Determination of natural radioactivity in products of animals fed with grass: A case study for Kars Region, Turkey.

In this research, the activity concentrations of 40 K, 232Th and 226Ra in 41 grass samples collected from Kars region, Turkey, were determined using gamma ray spectrometry. Natural radioactivity concentrations in animal food products were calculated based on activity concentrations of these radionuclides in pasture-grass samples and dry-grass consumption of animals. The average annual effective dose from these radionuclides for local consumers due to indirect ingestion of cow milk, sheep milk, poultry, mutton and beef consumption have been calculated as 9.01, 0.24, 1.76, 0.38 and 5.25 µSv y-1, respectively. Furthermore, the calculated average annual effective dose values for adults are within the values found in other countries worldwide. These results show that animal products can be safe for human consumption in terms of radiation exposure due to the natural radionuclides studied.

with animal species suitable for pasture type during grazing season. The grazing pattern of each animal species is different. They also have the characteristics of choosing the grasses that animals graze. Cattle eat grasses with their tongues at a height of 3 to 4 cm. When cattle first go to the pasture, they do not cause much damage to the pasture since they first eat the leaf ends of the plants 17 . Sheep and geese pick up plants very close to the soil surface. In other words, the amount of stubble remaining is very low 18 . If grazing is done frequently and with more animals, the pasture will be damaged. In the breeding of pastures which are disturbed by sheep and horses, it is also struggled by grazing with cattle 16,18 . Considering the grazing habits of the animals grazing in the pastures, the plants collected in our study were divided into three sections 19 .
Activity concentrations of natural radionuclides were determined by examining a total of 41 grass plant samples taken from 6 different stations in Kars province. Kars is located in the northeastern part of the country and shares part of its border with the Republic of Armenia. Geographical coordinates of Kars are 40° 25' 0″ north latitude and 43° 4′ 59″ east longitudes and have an average altitude of 1768 meters above sea level. Its surface area is 18557 km² and the total population is 288878 as of 2018. The province's economy is based on agriculture and animal husbandry because of the excess of pastures and meadows in Kars. Due to the proximity of the Metzamor Nuclear Facility in Armenia to the grasslands, the location of the sampling area of the selected grass was measured by a GPS instrument and the coordinates of the sampling sites were shown on the map with the Google Earth Map application (Fig. 1).
Approximately 5 kg of grassland plants were collected. After removal of any trace of soil, the plant samples were divided in three part as root, stem and leaves and dried in an oven at 105 °C for 12 hours. The dried plant samples were powdered and then ashed into a white ash at 425 °C in an electrical oven where the temperature gradually increased. Depending on the approximate grass samples presence, 0.07-0.09 kg of ashed plant sample was prepared and was transferred to cylindrical plastic containers with a diameter of 6 cm and a height of 5 cm. Containers of similar size and shape were used to maximize counting efficiency and accuracy and to minimize self-absorption for this particular geometry. Then samples were weighed and hermetically sealed to prevent the escape of radon gas. Samples were kept for 45 days to obtain a secular balance between 226 Ra ( 238 U daughter) and 232 Th daughter before the measurements 20 .
Activity determination. The activity of each sample was counted for a period of 86000 s using NaI(Tl) detector based on gamma-ray spectrometer system. The spectrum was analyzed using a PC (Personal Computer) based an MCA (Multi Channel Analyzer) system and Maestro software. The energy calibration and the relative efficiency calibration of the gamma spectrometer were performed using the certified reference material IAEA-375 Soil (originating from the area affected by the Chernobyl accident) 21 . For determining the activity concentrations in the soil samples and pasture-grass samples, suitable photopeaks at several energies were taken into account and the appropriate area (ROI) regions were selected for each peak. The activity concentrations of 40 K was evaluated from the 1460.8 keV gamma line. 226 Ra concentration was found out by measuring the 609.3, 1120.3 and 1764.5 keV gamma-rays from 214 Bi. Similarly, 583 keV and 2614.5 keV gamma-rays from 208 Tl were used to indicate the activity concentration of 232 Th. The net count rate under the most significant photo peaks of all radionuclides daughter peaks were determined by subtracting the background spectrum corresponding to the same count time. Afterwards the activity of radionuclide the background subtracted area, is calculated from the significant gamma ray energies 20 .
Annual effective dose from animal products. As calculated in similar studies the activity concentrations of 226 Ra, 232 Th and 40 K in each portion of pasture samples, daily dry matter feed intake rates and transition coefficients for animal products were used to calculate the activity concentrations of these mentioned radionuclides in some animal products (Eq. 1) 14 . www.nature.com/scientificreports www.nature.com/scientificreports/ where, A i is the calculated activity concentration in animal products, beef, milk and poultry in Bq kg −1 . A j is the average radionuclide concentration in grass. Q j is the average amount of dry-grass per day consumed by cattle, sheep and poultry (kg day −1 ), and R i is the fraction of the animal's daily intake by ingestion transferred to animal products (day kg −1 or day l −1 ) 22,23 . Calculation of the annual effective dose due to internal irradiation caused by the radionuclides present in foods ( 226 Ra, 232 Th and 40 K) is important for the assessment of the possible radiological risk for human health. Considering the IAEA's recommendations, radionuclide levels should be investigated separately in each of nutrients people consume in their daily life 24 . The mean ingestion dose should be calculated by considering the average activity levels of the natural radionuclides in each food. According to Till and Moore (1988), the annual effective dose from natural consumption of food is calculated by considering the radionuclide activity measured (Bq kg −1 ) in food, the amount of food consumed by people per year (kg y −1 ) and the effective dose conversion factor of each radionuclide 25 . Hence, the average annual effective dose for human due to ingested animal products could be calculated using Eq. (2).
where, A Ra , A Th and A K are activity concentration of 226 Ra, 232 Th and 40 K in Bq kg −1 , respectively. The dose conversion factor for 226 Ra, 232 Th and 40 K are DC Ra (0.28 µSv Bq −1 ), DC Th (0.23 µSv Bq −1 and DC K (0.0062 µSvBq −1 ), respectively 26 . U (kg y −1 ) is the annual consumption rate of animal products by people.

