Introduction

Humans have been continuously exposed to natural radiation since the Earth’s formation. The soil is the major source of radioactive nuclides in other materials, such as water, air, sediments and biological systems.[1] The levels of radiation are not the same in different parts of the world and depend on the concentration of radionuclides in the Earth’s crust. The study of naturally occurring radioactive nuclides and their significance in organisms is gaining popularity [2].

Natural concentrations of radionuclides in the soil are usually associated with the concentration of these atoms in the substratum [3]. The levels of natural radioactivity in the soil have attracted attention, because all populations around the globe are exposed to natural radioactivity, depending on the concentration of these radionuclides. A significant component of the background radiation is produced by natural radionuclides in the soil [4].

Natural radioactive materials have also become of great interest in the publications and reports of the International Atomic Energy Agency issued by the European Council Directive [3]. Knowledge of the radionuclide distribution is important, because it provides useful information for the observation of the natural environmental radioactivity and associated external exposure resulting from primary gamma radiation based on geological and geographical conditions. Such radiation can be observed at varied levels in the rocks in the different regions worldwide. The concentrations of the natural radionuclides 238U, 232Th and their daughters, as well as 40K, in the soil and rocks depend on the local geology and causes a diversity of dosages [5, 6]. This study was performed to identify the concentrations of the activities of 226Ra, 232Th, 40K and 137Cs in the soil samples along the Little Zab River Basin (LZRB) in Iraq and to assess their radiological impact in the region.

Materials and methods

Study area

The present study was carried out in several regions from the north to the south of the Little Zab River (LZR) in the Kurdistan Region of Iraq. The study area extends from 35°47′23.3" N to 36°11′28.4" N and from 44°10′26.3" E to 45°15′43.6" E and covers ~ 5,635 km2. The geographical position of the sampling locations is shown in Fig. 1.

Fig. 1
figure 1

Map of the study area and sampling sites

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The LZR, which is also known as the Lower Zab or Lesser Zab River, is the largest tributary of the Tigris River with ~ 71% of its basin situated in Iraq and the rest in Iran. The total length of the river is 456 km. It enters from Iran in the northeast of Iraq, follows many anticlines and meanders around the plunges, until it flows out of the mountainous area [7]. The average annual flow of water to the river reaches 7.17 km3, whilst its 5.07 km3 is being retarded by the Dukan Dam, which was built on the river path. Thus, the LZR is considered to be the main source for the annual filling of the dam. As the river flows into Iraq, it encounters several different geological structures, the ages of which go back to the Jurassic to Quaternary periods. For example, the upper part of the river is located within the Zagros suture zone, whereas its lower part is within the foothills with clastic unresisting rocks [8, 9].

Sample collection

Thirteen separate geological formations (laid at 23 sites) along the LZR were selected for the collection of soil samples to determine the activity concentration of naturally occurring radionuclides in the soil. Figure 1 illustrates the geographical positions of the sampling sites. A core method, with a core diameter of 15 cm and a depth of 20 cm, was used to collect the soil samples [10, 11]. Soil depth was considered to be particularly significant in areas with highly inhomogeneous distribution of radionuclides. After the stones and inorganic materials were removed, the soil samples were dried in an electric oven at approximately 105 °C, crushed and sieved through a 2 mm mesh sieve [12, 13]. For the gamma spectrometry device, 1 kg of each sample was packed in a 1 L Marinelli beaker, and the beaker was closed for 4 weeks to create a secular equilibrium between the Ra content of the samples and the radionuclides of its daughter [14, 15].

Counting of samples

The multichannel analyser and a high-purity germanium (HPGe) gamma-ray spectrometry system were used to count the gamma rays emitted from the soil samples. The activity concentrations of the radionuclides in the soil samples were determined by using a counting system made by Princeton Gamma Tech at Koya University. The HPGe coaxial detector had a relative efficiency of 73%, a peak-to-Compton ratio of 75/1 and crystal size with an active volume of 265 cm3. To secure the measurement station from the background radioactivity, the detector was placed in a lead well with a thickness of 10 cm.

The system was optimised for energy and resolution calibration by using three common sources, namely, 60Co, 137Cs and 226Ra. Efficiency calibration was achieved by following the same approach as that demonstrated by Ahmad [16] and Ahmad et al. [17]. The samples were put over the detector for at least 10 h. An empty beaker was counted within a 10-h measurement period to assess the background radiation in the detector location. The net peak region of gamma rays of the measured isotopes was corrected using the background spectra. After background calculation and subtraction, the naturally occurring radionuclides 226Ra, 232Th and 40K appeared in the new spectrum of the measured gamma ray [15].

