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[en] The activity of naturally occurring radionuclides in drinking and mineral waters in Bulgaria has been measured with high precision by means of nuclear and radiochemical methods. The survey is pointed at two radionuclides, which are most important from the point of view of public health in Bulgaria, because of their high toxicity and radiotoxicity in drinking water, namely radium-226 (226Ra) and lead-210 (210Pb). Twenty-four frequently used water sources were under investigation. The 226Ra activity ranges from ≤ 2.3 mBq/L to 90 mBq/L in drinking water and from 13 mBq/L to 5020 mBq/L in mineral water, both currently available to the public. Activity of 210Pb up to 17 mBq/L and 104 mBq/L was measured in drinking and mineral waters, respectively. Due to differences in the local geological structure of the aquifers, a large difference in values of the 226Ra and 210Pb activities was observed. The annual effective doses were calculated for all investigated waters for adult inhabitants assuming yearly consumption of 730 litres. Contributions to the annual effective doses from 226Ra and 210Pb varied from 2.56 µSv/y to 21 µSv/y and from 22 µSv/y to 1035 µSv/y for drinking and mineral waters, respectively. The results show that the annual effective dose of ingestion of these waters, excluding three mineral water samples, are below the individual dose criterion of 100 µSv/y according to the World Health Organization (WHO) recommendations. The data generated in this research may contribute to determine the base-line levels of natural radioactivity in drinking and mineral waters and help in the development of future guidelines for radiological protection of the Bulgarian population.
[en] The Department of Nuclear Medicine, St. Marina University Hospital in Varna, is equipped with a self-shielded BG-75 cyclotron and synthesis unit for production of single doses of 18F-FDG. The cyclotron is capable to deliver 7.5 MeV proton beam with maximal current of 5 μA. During irradiation, a small target volume of 18O-enriched water is bombarded with protons and the resulting neutron and gamma fields induce nuclear reactions within the target bulk, machine components and shielding, the walls of the cyclotron vault. The aim of the present work is to define the radiological characteristics of the activation products generated during the operation of the cyclotron that need to be considered for daily maintenance and for decommissioning of the facility in future. For that purpose Monte-Carlo calculations with detailed models of the vault and the machinery are used. Key words: cyclotron, target, vault, activation, radioactive waste, Monte-Carlo simulations
[en] Waters, as one of the most important components of the environment, are part of all food chains and a major route for the entry of radioactive substances into the human body. Therefore, regular and control studies of mineral water are necessary to guarantee that they have a low level of radioactivity. The objective of this study is to assess the 226Ra activity and the concentration of natural uranium (nat. U) in mineral waters from certain most frequently used sources in Bulgaria by means of nuclear and analytical methods. 226Ra is a naturally produced radioisotope with a period of half-life equal to 1600 years, whose specific activity in water was determined through analysis of the daughter product 222Rn. The measurements were carried out using a low level liquid scintillation (LS) counting in a Packard Tri-Carb 2770 TR/SL LS spectrometer. The concentration of nat. U was determined with a luminescent method based on the ability of six-valent uranium melted with sodium fluoride to produce light under UV illumination. The results show that the 226Ra activity is in the range 0.025‑0.211 Bq/L and the concentration of nat. U varies from less than 0.003 mg/L to 0.86 mg/L. Due to differences in the geological structure of the aquifers, a large difference in values of the 226Ra activity was observed, but no correlation with the nat. U concentration. The annual effective doses were calculated for all investigated waters for adult inhabitants assuming the yearly consumption of 730 L. The obtained new results are used to assess the radiation status of the investigated waters. They will support timely and adequate measures to reduce the harmful impact of ionizing radiation on the population in cases of increased radioactivity. Key words: 226Ra, natural uranium, mineral water, annual effective dose, liquid scintillation spectrometry
[en] Tritium, like the other two nonradioactive isotopes of hydrogen, is highly versatile. The expanding production and use of manmade tritium, especially in nuclear reactors, accelerators and neutron generators creates great concern about its safe processing, conditioning and long term storage. Therefore, the most important problem here is the immobilization of tritium in solid materials minimizing the leaching and the volume of the solid containment. The objective of this study is to assess the mobility of tritium from aqueous wastes fixed in solid samples. As a relatively simple and effective technique for immobilization of low and intermediate level radioactive wastes (LLW and ILW), cementation was selected for the experiments with the advantage of being a low temperature process utilizing inexpensive raw materials. The tritium radioactivity in simulated solutions was deposited in cement based matrices consisting of various types of zeolite/cement composites. Special attention was devoted to the explicit use of zeolite and cement materials of Bulgarian production. The tritium leaching was assessed by calculating the radioactivity diffusion rate from the solid sample into the water phase. Long term leaching tests were performed on different samples by measuring the corresponding tritium radioactivity in the water phase. The quantity of tritium was measured using the radioactive beta decay of tritium in helium-3 by means of liquid scintillation spectrometry. The laboratory experiments demonstrate that tritium aqueous waste could be successfully immobilized by sorption in zeolites followed by cementation. The obtained original results can be used in the development and optimization of technology procedures for long term conditioning and storage of tritium aqueous wastes. Key words: tritium aqueous waste, cementation, leaching test, zeolite, bentonite, liquid scintillation spectrometry
[en] The lifetimes of four low-lying levels of the yrast band in 168Yb, populated via the 154Sm(18O,4n) reaction, were measured for the first time by means of the recoil-distance Doppler-shift method. For the data-analysis we applied the differential decay-curve method in a version where gates are set on both shifted and unshifted components. The reduced transition probabilities indicate some reduction of the collectivity for states above the 6+ level, i.e. well below the band-crossing, which may be understood as the result of an interaction between the rotational ground-state band and yet unknown part of the two-quasiparticle ((ν13/2)) spin-aligned S-band or alternatively, as changes in the internal structure induced by the rotation.