Comparison of Natural Radioactivity of Commonly Used Fertilizer Materials in Egypt and Japan

Specific activities of U, Th, and K in the environment have been redistributed by the use of fertilizers in agriculture so their concentrations in fertilizermaterials should bemeasured to identify the safe utilization of fertilizers. In the presentwork, the specific activities of these radionuclides in five commonly used fertilizers in Egypt and five fertilizers used in Japan weremeasured byHPGe and γ-ray spectrometry. The average values of U, Th, and K in Japanese fertilizers were less than their values in Egyptian fertilizers but both had some sampleswith specific activities greater than the recommended limiting values.The radiological hazards of radium equivalent activity (Raeq), external (Hex) and internal (Hin) indexes, alpha and gamma indexes, and annual effective dose, due to the presence of these radionuclides, were calculated and compared with each other.


Introduction
Fertilizers play an important role in the agriculture sector to increase crop yields so fertilizer industries have spread out all over the world.Fertilizers are composed mainly of nitrogen (N), phosphorus (P), and potassium (K), which are essential elements for plants growth.The phosphorus portion is taken from phosphate rocks, which contain a relatively high concentration of naturally occurring 238 U, 232 Th, and 40 K and their radioactive daughters [1,2].Therefore, natural radioactivity in soil varies from one location to another due to the extensive use of fertilizer which is the main source of radioactivity in soil other than its natural origin [3,4].The extensive use of fertilizers can increase the amount of radionuclides in soils and in groundwater and consequential ingestion by humans through exposure routes such as drinking water and the food chain [4,5].Once deposited in bone tissue 226 Ra has a high potential for causing biological damage because of the continuous irradiation of the human skeleton over many years and it can induce bone sarcoma [4,5].
Factory workers that produce fertilizers and those who use fertilizers in agriculture are exposed to gamma radiation (external exposure) and alpha particles (internal exposure) emitted from the radionuclides of the 238 U series, 232 Th series, and 40 K. External exposure occurs directly by -rays, whereas internal exposure occurs by -particles that result from the inhalation of radon and its progenies.Consequently, the -particle dose is delivered directly to the bronchial tissue, creating a potential for radiogenic lung cancer [6][7][8].Therefore, radiation released from fertilizers has a potential of causing cancers in individuals exposed to significant levels so that monitoring of natural radioactivity in fertilizers has an importance from the viewpoint of radiation protection [9,10].
There is a significantly increasing international awareness of radiation hazards of fertilizer materials as a potential source of risk to workers, members of the public, and the environment.Japan as a developed country has a good radiation protection system so that, in the present work, the natural specific activities of 238 U, 232 Th, and 40 K, in commonly used   fertilizer materials in Egypt, were measured and their values were compared to those of Japanese fertilizers.In addition, the radiological hazards of radium equivalent activity (Ra eq ), external ( ex ) and internal ( in ) indexes, alpha and gamma indexes, and annual effective dose, due to the presence of those radionuclides, were calculated and compared with recommended limits.

Sample Preparation.
Samples of five commonly used fertilizers in Egypt and five fertilizers used in Japan as well were collected from different companies and factories, as shown in Tables 1 and 2. The selected samples were crushed and sieved through a 1 mm mesh size to remove the larger grains size and to become more uniform.Then, these samples were oven dried in a temperature controlled oven at 110 ∘ C for 24 h to ensure that the moisture is completely removed.After moisture removal, the samples were cooled down to room temperature in a desiccator.The dried homogenized samples were packed into airtight polyethylene containers (6 cm in diameter and 8 cm in height).The containers were carefully sealed with adhesive epoxy to prevent 222 Rn and 220 Rn from escaping.Each sample was stored in its sealed container for four weeks to achieve radioactive secular equilibrium.A similarly sealed empty container of the same geometry was left for the same time period in order to measure the radionuclide background [15].

