Transmission of Natural Radiation from Soil to Maize Plants and Radiological Hazards Resulting from Consumption in Upper Egypt

Radioactivity concentrations of natural radionuclides (Ra, Th and K) for some agricultural soil and foodstuff (maize) samples were measured by NaI(Tl) gamma spectrometer. The average activity concentrations in soil samples are 11 ± 1 to 37 ± 3 Bq kg, 7 ± 0.4 to 18 ± 2 Bq kg, and 101 ± 6 to 196 ± 9 Bq kg for Ra, Th and K respectively. The ranges of average activity concentrations for maize samples, collected from the same soil were found to be 5 ± 0.6 to 14 ± 0.7 Bq kg, 6 ± 1 to 11 ± 1 Bq kg and 154 ± 8 to 233.4 ± 12 Bq kg for Ra, Th and K respectively. The transfer factors (TFs) of natural radiation from soil to maize plants were also calculated. Additionally, the radiological hazards for farmers and populations were obtained. The obtained values are comparable to the internationally recommended values. The annual effective dose from maize consumption was also estimated, which was found to be in the range of between 254.4 and 511.5 μSv y.


INTRODUCTION
There are many naturally occurring radionuclides in the environment, existing in the soil, sediment, water, plants and air. 1 The consumption of food is generally the most important route by which natural radionuclides enter the human body.Therefore, it is important to assess the natural radionuclide levels in different foods and diets and to estimate the intake of these radionuclides. 2 The ratio of the radionuclide concentration in plants to the radionuclide concentration in soil per unit mass is referred to as the transfer factor (TF).The TF for a given type of plant and for a given radionuclide can vary considerably from one site to another, depending on several factors such as the chemical and physical properties of the soil, the environmental conditions, and the chemical form of the radionuclide in the soil. 3e aim of this study is to measure the concentrations of 232 Th, 226 Ra, and 40 K, potential radiological hazards for farmers and populations, to provide background data on natural radioactive isotopes for the study region, as well as to quantify the presence of long-lived gamma emitters in maize consumed in EL-Mynia governorate, Egypt to determine a TF for natural radionuclides from soil to maize and estimate annual effective doses to the general public due to this consumption.

Study Area
The geographical area of the EL-Mynia governorate is approximately 32,279 km 2 and has a population of approximately 4,481,879.The EL-Mynia governorate is located approximately 225 km south of Cairo and is one of the important agricultural and industrial regions in Egypt.EL-Mynia is mainly an agricultural governorate, as it includes approximately 6% of the total agricultural lands in Egypt, producing cotton, wheat, corn and potatoes. 4This study covered an area in the EL-Mynia governorate from location M2 (27°36'9.04"N; 30°48 '12.03"E) to M9 (28°41'52.23"N; 30°46'5.14"E).

Maize
Forty-one samples of maize were collected at harvest time from the same locations as the agricultural soil samples.The maize samples studied were of the strains Hitech Triple, Giza 311, Hybrid 314, Giza 101, Hybrid 101, Pioneer and Fine Seeds 101.The maize grains were both white and yellow.The consumption of maize per capita in the study area 5 is 67.3 kg y -1 .

Sampling and Sample Preparation
Eighty-two samples of soil and maize were collected from 14 locations (M1 to M14), with 3 samples of soil and 3 samples of maize from each location except M14, where 2 samples of soil and 2 samples of maize were collected.Soil samples were collected using a coring tool to a depth of 5 cm or to the depth of the plough line. 6Maize samples were collected at harvest time from the same locations as the agricultural soil samples.All samples were dried in an oven at approximately 110°C for 24 h to ensure that moisture was completely removed, while maize samples were oven dried at 95°C.
All soil samples were crushed, homogenised, and sieved through a 200-μm sieve, which is the optimum size for particles enriched in heavy minerals.Samples were placed in polyethylene beakers, 250 cm 3 each, and weighed.The beakers were completely sealed for 4 weeks to reach secular equilibrium for radium and thorium and their progenies. 7

