300 research outputs found
Searching for the statistically equilibrated systems formed in heavy ion collisions
Further improvements and refinements are brought to the microcanonical
multifragmentation model [Al. H. Raduta and Ad. R. Raduta, Phys. Rev. C {\bf
55}, 1344 (1997); {\it ibid.} {\bf 61}, 034611 (2000)]. The new version of the
model is tested on the recently published experimental data concerning the
Xe+Sn at 32 MeV/u and Gd+U at 36 MeV/u reactions. A remarkable good
simultaneous reproduction of fragment size observables and kinematic
observables is to be noticed. It is shown that the equilibrated source can be
unambiguously identified.Comment: Physical Review C, in pres
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
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
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
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Spinodal decomposition of expanding nuclear matter and multifragmentation
Density fluctuations of expanding nuclear matter are studied within a
mean-field model in which fluctuations are generated by an external stochastic
field. Fluctuations develop about a mean one-body phase-space density
corresponding to a hydrodinamic motion that describes a slow expansion of the
system. A fluctuation-dissipation relation suitable for a uniformly expanding
medium is obtained and used to constrain the strength of the stochastic field.
The distribution of the liquid domains in the spinodal decomposition is
derived. Comparison of the related distribution of the fragment size with
experimental data on the nuclear multifragmentation is quite satisfactory.Comment: 19 RevTex4 pages, 6 eps figures, to appear in Phys. Rev.
Long lifetime components in the decay of excited super-heavy nuclei
For nuclear reactions in which super-heavy nuclei can be formed, the essential difference between the fusion process followed by fission and non-equilibrium processes leading to fission-like fragments is there action time. Quite probable non-equilibrium
Signals of Bose Einstein condensation and Fermi quenching in the decay of hot nuclear systems
We report experimental signals of Bose-Einstein condensation in the decay of
hot Ca projectile-like sources produced in mid-peripheral collisions at
sub-Fermi energies. The experimental setup, constituted by the coupling of the
INDRA 4 detector array to the forward angle VAMOS magnetic spectrometer,
allowed us to reconstruct the mass, charge and excitation energy of the
decaying hot projectile-like sources. Furthermore, by means of quantum
fluctuation analysis techniques, temperatures and mean volumes per particle "as
seen by" bosons and fermions separately are correlated to the excitation energy
of the reconstructed system. The obtained results are consistent with the
production of dilute mixed (bosons/fermions) systems, where bosons experience a
smaller volume as compared to the surrounding fermionic gas. Our findings
recall similar phenomena observed in the study of boson condensates in atomic
traps.Comment: Submitted to Phys. Rev. Lett. (december 2014
Bimodality: a possible experimental signature of the liquid-gas phase transition of nuclear matter
We have observed a bimodal behaviour of the distribution of the asymmetry
between the charges of the two heaviest products resulting from the decay of
the quasi-projectile released in binary Xe+Sn and Au+Au collisions from 60 to
100 MeV/u. Event sorting has been achieved through the transverse energy of
light charged particles emitted on the quasi-target side, thus avoiding
artificial correlations between the bimodality signal and the sorting variable.
Bimodality is observed for intermediate impact parameters for which the
quasi-projectile is identified. A simulation shows that the deexcitation step
rather than the geometry of the collision appears responsible for the bimodal
behaviour. The influence of mid-rapidity emission has been verified. The two
bumps of the bimodal distribution correspond to different excitation energies
and similar temperatures. It is also shown that it is possible to correlate the
bimodality signal with a change in the distribution of the heaviest fragment
charge and a peak in potential energy fluctuations. All together, this set of
data is coherent with what would be expected in a finite system if the
corresponding system in the thermodynamic limit exhibits a first order phase
transition.Comment: 30 pages, 31 figure
Nuclear fission time measurements as a function of excitation energy - A crystal blocking experiment
CASFission times of lead and uranium nuclei have been measured at GANIL by the crystal blocking method. The inverse kinematics was used. Fragment atomic numbers and total excitation energies were determined. For data analysis, full Monte-Carlo trajectory calculations were used to simulate the blocking patterns. The effect of post-scission emissions, included in our simulations, is discussed. At high excitation energies, the scissions occur dominantly at times shorter than 10â19 s, whereas at low excitation energies (Eâ<250â300 MeV), scissions occurring at much longer times with sizeable probabilities are observed both for uranium and for lead nuclei, leading to average scission times much longer than those inferred from pre-scission emission
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