238U respectively [34]. Looking at the results, there is an agreement between the measured values and those from previous reports in different areas of the country given that the lowest activity concentrations of 40K generally fall below the global average in soil samples from the southwestern part of the country (Douala, Eseka, Lolodorf, Tiko, and Owe), and highest in samples from the North (Kitongo).

137Cs activity was detected in six samples out of fifteen. The presence of this radionuclide from anthropogenic origin in an environment can be attributed to the global fallout [35]. The 137Cs values detected in samples from Kitongo, Eseka, and Owe are higher than those reported in previous works in Cameroon [36] but extremely lower than the values measured in regions of eastern and central Europe (close to Chernobyl) and Japan (close to Fukushima), that have suffered severe nuclear power plant accidents [37, 38]. Furthermore, none of the eight samples from Douala gave any detectable 137Cs activity which could indicate: (1) the deposition of sea salt cations such as Mg2+ and Na+, replacing Cs ions fixed on soil particle surfaces [38] since Douala is a seaside City, and (2) the significantly high precipitation recorded in the city throughout the year, with the annual average of rainfall of over 3851.2 mm. Reported data (Table 3) in some regions affected by the Chernobyl or Fukushima nuclear power plant accident compared with these results show that 137Cs activity concentrations in Cameroon soils are extremely low.

Table 3 Atom ratios 236U/238U and 137Cs activity concentrations determined in different regions near Chernobyl and Fukushima
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Looking at uranium activity concentrations, the results obtained from mass spectrometry measurement on environmental soil in Cameroon serve as baseline data considering that this is the first work done on that aspect in the country. The results for 236U sources within the surface soils are expressed in atom ratios of 236U/238U (Table 2) show values ranging from 2.31 × 10− 8 to 4.95 × 10− 8. These values are within the range of those related to the global fallout summarized by [39]. This suggest that the 236U contents in environmental soils in Cameroon was caused by the global fallout although other factors may have contributed to these variations.

It is evident that enough interest has not been given to the investigation of the activity of 236U in surface soils in Africa. Presently, the only 236U measurement that has been conducted in Southern Africa [40] gave 236U level of (0.56 ± 0.21) ×106 atom/g which corresponds to (5.24 ± 1.96) ×10− 7 Bq kg− 1 in Maputo (Mozambique) and below detection limit in Durban (South Africa). In Lapalma Spanish Canary Islands on the western coast of Africa, 236U/238U ranged from 10− 7 at the near soil surface to 10− 9 with increasing depth [41].Thus, the 236U values are higher in Southern Africa (south hemisphere) compared to the ones measured in Cameroon (North hemisphere).

Regarding activity ratios 234U/238U, apart from two samples collected at the Kitongo with values of 1.22 and 1.58, the rest of the samples show 234U/238U activity ratios around 1 (secular equilibrium) i.e., from 0.97 to 1.11 with the average ± SD of 1.02 ± 0.04 (Table 4). Based on standards, the activity ratios in undisturbed environments are expected to be equal to 1. Value obtained in all soil samples from test areas have 234U/238U activity ratios within the secular equilibrium range except for two samples with higher values obtained from the Kitongo soil sample. The disequilibrium observed for Kitongo soils could be due to the deposition of dust from the mining activities operating in the area [17].

Table 4 Activity concentrations of 234U, atom and activity ratios 234U/238U
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Variance analysis between 238U and 226Ra activity concentrations

Activity ratios of 226Ra/238U carry out on all soil samples (Table 5) show concentrations ranging from 1.06 to 3.78, with the average ± SD of 2.23 ± 0.74, which indicates radioactive disequilibrium between the two radionuclides. A one-way ANOVA test performed on 238U and 226Ra activity concentrations show a significant difference between 238U activity concentrations and 226Ra activity concentrations, with a p = 0.001. This suggest that the two radionuclides are not in secular equilibrium in the soil samples in Cameroon and the variations maybe due to the difference in their geochemical properties. Nevertheless, a correlation test between the two radionuclides shows a strong correlation (Fig. 2), with a Pearson correlation coefficient of r = 0.99 (P < 0.01). A similar strong correlation has also been reported on the studies performed on granitoids samples in Greece [42], and in soil samples from Japan [43,44,45]. Therefore, a strong correlation coefficient observed and reported between these two radionuclides indicates that the two variables behave similarly in the natural environment (increasing together). Radioactive disequilibrium between 238U and 226Ra is common in the natural environments. This explains the reason why despite observing a strong correlation in the test samples, the two radionuclides are not in radioactive equilibrium in the soil samples. This might be attributed to the difference in their geochemical properties [46].

Table 5 Activity concentrations and activity ratios of 238U and 226Ra
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Fig. 2
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Correlation between 238U and 226Ra activity

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This disequilibrium has also been reported in most part of the world. For instance, in Egypt, 238U and 226Ra activity in desert rocks show no equilibrium between the two radionuclides [47], in Greece, the 226Ra/238U were closer to unity (unity means that the radionuclides are in radioactive secular equilibrium) in granitic rocks, with several samples showing 226Ra/238U out of unity [43]. Elsewhere, the activity ratios 226Ra/238U range from 0.64 to 0.72 [48]. Meanwhile in the USA, Rosholt Jr [46] observed radioactive disequilibrium of the uranium series on a total of 138 samples collected in various geological features across the continent [48]. also found out that, 44 samples had an excess uranium vs. radium and 94 had an excess radium vs. uranium form their samples population. Based on their interpretation, firstly, the deficiency of daughter product is theoretically uncommon, and can have three explanations, (a) either there is a deposition of uranium with a time less than 300,000 years less than that required time for equilibrium; or (b) there are greater leaching of daughter product than uranium, or (c) there are a combination of uranium deposition and daughter leaching I the environment. Such type of disequilibrium they observed in 44 samples is common with the pitchblende-type ores [46]. Secondly, the disequilibrium characterized with the excess of daughter nuclides found in 94 samples is in general the disequilibrium type found in samples taken from an oxidized environment as a result of leaching of uranium [46]. In our case, the excess of daughter product was found in the fifteen soil samples analyzed. This observation in the Cameroon soils [49] indicates that, samples are collected in oxidized environment and are characterized by a deficiency of uranium which one agree with Rosholt 94 samples taken from oxidized environment [48]. Thus, 238U in soil cannot be accurately determined by assuming radioactive secular equilibrium with 226Ra using gamma-ray spectrometry. The calculation of the activity ratios 226Ra/238U appears as a good tool to study the disequilibrium in uranium series and can be applied to detect the discharge of depleted uranium [50]. All the fifteen soil samples of the present study have the activity ratios 226Ra/238U > 1 which characterized a deficiency of 238U. This could also mean that there is no artificial deposition of 238U in the sampling areas.

Conclusion

Several radionuclides activity concentrations and 236U/238U atom ratios were measured in soil samples collected in Cameroon using mass spectrometry and gamma-ray spectrometry. Considering that this was a preliminary study of 236U in Cameroon soil samples, this can serve as reference in case of future accidental release of nuclear waste fuel in the environment. The results show that the source of 236U in Cameroon soils are mainly attributable to the global fallout. Low 137Cs activity concentrations ranging from below 0.1 to 6.9 Bq kg− 1 were recorded. Meanwhile, the study also shows that 238U activity concentrations cannot be accurately determined by gamma-ray spectrometry of 226Ra upon assuming secular equilibrium. All the fifteen soil samples display the activity ratios of 226Ra/238U> 1, indicating a deficiency of 238U.