Results and Discussions
The of 226 Ra, 232 Th and 40 K radioactivity concentrations in Bq kg −1 (dry weight) in different parts of pasture-grass samples are presented in  Tables 2 and 3, respectively.
The estimating values for the daily dry matter feed intakes of farm animals are given in Table 2 and transfer factors of 226 Ra, 232 Th and 40 K radionuclides from dry matter feed to animal products are shown in the Table 3.
Based on the concentration of 226 Ra, 232 Th and 40 K radionuclides in each portion of the pasture and the amount of grass eaten by the animals, it was found that the concentrations of 226 Ra, 232 Th and 40 K radionuclides calculated in each animal product would be different. www.nature.com/scientificreports www.nature.com/scientificreports/ in animal products in Bqkg −1 . The range of average 40 K concentration was computed to be 5.8-99.6 Bq kg −1 . Mutton, sheep milk and poultry are supplied from farm animals fed with stem and root parts of the grass from small ruminants and poultry, respectively. It was estimated that the concentration of 40 K in beef meat and cow milk obtained from bovine animals fed with the leaf part of the grass was higher than the concentration of 40 K in mutton, sheep milk and poultry. This is because 40 K accumulates mostly in the stem and leaf part of the grass where cattle are fed 20 .
The concentration of 226 Ra in animal product was ranged from 3.1 × 10 −2 Bq kg −1 (mutton) to 6.2 × 10 −1 Bq kg −1 (cow milk). As can be seen from Table 4, the highest 226 Ra values were found in beef and cow's milk compared to other farm animal products. This is because cattle can eat more grass than other farm animals. Although 232 Th accumulates mainly on the stems and leaves of grasses, it was found that the concentration of 232 Th was higher in animal products obtained from poultry fed by plants remaining in the soil after grazing of other animals 20 . Because the transfer factor from the 232 Th radionuclide feed to the poultry products is at least 100 times greater than the transfer factors of the same radionuclide to other animal products.     Table 5 shows the comparison of the results obtained with other reports, the mean values of 226 Ra in beef are higher than that those reported values , in Egypt, in United States, in Korea and in Taiwan 14,[27][28][29] . In this study, the average value of 40 K in beef is lower than the values reported in Korea by Choi et al. (2008), in Nigeria by Akinloye et al. (1999) and in Italy by Meli et al. (2013), while our obtained value is higher than the value reported by Harb et al. (2010) in Egypt 14,28,30,31 . Similarly, 40 K concentrations in milk were higher than concentration in milk from other countries 13,14,28,32,33 . Table 5 reveals some differences in activity concentrations in animal products compared to the countries reported. This may be explained by physical properties of soil according to geographical location, the characteristics of the growing grass, climatic condition during the growth of the grass, the race of grazing animals and their spending time on pasture for grazing.
In this study, the annual consumption rate of animal products for people are obtained from Turkish Meat and Milk Institution's report that are given in Table 6 34 . The radionuclide concentration values given in Table 4, the dose conversion factor for each radionuclide, and the annual consumption rate of animal products were used in Eq. 2 to evaluate the annual effective dose in the nutrients obtained from animals fed pasture-grass. Table 6 shows the calculated values of the average annual effective dose for 226 Ra, 232 Th and 40 K in animal products. In the annual effective dose calculations here, the animals were assumed to eat dry-grass containing 226 Ra or 232 Th. For a more detailed calculation, the dose assessment should be made considering that animals also eat foods containing a number of daughter radionuclides and that these would have different transfer factors.
The total annual dose due to internal irradiation caused by radiation emitted from the current 226 Ra, 232 Th and 40 K radionuclides in the investigated animal products was evaluated as 16.6 µSv. The contribution of 40 K radionuclide to total annual dose from animal products was 94.5%, while the contribution of 226 Ra and 232 Th radionuclides to the total annual dose was 4.2% and 1.3%, respectively.
There is no extra fertilization process in the pastures where the animals are grazed, but when the animals spend their times on pastures, they leave natural fertilizers rich in potassium to the pastures. Therefore, the highest contribution of 40 K radionuclide to the total annual dose from animal products can be explained by the fact that soil properties support the mobilization of potassium and subsequent migration to the grass.

conclusions
Ingestion of contaminated foods is one of the routes of uptake of potentially dangerous radionuclides for man in particular due to importance in human diets. The activity concentrations of 226 Ra, 232 Th and 40 K radionuclides were computed in food samples produced from animals consuming dry-grasses from 6 different grasslands of Kars region. The average annual effective dose from animal food consumption was figured out to be 16.6 µSv and the largest part of this dose was derived from the 40 K natural radionuclide. As a result of this study, it has been determined that there will be no negative effects on human health and environment. Since the dairy products and meat of the animals growing in the region are consumed by people living in both the local and other cities of the country, systems should also be put in place to monitor radionuclides in animal products in order to reduce human exposure to radiation. Radiological assessment of environmental health risk can be done using known radioactivity values of environmental samples as in this study.

Annual Consumption
Annual Effective Dose (µSv y -1 ) (  Table 6. The calculated annual effective dose values due to 226 Ra, 232 Th and 40 K (Bq kg −1 ) radionuclides in animal products and average annual consumption by adult people of Kars region.