Determination of 238U and 40K Radionuclide Concentrations in Some Granite Rocks by Gamma Spectroscopy and Energy Dispersive X-ray Analysis

The elemental potassium and uranium concentrations of the investigated samples were determined through the energy dispersion X-ray (EDX) unit of the electron scanning microscope (SEM) at the city of scientific research in Alexandria, Egypt. Uranium is a trace element in granites, making accurate detection with EDX difficult. Therefore, each sample was scanned by the electron microscope for multiple scans (with repeated measurements), targeting different regions of one sample to perform a comprehensive scan for each sample. The measuring time was selected to be the maximum time allowed by the instrument (30 min), while increasing the beam current as much as possible to ensure the accurate determination of the elemental concentration inside the sample. A schematic diagram of an EDX micro-analysis method is shown in Figure 4 [20].

The detection limit of an EDX system is affected by bremsstrahlung radiation—with lower concentrations, statistical errors and uncertainties are higher. Further, the detection limits for heavy elements (using the L or M lines) such as uranium tend to increase because the peak-to-background ratio is lower than it is for K lines [21]. The detection limit of the EDX system can be estimated from the following equation [22]:

where N(B) is the average background count, N(S) is the average count of the standard, and C_s is the concentration of the standard. The specific activity concentration of the radionuclides can be estimated theoretically from the elemental concentration given by EDX [23,24]:

where λ is the decay constant of the radionuclide and N is the number of radionuclides in a 1 kg granite sample, which can be given by [25]:

where C is the elemental concentration of mass percentage in the granite sample estimated from EDX, R is the isotopic abundance, N_A is the Avogadro’s number, and W is the atomic weight of the radionuclide.