THE ACTIVITY CONCENTRATIONS OF 40 K , 226 RA , 232 TH , 238 U AND 7 BE IN MOSS FROM SPAS IN EASTERN SERBIA IN THE PERIOD 2000-2012

In this work we present the activity concentrations of natural radionuclides 40K, 226Ra, 232Th, 238U and 7Be in mosses. One hundred and sixty-seven moss samples were collected between 2001 and 2012 from the territory of the Sokobanja, Banja Jošanica and Gamzigradska Banja spas. They were classified into 23 species. The activity concentrations (Bq/kg) in moss from Sokobanja spa were: 40K 25-427; 226Ra 0.3-36; 232Th 1.0-37; 238U 0.4-28 and 7Be 29-210; from Banja Jošanica spa they were: 40K 90-242; 226Ra 2.4-11.7; 232Th 2.0-12.7; 238U 1.6-11.3 and 7Be 142-212; Gamzigradska Banja spa: 40K 95-351; 226Ra 8.0-21; 232Th 5.1-19; 238U 6.7-18 and 7Be 20-144. The activity concentrations of dominant natural radionuclides (potassium, radium, thorium and uranium) in the moss samples were within the usual ranges for the territory of Serbia.


INTRODUCTION
Primordial radionuclides found on Earth have existed in their current form since before the Earth was formed and they are the most common and most significant source of ionizing radiation in the environment, both from the viewpoint of total irradiation of the population and local high radiation doses.They have a long half-life (10 5 -10 16 years) and differ significantly in physical and geochemical properties.These radionuclides are 235 U, 238 U, 232 Th, 226 Ra, 222 Rn and 40 K.The most abundant natural radionuclide in the lithosphere is 87 Rb, followed by 232 Th, 40 K and 238 U.
Uranium is naturally found as a mixture of three long-life isotopes: 238 U (99.28%), 235 U (0.71%) and 234 U (0.0006%).Uranium-238 serves as a raw material for obtaining 239 Pu, which is used as a fission ma-terial.The physical half-life of 238 U is 4.5x10 9 years, and the biological half-life is from 1-500 days (depending on the mobility of uranium compounds) (Kljajić et al., 1995).In the organism, uranium, regardless of its chemical form and exposure time, behaves as a toxin as it acts as a source of ionizing radiation and as a chemical toxic element.Uranium-238 and its products 226 Ra and 222 Rn, whose half-lives are 1 600 years and 3.8 days respectively, present the greatest danger to human health. 226Ra is also present in materials originating from coal and wood burning (forest fires).Radium-226 is found in different quantities in soils and rocks containing natural uranium.Its biological half-life is about 45 years (Eisenbud and Gesell, 1997).Due to its long physical half-life, increased chemical activity, low elimination from the organism, its amount in the organism increases.Thorium-232 is the most significant member of the thorium chain of radioac-tive decomposition.Natural thorium-232 is also a long-lived radionuclide (half-life 1.4x10 10 years).Its concentration is very small in the biosphere, due to its small specific activity and insolubility.Potassium-40 is a primordial natural radionuclide with a long half-life of 1.25x10 9 years, and a biological halflife of 58 days.Potassium is a monovalent lithophile element under natural conditions.In nature, 40 K is found in a mixture with stable potassium isotopes 39 K and 41 K ( 39 K-93.08%, 40 K-0.1119% and 41 K-6.9%).Among the naturally occurring primordial radionuclides, 40 K (T 1/2 =1.28x10 9 y) is very abundant in soil, as the molar fraction of 40 K is 0.0117.In a living organism, potassium is evenly distributed.Potassium is a chemical analogue to cesium.Among natural radionuclides, only 40 K is considered essential as it is part of the human organism and is under homeostatic control. 238U and its products 226 Ra and 222 Rn present the greatest risk for human health (Gržetić and Jelenković, 1995;Veselinović et al., 2004;Mason, 1996;United Nations Committee, 2000).
