Sur le comportement magnétique de l'acide desoxyribonucléique (ADN) entre 80 et 300°K

Detailed cryomagnetic study has been made on two samples of deoxyribonucleic acid (DNA), prepared from calf thymus; one of them was “simple”, the other highly polymerized and of critically high purity. The susceptibility of the system may be represented over the temperature interval 80–300°K by the formula\(\left( {\chi - \chi _0 } \right)T = a\left( {1 - \frac{5}{3}x} \right)\) whereχ ₀ andα are two significant constants, characteristic of the double structure of the DNA molecule,x varying from zero to 1, between two limiting values corresponding respectively to the simple and the highly polymerized state, the most frequent. The parameterx varies spontaneously, often during the same thermomagnetic process. Extrinsic conductivity might be expected from the magnetic behavior of the system, with a number of carriers per unit mass of the order of 10¹⁹ — the radicals of the bases serving as the carriers of impurity in the double helix.     

Etude des mouvements moléculaires du furanne en phases plastique et liquide par relaxation magnétique nucléaire

Les temps de relaxation spin-réseau du proton ont été mesurés dans les phases liquide et plastique du furanne pur et en solution dans le furanne perdeutéré, les relaxations nucléaires d'origine intra et inter-moléculaire ont été ainsi séparées. Des temps de corrélation pour le second harmonique sphérique τ₂ ont été déduits de ces mesures et comparés aux temps de corrélation du premier harmonique sphérique τ₁ obtenus par des mesures antérieures d'absorption micro-onde ; les valeurs du coefficient de diffusion translationnelle sont estimées en phase liquide.

Sur la base de ces résultats et en utilisant, pour la phase plastique, les donnés cristallographiques, un modèle est proposé pour dépeindre les mouvements réorientationnels des molécules dans chacune des deux phases condensées.

Dans le liquide, près du point triple, un modèle de type Ivanov s'applique avec des angles moyens de saut voisins de 50° et des durées de piegeage orientationnel de l'ordre de 1,3 ps. En augmentant la température, les temps de résidence tendent à devenir comparables aux durées de saut ou même inférieurs.

Dans la phase plastique, les sauts réorientationnels ont lieu de façon concertée (simultanée) entre les quatre orientations permises. Un modèle analytique décrivant les mouvements d'un ensemble de rotateurs dans un potentiel de symétrie rectangulaire permet d'expliquer les résultats expérimentaux en supposant que les sauts ont lieu de façon privilégiée entre puits de potentiel non adjacents. Un calcul numérique confirme l'existence de quatre puits de potentiel séparés par de faibles barrières et montre que les corrélations d'orientation ont principalement pour origine des intéractions de type Van der Waals entre atomes d'hydrogène de molécules voisines.     

Essai d'interprétation des déplacements et élargissements par la pression des raies du spectre de vibration-rotation des molécules diatomiques

An interpretation is given of the dependence on the rotational quantum number J of the pressure shift and broadening of the lines of the vibration-rotation band of a hydracide molecule perturbed by a noble gas. It is shown that, with the fundamental hypothesis of adiabatic collisions between the absorbing molecule and the noble gas atoms, the impact theory can describe the J dependence. This results from the fact that one introduces the average over the sine of the phase shift η, which is J dependent, and not the average of the phase shift itself which is vanishing. With a hard-sphere model good agreement is obtained between the calculated and the observed shifts. Further, a rough estimation of the breadth is made.     

Effets comparés des impuretés de terres rares sur la résonance ferrimagnétique et la relaxation nucléaire dans le grenat de fer et d’yttrium

The anomalous properties of impure yttrium iron garnet are related to the susceptibilities of the rare earth ions which it contains. The susceptibilities are calculated assuming a slow relaxation. It is shown that the success of the theory of “slow relaxation” requires the existence of a coupling between the transverse magnetization of the iron lattice on the one hand, and components of the magnetic moments of the rare earth ions, with non zero diagonal matrix elements, on the other hand. The temperature variations of the ferrimagnetic linewidth and the nuclear relaxation time are compared.     

