Experimental study of the isospin transport with $^{40,48}$Ca+ $^{40,48}$Ca reactions at 35 MeV/nucleon
Résumé
We investigate the isospin transport with 40,48 Ca+ 40,48 Ca reactions at 35 MeV/nucleon, measured with the coupling of the VAMOS high acceptance spectrometer and the INDRA charged particle multidetector. Using the quasi-projectile remnant measured with VAMOS and carefully selected light-charged-particles measured in coincidence with INDRA, a reconstruction method is applied to estimate the excited quasi-projectile (QP) on an event-by-event basis. The isospin diffusion is investigated using the isospin transport ratio with the asymmetry δ = (N − Z)/A of the projectile as an isospin-sensitive observable and the total transverse energy of Z ≤ 2 nuclei for experimental centrality sorting. The experimental isospin transport ratios present, for both the reconstructed QP and its remnant, a clear evolution towards isospin equilibration with increasing dissipation of the collision while the full equilibration is not reached. A smoother evolution with less discrepancies between the two mixed systems in the case of the reconstructed QP is also observed. The isospin migration is investigated using the neutron-to-proton ratio of the light-charged-clusters and the velocity of the QP remnant as a sorting parameter. More particularly, we focused on an angular region centered around the mid-rapidity of the reaction so as to characterize the low-density neck emissions. A systematic neutron-enrichment is observed and interpreted as a consequence of isospin migration, more particularly for the symmetric systems which present no isospin gradient between the projectile and the target. We also noticed that the ^{2}H and ^{4}He particles exhibit very close multiplicities independently of the sorting variable for the symmetric systems.
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