Measurement of Radionuclide
Activities with -Ray Spectrometry.Specific activities of 226 Ra, 232 Th, and 40 K in the samples were measured using an HPGe detector, ORTEC (model: GMX-70230 EG&G) with a volume of 190 cm 3 , a measured efficiency of 70%, and an energy resolution of 2.3 keV at 1332.5 keV.This was connected to a personal computer with a data acquisition system that has a Multichannel analyzer, model CANBERRA Multi Port II (4,096 channels).The data analysis was carried out via an APTEC MCA software program.
The HPGe detector's peak efficiency was determined using standard point sources of 60 Co, 133 Ba, 137 Cs, 22 Na, and a standard source of 226 Ra, maintained in the same container geometry as that used for the samples.Since radium ( 226 Ra) and its progenies produce 98.5% of the radiological effects of the uranium series, the activities of 238 U and the precursors of 226 Ra are normally ignored.Therefore, the reference for the 238 U series is often 226 Ra instead of 238 U [15].The 226 Ra activity was deduced from the -rays of energies of 351.9 keV associated with the decay of 214 Pb and 609.3 keV -rays associated with the decay of 214 Bi.The 186 keV photon peak of 226 Ra was not used because of the interfering peak of 235 U with energy of 185.7 keV.The 232 Th concentration was estimated from the -rays of energies of 911.1 keV associated with the decay of 228 Ac and the 583.4 keV line associated with the decay of 208 Tl.The 40 K concentration was obtained from 1460.8 keV -rays from the decay of 40 K itself [16][17][18][19][20][21].The specific activity concentration (Bq kg −1 ) of those radionuclides was calculated from (1) [15].For the calculations of specific radioactivity, coincidence-summing and self-absorption correction factors have not been applied.
where  is the net counts above the background,  is the absolute emission probability of the -ray decay,  is the net dry sample weight (kg),  is the measurement time (s), and  is the absolute efficiency of the detector.