Effets produits par l'irradiation avec des neutrons et avec des rayons gammas sur la série jaune et la série verte du spectre d'absorption excitonique de la cuprite

The effects of irradiations in a reactor and in a Cobalt 60 source on the green and yellow series of the excitonic absorption spectrum of cuprous oxide are studied. In the reactor irradiation experiments, the effects of fast neutrons are separately studied from those of thermal neutrons and of gamma rays. It is shown that the perturbations of the absorption spectra of Cu₂O by fast neutrons are very similar to those produced by an external electric field. They are also similar to those produced by quenching which creates probably internal strains. Charged defects occur probably in the crystal during bombardment and these charged defects produce an internal electric field and strains. It is not possible to study separately the effects of the electric field and the effects of strains. The electric field can be evaluated by comparison of its effects on the excitonic spectrum with those of an external field. An evaluation of the rate of creation of defects by fast neutrons in the crystal is deduced. It is also shown that the effects of gamma-rays are very different from those of fast neutrons. It is possible with gamma-ray irradiations to decrease residual internal fields which may exist in a crystal.     

Quelques problèmes posés par les découvertes de Pluton et de Neptune

This Note reviews the history of the discovery of the planets Pluto and Neptune. To cite this article: V. Kourganoff, C. R. Physique 4 (2003).     

Etude par résonance paramagnétique électronique de la variation en fonction de la température du couplage hyperfin du radical tert-butyle

Le radical tert-butyle obtenu par irradiation γ à 77°k du chlorure de tertiobutyle peut se présenter sous deux structures différentes. Une forme pyramidale, ayant pour origine les contraintes exercées par le réseau cristallin, et pour laquelle le couplage a ¹³C avec le carbone central est une fonction décroissante de la température. Une forme plane pour laquelle a ¹³C croît avec la température. L'étude du radical plan a permis de séparer les contributions relatives des vibrations, a ¹³C _v , et de la polarisation des électrons des couches internes et des liaisons, a ¹³C_pol, à la valeur du couplage a ¹³C. Le résultat obtenu confirme la relative insensibilité de a ¹³C_Pol, par rapport aux détails des substituants du carbone central, prévue par les calculs de Melchior.     

Sur la complétude des variétés pseudo-riemanniennes

We discuss completeness for pseudo-riemannian manifolds, and give new examples of non-complete compact manifolds. The former are simply connected, the latter locally homogeneous.     

Étude théorique des transferts couplés de chaleur et de masse lors de la désorption du couple zéolithe 13X-eau

A mathematical model based on scale changing has been established, in order to study heat and mass transfers in a zeolithe bed during water desorption. The model has been solved numerically by the finite-volume method, for the case of two-dimensional transfers. The numerical simulation is used to present the time-space evolution of the solid temperature, the pressure and the moisture content in the reactor and to determine the sensitivity to some parameters (heating temperature, outlet temperature, heat coefficients and reactor geometry). The effects of the particle diameter, the kinetic constant of desorption as well as the effective thermal conductivity are discussed.     

Facteur de bruit d'une chaı̂ne de cinématographie ultrarapide : application à la fusion par confinement inertiel

Inertial confinement fusion simulates in a laboratory the thermodynamic state of the center of stars, thus leading to the determination of stellar parameters. In order to reach that aim, high-speed cinematography brings up instruments specifically adapted to picosecond measurement, for which it is necessary to know the final precision. A model of the noise factor of the instruments under study is introduced and confronted to the experimental results obtained.     

Analyse de la conduction de la chaleur aux temps ultra-courts dans un solide par la thermodynamique irréversible étendue et la dynamique moléculaire

Instantaneous heat propagation and thermodynamic local equilibrium cannot be assumed when solving space and time microscale problems. Therefore, we reconsider the thermodynamics basis of the Fourier law in order to obtain the new heat conduction models: the hyperbolic heat equation (EH) and the modified hyperbolic equation (EHM). We have performed molecular dynamics (DM) experiments which are independent of any thermodynamic model, to test the macroscopic approaches. We show that the solutions of the EH and the EHM do not agree with the numerical experiments and that the MD results are strongly dependent on the way from which the macroscopic conditions are simulated in the microscopic point of view.     