Ionizing radiation reaching the surface of the Earth from the depth of cosmic space and the sun, has a large energy (0.01-10 GeV).These rays act on the environment in two ways: directly and indirectly, causing secondary radiation and forming cosmogenic radionuclides.Cosmogenic nuclides ( 3 H, 7 Be, 10 Be, 14 C, 26 Al, 32 Si, 36 Cl) are produced by cosmic rays in the atmosphere and are used in many research applications, such as paleoclimatology, solar activity reconstructions, dating methods, etc. (Kovaltsov and Usoskin, 2009).Due to their low concentrations, relatively short half-life and low radiation intensity, they are not very significant for total population irradiation.Berilium-7 forms under the action of cosmic radiation on oxygen and nitrogen in the atmosphere.About 75% of 7 Be is produced in the stratosphere, and about 25% in the higher layers of the troposphere (Masarik and Beer, 1999;Papastefanou et al., 1999). 7Be aerosols remain in the stratosphere for about a year, while the retention time in the troposphere is about 6 weeks.It reaches lower layers, surface waters and seawater by rainfall and is used as a tracer when investigating movement of air mass through the atmosphere.Changes of 7 Be concentrations in surface layers of air depend on several factors: change of air mass movement speed in the tropopause, variation of vertical mass movement in the troposphere, variation in horizontal transport of air mass from medium latitudes towards Polar Regions and on the fall amount.The 7 Be radioisotope has a half-life of 53.12 days and emits a photon energy of 477.6 keV, with a relative intensity of 10.56% (Masarik and Beer, 1999;Papastefanou et al., 1999).
Mosses are bioindicators of environment pollution for radionuclides and other pollutants.They are organisms with specific ecology and biological features as compared to vascular flora.Investigation of radionuclide activity levels in mosses provides a reliable insight into the contamination degree of ecological systems by radioactive isotopes (Chakrabortty and Paratkar, 2006;Basile et al., 2008;Čučulović and Veselinović, 2008;Čučulović and Veselinović, 2009;Čučulović et al., 2010;Čučulović et al., 2012;Čučulović et al., 2012;Čučulović et al., 2010;Čučulović et al., 2012).
Many factors influence the concentration of polluting substances in moss: microclimate, plant age and morphology, amount of emitted and settled polluting substances, physicochemical properties of polluting substances, regime and form of water supply, chemical composition and pH of the substrate on which they grow.Parameters which influence the level of activity of radionuclides in plant material are the amount of released and deposited radionuclides, physicochemical properties of the radionuclides, meteorological-climatic conditions, physicochemical properties of the soil where the plant grows, the species and physiological-morphological characteristics of the plant itself and the way it is grown.Various locations are contaminated differently with radionuclides due to varying biosphere contamination.The surface of the plant leaf is also very significant for radionuclide uptake: if it is bigger, more radionuclides are retained on it.The age of the plant is also important for radionuclide accumulation: younger plants absorb a 2-3 times greater amount of radionuclides than older ones.

MATERIALS AND METHODS
Samples of mosses were collected from 2000 to 2012 in the territory of Sokobanja, Banja Jošanica and Gamzigradska banja spas.Sokobanja is located in the central part of eastern Serbia and is part of the Zaječar region.Samples from the territory of the Sokobanja municipality were taken on the following locations: in the city of Sokobanja (hotel Sunce) (locality SB1), Lepterija (1.5 km east of the center of Sokobanja) (SB2), in Soko city (400 m from Lepterija) (SB3), Mt.Ozren (about 4.5 km from the center of Sokobanja) (SB4), the Ophthalmology Hospital (5 km from Sokobanja southeast, close to the Ozren Special Hospital for treating eye problems) (SB5).In the period 2000-2012, samples were taken as follows: locality SB1 6 samples, SB2 41, SB3 1, SB4 72 and SB5 1 sample, in total 121 samples.Banja Jošanica is located in the northwestern part of the Sokobanja valley, between the western part of Mt.Rtanj and the eastern slopes of Mt.Bukovik, close to Sokobanja.In the territory of Banja Jošanica, samples were taken in the following locations: in the park (BJ1), close to mineral water springs (BJ2) in the Jošanica river banks (BJ3).In the period 2009-2012, in location BJ1 5 samples were taken, in BJ2 3 samples and in BJ3 6 samples were taken, in total 14 samples.Gamzigradska Banja is located close to Zaječar.In the territory of Gamzigradska Banja samples were taken on the left (GB1) and right bank of the Crni Timok River (GB2), near the Gamzigrad Rehabilitation Center (GB3), near Hotel Kastrum (GB4), hotel annexes (GB5), hydroelectric power station (HE) Gamzigrad (GB6) and in the archeological locality of Felix Romuliana (GB7).In the period 2006-2012 in locality GB1 7 samples were taken, in GB2 1, in GB3 5, in GB4 3, in GB5 7, in GB6 4 and in locality GB7 2 samples were taken, in total 32 samples.Selections of localities were based on the presence of large enough amounts of mosses and the absence of immediate sources of pollution.
