E. De Filippo, A. Pagano, P. Russotto, F. Amorini, A. Anzalone, L. Auditore, V. Baran, I. Berceanu, B. Borderie, R. Bougault, M. Bruno, T. Cap, G. Cardella, S. Cavallaro, M. B. Chatterjee, A. Chbihi, M. Colonna, M. D'Agostino, R. Dayras, M. Di Toro, J. Frankland, E. Galichet, W. Gawlikowicz, E. Geraci, A. Grzeszczuk, P. Guazzoni, S. Kowalski, E. La Guidara, G. Lanzalone, G. Lanzanò, N. Le Neindre, I. Lombardo, C. Maiolino, M. Papa, E. Piasecki, S. Pirrone, R. Planeta, G. Politi, A. Pop, F. Porto, M. F. Rivet, F. Rizzo, E. Rosato, K. Schmidt, K. Siwek-Wilczynska, I. Skwira-Chalot, A. Trifirò, M. Trimarchi, G. Verde, M. Vigilante, J. P. Wieleczko, J. Wilczynski, L. Zetta, W. Zipper
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 $<$N/Z$>$ isospin asymmetry, stronger angular anisotropies and reduced odd-even staggering effects in neutron to proton ratio $<$N/Z$>$ 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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http://arxiv.org/abs/1206.0697
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