M.A. Taylor

Assessment of natural radioactivity levels and their relationship with soil characteristics in undisturbed soils of the northeast of Buenos Aires province, Argentina

Surface and depth profile concentrations (down to 50 cm) of ²³²Th chain, ²²⁶Ra, and ⁴⁰K radionuclides were determined in undisturbed coastal and inland soils of La Plata city region, Argentina, through their gamma-ray activity using a high-purity Ge detector spectrometer. These results were compared with superficial activities determined in soils from the surroundings of the Centro Atómico Ezeiza (Ezeiza Atomic Center) located in Ezeiza, Buenos Aires Province, Argentina. The hyperfine and magnetic Fe phases properties of soil profiles were characterized by Mössbauer spectroscopy, magnetic hysteresis loops and AC magnetic susceptibility. No dependence of the activity of the ²³²Th natural chain on depth was found, whereas variations for ²²⁶Ra and ⁴⁰K activities were observed. Positive correlations, determined by the Pearson correlation coefficients, were established between ⁴⁰K, ²²⁶Ra and ²³²Th activity concentrations for the whole set of soil samples. The annual external equivalent dose for adults was similar for La Plata and Ezeiza regions, with average values of 0.08 ± 0.01 mSv and 0.06 ± 0.02 mSv, respectively. The thermal dependence of the AC magnetic susceptibility revealed the existence of magnetite and hematite. The Mössbauer spectra of all soils were made up of signals associated with α-Fe₂O₃, a paramagnetic relaxation component, and Fe³⁺ and Fe²⁺ doublets. In addition, the spectra of inland soils revealed the presence of Fe₃O₄. A negative correlation was found between the activity concentrations and the α-Fe₂O₃ and Fe₃O₄ relative fractions, whereas a positive correlation was found between the Fe³⁺ relative fraction and the ⁴⁰K activity.

Inventories and concentration profiles of 137Cs in undisturbed soils in the northeast of Buenos Aires Province, Argentina.

Inventories and vertical distribution of (137)Cs were determined in La Plata region undisturbed soils, Argentina. A mean inventory value of 891 ± 220 Bq/m(2) was established, which is compatible with the values expected from atmospheric weapon tests fallout. The study was complemented with pH, organic carbon fraction, texture and mineralogical soil analyses. Putting together Southern Hemisphere (137)Cs inventory data, it is possible to correlate these data with the mean annual precipitations. The large differences in (137)Cs concentration profiles were attributed to soil properties, especially the clay content and the pH values. A convection-dispersion model with irreversible retention was used to fit the activity concentration profiles. The obtained effective diffusion coefficient and effective convection velocity parameters values were in the range from 0.2 cm(2)/y to 0.4 cm(2)/y and from 0.23 cm/y to 0.43 cm/y, respectively. These data are in agreement with values reported in literature. In general, with the growth of clay content in the soil, there was an increase in the transfer rate from free to bound state. Finally, the highest transfer rate from free to bound state was obtained for soil pH value equal to 8.

Thermal evolution of CaO-doped HfO2 films and powders

Solid solutions of ZrO2 and HfO2 are potential electrolyte materials for intermediate-temperature SOFC because both are oxygen-ion conductors. The main challenge for these compounds is to reduce the relatively high value of the activation energies vacancies diffusion, which is influenced by several factors. In this work the thermal evolution of CaO-HfO2 materials have been investigated. (CaO)y-Hf(1-y)O(2-y) (y = 0.06, 0.14 y 0.2) coatings and powders were synthesized by chemical solution deposition (CSD). Films were deposited onto alumina substrates by Dip Coating technique, the burning of organic waste was carried out at 500 ?C under normal atmosphere and then the films were thermally treated at intervals of temperature rising to a maximum temperature of 1250 ?C. By means Glazing Incidence X-ray Diffraction (?-2? configuration) the phases were studied in the annealed films. On the other hand, the thermal evolution and crystallization process of powders were analyzed in-situ by HT-XRD. The phenomena crystallization occurred in films and powders were analyzed. The activation energies of diffusion of oxygen vacancies of HfO2-14 mole% CaO and HfO2-20 mole% CaO films were measured from the thermal evolution of the relaxation constant measured by Perturbed Angular Correlation Technique.

TDPAC study in ZnTiF6·6H2O between room temperature and 1273 K

Using the time differential perturbed angular correlation technique, the hyperfine interactions as titanium sites in ZnTiF6·6H2O and its decomposition products were determined between room temperature (300 K) and 1273 K. The different steps in the decomposition pattern were identified and a simple model is given for it.

TDPAC study of hexafluorohafnates of transition metals

The hyperfine quadrupole interaction of CoHfF6.6H2O and ZnHfF6.6H2O was determined at room temperature for the first time. The thermal behaviour of the electric field gradient at hafnium sites in the compounds AHfF6.6H2O (where A is Co, Ni or Zn) determined using the time differential perturbed angular correlations (TDPAC) technique, is presented in the range from 16 K up to the dehydration temperature. The Bayer-Kushida model has proved to give a good account of the thermal evolution of the quadrupole interaction observed. Frequencies of 371.3 cm-1, 409.5 cm-1 and 325.4 cm-1 have been drawn, which could be associated to the asymmetric stretching mode of the (HfF6)2- ions of CoHfF6.6H2O, NiHfF6.6H2O and ZnHfF6.6H2O.

Approximate total Fe content determined by Mössbauer spectrometry: Application to determine the correlation between gamma-ray-emitter activities and total content of Fe phases in soils of the Province of Buenos Aires, Argentina

Pearson correlation coefficients between 40K, 226Ra and 232Th activities and the total Fe phase fractions yielded by Mössbauer spectroscopy have been calculated for soils of the Province of Buenos Aires, Argentina. Total fractions of Fe phases have been obtained from the relative fractions reported in previous works weighted by the Fe soil content and the recoilless-fraction of each Fe phase. An approximate method based on the relationship between the Mössbauer spectral absorption area (obtained from the 57Fe Mössbauer data) and the total Fe concentration (determined by colorimetric methods, after microwave assisted acid digestion of soil samples) has been used for the first time to determine the Fe concentration in soils with an accuracy of 15%. Protocol to extend the method for unknown samples is also discussed. The determined new coefficients differ from those reported previously. A significant and positive correlation between the total fraction of Fe2+ and the 40K activity values has been obtained. This result validates the hypothesis put forward in a previous work, i.e., that illite captures the 40K existing in the soil. In addition, with the new approximation, the Pearson correlation coefficients for the other natural radionuclides give values that indicate that the methodology reported here is appropriate to study the correlations between the activity values with the total fractions of Fe phases.

Local characterization of phases and phase transitions in

The hyperfine quadrupole interaction of was determined in the room-temperature to range. The study of the thermal behaviour of the hyperfine parameters identified an extended phase transition from to and the existence of a distorted low-temperature -phase.

PAC study on the thermal evolution of the 60ZrF4–40BaF2 glass

By using 181Ta probes, the PAC technique has been used to investigates the behaviour of the hyperfine interaction in 60ZrF4–40BaF2 glass from room temperature (RT) up to the glass temperature Tg (562 K). A first serie of measurements was obtained while heating the as-prepared sample from RT to Tg. A second serie was performed on the as-treated sample after cooling it down to RT and again heated to Tg. In both series the same three components of the quadrupole interaction were observed. Dynamical effects were detected in the main component and activation energies of 0.181 eV and 0.213 eV were obtained. The results are interpreted as dynamical effects associated to the movement in the fluorine ions during polyhedra rearrangements.