Results and Discussion
The specific activities of 226 Ra, 232 Th, and 40 K in commonly used fertilizer samples in Egypt and Japan are given in Tables 1  and 2. The respective average radionuclide activities of 226 Ra, 232 Th, and 40 K in the Egyptian fertilizer samples were 571 ± 20 Bq kg −1 , 19 ± 5 Bq kg −1 , and 182 ± 40 Bq kg −1 while, for the Japanese fertilizers, they were 325 ± 2 Bq kg −1 , 9 ± 1 Bq kg −1 , and 1500 ± 10 Bq kg −1 , as shown in Figure 1 and Tables 1 and  2. The radionuclide concentrations in Japanese fertilizer were less than those of Egyptian fertilizers except for potassium, as seen in Figure 1.The radionuclide concentration of 40 K is much higher in Japanese fertilizer samples and especially sample JF-1.Both Egyptian and Japanese fertilizers maintain radionuclide concentrations less than the recommended limits by UNSCEAR, 2008, [22].
To compare the radiation effect of different radionuclides in a sample UNSCEAR [22,23] has introduced the radium equivalent concentration (Ra eq ).
where  Ra ,  Th , and  K are activities of 226 Ra, 232 Th, and 40 K, respectively, in Bq kg −1 .Radium equivalent concentration was calculated based on the estimation 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 equivalent -ray dose.The value of Ra eq of fertilizer must be less than 370 Bq kg −1 to keep -ray dose below 1.5 mSv y −1 .The average radium equivalent concentration in Egyptian fertilizer was 613 ± 33 Bq kg −1 while for Japanese fertilizer, it was 454±5 Bq kg −1 , as shown in Tables 3 and 4. All Japanese fertilizers have radium equivalent concentrations   less than the recommended limit except JF5 (1277±8 Bq kg −1 ) while all Egyptian fertilizer samples have radium equivalent concentrations greater than recommended limit except EF5 (326 ± 16 Bq kg −1 ), as seen in Figure 1.
The external hazard index ( ex ) was determined from (3) [24,25]: where  Ra ,  Th , and  K are the activities of 226 Ra, 232 Th, and 40 K, respectively, in Bq kg −1 .The external hazard index should be less than unity in order to keep -radiation dose less than 1.5 mSv y −1 .The calculated external hazard index for Egyptian fertilizers had an average of 1.65 ± 0.04 (Table 3) while it was 1.22 ± 0.06 for Japanese fertilizer samples (Table 4).The external hazard index for Japanese fertilizers was less than the recommended limit except for the sample of JF5 (3.45 ± 0.09) while its value was higher than the recommended limit for Egyptian fertilizers except the one sample of EF-5 (0.88 ± 0.01), as shown in Figure 2(a).
In addition to the external hazard, radon and its shortlived products are also hazardous to respiratory organs.The internal exposure to radon and its progeny produced is quantified by an internal hazard index ( in ) which can be defined as [24,25] For the safe use of a material,  in should be less than unity.The average value of the internal hazard index was 3.20 ± 0.15 (Table 3 and Figure 2(b)) for Egyptian fertilizers, all of which are higher than the recommended limit.This means that the use of Egyptian fertilizers should be subject to precautions.On the other hand, Japanese fertilizers had a mean internal hazard index of 2.11 ± 0.13, with JF-1 and JF-3 having internal hazard index less than unity (Table 4 and Figure 2(b)).The use of fertilizers in agriculture is a possible source of exposure for the public.Elevated radionuclides exposure of the public might be expected, for example, to the workers in sites being developed for housing.[22].The absorbed dose () due to -rays emitted at 1 m of air above ground can be calculated from the following equation [24]: The absorbed dose was calculated for the samples as shown in Tables 5 and 6 and Figure 3(a).The radiation absorbed dose was varied from 141 ± 7 nGy h −1 (EF5) to 380 ± 26 nGy h −1 (EF1) with a mean value of 264 ± 15 nGy h −1 for Egyptian fertilizers.For Japanese fertilizers it varied from 38 ± 1 nGy h −1 (JF3) to 546 ± 4 nGy h −1 (JF5) with a mean value of 210 ± 2 nGy h −1 .All Egyptian fertilizers had absorbed radiation dose values greater than the limit recommended by UNSCEAR [23], of 59 nGy h −1 as also Japanese fertilizers except JF3, as given in Tables 5 and 6 and Figure 3(a).Therefore, except for one sample, all the Egyptian and Japanese fertilizers gave an absorbed dose larger than recommended so that these materials should be used with precautions.The annual effective dose () from -rays emitted from 226 Ra, 232 Th and 40 k in the samples was calculated from [22]  =  (nGy h −1 ) × 8760 (h y −1 ) ×  ×  (mSv/nGy) , (6) where  is the occupancy factor and  is the absorbed to effective dose conversion factor of 0.7 × 10 −6 Sv per Gy.The annual effective doses from -rays emitted by 226 Ra, 232 Th and 40 k in the samples varied from 173 ± 1 Sv y −1 (EF5) to 468 ± 3 Sv y −1 (EF1) with a mean value of 325 ± 2 Sv y −1 for the Egyptian samples.For Japanese samples they varied from 47 ± 1 Sv y −1 (JF3) to 672 ± 4 Sv y −1 (JF5) with a mean value of 258 ± 3 Sv y −1 , as shown in Tables 5 and 6 and Figure 3(b).The annual effective dose of all studied samples of Egyptian and Japanese fertilizers is less than the recommended limiting value of 480 Sv y −1 [22] except for the sample JF5.
The -ray radiation hazards associated with the natural radionuclides in fertilizer materials can be assessed by means of the radioactivity level index,   .According to the European of the Egyptian samples varied from 1.10 ± 0.06 for (EF-5) to 2.96 ± 0.20 for (EF-1) with a mean value of 2.06 ± 0.11.For the Japanese samples, it varied from 0.29 ± 0.03 for (JF3) to 4.26 ± 0.19 for (JF5) with a mean value of 1.63 ± 0.08, as can be seen in Tables 5 and 6 and Figure 4(a).All the measured samples had a radioactivity level index less than 6, so any of these samples can be used without special precautions [26].Alpha radiation due to the released radon from samples is called alpha index (  ) which can be calculated from (8), [26].Alpha index should be less than unity to reflect a radium concentration value less than 200 Bq kg −1 (the upper recommended value) which leads to a released radon concentration less than 200 Bq m −3 .
Alpha index of the Egyptian samples varied from 1.56 ± 0.07 (EF5) to 3.91 ± 0.12 (EF3) with a mean value of 2.86 ± 0.10.For Japanese samples, it varied from 0.13 ± 0.03 (JF1) to 6.32 ± 0.25 (JF5) with a mean value of 1.63 ± 0.11, as seen in Tables 5 and 6 and Figure 4(b).The values of alpha index for Egyptian fertilizers were more than unity while, for Japanese samples, it was less than unity except a sample of JF5.All these radiological indexes were actually initially developed and established for construction materials but they were used during this study and other previous studies in literature [11][12][13][14] to show how much the workers and public could receive radiation dose from fertilizer materials.Table 7 shows a comparison of the estimated radiological indexes values during this work and their values in previous studies in literature.Radium equivalent in the present work was less than its value for fertilizer used in Algeria and Brazil but it was greater than its value for fertilizer used in Saudi Arabia and Bangladesh.Gamma index behaved the trend as radium equivalent, as seen in Table 7.

