###### Abstract  
The specific activity and spatial distribution of \({}^{238}\)U, \({}^{232}\)Th and \({}^{40}\)K were determined in the surface soil from the Lisava uranium mining sector. This sector belongs to the Banat district, an historically important uranium mining area in Romania (an area with closed uranium mines and a radioactive waste dump). Gamma-ray spectrometry using a high-purity germanium (HPGe) detector was used to measure the activity of naturally occurring radionuclides in the soil. The average specific activities of \({}^{238}\)U, \({}^{232}\)Th and \({}^{40}\)K in the soil were 197.21 Bq/kg for \({}^{238}\)U, 16.21 Bq/kg for \({}^{232}\)Th and 543.21 Bq/kg for \({}^{40}\)K. The mineral contents of selected waste rock samples (sandstones) were examined using a scanning electron microscope (SEM), which revealed that brannerite, pitchblende and coffinite were the most important uranium-bearing minerals. The means of the radiological hazard parameters were calculated to be 262.22 Bq/kg radium equivalent activity (Raeq), 123.72 nGy/h absorbed gamma dose rates (DR), 0.7 external hazard index (Hex) and 1.8 representative level index (RLI). The spatial distribution of the risk assessment indices associated with the investigated soils exceeded the median values provided by UNSCEAR and reflected the geological settings and influences of anthropic activities such as uranium mining practices and the tipping of radioactive mining waste. Natural radionuclides; soil; spatial distribution; external exposure; radiation risk assessment; uranium mining +
Footnote †: journal: Applied Sciences  
1212263123312363  
rocks, uranium appears as U\({}^{4+}\) with crystallochemical properties similar to Th\({}^{4+}\) and the light rare earth elements. Under endogenic conditions, both parental radioactive elements (U\({}^{4+}\) and Th\({}^{4+}\)) crystallize together as late products of magmatic differentiation and are always concentrated in acidic rocks as accessory minerals. Under exogenic conditions, the chemical properties of uranium and thorium are markedly different. Uranium has a high geochemical mobility; it is oxidized easily from its tetravalent state to its hexavalent state [9; 10; 11]. Under these circumstances, uranium can be oxidized to uranyl, which is very soluble and easily transportable in groundwater and surface waters. Compared to uranium, thorium cannot be oxidized up to the hexavalent state and therefore has no analogue of the uranyl ion; any amount of thorium dissolved is adsorbed and precipitated as a hydrolyzed ion due to the potential of the tetravalent ions of thorium [10; 11]. Uranium is a substantial component of several minerals, the most notable of which are uraninite, brannerite and carnotite. Thorium may be found in a variety of minerals, including thorite and thorianite [12]. Both elements (thorium and uranium) coexist in accessory minerals such as zircon, monazite, apatite, allanite and titanite, which are found in igneous and metamorphic rocks, with zircon and monazite being the most resistant to weathering. Uranium may also combine in complex ways with organic materials and phosphate materials [13]. Potassium-40 is a naturally occurring radioactive potassium isotope that is found in trace amounts in all potassium-bearing minerals. Under normal conditions, potassium is a volatile lithophilic metal that is monovalent and has an average concentration of 2.8% in the upper continental crust [9]. Potassium is found in metamorphic and magmatic rocks as a major element of the feldspar mineral series, feldsparoids and micas. In sedimentary rocks, potassium can usually be found in sedimentary rocks due to their content in alkali and plagioclase feldspars [10]. Several natural occurrences and deposits of radioactive mineral ores with a high uranium content are known in Romania.