Extensive systematizations of theoretical and experimental nuclear densities and of optical potential strengths extracted from heavy-ion elastic scattering data analyses at low and intermediate energies are presented. The energy dependence of the nuclear potential is accounted for within a model based on the nonlocal nature of the interaction. The systematics indicates that the heavy-ion nuclear potential can be described in a simple global way through a double-folding shape, which basically depends only on the density of nucleons of the partners in the collision. The possibility of extracting information about the nucleon-nucleon interaction from the heavy-ion potential is investigated.

In this paper we analyze the nuclear fusion rate between equal nuclei for all five different nuclear burning regimes in dense matter (two thermonuclear regimes, two pycnonuclear ones, and the intermediate regime). The rate is determined by Coulomb barrier penetration in dense environments and by the astrophysical S-factor at low energies. We evaluate previous studies of the Coulomb barrier problem and propose a simple phenomenological formula for the reaction rate which covers all cases. The parameters of this formula can be varied, taking into account current theoretical uncertainties in the reaction rate. The results are illustrated for the example of the ^{12}C+^{12}C fusion reaction. This reaction is very important for the understanding of nuclear burning in evolved stars, in exploding white dwarfs producing type Ia supernovae, and in accreting neutron stars. The S-factor at stellar energies depends on a reliable fit and extrapolation of the experimental data. We calculate the energy dependence of the S-factor using a recently developed parameter-free model for the nuclear interaction, taking into account the effects of the Pauli nonlocality. For illustration, we analyze the efficiency of carbon burning in a wide range of densities and temperatures of stellar matter with the emphasis on carbon ignition at densities rho > 10^9 g/cc.Comment: 22 pages, 6 figures, accepted for publication in PR

We analyze thermonuclear and pycnonuclear fusion reactions in dense matter containing atomic nuclei of different types. We extend a phenomenological expression for the reaction rate, proposed recently by Gasques et al. (2005) for the one-component plasma of nuclei, to the multi-component plasma. The expression contains several fit parameters which we adjust to reproduce the best microscopic calculations available in the literature. Furthermore, we show that pycnonuclear burning is drastically affected by an (unknown) structure of the multi-component matter (a regular lattice, a uniform mix, etc.). We apply the results to study nuclear burning in a carbon_12-oxygen_16 mixture. In this context we present new calculations of the astrophysical S-factors for carbon-oxygen and oxygen-oxygen fusion reactions. We show that the presence of a CO lattice can strongly suppress carbon ignition in white dwarf cores and neutron star crusts at densities > 3e9 g cm^{-3} and temperatures T<1e8 K.Comment: 16 pages, 5 figures, to appear in Phys. Rev.

The temperature in the crust of an accreting neutron star, which comprises its outermost kilometre, is set by heating from nuclear reactions at large densities, neutrino cooling and heat transport from the interior. The heated crust has been thought to affect observable phenomena at shallower depths, such as thermonuclear bursts in the accreted envelope. Here we report that cycles of electron capture and its inverse, β(-) decay, involving neutron-rich nuclei at a typical depth of about 150 metres, cool the outer neutron star crust by emitting neutrinos while also thermally decoupling the surface layers from the deeper crust. This 'Urca' mechanism has been studied in the context of white dwarfs and type Ia supernovae, but hitherto was not considered in neutron stars, because previous models computed the crust reactions using a zero-temperature approximation and assumed that only a single nuclear species was present at any given depth. The thermal decoupling means that X-ray bursts and other surface phenomena are largely independent of the strength of deep crustal heating. The unexpectedly short recurrence times, of the order of years, observed for very energetic thermonuclear superbursts are therefore not an indicator of a hot crust, but may point instead to an unknown local heating mechanism near the neutron star surface.

We investigate the consequences of a new phenomenological model prediction of strongly reduced low-energy astrophysical S-factors for carbon and oxygen fusion reactions on stellar burning and nucleosynthesis. The new model drastically reduces the reaction rates in stellar matter at temperatures T < ∼ (3-10) × 10 8 K, especially at densities ρ > ∼ 10 9 g cm −3 , in a strongly screened or even pycnonuclear burning regime. We show that these modifications change the abundance of many isotopes in massive late-type stars and in particular strongly enhance the abundances of long-lived radioactive isotopes such as 26 Al and 60 Fe. The reduced reaction rates also significantly complicate carbon ignition (shift carbon ignition to higher temperatures and densities) in massive accreting white dwarfs exploding as type Ia supernovae and in accreting neutron stars producing superbursts. This would require much higher ignition densities for white dwarf supernovae and would widen the gulf between theoretical and inferred ignition depths for superbursts.

