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  • Author or Editor: M. Arribére 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  

The feasibility of validating data obtained in the determination of chlorine, potassium, and calcium by neutron capture activation analysis, through their determination using threshold reactions has been studied. To this purpose the authors experimentally determined the following fission neutron spectrum averaged cross sections (in mb):37Cl(n,p)37S, 0.234±0.015;41K(n,α)38Cl, 0.740±0.044;42Ca(n,p)42K, 3.09±0.22;43Ca(n,p)43K, 2.27±0.12;44Ca(n,p)44K, 0.074±0.013. The application of these values to the analysis of some reference materials are presented. Also, the problem of mutual interferences has been addressed for the following pairs:36S(n,Γ)37S and37Cl(n,p)37S;37Cl(n,Γ)38Cl and41K(n,α)38Cl;41K(n,ψ)42K and42Ca(n,p)42K.

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Abstract  

The235U fission neutron spectrum averaged cross section for the50Ti(n,)47Ca reaction was experimentally determined by irradiation of titanium with reactor neutrons. A value of (9.7±1.1) b was found for this cross section, using (307±11) b for the48Ti(n,p)48Sc spectrum-averaged cross section that was used as a standard. The50Ti(n,)47Ca spectrum-averaged cross section was also semiempirically evaluated by numerically integrating, through the ENDF/B-V representation of the235U fission neutron spectrum, the available experimental differential cross section data.

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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  

Five species of epiphytic lichens were sampled from urban and periurban areas of Bariloche, a non industrial city located on the East side of the Nahuel Huapi National Park, North-Western Patagonia, Argentina, and from pristine areas of the Park. Each individual lichen was analyzed using Instrumental Neutron Activation Analysis. The results of the analysis showed high concentrations of lithophile elements, with very good correlations among them. The relative concentration of these elements in all lichen species is in good agreement with the elemental composition of sediments collected in the Park. Elemental concentrations which are not related to detritical inputs are also presented.

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Abstract  

The fission neutron spectrum averaged cross-sections for the reactions70Ge(n,p)70Ga and74Ge(n,p)74Ga have been determined. The averages of four determinations are, respectively, (3.10±0.30) mb and (0.00938±0.00059) mb. The present values are, to the authors' knowledge, the first experimental data on the fission averaged cross-section for these reactions.

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Abstract  

We investigated the use of threshold reactions on magnesium, aluminum, silicon and phosphorus to determine elemental concentrations and also to correct analytical interferences, using the parametric method. The reactor neutron spectrum averaged cross sections of the threshold reactions on these elements were referred to the (111±3) mb value for the 58Ni(n,p)58g+mCo reaction cross section. The results of the cross section measurements are: 25Mg(n,p)25Na, 1.57±0.12 mb; 28Si(n,p)28Al, 5.68±0.25 mb; 29Si(n,p)29Al, 3.02±0.15 mb; 30Si(n,)27Mg, 0.1346±0.0090 mb; and 31P(n,)28Al, 1.912±0.083 mb. The values for the cross sections of the following reactions: 24Mg(n,p)24Na, 1.441±0.092 mb; 27Al(n,p)27Mg, 3.84±0.18 mb; and 27Al(n,)24Na 0.721±0.028 mb were re-measured to check the methodology. We present the application of these values to the analysis of some reference materials.

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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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