1,039 research outputs found
Comparison of Transfer-to-Continuum and Eikonal Models of Projectile Fragmentation Reactions
Spectroscopic properties of nuclei are accessible with projectile
fragmentation reactions, but approximations made in the reaction theory can
limit the accuracy of the determinations. We examine here two models that have
rather different approximations for the nucleon wave function, the target
interaction, and the treatment of the finite duration of the reaction. The
nucleon-target interaction is treated differently in the eikonal and the
transfer-to-continuum model, but the differences are more significant for light
targets. We propose a new parameterization with that in mind. We also propose a
new formula to calculate the amplitude that combines the better treatment of
the wave function in the eikonal model with the better treatment of the target
interaction in the transfer-to-continuum model.Comment: 21 pages, latex file including 3 tables. 5 figures. Submitted to
Phys. Rev.
Initial State Dependence of the Breakup of Weakly Bound Carbon Isotopes
The one-neutron nuclear breakup from the Carbon isotopes C and
C, is calculated as an example of application of the theory of transfer
to the continuum reactions in the formulation which includes spin coupling.
The effect of the energy sharing between the parallel and transverse neutron
momentum distributions is taken into account thus resulting in a theory which
is more general than sudden eikonal approaches. Both effects are necessary to
understand properly the breakup from not too weakly bound orbitals.
Breakup which leaves the core into an excited state below particle threshold is
also considered. The core-target interaction is treated in the smooth cut-off
approximation. By comparing to presently available experimental data we show
how to make some hypothesis on the quantum numbers and occupancy of the neutron
initial state. Possible ambiguities in the interpretation of inclusive cross
sections are discussed.Comment: 22 RevTeX pages,3 ps figures. Phys. Rev. C, accepte
Silk reinforced with graphene or carbon nanotubes spun by spiders
Here, we report the production of silk incorporating graphene and carbon
nanotubes directly by spider spinning, after spraying spiders with the
corresponding aqueous dispersions. We observe a significant increment of the
mechanical properties with respect to the pristine silk, in terms of fracture
strength, Young's and toughness moduli. We measure a fracture strength up to
5.4 GPa, a Young's modulus up to 47.8 GPa and a toughness modulus up to 2.1
GPa, or 1567 J/g, which, to the best of our knowledge, is the highest reported
to date, even when compared to the current toughest knotted fibres. This
approach could be extended to other animals and plants and could lead to a new
class of bionic materials for ultimate applications
Celebrating Alexander von Humboldt’s 250<sup>th</sup> anniversary: Exploring bio- and geodiversity in the Andes (IBS Quito 2019)
Molecular systematics and evolution of the Cyanocorax jays
Phylogenetic relationships were studied in the genus Cyanocorax (Aves: Corvidae) and related genera, Psilorhinus and Calocitta, a diverse group of New World jays distributed from the southern United States south to Argentina. Although the ecology and behavior of some species in the group have been studied extensively, lack of a molecular phylogeny has precluded rigorous interpretations in an evolutionary framework. Given the diverse combinations of plumage coloration, size, and morphology, the taxonomy of the group has been inconsistent and understanding of biogeographic patterns problematic. Moreover, plumage similarity between two geographically disjuct species, the Tufted Jay (Cyanocorax dickeyi) from western Mexico and the White-tailed Jay (C. mystacalis) from western Ecuador and Peru, has puzzled ornithologists for decades. Here, a phylogeny of all species in the three genera is presented, based on study of two mitochondrial and three nuclear genes. Phylogenetic trees revealed the non-monophyly of Cyanocorax, and the division of the whole assemblage in two groups: “Clade A” containing Psilorhinus morio, both species in Calocitta, Cyanocorax violaceus, C. caeruleus, C. cristatellus, and C. cyanomelas, and “Clade B” consisting of the remaining species in Cyanocorax. Relationships among species in Clade A were ambiguous and, in general, not well resolved. Within Clade B, analyses revealed the monophyly of the “Cissilopha” jays and showed no evidence for a sister relationship between C. mystacalis and C. dickeyi. The phylogenetic complexity of lineages in the group suggests several complications for the understanding biogeographic patterns, as well as for proposing a taxonomy that is consistent with morphological variation. Although multiple taxonomic arrangements are possible, recommendations are for recognizing only one genus, Cyanocorax, with Psilorhinus and Calocitta as synonyms.
Se estudiaron las relaciones filogenĂ©ticas en los gĂ©neros Cyanocorax, Psilorhinus y Calocitta (Aves: Corvidae), un grupo diverso de urracas del Nuevo Mundo cuyas especies se distribuyen desde el sur de los Estados Unidos hasta Argentina. Aunque la ecologĂa y el comportamiento de algunas especies en el grupo han sido estudiadas extensamente, la falta de una filogenia molecular ha impedido la interpretaciĂłn rigurosa de estos estudios en un marco evolutivo. Dadas las diversas combinaciones de coloraciĂłn de plumaje, tamaño y morfologĂa presentes en las especies del grupo, su taxonomĂa ha sido inconsistente y la interpretaciĂłn de sus patrones biogeográficos ha sido problemática. Mas aĂşn, la similitud de plumaje en especies que están geográficamente distantes, como Cyanocorax dickeyi del oeste de Mexico y C. mystacalis del oeste de Ecuador y PerĂş, ha sido difĂcil de interpretar. Se presenta una filogenia para todas las especies en los tres gĂ©neros, basada en el estudio de dos genes nucleares y dos genes mitocondriales. Los árboles filogenĂ©ticos mostraron la parafilia de Cyanocorax y la divisiĂłn de todas las especies en dos grupos: “Clado A” en el cual se encuentran Psilorhinus morio, ambas especies Calocitta, Cyanocorax violaceus, C. caeruleus, C. cristatellus, y C. cyanomelas, y “Clado B” en el cual se encuentran el resto de las especies de Cyanocorax. Las relaciones entre especies del Clado A fueron ambiguas y, en general, poco resueltas. En el Clado B, los análisis mostraron la monofilia de las especies en “Cissilopha”, pero no indicaron la monofilia de C. mystacalis + C. dickeyi. La complejidad filogenĂ©tica de los linajes en el grupo sugiere varias complicaciones en el entendimiento de su biogeografĂa y taxonomĂa. Con base en los resultados filogenĂ©ticos se reconoce un solo gĂ©nero, Cyanocorax, con Psilorhinus y Calocitta como sinĂłnimos
Core excitation in Coulomb breakup reactions
Within the pure Coulomb breakup mechanism, we investigate the one-neutron
removal reaction of the type A(a,b)X with Be and C
projectiles on a heavy target nucleus Pb at the beam energy of 60
MeV/nucleon. Our intention is to examine the prospective of using these
reactions to study the structure of neutron rich nuclei. Integrated partial
cross sections and momentum distributions for the ground as well as excited
bound states of core nuclei are calculated within the finite range distorted
wave Born approximation as well as within the adiabatic model of the Coulomb
breakup. Our results are compared with those obtained in the studies of the
reactions on a light target where the breakup proceeds via the pure nuclear
mechanism. We find that the transitions to excited states of the core are quite
weak in the Coulomb dominated process as compared to the pure nuclear breakup.Comment: Revtex format, five postscript figures included, to appear in Phys.
Rev.
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