3,616 research outputs found
Charge and energy dependence of the residence time of cosmic ray nuclei below 15 GeV/nucleon
The relative abundance of nuclear species measured in cosmic rays at Earth has often been interpreted with the simple leaky box model. For this model to be consistent an essential requirement is that the escape length does not depend on the nuclear species. The discrepancy between escape length values derived from iron secondaries and from the B/C ratio was identified by Garcia-Munoz and his co-workers using a large amount of experimental data. Ormes and Protheroe found a similar trend in the HEAO data although they questioned its significance against uncertainties. They also showed that the change in the B/C ratio values implies a decrease of the residence time of cosmic rays at low energies in conflict with the diffusive convective picture. These conclusions crucially depend on the partial cross section values and their uncertainties. Recently new accurate cross sections of key importance for propagation calculations have been measured. Their statistical uncertainties are often better than 4% and their values significantly different from those previously accepted. Here, these new cross sections are used to compare the observed B/C+O and (Sc to Cr)/Fe ratio to those predicted with the simple leaky box model
Source spectral index of heavy cosmic ray nuclei
From the energy spectra of the heavy nuclei observed by the French-Danish experiment on HEAO-3, the source spectra of the mostly primary nuclei (C, O, Ne, Mg, Si, Ca and Fe) in the framework of an energy dependent leaky box model (Engelmann, et al., 1985) were derived. The energy dependence of the escape length was derived from the observed B/C and sub-iron/iron ratios and the presently available cross sections for C and Fe on H nuclei (Koch-Miramond, et al., 1983). A good fit to the source energy spectra of all these nuclei was obtained by a power law in momentum with an exponent gamma = -2.4+0.05 for the energy range 1 to 25GeV/n (Engelmann, et al., 1985). Comparison with data obtained at higher energy suggested a progressive flattening of these spectra. More accurate spectral indices are sought by using better values of the escape length based on the latest cross section measurements (Webber 1984, Soutoul, et al., this conference). The aim is also to extend the analysis to lower energies down to 0.4GeV/n (kinetic energy observed near Earth), using data obtained by other groups. The only nuclei for which a good data base is possessed in a broad range of energies are O and Fe, so the present study is restricted to these two elements
Cryopreservation of cell suspensions and embryogenic calluses of Citrus using a simplified freezing process
Une méthode de congélation simplifiée a été expérimentée avec une suspension cellulaire de mandarine commune et des cals embryogènes de six variétés de #citrus$. Son efficacité est comparable à celle du protocole de congélation standard développé précédemment pour ces matériels, qui nécessitait l'emploi d'un congélateur programmable. La suspension cellulaire peut être congelée sans modifier les conditions originales (0,15 M saccharose +5 % DMSO). Les cals embryogènes de 5 variétés sur les 6 expérimentées résistent à la congélation avec le procédé simplifié. Les résultats optimaux sont obtenus en augmentant la concentration en DMSO à 10 ou 15%. (Résumé d'auteur
Cosmic ray spectral hardening due to dispersion in the source injection spectra
Recent cosmic ray (CR) experiments discovered that the CR spectra experience
a remarkable hardening for rigidity above several hundred GV. We propose that
this is caused by the superposition of the CR energy spectra of many sources
that have a dispersion in the injection spectral indices. Adopting similar
parameters as those of supernova remnants derived from the Fermi -ray
observations, we can reproduce the observational CR spectra of different
species well. This may be interpreted as evidence to support the supernova
remnant origin of CRs below the knee. We further propose that the same
mechanism may explain the "ankle" of the ultra high energy CR spectrum.Comment: 5 pages, 3 figures and 1 table. Updated with the diffusion
propagation model, accepted by Phys. Rev.
Blurring Borders: Innate Immunity with Adaptive Features
Adaptive immunity has often been considered the
penultimate of immune capacities. That system is now being
deconstructed to encompass less stringent rules that govern its
initiation, actual effector activity, and ambivalent results.
Expanding the repertoire of innate immunity found in all
invertebrates has greatly facilitated the relaxation of
convictions concerning what actually constitutes innate and
adaptive immunity. Two animal models, incidentally not on the line
of chordate evolution (C. elegans and
Drosophila), have contributed enormously to defining
homology. The characteristics of specificity and
memory and whether the antigen is pathogenic or nonpathogenic
reveal considerable information on homology, thus
deconstructing the more fundamentalist view. Senescence, cancer,
and immunosuppression often associated with mammals that possess
both innate and adaptive immunity also exist in invertebrates
that only possess innate immunity. Strict definitions become
blurred casting skepticism on the utility of creating rigid
definitions of what innate and adaptive immunity are without
considering overlaps
Correspondence : Chimpanzee helping is real, not a byproduct
In their recent study, Tennie et al.1 argue that positive instances of chimpanzees helping others can be a byproduct of testing methods1. The study includes a new task where chimpanzees can behave prosocially toward a conspecific either through an action (GO-condition) or by omission (NO-GO condition). The study further aims to test whether stimulus enhancement or carry-over effects from prior experiences explain previous results. We agree that a helping-by-omission task could in principle provide intriguing new evidence for chimpanzee helping. However, here we raise a number of crucial methodological issues that question the current interpretation of the study’s results. Furthermore, the study fails to consider the evidence from prior work addressing these concerns
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