M. Jurado Vargas

Radioactivity in bottled mineral waters.

Consumption of bottled mineral water is a growing practice and is sometimes a necessity rather than a choice. In this work, a study of the radioactive content of a wide selection of commercial bottled mineral waters for human intake was carried out. The origins of the analyzed waters were very different, coming from various locations in France, Portugal and Spain. Their total alpha and beta activity concentrations were determined and also gamma spectrometry was used to detect some radionuclides. In some cases, the waters presented high values of the total alpha and beta activity concentrations surpassing the reference levels established by the CSN, the Spanish. Regulatory Organization. In these cases, a determination of uranium and 226Ra was also performed by using low-level liquid scintillation counting. The results revealed a strong correlation between radioactive content and dry residue, and lead one to conclude that high radioactive content is mainly related to the mineralization in waters of underground origin.

Monte Carlo simulation of the self-absorption corrections for natural samples in gamma-ray spectrometry.

Gamma-ray self-attenuation corrections in the energy range 60-2000 keV were evaluated by means of Monte Carlo calculations for environmental samples in a cylindrical measuring geometry. The dependence of the full-energy peak efficiency on the sample density was obtained for some particular photon energies and, as a result, the corresponding self-attenuation correction factors were obtained. The calculations were performed by assuming that natural materials have mass attenuation coefficients very similar to those of water in the energy range studied. Three different HpGe coaxial detectors were considered: an n-type detector with 44.3% relative efficiency and two p-type detectors of relative efficiencies 20.0% and 30.5%. Our calculations were in very good agreement with the self-attenuation correction factors obtained experimentally by other workers for environmental samples of different densities. This work demonstrates the reliability of Monte Carlo calculations for correcting photon self-attenuation in natural samples. The results also show that the corresponding correction factors are essentially unaffected by the specific coaxial detector used.

SIMPLEX method for optimization of experiments Application to electrodeposition in alpha spectrometry

Abstract A common task in many fields of science is to search for the optimal values of variables giving the best results in an experiment which depends on several variables. A procedure based on the SIMPLEX minimization method is described which allows the simultaneous variation of several variables to obtain the best results while performing only a few experiments. The method is general and can be applied to any experimental work involving optimization of an unknown function depending on several variables. The application made in the present work is to finding the best result for the energy resolution in the electrodeposition process used in preparing sources to be measured by alpha spectrometry.

Testing efficiency transfer codes for equivalence.

Four general Monte Carlo codes (GEANT3, PENELOPE, MCNP and EGS4) and five dedicated packages for efficiency determination in gamma-ray spectrometry (ANGLE, DETEFF, GESPECOR, ETNA and EFFTRAN) were checked for equivalence by applying them to the calculation of efficiency transfer (ET) factors for a set of well-defined sample parameters, detector parameters and energies typically encountered in environmental radioactivity measurements. The differences between the results of the different codes never exceeded a few percent and were lower than 2% in the majority of cases.

Efficiency transfer in the calibration of a coaxial p-type HpGe detector using the Monte Carlo method.

Monte Carlo simulation was applied to the efficiency transfer exercise described in the EUROMET428 project (Appl. Radiat. Isot. 55 (2001) 493), evaluating the peak efficiencies in the energy range 60-2000 keV for a typical coaxial p-type HpGe detector and several types of source configuration: point sources located at various distances from the detector and a cylindrical box containing three matrices. The efficiency values were derived in two ways: (a) by direct calculation taking into account the physical dimensions of the detector provided by the supplier, and (b) by means of relative computation (efficiency transfer) taking also into consideration the known efficiency values for a reference point source. As expected, some significant discrepancies between the calculated and experimental values were found when a direct computation was made using the data provided by the supplier. On the contrary, the results for the peak efficiency derived by relative calculation by means of an efficiency transfer were in good agreement with the experimental values. The deviations found with this last procedure were generally below 5% for all the geometries considered, which is entirely satisfactory for the purposes of routine measurements.