The activity concentration of 226Ra was estimated using the gamma-ray lines of 351.9 keV (35.8%) gamma rays from the 214Pb decay and 609.3 keV (44.8%) and 1764.5 keV (15.36%) gamma rays from the 214Bi decay. The weighted average of the activity calculated using the gamma-ray lines of 238.6 keV (43%) from the 212Pb decay and 583 keV (84.5%) and 2614.5 keV (99.16%) from the 208Tl decay were used to calculate the activity concentration of 232Th. In addition, 1460.8 keV (10.7%) gamma-ray line was used to assess the activity concentration of 40K [18, 19]. 137Cs was also directly determined using the 661.7 keV (85.21%) gamma-rays line.

Calculations

Activity concentration

The activity concentration (A) of 226Ra, 232Th, 40K and 137Cs in the soil samples were calculated as follows [20,21,22]:

$$A=\left(\frac{N}{\varepsilon { I}_{\gamma } T M}\right)$$
(1)

where N is the net count, ε is the absolute gamma peak efficiency of the detector of this particular gamma-ray energy, Iγ is the decay intensity of the specific energy peak (including the decay branching ratio), T is the counting time of the measurement in seconds, M is the mass of the sample in kilogram. The relative combined standard deviation σA of the activity concentration is given by the formula [23]:

$$\frac{{\sigma }_{A}}{A}=\sqrt{\frac{{\sigma }_{N}^{2}}{N}+\frac{{\sigma }_{\varepsilon }^{2}}{\varepsilon }+\frac{{\sigma }_{{I}_{\gamma }}^{2}}{{I}_{\gamma }}+\frac{{\sigma }_{M}^{2}}{M}}$$
(2)

where σN is the standard deviation of the N net count rate per second, σε, \({\sigma }_{{I}_{\gamma }}\) and σM are the standard deviations of the ε, Iγ and M, respectively.

Efficiency calibration of the HPGe detector

The efficiency was calibrated by using the three standard gamma ray sources of 226Ra, 60Co and 137Cs. These samples were placed over the detector for at least 1 h. The spectra were evaluated by using a Thermo Scientific multi-channel analyser and the Princeton Gamma Tech computer software program QuantumGold 2001.

The measured data were well fitted by the RJS Graph 3.93.01 software to obtain the following power equation:

$$\varepsilon =8.5758 \times {E}_{\gamma }^{-0.896}$$
(3)

where ε is the absolute full peak efficiency of the detector, and Eγ is the energy of the gamma ray. Figure 2 shows the plotted graph of the absolute full peak efficiency and gamma-ray energy (Table 1).

Fig. 2
figure 2

Absolute full peak efficiency against the gamma-ray energy of the HPGe detector

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Table 1 Code of samples based on the soil geological formation
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Radium equivalent activity (Raeq)

The distribution of natural radionuclides in soils is not uniform. Therefore, the total exposure to radiation from 226Ra, 232Th and 40K nuclides was expressed by the Raeq) in (Bq kg−1). The Raeq in the soil samples was calculated as follows [22, 24]:

$$Ra_{eq} = \, \left( {A_{Ra} } \right) \, + \, \left( {1.43A_{Th} } \right) \, + \, \left( {0.077A_{K} } \right)$$
(4)

where ARa, ATh and AK represent the activity concentrations of 226Ra, 232Th, and 40K, respectively. According to the OECD of 1979 [25], the safe value of Raeq for any naturally occurring radioactive materials is less than 370 Bq kg−1.

Absorbed gamma dose rates (DRs)

To determine the uniform distribution of the naturally occurring radionuclides, the absorbed gamma DRs in the air at 1 m above the ground surface were calculated based on the guidelines provided by UNSCEAR (2000) [21]:

$$D_{R} \left( {nGy \, h^{ - 1} } \right) \, \left( {NORM} \right) \, = \, (A_{Ra} \times 0.462) \, + \, (A_{Th} \times 0.604) \, + \, (A_{K} \times 0.0417)$$
(5)

Annual effective dose (AED)

The AED) was calculated from the absorbed gamma DR by using the dose conversion factor of 0.7 Sv Gy−1 with an outdoor occupancy factor of 0.2 and 0.8 for indoor as given by UNSCEAR (2000) [21]. The AED was determined as follows:

$$AED\left( {\mu Sv \, y^{ - 1} } \right) \, = D_{R} \left( {nGy \, h^{ - 1} } \right) \times T \times F$$
(6)

where DR is the calculated dose rate (nGy h−1), T is the occupancy time and F is the conversion factor (0.7 Sv Gy−1 for environmental exposure to gamma rays of moderate energy as published in UNSCEAR 1993 [26] and UNSCEAR 2000). The outdoor occupancy factor T is approximately 20% of 8760 h y−1. The outdoor annual effective dose equivalent was determined given as follows:

$$AED_{outdoor} \left( {\mu Sv \, y^{ - 1} } \right) \, = D_{R} \left( {nGy \, h^{ - 1} } \right) \times (0.2 \times 8760 \, h \, y^{ - 1} ) \times 0.7 \, \left( {Sv \, Gy^{ - 1} } \right)$$
(7)

