Research Overview

A central goal of nuclear physics is to understand how protons and neutrons interact to form nuclei both here on earth as well as in extreme environments such as the crusts of neutron stars or in supernova explosions. Understanding the existence and properties of "pasta nuclei" in the crusts of neutron stars and element formation in supernova explosions requires knowledge of the thermodynamic properties of neutron-rich nuclear matter [Lattimer00]. Our present understanding of nucleonic matter, however, relies largely on the investigation of nuclei at or close to stability. Significant effort over the past two decades has been directed at understanding the behavior of highly excited nucleonic matter, leading to a reasonably good description in terms of a phase transition-like behavior of nuclear systems at high excitation [Pochodzalla95, Borderie01, Elliott02, Berkenbusch02, Natowitz02, Lopez05].

Properties of nuclear matter with a neutron-to-proton ratio away from stability is important in developing an improved understanding of astrophysical processes. Neutron star crusts, for example, are environments with exotic neutron-to-proton ratio [Lattimer04]. Theoretical calculations predict that, under appropriate conditions of temperature and density, nuclear matter may fractionate into a protonrich liquid and a neutron-rich gas [Muller95]. This thermodynamic result can also impact a short-lived nuclear collision in which thermodynamic equilibrium is not achieved [Colonna02, Sil04].

Cluster emission, Nucleon transport, and the density dependence of the symmetry energy

The fragmentation of a highly excited nuclear system is characterized by the rapid emission of clusters on a short timescale [Beaulieu00]. It has recently been proposed that as the nuclear system expands in response to its thermal excitation, both the amount of surface material and the importance of surface modes increases [Sobotka04, Toke05, Sobotka06]. The resulting rapid increase in the system's entropy results in increased cluster emission. Below the multifragmentation threshold, cluster emission is predominantly a surface phenomenon. Examining cluster production when nuclei are in the ground state or modestly excited provides a probe of in-medium correlations. While symmetric nuclei manifest enhanced alpha particle clustering [Macfarlane65, Liddick06], more neutron-rich systems should result in the enhanced production of neutron-rich clusters. Recent work suggests that the yield of neutron-rich clusters produced in a heavy-ion collision is sensitive to the density dependence of the symmetry energy [Li08]. Examination of the isotopic composition of clusters emitted from modestly excited nuclei with varying N/Z ratios should provide insight into how nucleon correlations in the nuclear surface evolve with excitation and N/Z. High resolution measurements of short-lived emitted clusters, which are unbound (e.g. 8Be*), can also provide insight into the surface properties of a nucleus. Interaction with the nuclear surface on a short timescale can modify the decay characteristics of these clusters and perhaps their structure [McIntosh07]. Finite nuclei are a useful laboratory in which a density gradient exists as one moves from the center of the nucleus to its low-density surface. The character of this sub-saturation density nuclear matter and how it evolves with increasing temperature [Sobotka04, Toke05, Sobotka06] and N/Z is intimately tied to the process of cluster formation.

A key issue in understanding the nuclear equation-of-state is the density dependence of the asymmetry term. Nuclear properties however are significantly impacted by the neutron-to-proton ratio of the system. In addition, it has been predicted that the stability of phases in a neutron star [Steiner08], the existence and nature of crustal vibrations [RocaMaza08], the density at which the core-crust transition occurs [Xu09], all depend sensitively on the density dependence of the symmetry energy. Radioactive beam facilities, either in existence or on the horizon, together with judicious choice of stable beam experiments, provide the means to examine the character of nucleonic matter as the neutron-to-proton ratio is varied.

For example, preferential neutron transport as compared to proton transport from high to low density can occur when two N/Z symmetric nuclei undergo a peripheral collision. Initial measurements support this physical picture [Thériault06]. Alternatively, cross-bombardment reactions can also be used to probe the density dependence of the symmetry energy [Tsang04, Chen05]. Investigating the density dependence of the asymmetry term in such reactions requires measurement of isotopically identified clusters and free nucleons (both neutrons and protons), at both projectile-velocity and midvelocity [Thériault06]. Comparison of recent measurements of the transverse neutron and proton emission with transport models indicates that the EOS is somewhat asy-soft in character [Famiano06]. Improved measurements, which allow consideration of total nucleon emission including those bound in both light (Z≤2) and heavy clusters (Z≥3), could reduce ambiguities in coalescence calculations.

Neutron star crusts and fusion dynamics for neutron-rich nuclei

Recent studies of neutron stars which accrete material from a companion star provide strong motivation to measure the fusion rates of light neutron-rich nuclei. Neutron-rich light nuclei, formed by electron capture reactions, can undergo either thermonuclear (temperature driven) or pycnonuclear (density or quantum zero point motion driven) fusion. The rate of these fusion reactions depends strongly on the nuclear atomic number Z. For example, carbon (and lower charged nuclei) may fuse at low densities before the nuclei become neutron-rich, while slightly higher atomic number nuclei, such as O or Ne, may not fuse until densities near 1011 g/cm3 where electron capture makes the nuclei neutron-rich. These latter fusion reactions and the depth at which they occur thus impacts the temperature profile of the star and consequently other observational properties.

Two examples in which such fusion heating may be important involve the initiation of X-ray superbursts and the cooling rate of crusts following X-ray bursts. Superbursts are gigantic X-ray bursts that are thought to involve the unstable ignition of 12C. However, some models have crust temperature profiles that are too cold to reach appropriate ignition conditions. Heat from neutron-rich O or Ne fusion could raise the temperature until carbon ignition is reached. The depth at which fusion heating occurs may also be important for the recently observed time scales for neutron star crusts to cool after extended outbursts. In summary, the interpretation of X-ray observations of accreting n stars may benefit significantly from a better knowledge of the fusion rates of light neutron-rich nuclei.

Fusion of light neutron-rich nuclei also allows one to address interesting nuclear structure and fusion dynamics questions. Simple barrier penetration models work well for predicting the fusion of light stable nuclei. Increasing the neutron number decreases the distance through which the nuclei must tunnel to achieve fusion. This trend should result in a systematic increase in the astrophysical S factor with increasing neutron number N. In addition, when multiple neutrons are added to the system, new modes of excitation of the neutron-rich skin could emerge. The excitation of these new modes could lead to a dramatic increase in the fusion cross-section over that predicted by static barrier penetration models. Indeed, the enhancement of sub-barrier fusion is observed for a number of heavy stable systems. Recent sub-barrier measurements by Loveland et al. show a fusion enhancement for 9Li+70Zn [Loveland06] attributable to the neutron-richness of the projectile. These considerations motivate a systematic program to measure how fusion rates increase with neutron number in light systems. Light systems present a good choice as the largest neutron excesses can be achieved and extreme N/Z will provide the most stringent tests of fusion models.

The first goal of the proposed studies is to observe a smooth increase with N expected from the increase in the nuclear radius. Quantifying this trend is important. Additionally, we will look for any dramatic changes with N that might signal new dynamics. Although calculations with coupled channel models are planned, such models may not provide the final answer as it is essential to know the channels involved in the fusion process. Systematic high-quality data are clearly needed.

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