1,123 research outputs found
Isospin fluctuations in spinodal decomposition
We study the isospin dynamics in fragment formation within the framework of
an analytical model based on the spinodal decomposition scenario. We calculate
the probability to obtain fragments with given charge and neutron number,
focussing on the derivation of the width of the isotopic distributions. Within
our approach this is determined by the dispersion of N/Z among the leading
unstable modes, due to the competition between Coulomb and symmetry energy
effects, and by isovector-like fluctuations present in the matter that
undergoes the spinodal decomposition. Hence the widths exhibit a clear
dependence on the properties of the Equation of State. By comparing two systems
with different values of the charge asymmetry we find that the isotopic
distributions reproduce an isoscaling relationship.Comment: 18 RevTex4 pages, 6 eps figure
Bimodality as a signal of Liquid-Gas phase transition in nuclei?
We use the HIPSE (Heavy-Ion Phase-Space Exploration) Model to discuss the
origin of the bimodality in charge asymmetry observed in nuclear reactions
around the Fermi energy. We show that it may be related to the important
angular momentum (spin) transferred into the quasi-projectile before secondary
decay. As the spin overcomes the critical value, a sudden opening of decay
channels is induced and leads to a bimodal distribution for the charge
asymmetry. In the model, it is not assigned to a liquid-gas phase transition
but to specific instabilities in nuclei with high spin. Therefore, we propose
to use these reactions to study instabilities in rotating nuclear droplets.Comment: 4 pages, 4 figures Accepted to PR
Mesure de l'énergie des ions lourds par la méthode des protons projetés
Un dispositif destinĂ© Ă la mesure de l'Ă©nergie des faisceaux d'ions lourds de 3 Ă 6 MeV/ uma a Ă©tĂ© construit. Le principe de la mĂ©thode est de mesurer l'Ă©nergie des protons projetĂ©s Ă zĂ©ro degrĂ© par collision Ă©lastique des ions incidents avec les noyaux d'hydrogĂšne d'une cible de formvar. L'incertitude calculĂ©e sur l'Ă©nergie ainsi mesurĂ©e pour les ions lourds est de + 0,45 %. Des mesures faites sur des faisceaux de 19F et 40Ca d'Ă©nergie bien connue, accĂ©lĂ©rĂ©s par un Tandem MP, ont montrĂ© un Ă©cart maximum de 0,3 % entre les Ă©nergies rĂ©elles et mesurĂ©es. Le dispositif permet de contrĂŽler ou calibrer des mĂ©thodes plus lourdes de dĂ©termination de l'Ă©nergie des ions lourds : dĂ©viation magnĂ©tique, temps de vol. Il se prĂȘte particuliĂšrement bien Ă la mesure des pertes d'Ă©nergie d'ions lourds dans des ralentisseurs solides
Mid-rapidity charge distribution in peripheral heavy ion collisions
The charge density distribution with respect to the velocity of matter produced in peripheral heavy ion reactions around Fermi energy is investigated. The experimental finding of enhancement of mid-rapidity matter shows the necessity to include correlations beyond BUU which was performed in the framework of nonlocal kinetic theory. Different theoretical improvements are discussed. While the in-medium cross section changes the number of collisions, it leads the transferred energy almost unchanged. In contrast the nonlocal scenario changes the energy transferred during collisions and leads to an enhancement of mid-rapidity matter. The renormalisation of quasiparticle energies is shown to be possible to include in nonlocal scenarios and and leads to a further enhancement of mid-rapidity matter distribution. This renormalisation is accompanied by a dynamical softening of the equation of state seen in longer oscillation periods of the excited compressional collective mode. We propose to include quasiparticle renormalization by using the Pauli-rejected collisions which circumvent the problem of backflows in Landau theory. Using the maximum relative velocity of projectile and target like fragments we associate experimental events with impact parameters of the simulations. For peripheral collisions we find a reasonable agreement between experiment and theory. For more central collisions the velocity damping is higher in one - body simulations than observed experimentally which is due to missing cluster formations in the used kinetic theory
Isospin diffusion in semi-peripheral + collisions at intermediate energies (I): Experimental results
Isospin diffusion in semi-peripheral collisions is probed as a function of
the dissipated energy by studying two systems + and
+ , over the incident energy range 52-74\AM. A close
examination of the multiplicities of light products in the forward part of
phase space clearly shows an influence of the isospin of the target on the
neutron richness of these products. A progressive isospin diffusion is observed
when collisions become more central, in connection with the interaction time
The prominent role of the heaviest fragment in multifragmentation and phase transition for hot nuclei
The role played by the heaviest fragment in partitions of multifragmenting
hot nuclei is emphasized. Its size/charge distribution (mean value,
fluctuations and shape) gives information on properties of fragmenting nuclei
and on the associated phase transition.Comment: 11 pages, Proceedings of IWND09, August 23-25, Shanghai (China
Dynamical effects in multifragmentation at intermediate energies
The fragmentation of the quasi-projectile is studied with the INDRA
multidetector for different colliding systems and incident energies in the
Fermi energy range. Different experimental observations show that a large part
of the fragmentation is not compatible with the statistical fragmentation of a
fully equilibrated nucleus. The study of internal correlations is a powerful
tool, especially to evidence entrance channel effects. These effects have to be
included in the theoretical descriptions of nuclear multifragmentation.Comment: 13 pages, 26 figures, submitted to Physical Review
Yield scaling, size hierarchy and fluctuations of observables in fragmentation of excited heavy nuclei
Multifragmentation properties measured with INDRA are studied for single
sources produced in Xe+Sn reactions in the incident energy range 32-50 A MeV
and quasiprojectiles from Au+Au collisions at 80 A MeV. A comparison for both
types of sources is presented concerning Fisher scaling, Zipf law, fragment
size and fluctuation observables. A Fisher scaling is observed for all the
data. The pseudo-critical energies extracted from the Fisher scaling are
consistent between Xe+Sn central collisions and Au quasi-projectiles. In the
latter case it also corresponds to the energy region at which fluctuations are
maximal. The critical energies deduced from the Zipf analysis are higher than
those from the Fisher analysis.Comment: 30 pages, accepted for publication in Nuclear Physics A, references
correcte
Pion radii in nonlocal chiral quark model
The electromagnetic radius of the charged pion and the transition radius of
the neutral pion are calculated in the framework of the nonlocal chiral quark
model. It is shown in this model that the contributions of vector mesons to the
pion radii are noticeably suppressed in comparison with a similar contribution
in the local Nambu--Jona-Lasinio model. The form-factor for the process
gamma*pi+pi- is calculated for the -1 GeV^2<q^2<1.6 GeV^2. Our results are in
satisfactory agreement with experimental data.Comment: 7 pages, 7 figure
- âŠ