J.S. Lilley

The threshold anomaly in 16O + 208Pb scattering

Abstract The empirically observed energy dependence of the optical potential at energies close to the Coulomb barrier (the threshold anomaly) in the collisions of 16 O on 208 Pb is investigated within the framework of coupled-reactions-channels (CRC) calculations. The motivation is to determine the extent to which the observed threshold anomaly can be accounted for by strong coupling of the elastic to the inelastic and transfer channels. CRC calculations have been performed at a range of energies above and below the Coulomb barrier, both for the complete set of reaction channels, and also, to see the effects more simply, for individual inelastic and transfer channels. An energy-dependent local polarisation potential has been extracted. It will be seen how this polarisation potential satisfies the dispersion relations, and how well it reproduces the elastic scattering and the fusion cross sections over a range of energies.

Heavy-ion fusion: Channel-coupling effects, the barrier penetration model, and the threshold anomaly for heavy-ion potentials*

We study the formal structure of the influence of channel coupling on near- and sub-barrier fusion. The reduction to a one-channel description is studied, with emphasis on the channel-coupling effects being manifest primarily as an energy dependence (the “threshold anomaly”) of the real nuclear potential. The relation to the barrier penetration model is examined critically. The results of large-scale coupled-channel calculations are used as “data” to illustrate the discussion. Particular emphasis is placed on the importance of reproducing correctly the partial-wave (or “spin”) distributions. The simple barrier penetration model is found to be adequate to exhibit the strong enhancements due to channel couplings when the threshold anomaly is taken into account, although there may be important corrections due to the long-ranged peripheral absorption, especially from Coulomb excitation.

Energy dependence of the 16O + 60Ni potential and the optical model dispersion relation

Abstract A systematic analysis of 16 O + 60 Ni elastic scattering data over a range of bombarding energies reveals strong variations of both the real and imaginary parts of the optical potential near the Coulomb barrier. The results are consistent with a dispersion relation interpretation of the effect which predicts a peak in the real surface potential close to the barrier.

Multistep effects in reactions of 17O with 208Pb near the coulomb barrier

Abstract The elastic scattering, inelastic scattering and single-neutron transfer cross sections in the collision of 78 MeV 17 O on 208 Pb are measured. It is shown that these cross sections can be understood in terms of a three-body model of the system comprising a 16 O core, a valence neutron and a 208 Pb target. The theoretical analysis points to the importance of multistep effects in all reaction processes. The inherent energy dependence of the 16 O 208 Pb elastic optical potential at energies close to the Coulomb barrier is also seen in 17 O 208 Pb elastic scattering. The effect of the valence-neutron channel coupling on the 17 O 208 Pb interaction potential is deduced, and is shown to be almost entirely imaginary. Implications for a possible isotope dependence of the sub-barrier fusion cross section are considered.

Magnetic substrate population in heavy-ion inelastic scattering at energies near the Coulomb barrier

The effect of nuclear reorientation coupling on the magnetic substate population in heavy ion inelastic scattering is discussed. Coupled channels calculation of the inelastic scattering of 56 MeV 16O projectiles by 92Zr leading to its first excited 2+ state is used as an illustration for the discussion. It is shown that at energies near the Coulomb barrier, the information on reorientation coupling can be extracted from the magnetic substrate population only by a careful analysis in the Coulomb-nucleus interference region.

Symmetric fission of 24Mg following inelastic scattering

Abstract Symmetric fission into two 12 C nuclei has been observed from states at high excitation energy in 24 Mg excited in inelastic scattering. Fission to the 10 B+ 4 N channel has also been observed and indirect evidence for the 16 O+ 8 Be channel noted. The same experiment also reveals fission into 16 O+ 12 C from states in 28 Si populated by alpha-particle pickup onto the 24 Mg projectile. The correlation of the states in 24 Mg observed in this experiment with those previously observed in 12 C+ 12 C scattering is discussed.

208Pb(7Li,6Li)209Pb reaction at 33 MeV and its effect on elastic scattering

Abstract Angular distributions of the differential cross section and the second-rank tensor analysing power T 20 have been measured for the one-neutron transfer reaction induced by polarized 7 Li ions on 208 Pb at 33 MeV leading to five states in 209 Pb. The experimental data were analysed simultaneously with data for elastic and inelastic scattering of 7 Li by means of CCBA and CRC calculations. A strong effect of the transfer channels on the elastic scattering was found. Coupling to the transfer channels cancelled out the effect of projectile excitation on the elastic-scattering cross section but was unable to account fully for the renormalization factor of the double-folding potential found previously in an optical-model analysis of 7 Li+ 208 Pb elastic scattering. The second-rank tensor analysing power for the elastic scattering was strongly affected by the transfer channels.

Symmetric and near-symmetric fission of 24Mg, 28Si and 32S

Abstract In several recent measurements the symmetric fission of 24 Mg has been reported. In this paper the results of a search for similar processes in 28 Si and 32 S are presented. In contrast to the results for 24 Mg, no evidence is seen for the fission of 28 Si to 16 O+ 12 C or 32 S to 16 O+ 16 O following inelastic excitation. However, these processes are observed to occur following pickup onto 24 Mg. The results are consistent with a simple model of the fission process developed by Harvey and suggest the existence of large scale cluster structure in all three nuclei.

16O+8Be breakup of 24Mg

Abstract 16O+8Be breakup of 24Mg has been identified following inelastic scattering of 24Mg projectiles. The spectrum of states observed is compared to that previously measured in the 12C+12C breakup channel to obtain information on the partial decay widths.

Evidence of projectile polarisation effects on the reaction 208Pb(17O, 16O)209Pb

Abstract Coupled reaction channels calculations (CRC) of the reactions 208 Pb( 17 O, 16 O) 209 Pb leading to different states of 209 Pb are compared with DWBA predictions at projectile energies of 78, 86 and 102 MeV. The calculations exhibit strong effects of multistep processes on Q -value and angular-momentum-mismatched transfer reactions. It is shown that the contribution to the transfer through the inelastic excitation of 17 O contains a major part of the multistep effect. A simple three-channel model comprising the elastic, inelastic and transfer channels is constructed which simulates the CRC effects on the transfer cross sections. The polarization effects of the eliminated channels give rise to effective potentials which are mainly imaginary.