The samples were cleaned, dried at room temperature and homogenized, then soaked in paraffin in Marinelli vessels (1L), and left for 30 days to reach the radioactive equilibrium.Activities of radionuclides were determined on an HPGe-ORTEC/ Ametek detector (relative efficiency 34%, resolution 1.65 keV at 1.33 MeV).The sample weight was about 0.1 kg.The total standard error of the method (including relative error in geometric efficiency estimation, photo peak counts estimation, sample volume determination, etc.) was estimated to about 20%.Spectral analysis was performed with the Gamma Vision 32 software package.The activities of 226 Ra and 232 Th were determined by their decay products 214 Bi (609.3 keV; 1120.3 keV and 1764.5 keV), 214 Pb (352 keV) and 228 Ac (338.4 keV; 911 keV and 968.9 keV), respectively.The activities of 40 K were determined from its 1460 keV γ-line.The activities of 7 Be were determined from its 477 keV γ-line.The average counting time interval was 6 x 10 4 s.Geometric calibration was performed with different radioactive reference materials, in the sampling geometry (Marinelli 1L): ( 1 Cs, 54 Mn, 113 Sn, 85 Sr, 88 Y, 980.0 g, 0.98 ± 0.01 g/cm 3 , 1000 ± 10 cm 3 , ref. date 1.4.2008).Nuclides were identified using a library-driven search routine and quantitative analyses were carried out using the ap-propriate detector calibration.Radionuclide results were reported in Bq/kg on a dry weight basis.
Other investigations have shown that the average activity concentrations of 232 Th in moss from: the Belgrade surrounding area were up to 45 Bq/kg (Grdović et al., 2010), Zlatibor 0.8-13.7 Bq/kg (Dragović et al., 2010)) and Vojvodina up to 20 Bq/kg (Krmar et al. 2013), while the average activity concentrations of 238 U in moss from the Belgrade surrounding area were up to 113 Bq/kg (Grdović et al., 2010), Zlatibor 1.7-25.1 Bq/kg (Dragović et al., 2010).The average activity concentrations of 7 Be in Hypnum cupressiforme moss collected on the territory of Vojvodina were from 201 to 920 Bq/kg.These depend on the season when the moss was sampled and are higher in the summer than the winter (Krmar et al. 2009;Krmar et al., 2013).
From this work, it follows that the lowest activity concentration of 232 Th was measured in Anomodon viticulosus moss (1.0 Bq/kg), while the highest value was measured in Brachythecium rutabulum (37 Bq/kg), growing on the territory of Sokobanja.It also follows that the lowest activity concentration of 238 U was measured in Neckera crispa moss (0.4 Bq/ kg), while the highest value was measured in Brachythecium rutabulum (28 Bq/kg), also from the ter-ritory of Sokobanja.The lowest activity concentration of 7 Be was measured in Pylaisia polyantha moss (20 Bq/kg), while the highest value was measured in Homalothecium sp. ( 14) moss (212 Bq/kg).