بررسی پرتوزایی طبیعی خاک های کشاورزی و بکر در شهرهای اراک و سربند در استان مرکزی

Introduction: Humans are constantly exposed to ionizing radiation. Most of the radiations originate from radioactive decay of natural radionuclides in the environment. Most of the ionizing radiations resulting from the decay of natural radioactive series of 238U, 235U, 232Th and 40K radionuclides in soil, rock and water. The average amount of uranium in the earth's crust is estimated 2.7 mg/kg that its amount increases up to 120 mg/kg in phosphate rocks. Phosphate rocks are main row material of chemical fertilizers production. Chemical fertilizers are chemical compounds that are used in order to increasing crops and improving properties of land. Yearly, many different types of fertilizers are produced such NPK, phosphate, etc.. Even though fertilizers can improve nutrient-deficient symptoms in farms but they contain heavy metals and radionuclides. Radionuclides in fertilizer enter to the soils and directly or indirectly can be absorbed by human body by inhalation of the radon gaseous or consumption of foodstuff. Radium salts with other salts are also dissolved in water and penetrate to deep layers of soil and can cause groundwater pollution. The consumption of chemical fertilizers containing radionuclides increases the radiological parameters of soils. Nuclear radiations is emitted from these elements are harmful to the body's tissues and may cause diseases such as cancer or genetic abnormalities. Therefore, the investigation of natural radioactivity contents in soils and radiological parameters are very important. In this research, the specific activities of natural radionuclides in agricultural and virgin soil samples determined using gamma ray spectrometry method in Arak and Saraband cities in Markazi province, Iran..For all samples, radiological parameters were calculated and compared with world average and some countries.
Materials and Methods: In this study, twenty two samples of agricultural and virgin soils were collected in different areas of Arak and Saraband cities of Markazi province in Iran, from surface up to 30 cm depth. The weight of each sample was about 2 kg. After drying the samples at room temperature, they were powdered by electric mill in the laboratory of Arak university. Soil samples were pulverized and passed through a 0.297mm sieve. They were kept in oven for 24 h at 100°C in order to remove the moisture content. After that, 950 g of each sample was packed in a Marinelli beaker container and sealed. Gamma ray spectra were registered after 50 days. The collected samples required particular care since radon is a short-lived gaseous nuclide and tends to escape from the samples. In this work, standard containers were sealed. After the minimum 50 days of preparing samples, gamma ray spectra were registered. This time is necessary for taking radioactive decay chain equilibrium, in which the decay rate of the daughters became equal to that of the parents. Specific activity measurements were performed by gamma ray spectrometry method employing high purity germanium (HPGe) P-type coaxial detector (GCD30195BSI) manufactured by Baltic Scientific Instrument LTD (005-Latvia) with 30% relative efficiency, which was connected to a multi-channel analyzer of 8192 channels. The energy resolution (full width at half maximum) of this detector is 1.95 keV for gamma energy line at 1332.520 keV due to 60Co and a Peak-to-Compton ratio of 60, and operating voltage was 3000 V. The detector and preamplifier are shielded in a chamber of three layers composed of 10 cm thick lead, 1.5 mm thick cadmium, and 3 mm thick copper. This shield serves to reduce background radiation.
Results and Discussion: Based on the results, the specific activities of 226Ra, 232Th and 40K radionuclides in soil samples varied in the range (39.60-51.94), (50.90 – 73.84) and (676.09 – 1094.50) in agricultural soils and (12.98 – 61.60), (18.15 – 60.98) and (257.48 – 866.58) in virgin soils, in Bq/kg. The mean specific activities of corresponding radionuclides were obtained 45.54, 69.09 and 926.71 Bq/kg for agricultural soils and 37.26, 43.17 and 604.04 Bq/kg for virgin soils. For all results calculated and have been tested variety of variance which show increasing in agriculture soil amount of 232Th and 40K compared with virgin soil. For 226Ra, the significant variation in probability level of 0.05 was not observed because of more mobility of radium salt than other salts and penetration into deeper layers. The increases the amounts of studied radionuclids in agricultural soils compared with average of worldwide virgin soils are 74.12, 72.30 and 131.68 percent, respectively. The average absorbed dose in air calculated for agricultural and virgin soils that obtained 105.22 and 70.59 in nGy/h, respectively. The average of Annual Gonadal Dose Equivalent (AGDE) and Excess Lifetime Cancer Risk (ELCR) for agricultural and virgin soils were obtained as (0.72 and 0.48) and (0.45×10-3 and 0.29×10-3), respectively. For virgin soil samples, radiological parameters were in good agreement with mean world value but in agricultural soil samples, increase was observed compared to virgin soil and agricultural soils of some countries.
Conclusions: The obtained data showed that the amount of mean specific radioactivity of natural radiouclides in agricultural soils were higher than some countries. The radiological parameters of agricultural soils in most samples were more than the world average of virigin soil values. The results of this research on virgin soil were in good agreement with the world average. This study showed that consumption of fertilizers in these cities was more than other countries. But all of radiological parameters of soil samples were less than maximum acceptable criteria and thus have no damaging effect for people health. However, the long term unsuitable usage of chemical fertilizers can have the effect of increasing of radioactivity in soils that can be harmful for the health of farmers and consumers of the crops.