External hazard index (Hex)

The external hazard index was calculated by assuming that 370 Bq kg−1of 226Ra, 259 Bq kg−1 of 232Th and 4810 Bq kg−1of 40K produce the same gamma dose rates. The following relation was used to evaluate the external hazard index [27, 28]:

$$H_{ex} = \left( {A_{Th} /259} \right) + \left( {A_{Ra} /370} \right) + \left( {A_{Ra} /4810} \right) \le 1$$
(8)

The external hazard index for 232Th,226Ra and 40K, was less than 1 mSv y−1, corresponding to the Raeq of 370 Bq kg−1 (OECD-1979) [25].

Result and discussion

Table 2 shows the activity concentrations of the natural radioactive nuclides 226Ra, 232Th and 40K and the artificial radionuclide 137CS in the samples in the different geological formations along the LZR. The mean values (range) of the activity concentrations of 226Ra, 232Th, 40K and 137CS in soils were 13.8 ± 0.5 (4.4–34.7), 6.5 ± 0.2 (1.5–13), 276.5 ± 4.4 (42–583) and 7.0 ± 0.2 (0.5–31.5) Bq kg−1, respectively. The variations in the activity concentrations of the naturally occurring radionuclides in the soil depended on the geological and geographical conditions of the area.

Table 2 Activity concentrations of radionuclides in the soil samples
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For 226Ra, the minimum activity concentration (4.4 ± 0.1 Bq kg−1) was observed in the Tanjaro Shiranish Formation (S12), and the maximum value (34.7 ± 0.6 Bq kg−1) was found in the Sargalu Formation (S10). For 232Th, the maximum activity concentration (13.3 ± 0.2 Bq kg−1) was found in the Quaternary sediment Dokan Conglomerate Formation (S14), and the minimum activity concentration (3.2 ± 0.1 Bq kg−1) was observed in the Sinjar and Kolosh Formation (S17). For 40K, the maximum activity concentration (583.9 ± 8.2 Bq kg−1) was found in the Shiranish Formation (S2), and the minimum specific activity concentration (42.1 ± 0.9 Bq kg−1) was found for the Tanjaro Shiranish Formation (S12). For 137CS, the maximum specific activity concentration (31.5 ± 0.3 Bq kg−1) was found in the Mukdadiya Formation (S20), and the minimum activity concentration (0.5 ± 0.1 Bq kg−1) for the sample Shiranish Formation (S1).

The activity concentrations of the 226Ra, 232Th and 40K radionuclides were not consistent with each other and did not record similar patterns in the different samples. Thus, these radionuclides were random in terms of their minimum and maximum values, but they are still generally within the limits of low and moderate levels of radioactivity. The fluctuations may be due to the topography of the region, which has hills and valleys.

The mountainous topography may allow the movement or the accumulation of radionuclide with the rainwater flow because of the ability of the chemical compounds to dissolve or undergo sedimentation and stagnation. Such assumptions could also be used to explain the 137Cs distribution by further assuming that the 137Cs was not naturally distributed. This compound may originally have a constant level as a fallout but became disturbed by environmental conditions and slightly accumulated in depressions and ponds.

Figure 3 shows the activity concentration of 40K in the soil samples. The obtained values were within the global average values [29]. The activity concentrations of the samples from the study area were compared with those from similar investigations in other countries (Table 3). The range of the activity concentrations of 226Ra was consistent with the results from Nineveh Province obtained by Najam et al. [30]. and most of other studies in different countries but lower than the results from Malaysia [31] and China [32]. The range of the activity concentrations of 232Th was in the level of the results from the Nineveh Province [30] and almost lower than those of all other studies. The 40K results were comparable with the results from other studies as shown in Table 3 and matched the world average. However, the maximum level of 40K in China [32] was almost twice the level in this study.

Fig. 3
figure 3

Activity concentration of 40K in the soil samples

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Table 3 Comparison of the ranges of activity concentrations of the naturally occurring radionuclides in the study area with those from similar investigations in other countries
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To compare the activity concentrations of 232Th, 226Ra and 40K in the soil samples, the Raeq as a common index was used to obtain the sum of activities and estimate the radiological hazards, external Hex, external DR and outdoor AED for the virgin soil samples in this study (Table 4).