Instrumentation and Calibration
Radioactivity measurements were performed by gamma ray spectrometer, employing a high-resolution scintillation detector NaI (Tl) crystal 3 × 3 inch.It had a hermetically sealed assembly including a NaI (Tl) crystal coupled with a PC-MCA Canberra Accuspec (US).
To reduce the gamma-ray background, a cylindrical lead shield (100 mm thick) with a fixed bottom and movable cover was used to shield the detector.The lead shield contained an inner concentric cylinder of copper (0.3 mm thick) to absorb X-rays generated in the lead. 8 determine the background distribution in the environment around the detector, an empty sealed beaker was counted in the same manner and in the same geometry as the samples.The measurement time of the activity or background was 43,200 s.The background spectra were used to correct the net peak area of the gamma rays of the measured isotopes.The dedicated software program Genie-000 9 was used.
The detection array was energy-calibrated using 60 Co (1173.2 and 1332.5 keV), 133 Ba (356.1 keV) and 137 Cs (661.9 keV).The efficiency calibration curve was constructed using different energy peaks covering the range up to ~2000 keV.Efficiency and energy calibrations for each sample measurement were performed to maintain the quality of the measurements.For quality control, the uncertainties of the measured values have been calculated from all parameters.All procedures are described in previous publications. 10e lower limit of detection (LLD) was calculated according to the IAEA directions 6 and is given by Equation 1: where F c = the Compton background in the region of the selected gamma line in the sample spectrum, ε = the system detection efficiency, P γ = the absolute transition probability by gamma decay, m = the sample mass in kilograms, and t = the counting time in seconds.
The lower limit of detection (LLD) in the case of soil samples was 2.4, 1.4 and 5.8 and for maize grains was 1.2, 1.3 and 5 (Bq kg -1 ) for 226 Ra, 232 Th and 40 K, respectively.Figure 2 presents an example of the energy spectra, indicating the gamma-ray lines of different origin compared with the background for soil.

Soil
The concentrations of 226 Ra, 232 Th, and 40 K in the collected samples of agricultural soil are listed in Table 1.The average values of the activity concentrations in soil varied from 11 ± 1 to 37 ± 3 Bq kg -1 , from 7 ± 0.4 to 18 ± 2 Bq kg -1 , and from 101 ± 6 to 196 ± 9 Bq kg -1 for 226 Ra, 232 Th and 40 K, respectively.
The variation in soils from different locations may be attributed to the wide variations in the geological formation of different types of soil.The higher 40 K activity concentrations compared with 226 Ra and 232 Th may be due to the widespread use of fertilisers. 11ese data show that the activity concentrations of 226 Ra, 232 Th and 40 K in the soil samples were below the world averages of 35, 35 and 370 Bq kg -1 for 226 Ra, 232 Th and 40 K, respectively, 12 except for the 226 Ra activity concentration (37 ± 3 Bq kg -1 ) in the samples from location M12.

Maize grain samples
The mean activities of the measured radionuclides in the 41 maize samples are given in Table 1.The results show that the mean activities of 226 Ra ranged from 5 ± 0.6 to 14 ± 0.7 Bq kg -1 , while the mean activities of 232 Th ranged from 5 ± 0.3 to 11 ± 1 Bq kg -1 .Finally, the 40 K concentrations ranged from 154 ± 8 to 233.4 ± 12 Bq kg -1 .Thus, 40 K showed the highest values among the maize samples despite having the lowest activity concentrations in the soil samples.This result may be attributed in part to the heavy use of chemical fertilisers to improve crop yields on the farms in the area. 13In addition, 40 K activities tend to decrease in the deep layers of agricultural soil.The decrease in 40 K with depth is due to the effect of irrigation water in dissolving thorium and potassium compounds.The solutions move towards the surface under the effect of heating by the sun and are deposited by evaporation. 14

