3.5 MeV triton activation for elements with Z<34

The sensitivities for the determination of 25 elements from Z=4 to Z=33, using 3.5 MeV triton activation, were calculated from experimentally measured yields for 50 radioisotopes obtained via (t, n), (t, d), (t, 2n) or (t, α) reactions. For an irradiation of 1 hour at 1 μA, the calculated detection limits are better than 100 ppb for B, N, O, F, Na, Mg, Al, Si an S. Because of the Coulomb barrier, triton activation is of considerable interest for non destructive analysis of low Z elements in medium and high Z matrices. Finally integrated activation curves of high sensitivity elements were determined.     

Triton reactions with some light and medium elements. Application to activation analysis

The excitation functions for nuclear reactions induced by tritons are determined up to about 3.5 MeV for some light and medium elements: fluorine, sodium, magnesium, aluminium, silicon, titanium and vanadium. Because of large cross-sections the activation technique was used with thin targets. Then the possibilities of using these reactions to analytical purposes are discussed.     

Contrôle de la pureté d'échantillons de niobium et de tantale par spectrométrie gamma directe après irradiation au moyen de protons de 10 MeV

Non-destructive analysis of samples of niobium and tantalum has been achieved by activation with 10-MeV protons and γ-ray spectrometry. Niobium and molybdenum have been detected and determined in tantalum, as well as molybdenum and tungsten in niobium. Upper limits of concentration have been established for over thirty other undetected elements; most of these limits are below the p.p.m. level, and some reach the p.p.b. level.     

Water waves as a spatial dynamical system; infinite depth case

We review the mathematical results on traveling waves in one or several superposed layers of potential flow, subject to gravity, with or without surface and interfacial tension, where the bottom layer is infinitely deep. The problem is formulated as a "spatial dynamical system," and it is shown that the linearized operator of the resulting reversible system has an essential spectrum filling the real line. We consider three cases where bifurcation occurs. (i) The first case is when, in moving a parameter, two pairs of imaginary eigenvalues merge into one pair of double eigenvalues, and then split into four symmetric complex conjugate eigenvalues. (ii) The second case is when one pair of imaginary eigenvalues meet in 0, and disappear; (iii) the third case is when the phenomenon described in (ii) is superposed to the presence of another pair of imaginary eigenvalues sitting at finite distance from 0. We give a physical example for each case and more specially study the solitary waves and generalized solitary waves, emphasizing the differences, in the methods and in the results, between these cases and the finite depth case.     

Analyse d'echantillons de cobalt par spectrometrie gamma directe apres irradiation au moyen de protons de 10 mev

Proton activation is applied to the analysis of cobalt samples; 41 elements were only occassionally detected. For the other elements, detection limits were irradiation with 10-MeV protons. Samples from two commercial Companies were analysed: Fe and Cu were always detected, while Ca, Ti, Cr, Zn, As, Se and Mo were only occasionally detected. For the other elements, detection limits were calculated; 33 limits are below the part per million level.     

The titration of carboxyl-terminated monolayers revisited: in situ calibrated fourier transform infrared study of well-defined monolayers on silicon

The acid-base equilibrium at the surface of well-defined mixed carboxyl-terminated/methyl-terminated monolayers grafted on silicon (111) has been investigated using in situ calibrated infrared spectroscopy (attenuated total reflectance (ATR)) in the range of 900-4000 cm (-1). Spectra of surfaces in contact with electrolytes of various pH provide a direct observation of the COOH <--> COO (-) conversion process. Quantitative analysis of the spectra shows that ionization of the carboxyl groups starts around pH 6 and extends over more than 6 pH units: approximately 85% ionization is measured at pH 11 (at higher pH, the layers become damaged). Observations are consistently accounted for by a single acid-base equilibrium and discussed in terms of change in ion solvation at the surface and electrostatic interactions between surface charges. The latter effect, which appears to be the main limitation, is qualitatively accounted for by a simple model taking into account the change in the Helmholtz potential associated with the surface charge. Furthermore, comparison of calculated curves with experimental titration curves of mixed monolayers suggests that acid and alkyl chains are segregated in the monolayer.     

High-energy limit of massless Dirac fermions in multilayer graphene using magneto-optical transmission spectroscopy

We have investigated the absorption spectrum of multilayer graphene in high magnetic fields. The low-energy part of the spectrum of electrons in graphene is well described by the relativistic Dirac equation with a linear dispersion relation. However, at higher energies (>500 meV) a deviation from the ideal behavior of Dirac particles is observed. At an energy of 1.25 eV, the deviation from linearity is approximately 40 meV. This result is in good agreement with the theoretical model, which includes trigonal warping of the Fermi surface and higher-order band corrections. Polarization-resolved measurements show no observable electron-hole asymmetry.     

Résonance 1 : 2 pour les champs de vecteurs réversibles1 : 2 resonance for reversible vector fields

We study the 1:2 resonance for reversible vector fields in $\text{R}{}^4$, depending on a parameter μ and we show the bifurcation of two families of periodic solutions and of two reversible solutions which are homoclinic to some of the periodic solutions belonging to one or the other of the previous families, depending on the sign of a coefficient. To cite this article: M. Barrandon, C. R. Acad. Sci. Paris, Ser. I 334 (2002) 7–10     

Accurate determination of lithium,boron, fluorine and sodium in some matrices using low energy alpha-particles induced gamma-rays

A method is described for analyzing some light elements with high accuracy. Prompt gamma-ray spectrometry with 3.5 MeV alpha-particles from a Van de Graaff accelerator was used: this technique is non-destructive, rapid and experimentally simple. Sensitivities are indicated and factors affecting the accuracy of analysis are considered. A range of different materials has been analyzed and analytical results are presented.