A classical dynamical model that treats breakup stochastically is presented for low energy reactions of weakly bound nuclei. The three-dimensional model allows a consistent calculation of breakup, incomplete, and complete fusion cross sections. The model is assessed by comparing the breakup observables with continuum discretized coupled-channel quantum mechanical predictions, which are found to be in reasonable agreement. Through the model, it is demonstrated that the breakup probability of the projectile as a function of its distance from the target is of primary importance for understanding complete and incomplete fusion at energies near the Coulomb barrier. DOI: 10.1103/PhysRevLett.98.152701 PACS numbers: 25.70.Jj, 25.70.Mn Recent developments of radioactive isotope accelerators provide an opportunity to investigate on Earth the fusion reactions that form heavy elements in the cosmos. These involve reactions of nuclei far from stability, the most exotic of which are often very weakly bound. Breakup of weakly bound nuclei is thus an important process in their collisions with other nuclei. A major consequence of breakup is that not all the resulting breakup fragments might be captured by the target, termed incomplete fusion (ICF); capture of the entire projectile by the target is called complete fusion (CF). Such ICF processes can dramatically change the nature of the reaction products, as has been investigated in detail for the stable weakly bound nuclei 9 Be and 6;7 Li [1]. There, at energies above the fusion barrier, CF yields were found to be only 2=3 of those expected, the remaining 1=3 being in ICF products. Events where the projectile breaks up and none of the fragments are captured provide an important diagnostic of the reaction dynamics. This we call no-capture breakup (NCBU), also referred to as elastic breakup.In a conventional picture of fusion, two colliding nuclei will fuse if they overcome the potential barrier due to their mutual Coulomb and nuclear interactions. The additional breakup degrees of freedom when one of the colliding nuclei is weakly bound makes the process very much more complicated. An outstanding theoretical challenge is to model the CF and ICF processes in such collisions, since this separation is crucial to understand the effects of breakup on fusion [1,2]. Quantum mechanical few-body approaches, such as the continuum discretized coupledchannel (CDCC) method [3,4] and the time-dependent wave packet method [5], cannot separate incomplete and complete fusion contributions to their absorptive cross sections [6], since both result in depletion of the total few-body wave function. The CDCC method can, however, make reliable predictions of the NCBU process [7], as will be exploited here. What, then, are the alternatives to the above models? A novel optical decoherence model has been suggested [6] but has yet to be implemented. Another approach is to use the concept of classical trajectories which allow CF and ICF events to be separated, as in the two-dimensional model of Ref. [8]...

X-ray observations of transiently accreting neutron stars during quiescence provide information about the structure of neutron star crusts and the properties of dense matter. Interpretation of the observational data requires an understanding of the nuclear reactions that heat and cool the crust during accretion, and define its nonequilibrium composition. We identify here in detail the typical nuclear reaction sequences down to a depth in the inner crust where the mass density is ρ = 2 × 10 12 g cm −3 using a full nuclear reaction network for a range of initial compositions. The reaction sequences differ substantially from previous work. We find a robust reduction of crust impurity at the transition to the inner crust regardless of initial composition, though shell effects can delay the formation of a pure crust somewhat to densities beyond ρ = 2 × 10 12 g cm −3 . This naturally explains the small inner crust impurity inferred from observations of a broad range of systems. The exception are initial compositions with A ≥ 102 nuclei, where the inner crust remains impure with an impurity parameter of Q imp ≈ 20 due to the N = 82 shell closure. In agreement with previous work we find that nuclear heating is relatively robust and independent of initial composition, while cooling via nuclear Urca cycles in the outer crust depends strongly on initial composition. This work forms a basis for future studies of the sensitivity of crust models to nuclear physics and provides profiles of composition for realistic crust models.

Above-barrier cross sections of α-active heavy reaction products, as well as fission, were measured for the reactions of 10,11 B with 209 Bi. Detailed analysis showed that the heavy products include components from incomplete fusion as well as complete fusion (CF), but fission originates almost exclusively from CF. Compared with fusion calculations without breakup, the CF cross sections are suppressed by 15% for 10 B and 7% for 11 B. A consistent and systematic variation of the suppression of CF for reactions of the weakly bound nuclei 6,7 Li, 9 Be, 10,11 B on targets of 208 Pb and 209 Bi is found as a function of the breakup threshold energy.