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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

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.

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.

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