Distribution of uranium and thorium in sediments and plants from a granitic fluvial area

Abstract A study of the presence of natural uranium and thorium isotopes in sediments and plants belonging to a granitic fluvial region of the Ortigas river (west of Spain) has been carried out. The existence of two uranium mines in the neighbourhood of the sampled sites and the granitic characteristics of the zone produce significant concentrations of natural radionuclides. Temporal and spatial variations of uranium and thorium concentrations and the activity ratios 234 U/ 238 U, 228 Th/ 232 Th and Th/U were studied to better understand the mobilization mechanisms such as leaching and transport at play in the studied system. These determinations were made using alpha-particle spectrometry with silicon detectors. The measurements were also compared with the results previously found for water of this fluvial area. Uranium in sediments showed variations due to changes in rainfall, but thorium content was nearly constant. Uranium and thorium concentrations in plants were lower after rainfall. Incorporation of uranium into the plants seemed to be mainly from water, whereas incorporation of thorium seemed to be from both sediments and water.

Yields and losses at each step in preparing uranium and thorium samples for alpha spectrometry

Abstract The percentages of activity lost at each step in a common method for determination of uranium and thorium in aqueous environmental samples were investigated. The method studied involves the concentration of the sample (in which the actinides are co-precipitated with an iron hydroxide carrier), extraction by di-isopropylether, separation of uranium and thorium using an ion-exchange procedure, and, finally, electrodeposition using sulphuric acid as electrolyte. The results suggest that the main optimization of the total yield must be done at the concentration step, because around 50% of initial activity is lost here.

Application of ion transport simulation to the backscattering in α-particle sources

Abstract The standardization of α-particle emitting sources with 2π detectors requires corrections for backscattering from the backing material. The Monte Carlo computer code SRIM, developed to simulate the transport of ions in matter, was applied to the study of the backscattering of α-particles in different materials. Simulations of 5.3 MeV α-particles from 210 Po were performed for various backing materials with atomic numbers ranging from 13 to 79. In addition, some experiments using a 2π detector and a well-defined low solid angle chamber with a silicon detector were also performed. The experimental backscattering coefficients for 241 Am α-particle sources in Pt and Ag supports and for 239 Pu αs in stainless steel and quartz backing materials were determined. There was reasonable agreement between calculations and experimental results (carried out by us and by other laboratories). This work shows that Monte Carlo simulation of the transport of ions can be a very useful tool to evaluate the backscattering corrections in the standardization of α-particle sources.

Gamma and alpha spectrometry for natural radioactive nuclides in the spa waters of Extremadura (Spain)

Abstract Gamma and alpha spectrometry of waters from several spas of Extremadura (Spain) were performed in order to determine their radioactivity. For the gamma spectrometry, the sample was measured directly using an HpGe detector with a Marinelli beaker. The nuclide 222Rn was determined by measuring the emissions from its daughters 214Pb and 214Bi and assuming transient equilibrium. Each sample was measured for several days in order to check that 214Pb and 214Bi decay with the half-life of 222Rn. Concentrations of 222Rn ranged from 3.5 to 234 Bq/l. Alpha spectrometry of uranium, thorium and 226Ra was performed by chemical separation of these elements from the samples and then using PIPS detectors for the measurement of the sources. Uranium concentrations were in general below 1 μg/l (with one exception), thorium specific activities were of the order of 10−5 Bq/l, and 226Ra activity ranged from 4.5 to 71 mBq/l. These results allow some conclusions to be established about the environmental behaviour of these radionuclides.

A rapid method for determination of the isotopic composition of uranium samples by alpha spectrometry

Abstract A simple method of analyzing alpha spectra from natural and enriched or depleted uranium samples is developed. The procedure is non-iterative, and takes into consideration low-energy tail and branching-ratio corrections to accurately calculate the area corresponding to each uranium isotope ( 234 U, 235 U, 236 U, 238 U) in the spectrum, and then the isotopic composition of the sample. A BASIC computer program, called ENURA, has been developed to perform all the necessary calculations to give the results together with their uncertainties. Several samples were prepared with different uranium concentrations made from standard solutions with known compositions, and the method was checked against the experimental measurements from these samples. Other series of uranium spectra were theoretically constructed using a given line shape in order to cover the required range of enriched or depleted uranium.