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  • Author or Editor: S. Ribeiro Guevara x
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Abstract  

Historical records of short lived (140Ba, 131I, 103Ru and 95Zr+95Nb) and long-lived (137Cs and 90Sr) fission products by fallout measurements performed in Argentina since 1959 were analyzed in order to define the main characteristics of 137Cs fallout time evolution in the Nahuel Huapi National Park, Patagonia, Argentina. Sedimentary cores were sampled from Lake Nahuel Huapi and Lake Morenito, which are located within Nahuel Huapi National Park. 137Cs specific activity profiles were measured and 210Pb dating was performed in each core. The time evolution of 137Cs fallout shows different characteristics than records taken in the Northern Hemisphere.137Cs specific activity profiles of the cores studied reproduce the fallout time sequence observed in the historical records, and the chronology obtained shows excellent agreement with 210Pb dating.

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Abstract  

Mercury and other elements of interest are determined inlichens collected in Nahuel Huapi National Park, Northern Patagonia, Argentina. Pooled samples are analyzed by instrumental neutron activation analysis. Mercury contents in Usnea sp. collected from undisturbed sites range from 0.0558±0.0083 to 1.38±0.18 µg·g−1. Other potential pollutants are identified by the analysis of Usnea sp. samples, namely Sb, As, Br, Zn, and Se. Previous experiments with foliose and fruticose lichens are also discussed. The analysis of mercury contents of foliose lichens sampled from urban and periurban sites of Bariloche city, and from undisturbed regions, demonstrate that the atmosphere of Bariloche city is enriched in mercury compared to the surroundings. The result is confirmed by transplantation experiments from undisturbed zones to urban sites.

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Abstract  

The reaction cross sections averaged over a 235U fission neutron spectrum have been measured for the 54Fe(n, 2n)53gFe and 54Fe(n, 2n)53mFe threshold reactions. The values found are, respectively: (1.14±0.13) mb, and (0.52±0.16) mb. The measured cross sections are referred to the (111±3) mb standard cross section of the 58Ni(n, p)58m+gCo reaction. The (81.7±2.2) mb standard cross section value for the 54Fe(n, p)54Mn reaction, was also used as a monitor to check the results obtained with the Ni standard, leading to an excellent agreement.

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Abstract  

For three high threshold reactions, we have measured the cross sections averaged over a 235U fission neutron spectrum. The measured reactions, and corresponding averaged cross sections found, are: 127I(n,2n)126I, (1.36±0.12) mb; 90Zr(n,2n)89mZr, (13.86±0.83) mb; and 58Ni(n,d+np+pn)57Co, (274±15) b; all referred to the well known standard of (111±3) mb for the 58Ni(n,p)58m+gCo averaged cross section. The measured cross sections are of interest in nuclear engineering for the characterization of the fast neutron component in the energy distribution of reactor neutrons.

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Summary  

Heavy metal profiles of short sediment cores sampled from lakes located in Nahuel Huapi National Park, Northern Patagonia, Argentina, were determined by INAA. Core dating was performed by measuring natural 210Pb and anthropogenic 137Cs, and by identification of tephra layers. No evidence was found for the input increase of the trace elements Sb, Ba, Cs, Zn, Co, Hf, Ni, Se, Sr, Ti, U, and V in the lake environment. High As concentrations (up to 250 μg . g-1) were found in relation with the diffusion processes of Mn and Fe oxides. Strong correlation was observed between the Br concentration and organic matter content, but no evidence was found for the relevant increase of Br inputs in recent years. Ag concentrations were found to be enriched in the upper core layers over the baseline values determined for deep layers ranging from 0.1 to 0.2 μg . g-1. Hg concentration profiles were also enriched in the upper core layers over the baseline (from less than 0.07 to 0.2 μg . g-1 for pre-industrial time, to 0.1 to 0.3 μg . g-1 for modern times) but in relation with the increased organic matter content of the sediment.

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Summary  

Volcanic ashes erupted by recent magmatic events were collected from one sedimentary sequence of lake Nahuel Huapi (Patagonia, Argentina) and then analyzed by Instrumental Neutron Activation Analysis. Primary volcanic components, including white and brownish pumice fragments, scoriae and glass shards, as well as bulk sediments from four tephra layers identified in the sedimentary sequence, were characterized by their major and trace element contents. The two tephra layers being in the upper section of the sequence can be linked to the eruptions of Cordon Caulle fissure (Puyehue-Cordon Caulle region) occurred in 1921-1922 and 1960, according to core dating. The third tephra layer, which is clearly associated to an event occurred in the 1890-1900 decade, has different compositional patterns showing a different source from those upper tephras. Whereas, the fourth, which could be associated to an event occurred at the end of the 18 th century, shows the same composition patterns as those registered for the Cordon Caulle fissure.

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Abstract  

Unsupported 210Pb fluxes were determined from sediment core inventories in lakes located in Northern Patagonia, Argentina. Total 210Pb, 226Ra, associated with supported 210Pb, and 137Cs specific activity profiles were measured by gamma-ray spectrometry. Unsupported 210Pb fluxes showed very low values when compared to other regions, with a 12 fold variation, ranging from 4 to 48 Bq m–2.y–1. The linear correlation observed between the 210Pb fluxes and137Cs cumulative fluxes in sediment cores sampled from water bodies within a zone with similar precipitation demonstrated that both radioisotopes behave in the same manner in these systems concerning the processes occurred from fallout to sediment deposition, and that there are no appreciable local or regional sources of unsupported 210Pb. Positive correlation of 210Pb fluxes with organic matter contents of the uppermost sediment core layers was also observed.

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Journal of Radioanalytical and Nuclear Chemistry
Authors:
G. Capote Rodriguez
,
R. Padilla Alvarez
,
G. Pérez Zayas
,
A. Hernández Rivero
,
M. López Reyes
,
S. Ribeiro Guevara
, and
J. Molina Insfrán

Abstract  

Thirty elements were determined by INAA in Cuban soil samples collected in 17 zones from western regions of Cuba, which were affected by neuropathy epidemic in 1992. The presence of toxic elements was evaluated to provide data for the analysis of possible paths of intoxication through local agriculture product consumption. The results obtained for As, Th and U are compared with similar data reported on literature.

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Journal of Radioanalytical and Nuclear Chemistry
Authors:
E. L. Dorval
,
M. A. Arribére
,
S. Ribeiro Guevara
,
I. M. Cohen
,
A. J. Kestelman
,
R. A. Ohaco
,
M. S. Segovia
,
A. N. Yunes
, and
M. Arrondo

Summary  

We have measured the cross sections, averaged over a 235U fission neutron spectrum, for the two high threshold reactions: 75As(n,p)75mGe and 75As(n,2n)74As. The measured averaged cross sections are 0.292±0.022 mb, referred to the 3.95±0.20 mb standard for the 27Al(n,p)27Mg averaged cross section, and 0.371±0.032 mb referred to the 111±3 mb standard for the 58Ni(n,p)58m+gCo averaged cross section, respectively. The measured averaged cross sections were also evaluated semi-empirically by numerically integrating experimental differential cross section data extracted for both reactions from the current literature. The calculations were performed for four different representations of the thermal-neutron-induced 235U fission neutron spectrum. The calculated cross sections, though depending on analytical representation of the flux, agree with the measured values within the estimated uncertainties.

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