TFs for Natural Radioactivity
TFs were calculated as the ratio of the radionuclide concentration in plants (Bq kg -1 plant) to the concentration in the soil (Bq kg -1 soil), as shown in Equation 2: where P = the radionuclide concentration in maize grains (Bq kg -1 dry wt.) S = the corresponding concentration in soil (Bq kg -1 dry wt.).
The soil to maize grain TFs for the radionuclides studied are given in Table 2 and compared with the default values of 0.04, 0.05 and 1 for 226 Ra, 232 Th and 40 K, respectively. 15     The radium equivalent activity was used to obtain the sum of activities to compare the activity concentrations of the soil samples, which contain 226 Ra, 232 Th and 40 K.The radium equivalent activities (Ra eq ) were calculated based on the estimations that 370 Bq kg -1 of 226 Ra, 259 Bq kg -1 of 232 Th and 4810 Bq kg -1 of 40 K produce the same gamma ray dose rate; therefore the Ra eq is given by: 16 Ra eq = A Ra + 1.43A Th + 0.077A k ( 3 )   where A Ra , A Th and A K are the activities of 226 Ra, 232 Th and 40 K, respectively, in Bq kg -1 .Column 2 of Table 3 gives the radium equivalent activities (Ra eq ) for agricultural soil.

Absorbed gamma dose rate (D)
The absorbed dose rates due to gamma radiation in the air at 1 m above the ground surface for the uniform distribution of the naturally occurring radionuclides ( 226 Ra, 232 Th and 40 K) were calculated based on guidelines provided by The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR). 18e conversion factors used to compute the absorbed gamma dose rate (D) in air per unit activity concentration 19 in Bq kg -1 (dry weight) corresponds to 0.462 nGy h -1 for 226 Ra, 0.604 nGy h -1 for 232 Th and 0.042 nGy h -1 for 40 K. Therefore, D can be calculated as follows: where A Ra , A Th and A K have the same meaning as in Equation 1.
The average absorbed dose rates in Table 3  The annual effective dose rate outdoors, in units of μSv y -1 , is calculated by the following formula: 20 Annual effective dose (AED) rate = D×T×F (5)   where D = the calculated dose rate in nGy h -1 , T = the outdoor occupancy time (0.2 × 24 h × 365.25 d ≈ 1753 h y -1 ), and F = the conversion factor (0.7 × 10 -6 SvG y -1 ).
The AED rates vary from 16.3 to 39.2 µSv y -1 .These values are lower than the world average values 21 at 70 Sv y -1 , as shown in Table 3, column 4. The relative contribution of radium to the absorbed dose and AED are higher than the relative contributions of both thorium and potassium, as shown in Figure 8. Figure 9 shows the distributions of dose rate and AED for agricultural soil.

3.4.4
The ext formula where C Bq kg -1 .

Internal hazard index (H in )
The internal hazard index (H in ) describes the internal exposure to carcinogenic radon and its short-lived progeny 23 and is given by the following formula: 17,21 H in = (A Ra /185 + A Th /259 + A K /4810) ≤ 1 (7)   where A Ra , A Th and A K have the same meaning as in Equation 1.
Table 3 shows that the calculated average values of the internal hazard index (H in ) for all samples are less than unity.

Gamma radiation hazard index (I γr )
Another radiation hazard index called the representative level index, I γr , is defined by the following formula, 24 where A Ra , A Th and A K have the same meaning as in Equation 1: The calculated I γr values for the samples under investigation are given in Table 4.
It is clear that the agricultural soil samples have results lower than unity. 18Figure 10 shows the relative contributions of 226 Ra, 232 Th and 40 K to I γr in agricultural soil, while Figure 11 shows the distribution of the representative level index I γ .Excess lifet lifetime ca n: 25 the annual e is a risk fact 0 uses values

Annual gonadal dose equivalent (AGDE)
According to UNSCEAR, 26 the gonads, active bone marrow and bone surface cells are considered the organs of interest.Therefore, the annual gonadal dose equivalent (AGDE, μSv y -1 ) due to the specific activities of 226 Ra, 232 Th and 40 K for farmers was calculated using the following formula, 27 where A Ra , A Th and A K have the same meaning as in Equation 1: AGDE = 3.09A Ra + 4.18A Th 0.314A K (10)   The average values of AGDE are presented in Table 4 (column 4).As shown, the highest average value is 223.4 (μSv y -1 ) in samples from location (12), which is attributable to the use of mixed fertiliser (nitrogen-phosphorus).

