Multifragmentation reactions and properties of stellar matter at subnuclear densities

We point out the similarity of thermodynamic conditions reached in nuclear multifragmentation and in supernova explosions. We show that a statistical approach previously applied for nuclear multifragmentation reactions can also be used to describe the electroneutral stellar matter. Then properties of hot unstable nuclei extracted from the analysis of multifragmentation data can be used to determine a realistic nuclear composition of hot supernova matter.

Figure 1

Entropy per nucleon for different disintegration channels of ${}^{238}\mathrm{U}$ as a function of excitation energy per nucleon. Calculations are performed within the grand-canonical version of the SMM [12, 21] at the freeze-out density $\rho ={\rho }_0/3$. Dashed and dotted lines correspond to compound nucleus and vaporization ($A\le 4$) channels, respectively. Solid line is the total entropy, obtained by summing over all breakup channels.

Figure 2

Mean $N/Z$ vs charge of fragments produced in breakup of ${}^{238}\mathrm{U}$ with energy 1 GeV/nucleon on Pb and Ti targets. Points are experimental data obtained on fragment separator (FRS) at GSI [24]. Dashed line is SMM calculation for primary hot fragments; solid and dot-dashed lines are fragments after secondary deexcitation. Dotted line 1 corresponds to stable nuclei, dotted line 2 is the EPAX phenomenological parametrization for nuclei produced by spallation. Solid and dashed lines are calculations at the standard symmetry energy parameter $\gamma =25$ MeV; dot-dashed line is for reduced $\gamma =15$ MeV.

Figure 3

Mean charge-to-mass ratios (left top panel), and mass distributions of hot primary fragments (other panels) calculated with the SMM generalized for supernova conditions. Left panels are calculations for temperature $T=3$ MeV and fixed lepton fraction $Y_L=0.2$ per nucleon. Right panels, for temperature $T=1$ MeV and fixed electron fractions $Y_e$. Lines correspond to baryon densities shown in the figure. Solid, dashed, and dotted lines are calculations at the symmetry energy parameter $\gamma =25$ MeV; dot-dashed lines are for reduced $\gamma =15$ MeV.