Monitoring the concentration of 238U, 232Th, 40K and 137Cs of soil in Anarak-Khour district, Central Iran

[Research]

Monitoring of the concentration of ²³⁸U, 232Th, ⁴⁰K and ¹³⁷Cs of soil in Anarak-Khour district, central Iran

E. Ehsanpour¹, M. R. Abdi¹*, M. Mostajaboddavati², H. Bagheri³

1-Dept. of physics, Faculty of science, University of Isfahan, Iran

2- Dept. of Nuclear Engineering, Faculty of Advance Sciences and Technologies, University of Isfahan, Iran

3-Dept. of Geological Science, Faculty of science, University of Isfahan, Iran

*Corresponding author’s E-mail: r.abdi@phys.ui.ac.ir

ABSTRACT

This study was conducted to determine the radioactivity concentration in soil samples of Anarak-Khour district located at central Iran. 48 soil samples from a depth of 10-15 cm were taken from seven different sites for monitoring. The radioactivity concentration of ²³⁸U, ²³²Th, ¹³⁷Cs and ⁴⁰K in soil samples was determined, using a high resolution (HPGe detector, p-type) γ-spectrometry by detectors with 55% and 38.5% relative efficiencies. The results showed that the radioactivity concentration ranged from 9.91 ± 0.11 to 4479.95 ± 0.15 Bq.kg^-1 for ²³⁸U, from 5.95 ± 0.14 to 389.72 ± 0.10 Bq.kg^-1 for ²³²Th, ≤MDA to 8.89 ± 0.13 Bq.kg^-1 for ¹³⁷Cs and from 123.18 ± 0.27 to 5201.42 ± 0.27 Bq.kg^-1 for ⁴⁰K in soil samples. The calculated internal and external hazard indices were more than 1 in some sites which showed a high dose for this area. These results could be used as a database of this area because it might be used as a nuclear waste site in the future.

INTRODUCTION

The intersection of Uroomieh-Dokhtar Magmatic Belt (UDMB) and the major Great Kavir-Doruneh fault (GKDF) are two important structural traits in the Anarak-Khour area in central Iran (Fig. 1). This region is in the north-western corner of the Central-Eastern Iranian micro plate. This terrene is an approximately 2300 km² region of medium relief surrounded by fold and thrust belts within the Alpine-Himalayan orogenic system of western Asia (Bagheri et al., 2006a). In the Anarak-Khour area there are Cu, Ni, Co, As, U, Pb, Zn, Au and Ag deposits which are localized in the same area under similar geological environment along the north-western and western surroundings of Anarak-Khour massif (Badham et al., 1976). The sediments of Meskani and Talmessi in the Anarak mining region which are 7 km apart were mined for copper and nickel until 1960 (Bagheri et al., 2006b). Then mining was discontinued in this area. Recently, exploration activities were continued by the atomic energy organization of Iran due to uranium exploration. The most important active mine in the area is Nakhlak Lead Sediment located in 40 Km east of Anarak. Places that have a greater radioactivity concentration were selected as sample sites. Then the soil samples of this area were collected from 33°15'38.10"N (with 53°37'33.03"E) to 34°12'37.60"N (with 55°18'43.45"E). The studied area is located within the rectangular area and is divided to seven sites as it is shown in Fig. 2.

Fig 1. Main structural lineaments in Central Iran and location of the study area (modified from Bagheri et al., 2007)

Fig 2. Sampling sites

There are a lot of researches which deal with determining the radioactivity concentration throughout Iran (Abdi et al., 2009, 2006a, 2006b, 2008; Pourahmad et al., 2008; Zare et al., 2012). But there is not enough information about Anarak-Khour area which might be used as a site for nuclear waste. The objective of this study was to determine of radioactivity concentration of ²³⁸U, ²³²Th, ¹³⁷Cs and ⁴⁰K in Anarak-Khour area and to estimate of possible radiation hazard for the people, which possibly might work in the investigated area in future.

MATERIAL AND METHODS

SAMPLING AND PREPARATIONS

48 soil samples from a depth of 10-15 cm were taken from seven different sites. The sample places were chosen in terms of degree-minute-second latitudinal and longitudinal position using a hand-held Global Positioning System (GPS) unit in Table 1. Places that would likely have a greater radioactivity concentration were selected as sample sites so the numbers of samples collected in each site were different. The soil samples were collected from 3 separate holes, with 1 meter width and 10-15 cm depth. The soil samples were dried by spreading them on polythene layers at room temperature for 2 weeks under a controlled environment to avoid local dust pollution. Then, the soil samples were oven dried at 110° C for 24 hours. They were passed through a 50 mesh sieve. 950 g of each sample was transferred to Marinelli-beakear. Then, the Marinelli-beakers were sealed and kept for at least 5 weeks. During this time, the daughter of radon achieved to equilibrium with ²²⁶Ra and the samples were ready to be analyzed by gamma spectroscopy (Abdi et al., 2008, 2009).