Effective Dose due to Ingestion (M)
The annual effective dose from the consumption of maize was calculated using the following formula: 12 M = AEI (11)   where M = the annual effective dose (Sv y -1 ) A = the activity concentration for the radionuclide (Bq kg -1 ) E = the dose conversion factor for the radionuclide (Sv Bq -1 ) I = the annual intake of maize (kg) The values for E (0.28, 0.23 and 0.0062 Sv Bq -1 for 226 Ra, 232 Th and 40 K, respectively) were selected based on the International Commission on Radiological Protection (ICRP) classifications for adults. 28The values of I were taken to be 67.3 kg y -1 , according to the Egyptian Ministry of Agriculture and Land Reclamation Report (2013) 5 .The results of the annual effective dose M are presented in Table 3.
Table 5 shows that the values of the annual effective dose (Sv y -1 ) from the consumption of maize by adults were found to be of several orders of magnitude higher than the 290 Sv y -1 world average of ingestion exposure from natural sources reported in UNSCEAR (2000), 12 except for 254.4 Sv y -1 in the samples from location M1.aize 0.07 NM NM U.S.A (New York) 43 Maize 56.8 NM NM NM = Not measured BDL= Below detection limit

CONCLUSION
The activity concentrations of naturally occurring radionuclides in soil samples from all studied locations were below the world average ranges of 35, 35 and 370 Bq kg -1 for 226 Ra, 232 Th and 40 K, respectively, 12 except for one location (M12), in which the activity concentration of 226 Ra is slightly higher, at 37 Bq kg -1 .The radiological hazards for all soil samples were lower than the world average, so it is safe for farmers and the population and can be used to build raw materials or other human activities without any radiological risk.
The TFs for 226 Ra, 232 Th and 40 K from soil to maize are higher than the default values of 0.04, 0.05 and 1 for 226 Ra, 232 Th and 40 K, respectively. 15The annual effective dose from the consumption of maize was calculated for adults and found to be of several orders of magnitude higher than the 0.29 mSv y -1 world average ingestion exposure from natural sources reported in UNSCEAR (2000). 12

ACKNOWLEDGEMENT
This work was carried out using the nuclear analytical facilities at Physics Department, Faculty of Sciences, Al-Azhar University, Assiut, Egypt. 6.

Figure 1 :
Figure 1: Map of sample locations of the studied area.

Figure 2 :
Figure 2: Typical gamma-ray lines spectrum of soil sample and background.

*
Mean ± uncertainty, (range) Figures 3, 4 and 5 show the average values of the 226 Ra, 232 Th and40 K activity concentrations (Bq kg -1 ) in soil and maize and the TFs from soil to maize.

Figure 3 :Figure 4 :
Figure 3: Average values of 226 Ra activity concentration (Bq kg -1 ) in agricultural soil, maize and TF from soil to maize.

Figure 5
Figure 5 Average values of 40 K activity concentrations (Bq kg -1 ) in agricultural soil, maize and TF from soil to maize.

Figure 7 :
Figure 7: The relative contribution of 226 Ra, 232 Th and 40 K to Ra eq in agricultural soil.

Figure 8 :
Figure 8: The relative contribution of 226 Ra, 232 Th and 40 K to dose rate and AED in agricultural soil.

Figure 10 :
Figure 10: The relative contribution of 226 Ra, 232 Th and 40 K to I γr in agricultural soil.

Table 2 :
Soil to maize transfer factors.

Table 3 :
The equivalent radium (Ra eq ), dose rate, annual effective dose (AED), external hazard (H ex ) and internal hazard (H in ) for agricultural soil.

Table 5 :
Annual effective dose (Sv y -1 ) from consumption of maize for adults.

Table 6 :
Comparison of the average activity concentrations in the present study and from different studies.