Conclusion
Natural radioactivity of 226 Ra, 232 Th, and 40 K in different types of fertilizers used in Egypt and in Japan was measured using a high purity germanium detector.The specific activities in Egyptian samples ranged from 312 ± 14 to 782 ± 24 Bq kg −1 for 226 Ra, ND to 67 ± 13 Bq kg −1 for 232 Th, and 88 ± 22 to 251 ± 94 Bq kg −1 for 40 K.In Japanese samples these specific activities ranged from 25 ± 1 to 1265 ± 5 Bq kg −1 for 226 Ra, 5 ± 2 to 15 ± 1 Bq kg −1 for 232 Th, and 31 ± 5 to 3909 ± 21 Bq kg −1 for 40 K.The specific radioactivities of Egyptian fertilizers are much higher than their values for Japanese fertilizers but still in the range of recommended limits of UNSCEAR 2008.The radiological hazard indexes, of radium equivalent activities (Ra eq ), external and internal indexes, gamma index, absorbed radiation, and annual effective doses of the Egyptian fertilizers were higher than the values for Japanese fertilizers and higher than the respective safely values of 370 Bq kg −1 , unity, 59 nGy h −1 , and 480 Sv y −1 .From all of these results, we deduce that the amount of fertilizers that showed high radioactivities should be decreased and used with precautions.

− 1 )Figure 1 :
Figure 1: Comparison of natural radioactivities and radium equivalent of Egyptian and Japanese fertilizers.

Figure 2 :
Figure 2: Comparison of external and internal indexes of Egyptian and Japanese fertilizers.

Figure 3 :
Figure 3: Comparison of absorbed and annual effective doses of Egyptian and Japanese fertilizers.

Figure 4 :
Figure 4: Comparison of gamma and alpha indexes of Egyptian and Japanese fertilizers.

Table 1 :
Activity concentrations of 226 Ra, 232 Th, and 40 K in Egyptian fertilizers.
ND: nondetectable value (the measured value less than thorium detection limit of 3 Bq kg−1

Table 2 :
Activity concentrations of 226 Ra, 232 Th, and 40 K in Japanese fertilizers.

Table 3 :
The Radium equivalent and hazard index in Egyptian fertilizers.

Table 4 :
The Radium equivalent and hazard index in Japanese fertilizers.

Table 5 :
Absorbed and annual effective dose and hazard indexes of fertilizers used in Egypt.

Table 6 :
Absorbed and annual effective dose and hazard indexes of fertilizers used in Japan.

Table 7 :
Comparison of radiological indexes in the present work and their values in literature.