Diffusion coulombienne multiple a haute énergie dans l'emulsion nucléaire

L'étude de la diffusion coulombienne multiple à haute énergie et à grandes longueurs de cellule pose le problème de la validité du facteur de diffusion K introduit de manière théorique par Voyvodic et Pickup. Les émulsions nucléaires K 5, exposées à des faisceaux de muons de haute énergie (6 GeV et 12 GeV) constituent un matériel de choix pour une telle étude. Nous avons ainsi pu déterminer expérimentalement la valeur de K pour des cellules de 1 mm à 15 mm et compléter des résultats encore parcellaires. Dans ce travail, nous étudions également plusieurs méthodes de mesure par diffusion coulombienne multiple

Ralstonia syzygii, the Blood Disease Bacterium and Some Asian R. solanacearum Strains Form a Single Genomic Species Despite Divergent Lifestyles

The Ralstonia solanacearum species complex includes R. solanacearum, R. syzygii, and the Blood Disease Bacterium (BDB). All colonize plant xylem vessels and cause wilt diseases, but with significant biological differences. R. solanacearum is a soilborne bacterium that infects the roots of a broad range of plants. R. syzygii causes Sumatra disease of clove trees and is actively transmitted by cercopoid insects. BDB is also pathogenic to a single host, banana, and is transmitted by pollinating insects. Sequencing and DNA-DNA hybridization studies indicated that despite their phenotypic differences, these three plant pathogens are actually very closely related, falling into the Phylotype IV subgroup of the R. solanacearum species complex. To better understand the relationships among these bacteria, we sequenced and annotated the genomes of R. syzygii strain R24 and BDB strain R229. These genomes were compared to strain PSI07, a closely related Phylotype IV tomato isolate of R. solanacearum, and to five additional R. solanacearum genomes. Whole-genome comparisons confirmed previous phylogenetic results: the three phylotype IV strains share more and larger syntenic regions with each other than with other R. solanacearum strains. Furthermore, the genetic distances between strains, assessed by an in-silico equivalent of DNA-DNA hybridization, unambiguously showed that phylotype IV strains of BDB, R. syzygii and R. solanacearum form one genomic species. Based on these comprehensive data we propose a revision of the taxonomy of the R. solanacearum species complex. The BDB and R. syzygii genomes encoded no obvious unique metabolic capacities and contained no evidence of horizontal gene transfer from bacteria occupying similar niches. Genes specific to R. syzygii and BDB were almost all of unknown function or extrachromosomal origin. Thus, the pathogenic life-styles of these organisms are more probably due to ecological adaptation and genomic convergence during vertical evolution than to the acquisition of DNA by horizontal transfer

DIFFUSION INELASTIQUE DE MUONS DE 12 GeV/c

Nous avons étudié la diffusion inélastique de muons de 12 GeV/c, à l'aide d'émulsions nucléaires K5. La section efficace totale de muo-production obtenue est de (13,4 ± 1,2) µb par nucléon. Ce résultat expérimental ne semble pas s'interprêter avec la contribution moyenne des photons scalaires déterminée à partir des électrons (7).Inelastic interactions induced by 12 GeV/c muons were studied by using K5 emulsions. The total muoproduction cross-section is determined to be (13.4 ± 1.2) µb per nucleon. It seems it cannot be explained with the average scalar photon contribution obtained from electrons (7)