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.
Authors:S. Ribeiro Guevara, M. Arribére, and A. Kestelman
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.
Authors:S. Ribeiro Guevara, D. Bubach, and M. Arribére
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.
Authors:R. Daga, S. Ribeiro Guevara, M. L. Sánchez, and M. Arribére
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.
Authors:M. Arribére, S. Guevara, A. Kestelman, and I. Cohen
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.
Authors:M. Arribére, I. Cohen, A. Kestelman, and S. Guevara
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.
Authors:Ribeiro Guevara S, A. Rizzo, R. Sánchez, and M. Arribére
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.
Authors:P. Suarez, S. Guevara, M. Arribére, and A. Kestelman
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.
Authors:S. Ribeiro Guevara, A. Rizzo, R. Sánchez, and M. Arribére
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.