GAMMA-RAY DETECTION SYSTEM

The activity of ²³⁸U (²²⁶Ra), ²³²Th, ⁴⁰K and ¹³⁷Cs in the samples were measured using two P-type coaxial HPGe detectors. The relative efficiency of the detectors was 55% with energy resolution of 1.98 keV and 38.5% with energy resolution of 1.8keV at 1.33 MeV of ⁶⁰Co. The detectors were kept in a vertical position and shielded by 10 cm thickness lead wall, 2 mm cadmium and 3 mm copper to reduce background radiation (Debertin et al., 1988). Spectrum acquisition was done using the computer software MAESTRO with a multichannel analyzer (4096-channel) and spectrum analysis was done by the OMNIGAM software. In order to measure the activity of a sample, it is necessary to know the detection efficiency of the system by standard sources which have physical dimensions, chemical composition and density similar to the samples. If standard Marinelli beakers are used for standards and samples with similar geometry, the deviation can be reduced almost to zero (Mostajaboddavati et al., 2006).

The reference material IAP-mixed gamma soil standard sources (Institute of Atomic Energy POLATOM, Radioisotope Center) containing ²⁴¹Am (0.49 ± 0.004 Bq.kg^-1), ¹³⁷Cs (0.09 ± 0.001 Bq.kg^-1) and ¹⁵²Eu (0.11 ± 0.001 Bq.kg^-1) were used to obtain efficiency curve. The absolute photopeak efficiencies and activities were determined using following equation (Abbas, 2001):

(1)

(2)

Where N, t, m, p and ɛ (E) are net area counts, time, intensity, sample weight and absolute photo peak efficiency at specific energy, respectively (Faghihian et al., 2012). The specific activity of ²³⁸U and ²²⁶Ra was determined by gamma-ray lines of ²¹⁴Bi at 609.3, 1120.3 and 1764.5 keV and ²¹⁴Pb at 295 and 351 keV, while the specific activity of ²³²Th was evaluated by gamma-ray lines of ²²⁸Ac at 338.4, 911.1 and 968.9 keV. The specific activity of ⁴⁰K and ¹³⁷Cs were measured by their 1460.8 and 661.6 keV gamma-ray lines (Abbas, 2001).

The absorbed dose rates (D) are calculated according to UNSCEAR (2000) due to gamma radiation in air, 1m above the ground level for ²³⁸U, ²³²Th and ⁴⁰K radio nuclides as following:

The annual effective outdoor dose rate in units of mSv.year⁻¹ is calculated using the following formula (Farai et al., 2005):

The radium equivalent activity is a sum of the studied natural radio nuclides and is based on the hypothesis that 370 Bq.L^-1 of ²²⁶Ra, 259    Bq.L ^-1 of ²³²Th, and 4810 Bq.L^-1 of ⁴⁰K produce the same gamma radiation dose rate (Farai et al., 2005). The maximum value of Ra_eq must be less than 370 Bq.L^-1 for safe use (Abbady, 2004). It is defined as follows:

Where A_Ra, A_Th and A_K are the radioactivity concentrations of ²²⁶Ra, ²³²Th and ⁴⁰K, respectively.

A hazard index called the external hazard index H_ex is defined as follows (UNSCEAR, 2000):

In addition to the external hazard index, radon and its short-lived progeny are hazardous to the respiratory organs. The internal exposure to radon and its daughter progenies is evaluated by the internal hazard index H_in, which is given in the following equation (UNSCEAR, 2000):

Table 1. Sampling spots information.

RESULTS AND DISCUSSION

In this study, duplication of the analysis samples, the blank samples and a standard mixed source containing ²⁴¹Am, ¹⁰⁹Cd, ⁵⁷Co, ¹³³Ba, ¹³⁷Cs and ⁶⁰Co (Institute of Atomic Energy POLATOM, Radioisotope Center) which have a geometry identical to those of the soil samples have been used for quality control procedures. The results of the standard mixed source are shown in Table 2. Also, a routinely daily checking such as energy calibration of the gamma spectroscopy system had been applied. Each sample was analyzed two times and the mean value of them were calculated and reported here, the recovery in many elements was about 98%.

Table 2. Standard mixed source data.

