459 research outputs found
Microscopic calculations of double and triple Giant Resonance excitation in heavy ion collisions
We perform microscopic calculations of the inelastic cross sections for the
double and triple excitation of giant resonances induced by heavy ion probes
within a semicalssical coupled channels formalism. The channels are defined as
eigenstates of a bosonic quartic Hamiltonian constructed in terms of collective
RPA phonons. Therefore, they are superpositions of several multiphonon states,
also with different numbers of phonons and the spectrum is anharmonic. The
inclusion of (n+1) phonon configurations affects the states whose main
component is a n-phonon one and leads to an appreacible lowering of their
energies. We check the effects of such further anharmonicities on the previous
published results for the cross section for the double excitation of Giant
Resonances. We find that the only effect is a shift of the peaks towards lower
energies, the double GR cross section being not modified by the explicity
inclusion of the three-phonon channels in the dynamical calculations. The
latters give an important contribution to the cross section in the triple GR
energy region which however is still smaller than the experimental available
data. The inclusion of four phonon configurations in the structure calculations
does not modify the results.Comment: Revtex4, to be published in PR
Anharmonic vibrations in nuclei
In this letter, we show that the non-linearitites of large amplitude motions in atomic nuclei induce giant quadrupole and monopole vibrations. As a consequence, the main source of anharmonicity is the coupling with configurations including one of these two giant resonances on top of any state. Two-phonon energies are often lowered by one or two MeV because of the large matrix elements with such three phonon configurations. These effects are studied in two nuclei, 40Ca and 208Pb
Reactor monitoring and safeguards using antineutrino detectors
Nuclear reactors have served as the antineutrino source for many fundamental
physics experiments. The techniques developed by these experiments make it
possible to use these very weakly interacting particles for a practical
purpose. The large flux of antineutrinos that leaves a reactor carries
information about two quantities of interest for safeguards: the reactor power
and fissile inventory. Measurements made with antineutrino detectors could
therefore offer an alternative means for verifying the power history and
fissile inventory of a reactors, as part of International Atomic Energy Agency
(IAEA) and other reactor safeguards regimes. Several efforts to develop this
monitoring technique are underway across the globe.Comment: 6 pages, 4 figures, Proceedings of XXIII International Conference on
Neutrino Physics and Astrophysics (Neutrino 2008); v2: minor additions to
reference
A non-perturbative approach to halo breakup
The theory of weakly bound cluster breakup, like halo nucleus breakup, needs
an accurate treatment of the transitions from bound to continuum states induced
by the nuclear and Coulomb potentials. When the transition probability is not
very small, a non-perturbative framework might be necessary. Nuclear excitation
dominates at small impact parameters whereas the Coulomb potential being long
range acts over a larger impact parameter interval. In this article, we propose
an effective breakup amplitude which meets a number of requirements necessary
for an accurate quantitative description of the breakup reaction mechanism.
Furthermore our treatment gives some insight on the interplay between time
dependent perturbation theory and sudden approximation and it allows to include
the nuclear and Coulomb potentials to all orders within an eikonal-like
framework.Comment: 22 Latex pages, 1 table, 8 eps figures. Accepted for publication on
Nucl. Phys.
Anharmonic vibrations in nuclei
We show that the non-linearities of large amplitude motions in atomic nuclei induce giant quadrupole and monopole vibrations. As a consequence, the main source of anharmonicity is the coupling with configurations including one of these two giant resonances on top of any state. Two-phonon energies are often lowered by one or two MeV because of the large matrix elements with such three phonon configurations. These effects are studied in two nuclei, 40Ca and 208Pb
Les Jessour dans le Sud-Est tunisien : un système hydro-agricole ancestral dans un milieu aride
Malgré un bilan hydrique largement déficitaire dans le Sud-Est tunisien à climat aride, l'Homme a pu pratiquer l'agriculture en développant la technique des Jessour. Cette technique est confrontée aujourd'hui au risque d'abandon suite aux changements socio-économiques qu'a connus cette région au cours des cinq dernières décennies. Cet article fait le point de l'intérêt hydro-agricole des Jessour et présente les grandes lignes d'une recherche visant à évaluer le bilan hydrique de parcelles utilisant cette technique. Le but est de participer à la conservation de ce savoir-faire ancestral et d'optimiser son utilisation
Emergence of Biofuel Production Chain in Mali and Madagascar: Governance and Risk Management
La biomasse a suscité, ces dernières années, un intérêt renouvelé pour la
production d’énergie. L’attention s’est focalisée, entre autres, sur les
agrocarburants et sur les pays du Sud pour la production de biomasse. En
Afrique, la majorité des filières agrocarburants sont à l’heure actuelle dans
une phase d’émergence. Au Mali et à Madagascar, ces dernières visent
principalement la valorisation des graines de jatropha, pour l’électrification
rurale ou le transport (biodiesel pour le marché national, ou huile végétale
pure pour l’export).
