Introduction

Man-made and natural sources of radiation are the cause of radiation that human beings are continuously exposed to [2]. The level of activity concentration of radionuclides in phosphate fertilizer and soil samples provides useful information in the monitoring of environmental contamination [3]. The primordial radionuclides of interest in soil and fertilizers are 40K, 238U, 232Th and the progeny of the uranium and thorium decay series [4, 5]. Processes like mining and transportation of phosphate ores, production and usage of phosphate fertilizers contribute to enhanced exposure of workers, public and the environment to these radionuclides [6].

Radium equivalent activity is an index that has been introduced to represent the specific activities of 226Ra, 232Th and 40K by a single quantity, which takes into account the radiation hazards associated with them [7]

$$ {\text{Ra}}_{\text{eq}} = \, C_{\text{Ra}} + { 1}. 4 3C_{\text{Th}} + \, 0.0 7 7C_{\text{K}} $$
(1)

Where C Ra, C Th and C K are the specific activities of 226Ra, 232Th and 40K in Bq kg−1. The external hazard index is defined as:

$$ H_{\text{ex}} = \left( {C_{\text{Ra}} /370} \right) + \left( {C_{\text{Th}} /259} \right) + \left( {C_{\text{K}} /4810} \right) \le 1 $$
(2)

Representative level index I γ used to estimate the levels of γ-radiation hazard associated with the natural radionuclides in specific materials is defined as [8]:

$$ I_{\gamma } = \left( {C_{\text{Ra}} /150} \right) + \left( {C_{\text{Th}} /100} \right) + \left( {C_{\text{K}} /1500} \right) $$
(3)

The total air absorbed dose rate (nGy h−1) due to the mean activity concentrations of 238U, 232Th and 40K (in Bg kg−1) can be calculated using the formula (1), [9]:

$$ D = 0.429C_{\text{U}} + 0.666C_{\text{Th}} + 0.042C_{\text{K}} $$
(4)

To estimate the annual effective doses, account must be taken of the conversion factor from absorbed doses in air to effective dose and the outdoor occupancy factor (1), [10]:

$$ {\text{Annual effect}}.{\text{ dose }}\left( {{\text{mSvy}}^{ - 1} } \right) \, = {\text{ Dose rate }}\left( {{\text{nGy h}}^{ - 1} } \right) \times 8 7 60 \, \left( {\text{h}} \right) \times 0. 2\times 0. 7 { }\left( {{\text{Sv Gy}}^{ - 1} } \right) $$
(5)

The soil-to-plant transfer factor (TF), or the ratio of the concentration of radioactivity in the crop to the radioactivity per unit mass of the soil, is a value used in evaluation studies on the impact of releases of radionuclides into the environment. Transfer factor is retained and used for the application of fertilizers at rates used in routine agriculture [11, 12].

Since the Vojvodina region in Pannonian Plain is agricultural region, the contents of natural radionuclides (226Ra, 232Th and 40K) in 33 fertilizers samples and in 50 soil samples were measured by low level gamma spectrometry. Based on the measured values of the activity concentration of radionuclides in soils and TF given in IAEA TRS 472 [12] for most commonly grown crops in Vojvodina, the activity concentration of radionuclides in plants were calculated.

Materials and methods

Thirty three samples of fertilizers were collected from importer and local markets and tested for radioactivity by low-level gamma-spectrometry. A typical sample weight was about 400 g. After homogenization, they were transferred to sample holders, cylindrical containers (67 mm diameter and 62 mm height), and sealed. Measurements started at least a month after sealing in order to ensure radon equilibrium. Fifty soil samples from Vojvodina region were collected. Sampling locations are presented on Fig. 1. From each location of an approximately 10 × 10 m area, ten subsamples were collected. The sub sample material was thoroughly mixed and homogenized. The homogenized soil samples were dried at 105 °C to constant mass and transferred to sample holders.

Fig. 1
figure 1

Sampling locations of agriculture soil from Vojvodina region

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The radionuclide content of the samples was measured using the HPGe extended range ORTEC GMX type detector (10 keV–3 MeV) with nominal efficiency of 32 % and resolution of 1.9 keV. The detector was shielded with the cylindrical 12 cm thick lead shield. The gamma spectra were acquired and analyzed using the Canberra Genie 2000 software. All measurement uncertainties are presented at 95 % confidence level. The detector was calibrated by means of a reference radioactive material in cylindrical geometry (NBS Standard Reference Material 4350B). Self absorption effects due to different densities were taken into account using the ANGLE computer code based on the concept of the effective solid angle. Such careful calibration was necessary in order to ensure low calibration error (<10 %) in the low-energy region (below 100 keV) where the strongest analytical lines of 234Th (direct 238U descendant) are located.

