Fission des noyaux lourds et superfluidité nucléaire

École thématiqu

The Quasi-Molecular Stage of Ternary Fission

We developed a three-center phenomenological model,able to explain qualitatively the recently obtained experimental results concerning the quasimolecular stage of a light-particle accompanied fission process. It was derived from the liquid drop model under the assumption that the aligned configuration, with the emitted particle between the light and heavy fragment, is reached by increasing continuously the separation distance, while the radii of the heavy fragment and of the light particle are kept constant. In such a way,a new minimum of a short-lived molecular state appears in the deformation energy at a separation distance very close to the touching point. This minimum allows the existence of a short-lived quasi-molecular state, decaying into the three final fragments.The influence of the shell effects is discussed. The half-lives of some quasimolecular states which could be formed in the $^{10}$Be and $^{12}$C accompanied fission of $^{252}$Cf are roughly estimated to be the order of 1 ns, and 1 ms, respectively.Comment: 12 pages, 6 epsf, uses ws-p8-50x6-00.cl

Evidence for nucleon pair breaking even in the coldest scission configurations of 234U and 236U

Primary fragment mass yields, measured at the highest values of the kinetic energy released in the thermal neutron induced fission of 233U and 235U, do not reveal any preference for splits into two even-mass nuclei. Moreover our data suggest that the mechanism of pair breaking would take place before scission, namely that the low energy fission would be a strongly dissipative process.Les rendements des masses des fragments primaires, mesurés pour les valeurs les plus élevées de l'énergie cinétique libérée dans la fission de 233U et 235U induite par neutrons thermiques, ne révèlent aucune préférence pour les fragmentations en deux noyaux de masse paire. Nos données suggèrent en outre que le mécanisme de brisure de paires interviendrait avant la scission, ce qui revient à dire que la fission à basse énergie serait un processus fortement dissipatif

Herbaceous angiosperms are not more vulnerable to drought-induced embolism than angiosperm trees

The water transport pipeline in herbs is assumed to be more vulnerable to drought than in trees due to the formation of frequent embolisms (gas bubbles), which could be removed by the occurrence of root pressure, especially in grasses. Here, we studied hydraulic failure in herbaceous angiosperms by measuring the pressure inducing 50% loss of hydraulic conductance (P50) in stems of 26 species, mainly European grasses (Poaceae). Our measurements show a large range in P50 from 20.5 to 27.5 MPa, which overlaps with 94% of the woody angiosperm species in a worldwide, published data set and which strongly correlates with an aridity index. Moreover, the P50 values obtained were substantially more negative than the midday water potentials for five grass species monitored throughout the entire growing season, suggesting that embolism formation and repair are not routine and mainly occur under water deficits. These results show that both herbs and trees share the ability to withstand very negative water potentials without considerable embolism formation in their xylem conduits during drought stress. In addition, structure-function trade-offs in grass stems reveal that more resistant species are more lignified, which was confirmed for herbaceous and closely related woody species of the daisy group (Asteraceae). Our findings could imply that herbs with more lignified stems will become more abundant in future grasslands under more frequent and severe droughts, potentially resulting in lower forage digestibility.

Acquisition de données sur bande magnétique compatible ordinateur

A recent system of statistical multiparametric data acquisition on magnetic tape is described. The buffer store can accomodate 72 digits at most. The use of a incremental magnetic recorder allows one to obtain a computer magnetic tape directly and to improve by a factor 30 the acquisition rate of classical paper tape punchers.Nous décrivons un ensemble récent d'acquisition directe sur bande magnétique de données statistiques multiparamétriques (72 digits maximum). L'utilisation d'un enregistreur magnétique incrémental permet de créer des bandes compatibles avec les calculateurs et de multiplier par 30 le taux d'acquisition des classiques perforateurs de bande de papier

Acquisition de données sur bande magnétique compatible ordinateur

Nous décrivons un ensemble récent d'acquisition directe sur bande magnétique de données statistiques multiparamétriques (72 digits maximum). L'utilisation d'un enregistreur magnétique incrémental permet de créer des bandes compatibles avec les calculateurs et de multiplier par 30 le taux d'acquisition des classiques perforateurs de bande de papier

Sélecteur mécanique pour éliminer la contamination due aux diffractions d'ordre supérieur, au spectromètre a cristal

A mechanical velocity selector has been calculated and constructed to be use d in connection with a neutron crystal spectrometer, to remove the higher order coherently scattered neutrons. Total cross sections of Neodymium, Iridium and Thorium have been measured with this equipment.Nous avons calculé et réalisé un sélecteur mécanique destiné à éliminer les neutrons pouvant subir les réflexions de Bragg d'ordre supérieur à 1. Cet appareil a permis des mesures de sections efficaces totales du Néodyme, Iridium et Thorium dans le domaine des basses énergies