Radioactivity of Natural Waters in Nurabad District of Samarkand Region

Abstract

The 222 Rn content in natural waters is determined, as a rule, according to the α-activity of the gas emitted from the water and all other radionuclides in the uranium-thorium families and 40 K by the gamma-spectrometric method and by radiochemical separation the isotopes being analyzed or evaporation of water samples to dry residue [1].In the present work, gamma spectrometry was used to investigate the content of natural radionuclides in water samples from springs, dug wells, boreholes, rivers, lakes, and water reservoirs in the Nurabad district of Samarkand region.Sample Preparation and Measurement Procedure. Samples were obtained from 43 natural sources located in different parts of the Nurabad district, which differ by the geophysical characteristics of the locale. To determine the specific activity of 222 Rn, the water samples were hermetically packed in a Marinelli vessel immediately after extraction (samples s).The evaporation losses of radon in the process of extraction and packaging of the samples s did not exceed 10% (Fig. 1) [2]. Concentrated samples prepared by evaporating water samples with volumes ranging from 10-50 liters to 1 liter at temperature ~70°C to a dry residue (samples p) were used to determine the specific activity of long-lived 226 Ra, 232 Th, and 40 K. The radioactive equilibrium of 226 Ra was restored in the p samples after a holding period of approximately 40 days. Samples ƒ of distilled water were prepared to determine the background component in the instrument spectra. All samples were packed in standard one-liter Marinelli vessels.The investigations were performed using a gamma spectrometer with a 63 × 63 mm NaI(Tl) detector, placed inside 10-cm thick lead shielding, with energy resolution ~7% on the 1332 keV line. The gamma spectrometer was calibrated in terms of efficiency using the standard 226 Ra, 232 Th, and 40 K sources from the OMASN set with filler density ρ 1 = 0.86-0.96 and ρ 2 = 1.64-1.86 kg/liter. The accumulation and processing of the instrument spectra were accomplished using standard spectrometric apparatus and software package. The γ-spectra were processed by the procedure of [3], which makes it possible to adjust the instability of the characteristics of the analog spectrometric apparatus.The determination of the activity of the natural radionuclides in the water samples by the gamma-spectrometric method is complicated by the fact that the instrument spectrum of the background component is formed by the same radionuclides as in the samples used for analysis. In addition, as a rule, the intensity of the background spectrum is much greater than that of the analyzed radionuclides. These difficulties can be overcome to a large extent by systematic decomposition of the initial spectra into individual components (Fig. 2).The γ-spectrum of the s samples is a superposition of the spectra of 226 Ra, 232 Th, 235 U, and 40 K as well as the background spectrum F: S 0 = S(Rn) + S(Ra) + S(U) + S(F) + S(K) + S(F).