Bimodality as a Signal of a Liquid-Gas Phase Transition in Nuclei?

We use the heavy-ion phase-space exploration model to discuss the origin of the bimodality in charge asymmetry observed in nuclear reactions around the Fermi energy. We show that it may be related to the important angular momentum (spin) transferred into the quasiprojectile before secondary decay. As the spin overcomes the critical value, a sudden opening of decay channels is induced and leads to a bimodal distribution for the charge asymmetry. In the model, it is not assigned to a liquid-gas phase transition but to specific instabilities in nuclei with high spin. Therefore, we propose to use these reactions to study instabilities in rotating nuclear droplets.

Figure 1

Correlation between the largest fragment $Z_1$ and the charge asymmetry ${\eta }_Z$ for different bins of $E_{t12}^{\mathrm{QT}}$ (contour levels are in log scale).

Figure 2

Correlation between the atomic number (top row), the thermal energy (middle row), and spin (bottom row) of the QP source for events of Fig. 1c. The left column corresponds to the ER case and the right to the MF case (contour levels are in log scale).

Figure 3

${\eta }_Z$ distributions for the statistical deexcitation of an ${}^{120}\mathrm{Sn}$ nucleus for different initial thermal energies $E_{\mathrm{th}}$ (from left to right) and spins $J$ (from top to bottom).

Figure 4

Correlation between the largest ($Z_1$) and the second largest fragment ($Z_2$) obtained with the standard HIPSE (left panel) and SMM decay (right panel). Both simulations are made with the same set of $(Z,A,E^{*})$ given by the preequilibrium stage of HIPSE (contour levels are in log scale).