Radius of curvature of the S factor maximum in sub-barrier fusion hindrance

A maximum of the S(E) factor is evidence for an onset of sub-barrier fusion hindrance and it can be well described by a radius-of-curvature expression near the maximum. The systematics of this radius of curvature has been studied over a wide range of projectile-target combinations. It follows a tentative general trend as a function of the parameter ζ=Z1Z2μ, and is strongly affected by effects associated with the nuclear structure of the nuclei in the entrance channel. It also explains the reason why the S factor maximum is not easily recognized visually for lighter, astrophysically interesting fusion systems

Expectations for C12 and O16 induced fusion cross sections at energies of astrophysical interest

The extrapolations of cross sections for fusion reactions involving C12 and O16 nuclei down to energies relevant for explosive stellar burning have been reexamined. Based on a systematic study of fusion in heavier systems, it is expected that a suppression of the fusion process will also be present in these light heavy-ion systems at extreme sub-barrier energies due to the saturation properties of nuclear matter. Previous phenomenological extrapolations of the S factor for light heavy-ion fusion based on optical model calculations may therefore have overestimated the corresponding reaction rates. A new "recipe" is proposed to extrapolate S factors for light heavy-ion reactions to low energies taking the hindrance behavior into account. It is based on a fit to the logarithmic derivative of the experimental cross section which is much less sensitive to overall normalization discrepancies between different data sets than other approaches. This method, therefore, represents a significant improvement over other extrapolations. The impact on the astrophysical reaction rates is discussed

Survey of heavy-ion fusion hindrance for lighter systems

A survey of heavy-ion fusion cross sections at extreme sub-barrier energies has been carried out for lighter systems with positive Q values. A general parametrization is proposed, which describes excitation functions for a wide range of light systems at low energies. This parametrization is then applied to a calculation of excitation functions and S factors for the system O16+O16, which has recently been investigated with various other theoretical approaches. It is suggested that this parametrization is useful for estimating sub-barrier fusion cross sections with exotic neutron-rich partners which cannot be studied in the laboratory

Systematics of heavy-ion fusion hindrance at extreme sub-barrier energies

The recent discovery of hindrance in heavy-ion induced fusion reactions at extreme sub-barrier energies represents a challenge for theoretical models. Previously, it has been shown that in medium-heavy systems, the onset of fusion hindrance depends strongly on the "stiffness" of the nuclei in the entrance channel. In this work, we explore its dependence on the total mass and the Q-value of the fusing systems and find that the fusion hindrance depends in a systematic way on the entrance channel properties over a wide range of systems

Analysis of heavy-ion fusion reactions at extreme sub-barrier energies

A coupled-channels analysis has been carried out for fusion reactions in the system [Formula Presented]. It demonstrates that conventional coupled-channels calculations are unable to reproduce the unexpected steep falloff of the recently measured cross sections at extreme sub-barrier energies. Heavy-ion fusion excitation functions are also analyzed in terms of the [Formula Presented] factor, as this offers a pragmatic way to study fusion behavior in the energy regime of interest. It is shown that the steep falloff in cross section observed in several heavy-ion systems translates into a maximum of the [Formula Presented] factor. The energies where the maximum occurs can be parametrized with a simple empirical formula. The parametrization, which is derived here for rather stiff heavy-ion systems, provides an upper limit for reactions involving softer nuclei

Measurement Near Threshold of 9-Be(3-He, Pi) to the A = 12 Isobaric Triplet by Recoil Detection

This work was supported by the National Science Foundation Grant NSF PHY 81-14339 and by Indiana Universit

Origin and consequences of C12+C12 fusion resonances at deep sub-barrier energies

Previous explanations for the resonance behavior of C12+C12 fusion at low energies were based on a nonresonant compound-nucleus background and an additional contribution from a series of resonances. This separation into "resonance" and "background" contributions of the cross section is artificial. We propose to explain this phenomenon through the impact on the cross section of the relatively large spacings and the narrow widths of Mg24 compound levels in the corresponding excitation-energy region

Do we understand heavy-ion fusion reactions of importance in stellar evolution?

Since the first observation of hindrance in heavy-ion fusion, many extrapolated cross sections for astrophysically interesting fusion reactions, such as 12C + 12C, 12C + 16O, 16O + 16O, 24O etc. need to be reexamined. In this contribution, the effects of fusion hindrance at extreme low energies are discussed

Preparations for Recoil Detection System at the Cooler T-Site

This research was sponsored by the National Science Foundation Grant NSF PHY-931478

Ru48 single-nucleon transfer at the barrier

Single-nucleon transfer cross sections have been measured for the 48Ti+104Ru reaction over a large angular range at an energy near the Coulomb barrier. Evidence has been found previously in -ray studies for superdeformed shapes in the compound system (152Dy) reached by this reaction. Reaction channels which couple to these shapes may experience interaction time delays, which would be revealed experimentally by broadened angular distributions. Although an enhancement is found in the forward angle Ti49 yields, this enhancement is small and may reflect uncertainties in the analysis