Dans la littérature en économie des filières, la plupart des analyses
s’intéressent au mode de gouvernance et aux stratégies de gestion du risque
dans des filières d’ores et déjà établies. L’exemple des agro carburants
produits à partir du Jatropha offre l’opportunité de s’interroger sur les
processus de construction des filières. Il permet d’analyser les stratégies de
gestion de risque dans un contexte de nombreuses incertitudes, liées non
seulement à la nécessaire coordination d’activités de production agricoles
et de distribution d’énergie, mais aussi au caractère méconnu de la culture,
à la mise au point de procédés de conversion énergétique adaptés aux
contextes africains et à l’adoption de nouvelles pratiques par les distributeurs
d’énergie ou les consommateurs finaux.
Sur la base de l’analyse des dynamiques en cours au Mali et à Madagascar,
l’objectif de l’article est double. Il vise tout d’abord à identifier les
opérateurs qui jouent un rôle moteur dans le développement des filières
Jatropha et à analyser leurs outils de gouvernance. Il étudie ensuite la façon
dont ces opérateurs « moteurs » mobilisent ces outils de gouvernance
pour gérer les risques de production, de transformation et de mise en
marché
Probing pre-formed alpha particles in the ground state of nuclei
In this Letter, we report on alpha particle emission through the nuclear
break-up in the reaction 40Ca on a 40Ca target at 50A MeV. It is observed that,
similarly to nucleons, alpha particles can be emitted to the continuum with
very specific angular distribution during the reaction. The alpha particle
properties can be understood as resulting from an alpha cluster in the daughter
nucleus that is perturbed by the short range nuclear attraction of the
collision partner and emitted. A time-dependent theory that describe the alpha
particle wave-function evolution is able to reproduce qualitatively the
observed angular distribution. This mechanism offers new possibilities to study
alpha particle properties in the nuclear medium.Comment: 4 pages, 3 figure
Structural and magnetic dynamics of a laser induced phase transition in FeRh
We use time-resolved x-ray diffraction and magnetic optical Kerr effect to
study the laser induced antiferromagnetic to ferromagnetic phase transition in
FeRh. The structural response is given by the nucleation of independent
ferromagnetic domains (\tau_1 ~ 30ps). This is significantly faster than the
magnetic response (\tau_2 ~ 60ps) given by the subsequent domain realignment.
X-ray diffraction shows that the two phases co-exist on short time-scales and
that the phase transition is limited by the speed of sound. A nucleation model
describing both the structural and magnetic dynamics is presented.Comment: 5 pages, 3 figures - changed to reflect version accepted for PR
Nuclear break-up of 11Be
The break-up of 11Be was studied at 41AMeV using a secondary beam of 11Be
from the GANIL facility on a 48Ti target by measuring correlations between the
10Be core, the emitted neutrons and gamma rays. The nuclear break-up leading to
the emission of a neutron at large angle in the laboratory frame is identified
with the towing mode through its characteristic n-fragment correlation. The
experimental spectra are compared with a model where the time dependent
Schrodinger equation (TDSE) is solved for the neutron initially in the 11 Be. A
good agreement is found between experiment and theory for the shapes of neutron
experimental energies and angular distributions. The spectroscopic factor of
the 2s orbital is tentatively extracted to be 0.46+-0.15. The neutron emission
from the 1p and 1d orbitals is also studied
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