164 research outputs found
Measurements of sideward flow around the balance energy
Sideward flow values have been determined with the INDRA multidetector for
Ar+Ni, Ni+Ni and Xe+Sn systems studied at GANIL in the 30 to 100 A.MeV incident
energy range. The balance energies found for Ar+Ni and Ni+Ni systems are in
agreement with previous experimental results and theoretical calculations.
Negative sideward flow values have been measured. The possible origins of such
negative values are discussed. They could result from a more important
contribution of evaporated particles with respect to the contribution of
promptly emitted particles at mid-rapidity. But effects induced by the methods
used to reconstruct the reaction plane cannot be totally excluded. Complete
tests of these methods are presented and the origins of the
``auto-correlation'' effect have been traced back. For heavy fragments, the
observed negative flow values seem to be mainly due to the reaction plane
reconstruction methods. For light charged particles, these negative values
could result from the dynamics of the collisions and from the reaction plane
reconstruction methods as well. These effects have to be taken into account
when comparisons with theoretical calculations are done.Comment: 27 pages, 15 figure
Study of intermediate velocity products in the Ar+Ni collisions between 52 and 95 A.MeV
Intermediate velocity products in Ar+Ni collisions from 52 to 95 A.MeV are
studied in an experiment performed at the GANIL facility with the 4
multidetector INDRA. It is shown that these emissions cannot be explained by
statistical decays of the quasi-projectile and the quasi-target in complete
equilibrium. Three methods are used to isolate and characterize intermediate
velocity products. The total mass of these products increases with the violence
of the collision and reaches a large fraction of the system mass in mid-central
collisions. This mass is found independent of the incident energy, but strongly
dependent on the geometry of the collision. Finally it is shown that the
kinematical characteristics of intermediate velocity products are weakly
dependent on the experimental impact parameter, but strongly dependent on the
incident energy. The observed trends are consistent with a
participant-spectator like scenario or with neck emissions and/or break-up.Comment: 37 pages, 13 figure
Emission time scale of light particles in the system Xe+Sn at 50 AMeV. A probe for dynamical emission ?
Proton and deuteron correlation functions have been investigated with both
impact parameter and emission source selections. The correlations of the system
(129Xe + natSn) at 50 AMeV have been measured with the 4 pi INDRA which
provides a complete kinematical description of each event. The emission time
scale analyzed with a quantum model reveals the time sequence of the light
particles emitted by the projectile-like fragment. The short and constant
emission time of the proton, independent of the impact parameter, can be
attributed to a preequilibrium process.Comment: 20 pages, with 11 included figures; Accepted by European Physics
Journal
New method for the discrimination of single-source events in heavy-ion collisions
MatiÚre NucléaireThis paper introduces a new method for the selection of central single-source events, based on classical multivariate techniques. The resulting discriminating variable is shown to be valid for different hypotheses on the nuclear source deexcitation mechanism. It enables the selection of events which are representative of the whole set of single-source events. Application to the Ni+Ni at 32A MeV system measured with the INDRA multidetector has allowed the determination of the fusion probability as a function of the impact parameter and the evaluation of the corresponding cross section
Trees WantedâDead or Alive! Host Selection and Population Dynamics in Tree-Killing Bark Beetles
Bark beetles (Coleoptera: Curculionidae,
Scolytinae) feed and breed in dead or severely weakened
host trees. When their population densities are high, some species aggregate on
healthy host trees so that their defences may be exhausted and the inner bark
successfully colonized, killing the tree in the process. Here we investigate
under what conditions participating with unrelated conspecifics in risky mass
attacks on living trees is an adaptive strategy, and what this can tell us about
bark beetle outbreak dynamics. We find that the outcome of individual host
selection may deviate from the ideal free distribution in a way that facilitates
the emergence of tree-killing (aggressive) behavior, and that any heritability
on traits governing aggressiveness seems likely to exist in a state of flux or
cycles consistent with variability observed in natural populations. This may
have implications for how economically and ecologically important species
respond to environmental changes in climate and landscape (forest) structure.
The population dynamics emerging from individual behavior are complex, capable
of switching between âendemicâ and âepidemicâ regimes
spontaneously or following changes in host availability or resistance. Model
predictions are compared to empirical observations, and we identify some factors
determining the occurrence and self-limitation of epidemics
- âŠ