Table 4 External absorbed gamma dose rates (DR), External hazard index (Hex), Outdoor annual effective dose (AED) and Radium equivalent activity (Raeq) for soils
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The calculated value of Raeq in the virgin soil samples varied in the range of 10–70.6 Bq kg−1, with the average value of 44.4 Bq kg−1. Variations in the Raeq values in the different soil samples depended on the type and content of the natural radionuclide. Figure 4 shows that the Raeq in the soil sample was lower than the global average value. The obtained Raeq values in the soil samples were within the recommended limit of 370 Bq kg−1.

Fig. 4
figure 4

External absorbed gamma dose rates (DR), radium equivalent activity (Raeq) and outdoor annual effective dose (AED) for the soil samples in the study area

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Absorbed gamma DRs were found to be in the range of 4.9–37.2 nGy h−1, with an average value of 22.6 nGy h−1. The estimated average value of the absorbed gamma DRs was lower than the worldwide average value of 59 nGy h−1 as reported by UNSCEAR 2000 [21]. The measured value of the outdoor AED was in the range of 5.97–45.65 Sv y−1, with an average value of 27.7 Sv y−1. As illustrated in Fig. 5, the calculated values of the absorbed gamma DRs and annual effective dose equivalent for the virgin soil samples were also lower than the global average value.

Fig. 5
figure 5

Intercomparison of the average activities of a 226Ra, 232Th and 137Cs and b 40K in the four regions of the study area, with the weighted average of errors of each region

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The external Hex was found to be in range of 0.02–0.18 mSv y−1. The external Hex values for the soil samples in the study area were lower than the recommended standard of 1 mSv y−1 by OECD in 1979 [25].

Intercomparison of the activity levels of the radionuclides in the sampled locations

The activity levels of the 226Ra, 232Th and 137Cs (Fig. 5a) and 40K (Fig. 5b) were compared using the average values from four groups of samples. These groups were divided according to the administrative division of the region. The subregions were Pshdar, Betwen, Lower Dokan and Taq Taq. Pshdar generally had the highest level of activity for the four radionuclides, whilst Lower Dokan showed the lowest levels for 226Ra, 232Th and 40K. The radioactivity level of 137Cs in Pshdar was more than four times its level in Taq Taq. Pshdar and Taq Taq showed higher levels of 40K activity than the other two regions. The diversity in 137Cs activity could be explained by considering the following: the level of the regions above sea level; the direction of the slope of region, where the eastern side is expected to receive a higher level of the 137Cs fallout than the western side; and whether the region is stagnant, which keeps the accumulated 137Cs, or a sloping area, which loses 137Cs by being washed away by rainwater.

Histograms of the normality distribution of the primordial radionuclides

A histogram can be used to evaluate visually whether the data have symmetrical, normal, or Gaussian distribution or whether the distribution is asymmetrical or skewed. When the distribution is not normal, it cannot be accurately described by mean and standard deviation, but the median, quartiles and percentiles should be used. Figure 6 shows the frequency distributions of the primordial radionuclides (226Ra, 232Th and 40K) and the man-made radionuclide (137Cs). Figures 6a, b and c correspond to the distributions of 226Ra, 232Th and 40K. The curves show that the three radionuclides had asymmetric distributions about the mean values, with positive skewness. The asymmetric tail were extending toward values that were higher than the mean values. Nevertheless, the distributions of these nuclei maintained a considerable normality. This result is in agreement with that of Sivakumar et al. [38]. 137Cs showed a large positive skewness, indicating that its distribution was asymmetric about the mean value, and the asymmetric tail was extending toward values that were higher than the mean value to a certain extent. This phenomenon rejects the normal distribution and removes the normality from this peak.

Fig. 6
figure 6

Histograms of the activity concentrations of a 226Ra, b 232Th, c 40K and d 137Cs indicating the test for the normality of the distribution of these activities in the tested samples

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Conclusion

HPGe was used to measure the activity concentrations of 232Th, 226Ra,40K and 137Cs in the soil samples collected from the LZRB in Iraq. The mean values of the activity concentrations of 226Ra, 232Th, 40K and 137Cs varied in the ranges from 4.4 ± 0.1 to 34.7 ± 0.6, from 1.5 ± 0.1 to 13.3 ± 0.2, from 42.1 ± 0.9 to 583.9 ± 8.2 and from 0.5 ± 0.1 to 31.5 ± 0.3 Bq kg−1, respectively. The measured radioactivity of the soil in the study area was below the worldwide average and posed no risk to the health of the population. Pshdar showed the highest level of radioactivity, whilst Taq Taq showed the lowest level. The regions that are more elevated above sea level had higher 137Cs concentration than those areas at lower elevation above sea level.