62 research outputs found
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
Nuclear multifragmentation time-scale and fluctuations of largest fragment size
Distributions of the largest fragment charge, Zmax, in multifragmentation
reactions around the Fermi energy can be decomposed into a sum of a Gaussian
and a Gumbel distribution, whereas at much higher or lower energies one or the
other distribution is asymptotically dominant. We demonstrate the same generic
behavior for the largest cluster size in critical aggregation models for small
systems, in or out of equilibrium, around the critical point. By analogy with
the time-dependent irreversible aggregation model, we infer that Zmax
distributions are characteristic of the multifragmentation time-scale, which is
largely determined by the onset of radial expansion in this energy range.Comment: Accepted for publication in Physical Review Letters on 8/4/201
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
Coulomb chronometry to probe the decay mechanism of hot nuclei
In 129 Xe+ nat Sn central collisions from 8 to 25 MeV/A, the three-fragment
exit channel occurs with a significant cross section. We show that these
fragments arise from two successive binary splittings of a heavy composite
system. The sequence of fragment production is determined. Strong Coulomb
proximity effects are observed in the three-fragment final state. A comparison
with Coulomb trajec-tory calculations shows that the time scale between the
consecutive break-ups decreases with increasing bombarding energy, becoming
quasi-simultaneous above excitation energy E * = 4.00.5 MeV/A. This
transition from sequential to simultaneous break-up was interpreted as the
signature of the onset of multifragmentation for the three-fragment exit
channel in this system.Comment: 12 pages; 13 Figures; 4 Table; Accepted for publication in Physical
Review
The ASY-EOS experiment at GSI: investigating the symmetry energy at supra-saturation densities
The elliptic-flow ratio of neutrons with respect to protons in reactions of
neutron rich heavy-ions systems at intermediate energies has been proposed as
an observable sensitive to the strength of the symmetry term in the nuclear
Equation Of State (EOS) at supra-saturation densities. The recent results
obtained from the existing FOPI/LAND data for Au+Au collisions
at 400 MeV/nucleon in comparison with the UrQMD model allowed a first estimate
of the symmetry term of the EOS but suffer from a considerable statistical
uncertainty. In order to obtain an improved data set for Au+Au collisions and
to extend the study to other systems, a new experiment was carried out at the
GSI laboratory by the ASY-EOS collaboration in May 2011.Comment: Talk given by P. Russotto at the 11th International Conference on
Nucleus-Nucleus Collisions (NN2012), San Antonio, Texas, USA, May 27-June 1,
2012. To appear in the NN2012 Proceedings in Journal of Physics: Conference
Series (JPCS
BMC Psychol
Background Preschoolers regularly display disruptive behaviors in child care settings because they have not yet developed the social skills necessary to interact prosocially with others. Disruptive behaviors interfere with daily routines and can lead to conflict with peers and educators. We investigated the impact of a social skills training program led by childcare educators on childrenâs social behaviors and tested whether the impact varied according to the childâs sex and family socio-economic status. Methods Nineteen public Child Care Centers (CCC, nâ=â361 children) located in low socio-economic neighborhoods of Montreal, Canada, were randomized into one of two conditions: 1) intervention (nâ= 10 CCC; 185 children) or 2) wait list control (nâ=â9 CCC; 176 children). Educators rated childrenâs behaviors (i.e., disruptive and prosocial behaviors) before and after the intervention. Hierarchical linear mixed models were used to account for the nested structure of the data. Results At pre-intervention, no differences in disruptive and prosocial behaviors were observed between the experimental conditions. At post-intervention, we found a significant sex by intervention interaction (ÎČ intervention by sexâ=âââ1.19, pâ=â0.04) indicating that girls in the intervention condition exhibited lower levels of disruptive behaviors compared to girls in the control condition (f2 effect sizeâ=âââ0.15). There was no effect of the intervention for boys. Conclusions Girls may benefit more than boys from social skills training offered in the child care context. Studies with larger sample sizes and greater intervention intensity are needed to confirm the results
Ătude de l'Ă©nergie de symĂ©trie dans les collisions Ca+Ca Ă 35 MeV/A.
ExpĂ©rience GANIL/INDRA-VAMOSSymmetry term (Csym) inside the nuclear equation of state is studied with the help of the isoscaling parameters and the isotopic distributions widths for 40,48Ca+40,48Ca reactions at an incident energy of 35 MeV/A. Peripheral et semi-peripheral collisions are studied. A coupling between two experimental setups, VAMOS spectrometer and INDRA multidetector was done. VAMOS was able to measure the charge and mass of the projectile like fragment (PLF) with a high precision. Light charged particles were measured in coincidence with INDRA, and they give us an impact parameter and excitation energy estimations They are also used in the primary fragment reconstruction, which are the PLF parent. PLF study shows the presence of an important odd-even effect, which can be associated to the secondary decay effects. The extracted values of the symmetry energy term directly from the PLF are not reliable. Therefore, it is necessary to reconstruct the primary fragment in order to take into account the secondary decay effects. By studying primary fragments, we observe an evolution of the parameter Csym/T as a function of the charge. This evolution is attributed to the fragment excitation energy. Also, global distributions of Csym/T give us an information on the surface over the volume contributions to the symmetry energy ratio. Those ratios show important surface contributions, same results given by simulations. With extracted temperatures from proton energy spectra, a Csym value can be obtained. Those values, around 30 MeV, are coherent with values around the saturation density.Le terme d'Ă©nergie de symĂ©trie (Csym) contenu Ă l'intĂ©rieur de l'Ă©quation d'Ă©tat de la matiĂšre nuclĂ©aire est Ă©tudiĂ© via l'isoscaling et les formes des distributions isotopiques pour les rĂ©actions 40,48Ca+40,48Ca Ă une Ă©nergie incidente de 35 MeV/A. Les collisions pĂ©riphĂ©riques et semi-pĂ©riphĂ©riques sont Ă©tudiĂ©es. Un couplage entre deux appareils expĂ©rimentaux, le spectromĂštre VAMOS et le multidĂ©tecteur INDRA, a Ă©tĂ© fait. VAMOS a permis de mesurer la charge et la masse des rĂ©sidus d'Ă©vaporation du quasi-projectile (PLF) avec grande prĂ©cision. Les particules lĂ©gĂšres chargĂ©es ont Ă©tĂ© mesurĂ©es simultanĂ©ment avec INDRA, permettant d'estimer le paramĂštre d'impact et l'Ă©nergie d'excitation ainsi que de reconstruire le fragment primaire, parent du PLF. L'Ă©tude des PLF montre la prĂ©sence d'un effet pair-impair associĂ© aux effets de dĂ©sintĂ©grations secondaires. L'extraction d'une information concernant Csym par l'Ă©tude des PLF, dans le cadre de cette thĂšse, est donc difficile, d'oĂč la nĂ©cessitĂ© de reconstruire le fragment primaire. En Ă©tudiant les fragments primaires reconstruits, nous observons une Ă©volution du paramĂštre Csym/T en fonction de la charge. Cette Ă©volution est attribuĂ©e au degrĂ© d'excitation des fragments. De plus, la forme des distributions globales de Csym/T nous permet d'extraire le rapport des contributions de surface et de volume Ă l'Ă©nergie de symĂ©trie. Ces rapports nous montrent des effets de surface importants, appuyĂ©s par les calculs thĂ©oriques. Ă partir des tempĂ©ratures extraites par les spectres en Ă©nergie des protons, une valeur de Csym est extraite. Ces valeurs, autour de 30 MeV, sont cohĂ©rentes avec des valeurs autour de la densitĂ© de saturation
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