Polarisation de la lumière diffusée par des particules agrégées fractales

Some of the dust particles present in the solar system are likely to be aggregates. The study of polarization of the light scattered by such aggregated particles is performed for the six main formation processes. The analysis of the results shows that an irregular and fluffy structure of the particles allows a better understanding of the light scattering observations.     

Comparaison des fonctions de corrélation mono- et multimoléculaires des liquides et des solutions

Le théorème de corrélation microscopique-macroscopique correspondante est redérivé à l'aide de la théorie des groupes. La démonstration est fondée sur l'invariance de l'hamiltonien d'un échantillon de liquide sous l'effet des permutations des molécules identiques. Les conséquences et les limitations du théorème sont examinées en détail.     

Etude de l'énergie de désorption des substances polaires

Polar substance energy desorption study. This study suggests an extension of the Dubinin-Radushkevich and Gibbs-Helmholtz equations, by using adsorption forces in microporous composites. These forces, mainly those of Van der Waals, have been set up in three groups of components according to their effects :

• •unspecified forces,
• •dispersion-perturbation forces,
• •quadrupole and dipole interaction forces.

The suggested models take into account the different forces and present a good coherence with the experiment.     

Apports de la colorimétrie et de la spectroradiométrie à la caractérisation in situ des peintures paléolithiques de la grotte Chauvet (Ardèche,France)

The colour of the Palaeolithic paintings on cave walls has been studied primarily in a qualitative way and, more rarely, from the physical angle. Our approach is based on the production of colorimetric measurements in caves, by means of the acquisition of energy radiance spectra with a portable spectroradiometer that can function without contact, several metres away from the decorated walls being studied, thus respecting the constraints of conservation. The colour of parietal works is an element that needs to be taken into account for their archaeological study and conservation. But observation is insufficient and only the measurement of colour makes it possible to compare distant works on rocky supports with different colourings and surface conditions.This article shows the methodology used, and it takes stock of the experience acquired in the research programme on the Chauvet Cave. It also attempts to show the benefits of taking spectroradiometric measurements on prehistoric artworks. However, the in-situ study of their colour cannot be a substitute for chemical and mineralogical analyses and the characterization of the materials, either sampled or in place. Its interest lies in its capacity for detecting even minimal changes in condition and composition. So our objective has been to establish comparisons between the graphic entities to identify the origins of the variations detected in their optical signatures. These variations can be linked to the nature of the colouring materials, but also to their position on the wall, a consequence of the application methods and sometimes also of later removals through taphonomic processes such as water run-off. Alterations (patinas) and coverings (notably veils of calcite or clay) have also been taken into account in our analysis of the data. Hence, we carried out experiments in calcification of surfaces to evaluate the case – quite frequent in caves – of calcite covering a coloured surface, outside of any other phenomenon of alteration. The first results lead us assume that it is possible to identify the signature of a veil of calcite on given colouring materials and also to present hypotheses about the nature of colouring materials covered by a veil of calcite.     

Transfert de chaleur par rayonnement dans un four d'incinération industriel: application de la méthode des zones

In the context of resource conservation, incineration of domestic wastes is a technique allowing an interesting potential in energy valorization. However, because of the variability (annual, monthly) in quality and quantity of raw material, one of the characteristics of waste incineration cogeneration power plants is their frequent partial or excess loading. In order to describe their behaviour at these non-nominal loads, there is a need for simulation tools which predict with accuracy the behaviour of the principal components of the plant at these loads. This paper presents an original approach for predicting the heat transfer in a waste incinerator. In particular, the model developed is applied to and validated with measurements from a waste incinerator located at the Cottendart waste incineration cogeneration power plant in Switzerland. The part of the incinerator studied is characterized by high gas temperatures (1 000-500 °C) for which radiative heat transfer represents the most important part of the global heat exchange. A three-dimensional model using nodal analysis is proposed. The radiative heat exchange is treated using the zone method. With a fine enough mesh, the behaviour of the system in regions far from the nominal point can be predicted with very high accuracy. This paper describes the model and the simulation results.