Fusion of light proton-rich exotic nuclei at near-barrier energies

We study theoretically fusion of the light proton-rich exotic nuclei $^{17}$F and $^8$B at near-barrier energies in order to investigate the possible role of breakup processes on their fusion cross sections. To this end, coupled channel calculations are performed considering the couplings to the breakup channels of these projectiles. In case of $^{17}$F, the coupling arising out of the inelastic excitation from the ground state to the bound excited state and its couplings to the continuum have also been taken into consideration. It is found that the inelastic excitation/breakup of $^{17}$F affect the fusion cross sections very nominally even for a heavy target like Pb. On the other hand, calculations for fusion of the one-proton halo nucleus $^8$B on a Pb target show a significant suppression of the complete fusion cross section above the Coulomb barrier. This is due to the larger breakup probability of $^8$B as compared to that of $^{17}$F. However, even for $^8$B, there is little change in the complete fusion cross sections as compared to the no-coupling case at sub-barrier energies.Comment: 11 pages, 3 figures, Revtex.st

Fusion barrier distributions in systems with finite excitation energy

Eigen-channel approach to heavy-ion fusion reactions is exact only when the excitation energy of the intrinsic motion is zero. In order to take into account effects of finite excitation energy, we introduce an energy dependence to weight factors in the eigen-channel approximation. Using two channel problem, we show that the weight factors are slowly changing functions of incident energy. This suggests that the concept of the fusion barrier distribution still holds to a good approximation even when the excitation energy of the intrinsic motion is finite. A transition to the adiabatic tunneling, where the coupling leads to a static potential renormalization, is also discussed.Comment: 9 pages, 4 figures, Submitted to Physical Review

Effect of continuum couplings in fusion of halo 11^{11}Be on 208^{208}Pb around the Coulomb barrier

The effect of continuum couplings in the fusion of the halo nucleus $^{11}$Be on $^{208}$Pb around the Coulomb barrier is studied using a three-body model within a coupled discretised continuum channels (CDCC) formalism. We investigate in particular the role of continuum-continuum couplings. These are found to hinder total, complete and incomplete fusion processes. Couplings to the projectile $1p_{1/2}$ bound excited state redistribute the complete and incomplete fusion cross sections, but the total fusion cross section remains nearly constant. Results show that continuum-continuum couplings enhance the irreversibility of breakup and reduce the flux that penetrates the Coulomb barrier. Converged total fusion cross sections agree with the experimental ones for energies around the Coulomb barrier, but underestimate those for energies well above the Coulomb barrier.Comment: 15 pages, 7 figures, accepted in Phys. Rev.

Fusion of light exotic nuclei at near-barrier energies : effect of inelastic excitation

The effect of inelastic excitation of exotic light projectiles (proton- as well as neutron-rich) $^{17}$F and $^{11}$Be on fusion with heavy target has been studied at near-barrier energies. The calculations have been performed in the coupled channels approach where, in addition to the normal coupling of the ground state of the projectile to the continuum, inelastic excitation of the projectile to the bound excited state and its coupling to the continuum have also been taken into consideration. The inclusion of these additional couplings has been found to have significant effect on the fusion excitation function of neutron-rich $^{11}$Be on $^{208}$Pb whereas the effect has been observed to be nominal for the case of proton-rich $^{17}$F on the same target. The pronounced effect of the channel coupling on the fusion process in case of $^{11}$Be is attributed to its well-developed halo structure.Comment: 9 pages, 3 figures, Revtex.st

Quantum Tunneling in Nuclear Fusion

Recent theoretical advances in the study of heavy ion fusion reactions below the Coulomb barrier are reviewed. Particular emphasis is given to new ways of analyzing data, such as studying barrier distributions; new approaches to channel coupling, such as the path integral and Green function formalisms; and alternative methods to describe nuclear structure effects, such as those using the Interacting Boson Model. The roles of nucleon transfer, asymmetry effects, higher-order couplings, and shape-phase transitions are elucidated. The current status of the fusion of unstable nuclei and very massive systems are briefly discussed.Comment: To appear in the January 1998 issue of Reviews of Modern Physics. 13 Figures (postscript file for Figure 6 is not available; a hard copy can be requested from the authors). Full text and figures are also available at http://nucth.physics.wisc.edu/preprints

The binding of tritiated ouabain to mammalian non-myelinated nerve fibres

1. A study has been made of the binding of radioactively labelled ouabain by desheathed rabbit vagus nerves, which consist mainly of non-myelinated fibres. The corresponding inhibition of the electrogenic sodium pump was also measured. 2. By varying the ouabain concentration and the external potassium concentration two kinds of binding sites could be distinguished: a first site specifically associated with pumping and whose ability to bind ouabain is dependent on the external presence of potassium; and a second site not associated with pumping and unaffected by external potassium. 3. Just complete inhibition of the sodium pumping mechanism is associated with a specific binding of ouabain of about 4·3 p-mole/mg dry nerve. 4. This gives an upper limit for the density of sodium pumping sites of about 750 per square micron. 5. The turnover rate (i.e. (cation pumped)/(number of sites)) at 20° C is about 22 sec(-1)

Optical studies on the kinetics of the sodium pump in mammalian non-myelinated nerve fibres

1. A study has been made of the changes in the fluorescence of desheathed rabbit cervical vagus nerves that occur during and after electrical stimulation of its non-myelinated fibres. 2. Stimulation for 5 sec at 30 shocks/sec produces a maximal decrease, of about 1% of the resting fluorescence. Stimulation for less than 0·5 sec fails to produce responses visible above the inherent noise in the recording system. 3. A pharmacological dissection (with ouabain, metabolic inhibitors, and calcium) has revealed four phases of fluorescence change: (a) under conditions where the sodium pump is functioning, there is a prolonged decrease in the fluorescence following electrical activity; (b) even in the absence of pumping the mere entry of sodium into the nerve causes an initial decrease in fluorescence; (c) the entry of calcium ions with electrical activity also causes an initial rapid decrease in fluorescence; (d) following these phases of decreased fluorescence there is a phase of increased fluorescence. 4. These changes in fluorescence are related to changes in the NADH concentration in the nerve resulting from: (a) the splitting of ATP during sodium extrusion; (b) the initial binding of sodium to the sodium- and potassium-dependent ATPase, which is the sodium pump; (c) the stimulation of mitochondrial respiration by calcium that has entered during the spike; and (d) an increased glycogenolysis as a result of the calcium entry during activity