15 pages, 6 figures, accepted in Physics Letters BInternational audienceThe fragmentation of quasi-projectiles from the nuclear reaction $^{40}$Ca+$^{12}$C at 25 MeV per nucleon bombarding energy was used to produce $\alpha$-emission sources. From a careful selection of these sources provided by a complete detection and from comparisons with models of sequential and simultaneous decays, evidence in favor of $\alpha$-particle clustering from excited $^{16}O$, $^{20}Ne$ and $^{24}Mg$ is reported
We present a new experimental method to correlate the isotopic composition of intermediate mass fragments (IMF) emitted at mid-rapidity in semi-peripheral collisions with the emission timescale: IMFs emitted in the early stage of the reaction show larger values of isospin asymmetry, stronger angular anisotropies and reduced odd-even staggering effects in neutron to proton ratio distributions than those produced in sequential statistical emission. All these effects support the concept of isospin "migration", that is sensitive to the density gradient between participant and quasi-spectator nuclear matter, in the so called neck fragmentation mechanism. By comparing the data to a Stochastic Mean Field (SMF) simulation we show that this method gives valuable constraints on the symmetry energy term of nuclear equation of state at subsaturation densities. An indication emerges for a linear density dependence of the symmetry energy.
The fragmentation of quasi-projectiles from the nuclear reaction 40 Ca+ 12 C at 25 MeV/nucleon was used to produce excited states candidates to αparticle condensation. Complete kinematic characterization of individual decay events, made possible by a high-granularity 4π charged particle multidetector, reveals that 7.5±4.0 % of the particle decays of the Hoyle state correspond to direct decays in three equal-energy α-particles.
The fully energy-damped yields from the 35 Clϩ 12 C reaction have been systematically investigated using particle-particle coincidence techniques at a 35 Cl bombarding energy of ϳ8 MeV/nucleon. The fragmentfragment correlation data show that the majority of events arises from a binary-decay process with rather large numbers of secondary light-charged particles emitted from the two excited exit fragments. No evidence is observed for ternary break-up events. The binary-process results of the present measurement, along with those of earlier, inclusive experimental data obtained at several lower bombarding energies are compared with predictions of two different kinds of statistical model calculations. These calculations are performed using the transition-state formalism and the extended Hauser-Feshbach method and are based on the available phase space at the saddle point and scission point of the compound nucleus, respectively. The methods give comparable predictions and are both in good agreement with the experimental results thus confirming the fusionfission origin of the fully damped yields. The similarity of the predictions for the two models supports the claim that the scission point configuration is very close to that of the saddle point for the light 47 V compound system. The results also give further support for the specific mass-asymmetry-dependent fission barriers needed in the transition-state calculation. ͓S0556-2813͑96͒02407-7͔
Matière Nucléaire NIMSemiperipheral collisions in the $^{124}$Sn+${64}Ni reaction at 35 MeV/nucleon were studied using the forward part of the Charged Heavy Ion Mass and Energy Resolving Array. Nearly completely determined ternary events involving projectilelike fragments (PLF), targetlike fragments (TLF), and intermediate mass fragments (IMF) were selected. A new method of studying the reaction mechanism, focusing on the analysis of the correlations between relative velocities in the IMF+PLF and IMF+TLF subsystems, is proposed. The relative velocity correlations provide information on the time sequence and time scale of the neck fragmentation processes leading to production of IMFs. It is shown that the majority of light IMFs are produced within 40–80 fm/c after the system starts to reseparate. Heavy IMFs are formed at times of about 120 fm/c or later and can be viewed as resulting from two-step (sequential) neck rupture processes
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