The radioactivity concentration of ²³⁸U (²²⁶Ra), ²³²Th, ⁴⁰K, and ¹³⁷Cs in the soil samples collected from different parts of the studied area are given in Table 3. The radioactivity concentration ranged from 9.91 to 4479.95 Bq.kg^-1 for ²³⁸U (²²⁶Ra); from 5.95 to 389.72 Bq/kg for ²³²Th; from 123.18 to 5201.42 Bq.kg^-1 for ⁴⁰K and from -1 for ¹³⁷Cs. A part of Irakan Zone (Site No.7) has the high radioactivity concentration of ²³⁸U and ²³²Th which might be attributed to the geological structure of the region (Ayoubi et al., 2012). Also, the maximum radioactivity concentration of ⁴⁰K is found in sample No. 48 (Irakan Zone). The radioactivity concentration of ¹³⁷Cs in site No. 7 (Irakan Zone) is less than minimum detection analysis (MDA) which is maybe related to the altitude of this area. Less than MDA radioactivity concentration of ¹³⁷Cs in site No.7 is not related to the erosion (Ayoubi et al., 2012; Bagheri et al., 2013). However, because of that this site has the almost the lowest altitude as compared to other sites, accumulation of rainwater in this area caused the ¹³⁷Cs deposited in the depth of 10-15 cm which our samples were being collected. Fig. 3 shows the distribution of ²³⁸U, ²³²Th, ¹³⁷Cs and ⁴⁰K radioactivity concentrations of soil samples in seven sites.  To evaluate the studied area the radioactivity concentration of ²³⁸U (²²⁶Ra), ²³²Th, ⁴⁰K, and ¹³⁷Cs were compared with those reported from the other parts of Iran and then compared with those that were reported from other countries. By comparing our results with other parts of Iran, it is clear that the average radioactivity concentration of ²³⁸U (²²⁶Ra) and ⁴⁰K in the studied area is relatively high. The radioactivity concentration of ²³²Th and ¹³⁷Cs reported in the north of Iran is higher than the studied area. For example, average radioactivity concentration of ¹³⁷Cs is higher in Guilan (Sadremomtaz et al., 2010).

It is maybe due to the fact that the north of Iran is an agricultural area in which fertilizers that contain of Th element (Rezaee, 2009). Also our results have been compared with results which were reported in other parts of the center of Iran by Ayoubi, Afshar and Rahimi. It shows the radioactivity concentration of ¹³⁷Cs in this area is less than other parts of center of Iran (Afshar et al., 2010; Ayoubit et al., 2012; Rahimi et al., 2013). The average radioactivity concentration of ²³⁸U (²²⁶Ra) of our samples is less than those reported from Malaysia but more than other countries for example China, Egypt, Greece, Kuwait and South Carolina (Yanga et al., 2005; Morsy et al., 2012; Travidon et al., 1996; Saad et al., 2002; Nasirian et al., 2008; Khan et al., 2011; Powell et al., 2007; Kam et al., 2007, 2010 and UNSCEAR, 2000). The radioactivity concentration of ²³⁸U (²²⁶Ra) of our samples is more than acceptable value (35 Bq.kg^-1).

The radioactivity concentration of ²³²Th of our samples is less than those reported from Malaysia and is approximately equal to acceptable value of 35 Bq.kg^-1. The radioactivity concentration of ⁴⁰K of our samples is approximately equal to those reported from other countries (Table 4). The acceptable value of ⁴⁰K is 370 Bq.kg^-1 which shows that our data are more than the acceptable amount. The absorbed dose rates (D), annual effective outdoor dose rate, radium equivalent activity (Ra_eq), H_ex and H_in of our samples are calculated and their results are shown in the Table 5. The calculated total gamma dose rate varied from 13.41 to 2284.27 ­nGy.h^-1 among soil samples due to primordial radio nuclides.

In average, the absorbed dose rate (nGy.h^-1) of seven sites is reported in Table 5. The average total gamma dose rate is more than the worldwide average of 55 nGy.h^-1 in some sites (Abdi et al., 2009). The annual effective dose obtained for the seven sites ranged from 0.02 mSv in site No.2 to 2.81 mSv in site No.7 for the background area.

If the value of internal or external radiation hazard index is found to be less than unity, then there is no potential internal or external radiation hazard. Both the external radiation hazard index (H_ex) and the internal radiation hazard index (H_in) in some sites are more than unit. This indicates that the soils of Anarak-khour area are not free from the radiation hazards.

Fig 3. Distribution of radioactivity concentrations of ²³⁸U, ²³²Th, ⁴⁰K and ¹³⁷Cs in soil samples

Table 4. Comparison of radioactivity of soils with other areas of the world.

Table 3. Radioactivity concentration of ²³⁸U, ²³²Th, ¹³⁷Cs and ⁴⁰K in soil samples.

Table 5. Calculated values of absorbed dose rate and annual effective dose, radium equivalent activity, external and internal radiation hazard.