Sub-barrier Fusion Reactions Research Articles

Anomalously broad spin distributions in sub-barrier fusion reactions.

The mean square spin value of the compound nucleus $^{248}\mathrm{Cf}$ has been determined from fissionfragment angular distributions for the $^{12}\mathrm{C}$ + $^{236}\mathrm{U}$ and $^{16}\mathrm{O}$ + $^{232}\mathrm{Th}$ reactions. The anisotropy and, hence, the mean square spin values are much larger than expected. Models which reproduce the enhancement observed in the sub-barrier fusion excitation functions fail to account for the spin-distribution observations.

Alignment effects in subbarrier fusion reactions

Subbarrier fusion reactions in which one constitutent is deformed and aligned are considered in the framework of a coupled-channel model with incoming-wave boundary conditions. The effect of alignment on the fusion cross section is estimated to be 40% for aligned 165Ho. The semiclassical analog to the coupled-channel equations is developed, yielding an expression for the cross section in terms of a weighted average over all orientations of the deformed nucleus.

Influence of neck formation on heavy ion subbarrier fusion.

We calculate the effect of the neck degree of freedom on subbarrier fusion reaction cross sections. The potential energy and mass tensor are calculated on the basis of a two-dimensional representation of the liquid drop model. The WKB and Hill-Wheeler approximations are used to calculate the tunneling amplitudes for motion along the direction of the neck size degree of freedom. The resulting cross section for collisions between two identical Ni isotopes show a marked enhancement at low energies compared with the standard one-dimensional radial penetration calculations.

Open problems in sub-barrier fusion reactions

The magnitude and dependence on mass number displayed by sub-Coulomb fusion cross sections is wrongly predicted by standard penetrability calculations. Qualitatively the observed unsystematic A-dependence can be correlated with specific two-nucleon transfer processes, while the large measured cross sections can be understood in terms of zero point fluctuations. Schematic model calculations indicate that the coupling of the entrance channel to inelastic and transfer channels act to enhance the transmission through the barrier at low bombarding energies and decrease it at energies above the barrier. To become more quantitative one has to treat these couplings in detail making use of the specific formfactors and taking care of the associated Q-value effects.