Results and discussion

Activity concentrations of 226Ra, 232Th, 40K and 238U, Raeq, H ex, I γ , D in 33 measured different fertilizer samples (different chemical compositions and different origin) are presented in Tables 1, 2 and Fig. 2. 40K in fertilizer samples is in the range from 43 (12) to 10,800 (300) Bq kg−1, Fig. 2a. The largest activity concentration of 226Ra is (452 (19) Bq kg−1) and the lowest is (4 (3) Bq kg−1), Fig. 2a. The largest concentration of 232Th is (226 (18) Bq kg−1) and the smallest is (8 (3) Bq kg−1), Fig. 2a. 238U in fertilizer samples is in the range from 31 (20) to 2,640 (110) Bq kg−1, Fig. 2a. The obtained results show that the averages of radiation hazard parameters, calculated according to Eqs. 1–5, for most fertilizers samples are higher than acceptable level 370 Bg kg−1 Raeq, 1 for level index I γr , the external hazard index H ex ≤ 1 and 59 nGy h−1 for absorbed dose rate, Table 2 and Fig. 2b [1].

Table 1 Mean values, standard deviations, minimum and maximum activity concentrations of radionuclides in fertilzers samples
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Table 2 Mean values, standard deviations, minimum and maximum values for radium equivalent activity (Raeq), representative level index (I γ ), external hazard index (H ex) and absorbed dose rate (D) in fertilzers samples
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Fig. 2
figure 2

a Activity concentrations of 226Ra, 232Th, 40K and 238U in fertilizer samples b Raeq, H ex, I γ , D in fertilizer samples

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The radiations can effect human beings internally through the ingestion of radioactive materials into the body [13]. Therefore it is necessary to determine the content of radionuclides in the environment and calculate the potential radiation risk to humans. Mean values, standard deviations, minimum and maximum activity concentrations of radionuclides in 50 soil samples are presented in Table 3. The accumulation of radionuclides in farm crops varies considerably for different soils. The difference in TFs to farm crops for different soils may be one or two orders of magnitude. Soil properties that are likely to affect radionuclide transfer values include mineralogical and granulometric composition, organic matter content, pH and fertility [12]. Soil fertility, the duration of the vegetative period and the character of the root distribution in soil also influence radionuclide TFs. Radionuclides often transfer in greater concentrations to leaves and stems, and in much lower concentrations to the generative parts of plants [12]. Based on the values of TF given in IAEA TRS 472 [12] for most commonly grown crops in Vojvodina (Table 4), the activity concentration of radionuclides in plants were calculated and presented in Table 5. During the calculations the average values of TF for all group of soil were used.

Table 3 Mean values, standard deviations, minimum and maximum activity concentrations of 40K, 226Ra, 232Th and 238U in soil samples
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Table 4 Soil to plant transfer factors TF for for most commonly grown crops in Vojvodina for temperate environment (TF taken from [12])
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Table 5 Calculated average activity concentrations of 40K, 226Ra, 232Th and 238U in some agricultural products
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Conclusion

In the paper the activity concentrations of the natural radionuclides (226Ra, 232Th, 40K and 238U) in 33 fertilizers samples and in 50 soil samples from Vojvodina region, Serbia, were determinate by low level gamma spectrometry. The obtained results show that the averages of radiation hazard parameters for most fertilizers samples are higher than acceptable level 370 Bg kg−1 Raeq, 1 for level index I γr , the external hazard index H ex ≤ 1 and 59 nGy h−1 for absorbed dose rate [1]. Activity concentration of 40K in agriculture soil samples is within values characteristic of the region [14]. Its higher concentration can be explained by the use of fertilizers with large content of potassium. Activity concentration of 238U in soil samples are within the typical values of Vojvodina soil [14]. Slight deviations and increased levels have been observed in some samples could be explained by phosphate fertilizers used. Based on the values of TFs [12] for most commonly grown crops in Vojvodina, the activity concentration of radionuclides in plants were calculated. General conclusion of this study is that in soil samples and analyzed agriculture products did not find increase radioactivity, which could threaten food production or give negative influence on human health.