1,300 research outputs found
Investigation of collective radial expansion and stopping in heavy ion collisions at Fermi energies
We present an analysis of multifragmentation events observed in central Xe+Sn
reactions at Fermi energies. Performing a comparison between the predictions of
the Stochastic Mean Field (SMF) transport model and experimental data, we
investigate the impact of the compression-expansion dynamics on the properties
of the final reaction products. We show that the amount of radial collective
expansion, which characterizes the dynamical stage of the reaction, influences
directly the onset of multifragmentation and the kinematic properties of
multifragmentation events. For the same set of events we also undertake a shape
analysis in momentum space, looking at the degree of stopping reached in the
collision, as proposed in recent experimental studies. We show that full
stopping is achieved for the most central collisions at Fermi energies.
However, considering the same central event selection as in the experimental
data, we observe a similar behavior of the stopping power with the beam energy,
which can be associated with a change of the fragmentation mechanism, from
statistical to prompt fragment emission.Comment: 15 page
Pseudo-critical clusterization in nuclear multifragmentation
In this contribution we show that the biggest fragment charge distribution in
central collisions of Xe+Sn leading to multifragmentation is an admixture of
two asymptotic distributions observed for the lowest and highest bombarding
energies. The evolution of the relative weights of the two components with
bombarding energy is shown to be analogous to that observed as a function of
time for the largest cluster produced in irreversible aggregation for a finite
system. We infer that the size distribution of the largest fragment in nuclear
multifragmentation is also characteristic of the time scale of the process,
which is largely determined by the onset of radial expansion in this energy
range.Comment: 4 pages, 3 figures, Contribution to conference proceedings of the
25th International Nuclear Physics Conference (INPC 2013
Freeze-out volume in multifragmentation - dynamical simulations
Stochastic mean-field simulations for multifragmenting sources at the same
excitation energy per nucleon have been performed. The freeze-out volume, a
concept which needs to be precisely defined in this dynamical approach, was
shown to increase as a function of three parameters: freeze-out instant,
fragment multiplicity and system size.Comment: Submitted to Eur. Phys. J. A - march 200
Sequential fissions of heavy nuclear systems
In Xe+Sn central collisions from 12 to 20 MeV/A measured with the INDRA
4 multidetector, the three-fragment exit channel occurs with a significant
cross section. In this contribution, we show that these fragments arise from
two successive binary splittings of a heavy composite system. Strong Coulomb
proximity effects are observed in the three-fragment final state. By comparison
with Coulomb trajectory calculations, we show that the time scale between the
consecutive break-ups decreases with increasing bombarding energy, becoming
compatible with quasi-simultaneous multifragmentation above 18 MeV/A.Comment: 6 pages, 5 figures, contribution to conference proceedings of the
Fifth International Workshop on Nuclear fission and Fission-Product
Spectroscop
Fragment properties of fragmenting heavy nuclei produced in central and semi-peripheral collisions
Fragment properties of hot fragmenting sources of similar sizes produced in
central and semi-peripheral collisions are compared in the excitation energy
range 5-10 AMeV. For semi-peripheral collisions a method for selecting compact
quasi-projectiles sources in velocity space similar to those of fused systems
(central collisions) is proposed. The two major results are related to
collective energy. The weak radial collective energy observed for
quasi-projectile sources is shown to originate from thermal pressure only. The
larger fragment multiplicity observed for fused systems and their more
symmetric fragmentation are related to the extra radial collective energy due
to expansion following a compression phase during central collisions. A first
attempt to locate where the different sources break in the phase diagram is
proposed.Comment: 23 pages submitted to NP
Development and Validation of a Sensitive Entropy-Based Measure for the Water Maze
In the water maze, mice are trained to navigate to an escape platform located below the water's surface, and spatial learning is most commonly evaluated in a probe test in which the platform is removed from the pool. While contemporary tracking software provides precise positional information of mice for the duration of the probe test, existing performance measures (e.g., percent quadrant time, platform crossings) fail to exploit fully the richness of this positional data. Using the concept of entropy (H), here we develop a new measure that considers both how focused the search is and the degree to which searching is centered on the former platform location. To evaluate how H performs compared to existing measures of water maze performance we compiled five separate databases, containing more than 1600 mouse probe tests. Random selection of individual trials from respective databases then allowed us to simulate experiments with varying sample and effect sizes. Using this Monte Carlo-based method, we found that H outperformed existing measures in its ability to detect group differences over a range of sample or effect sizes. Additionally, we validated the new measure using three models of experimentally induced hippocampal dysfunction: (1) complete hippocampal lesions, (2) genetic deletion of αCaMKII, a gene implicated in hippocampal behavioral and synaptic plasticity, and (3) a mouse model of Alzheimer's disease. Together, these data indicate that H offers greater sensitivity than existing measures, most likely because it exploits the richness of the precise positional information of the mouse throughout the probe test
Estimate of average freeze-out volume in multifragmentation events
An estimate of the average freeze-out volume for multifragmentation events is
presented. Values of volumes are obtained by means of a simulation using the
experimental charged product partitions measured by the 4pi multidetector INDRA
for 129Xe central collisions on Sn at 32 AMeV incident energy. The input
parameters of the simulation are tuned by means of the comparison between the
experimental and simulated velocity (or energy) spectra of particles and
fragments.Comment: To be published in Phys. Lett. B 12 pages, 5 figure
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
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