Investigating the possible existence of hyper-heavy nuclei in a neutron-star environment

The synthesis of hyper-heavy elements is investigated under conditions simulating neutron star environment. The constrained molecular dynamics approach is used to simulate low energy collisions of extremely $n$-rich nuclei. A new type of the fusion barrier due to a “neutron wind” is observed when the effect of neutron star environment (screening of Coulomb interaction) is introduced implicitly. When introducing also a background of surrounding nuclei, the nuclear fusion becomes possible down to temperatures of ${10}^8$ K and synthesis of extremely heavy and $n$-rich nuclei appears feasible. A possible existence of hyper-heavy nuclei in a neutron star environment could provide a mechanism of extra coherent neutrino scattering or an additional mechanism, resulting in x-ray burst or a gravitational wave signal and, thus, becoming another crucial process adding new information to the suggested models on neutron star evolution.

Figure 1

Typical evolution of nucleonic density for central collision ${}^{140}\mathrm{Ni}+{}^{460}\mathrm{U}$ calculated using the CoMD code at beam energy $1.25\mathrm{MeV}/\mathrm{nucleon}$ (left panel) and $1.5\mathrm{MeV}/\mathrm{nucleon}$ (right panel). A Coulomb interaction cutoff 2 fm, incompressibility $K_0=254\mathrm{MeV}$ and a soft density dependence of symmetry energy were used. Scattering due to a “neutron wind” dominates at lower beam energy (left panel), while fusion dominates at higher beam energy (right panel).

Figure 2

Evolution of the system ${}^{140}\mathrm{Ni}+{}^{460}\mathrm{U}$ in the nucleon bath with 10% proton concentration calculated using the CoMD code at beam energy $0.01\mathrm{MeV}/\mathrm{nucleon}$ and Coulomb interaction cutoff at 10, 7, 5, 4, 3, and 2 fm (from top to bottom and left to right), corresponding to nucleon bath densities ${\rho }_0/100, {\rho }_0/50, {\rho }_0/30, {\rho }_0/17, {\rho }_0/10$, and ${\rho }_0/5$, respectively. Incompressibility is $K_0=254\mathrm{MeV}$ and a soft density dependence of symmetry energy were used. The initial distance is $25\mathrm{fm}$. On the time scale up to 25000 $\mathrm{fm}/c$ the resulting nucleus appears to dissolve in the nucleon bath with density above ${\rho }_0/30$, even sooner with increasing density.