S. A. Goncharov

Rainbows in Nuclear Reactions and the Optical Potential

Nucleus-nucleus scattering and charge-exchange reactions are studied in a region where rainbow-like phenomena are seen. Optical potentials are found which give fits to the cross sections, but in spite of the rainbow-like phenomena and contrary to apparently accepted expectations, not determined unambiguously. Other possibilities of resolving the ambiguities are discussed.

Pronounced Airy structure in elastic16O+12Cscattering atElab=132MeV

Measurement of elastic {sup 16}O+{sup 12}C scattering at E{sub lab}=132 MeV has been performed over the angular range 6{degree}{lt}{Theta}{sub c.m.}{lt}125{degree}, which covers both the diffractive and refractive regions. A prominent minimum has been observed at {Theta}{sub c.m.}{approx}86{degree}, which can be identified as an Airy minimum preceding the rainbow maximum. It thus provides the first clear experimental evidence for the refractive (rainbow) scattering pattern in the {sup 16}O+{sup 12}C system. This Airy structure can be well described by discrete sets of optical potentials with a relatively weak absorption and a deep real potential. Candidates for the realistic family of {sup 16}O+{sup 12}C optical potentials at E{sub lab}=132 MeV are discussed; those include the semimicroscopic potential given by the double-folding model. {copyright} {ital 1998} {ital The American Physical Society}

Scattering of 3He on 12C and the inelastic form factor

Abstract Measurements of inelastic scattering of 3He on 12C at EHe = 72 MeV with excitation of low lying 2+, 3−, 0+ states of the target are reported. The data were analyzed both by DWBA and coupled-channels approximation, the latter leading to only minor changes, and we argue that nuclear rainbow effects are present. It is possible to reproduce the main features of the 2+ and 3− inelastic angular distributions by means of the optical model potential previously selected for the reaction (3He, t) on 13,14C targets, only if the derivative excitation form factors are modified in the vicinity of the strong absorption radius. The fit is improved in detail and can be made nearly perfect if more flexible model-independent form factors are employed. An alternative potential is also discussed.

Observation of abnormally large radii of nuclei in excited states in the vicinity of neutron thresholds

Differential cross sections for inelastic scattering leading to the excitation of some nuclear states situated near neutron-emission thresholds were analyzed. With the aid of a modified diffraction model, abnormally large radii were found for the 1/21+ state of the 13C nucleus at 3.09 MeV, for the first levels of positive-parity rotational bands in the 9Be (1/2+ level at 1.68 MeV and 5/2+ level at 3.05 MeV) and 11Be (5/2+ level at 1.78 MeV and 3/2+ level at 3.41 MeV) nuclei, and for the 21+ state of the 14Be nucleus at 1.54 MeV and 11− state of the 12Be nucleus at 2.7 MeV. All of these states possess signatures typical of neutron halos.

Nuclear rainbow in scattering and reactions and nucleus-nucleus interaction at small distances

A review of recent experiments on the study of the nuclear rainbow phenomenon in scattering and some reactions induced by light heavy ions is given. It includes (i) the study of the differential cross sections of the 16O + 12C elastic scattering at seven 16O energies between 130 and 281 MeV; (ii) finding of the phenomenological potential deeper than that of folding model; (iii) the first data on 16O + 14C elastic scattering; (iv) dispersion relation analysis of the obtained data and observation of abnormal nuclear dispersion; and (v) use of the charge-exchange 14C(6Li,6He)14N reaction to search for pion-condensation effects.

Study of rlastic and inelastic 11B +α scattering and search for cluster states of enlarged radius in 11B

The differential cross sections for elastic and inelastic 11B + α scattering were measured at the alpha-particle energy of 65 MeV, the inelastic-scattering processes leading to the excitation of known states of 11B up to excitation energies of about 14 MeV. Data on elastic scattering were analyzed together with those that were published earlier for lower energies. The cross sections for inelastic scattering were analyzed on the basis of the distorted-wave method. A modified diffractionmodel was used to determine the root-mean-square radii of excited states. The radii of states whose excitation energies were below about 7MeV were found to agree with radius of the ground state to within 0.1 to 0.15 fm. This result complieswith the traditional idea that the low-lying states of 11B have a shell structure. The possibility that these states belong to the predicted rotational bands, which, if any, are truncated to three states, cannot be ruled out either. The majority of the observed highly excited states are distributed among four rotational bands. The moments of inertia of these bands are close; for the band based on the 3/2− state at E* = 8.56 MeV, they are even higher than those of the Hoyle state in the 12C nucleus. The measured radii of states associated with these bands of 11B are larger than the ground-state radius by 0.7 to 1.0 fm and are also close to the radius of the Hoyle state. The results of the present study agree with the existing predictions that the cluster structure of the 11B nucleus is diverse at high excitation energies. The hypothesis that the 11B nucleus features a “giant” state of size commensurate with those in heavy nuclei was not confirmed.

Nucleus-nucleus potential within the semimicroscopic dispersive model on the basis of a corrected folding-model potential

A procedure is proposed for correcting the energy dependence of the mean-field component of the optical-model potential, this component being calculated within the double-folding model with allowance for exchange effects in the approximation of one-nucleon knockout exchange. The procedure is based on employing, for the mean field, an empirical energy dependence that is obtained from a dispersion analysis of integrated features of phenomenological optical-model potentials and is tested by applying it to calculating the mean field and to analyzing data on the elastic scattering of alpha particles on oxygen nuclei within the semimicroscopic dispersive optical model.

Dispersive semimicroscopic analysis of nuclear-nuclear collisions on the basis of a corrected folding-model potential

A procedure for correcting the energy dependence determined for the mean-field component of the optical potential on the basis of the double-folding model with allowance for exchange effects in the approximation of single-nucleon exchange knockout is used to calculate the mean field and to analyze data on elastic scattering within the dispersive semimicroscopic optical-potential model. The results of an analysis of all data (including those that were obtained quite recently) on 16O + 12C and 16O + 14C interactions at energies of up to 100 MeV per nucleon are presented. The energy dependence of the resulting potentials agrees well with that which was obtained previously within the phenomenological approach. A systematics of the positions of Airy minima in angular distributions versus the reduced mass in the range from 2 to 8 amu is constructed.

Theoretical Analysis of the Elastic and Inelastic Scattering 8He + p at E = 72 MeV/u

We study the excited-state quantum numbers in the extremely neutron-rich nucleus 8He and show that the existing experimental data admit spin-parity Jπ = 2+, which is standard for first-excited states in nuclei with even numbers of protons and neutrons, as well as the unique case of Jπ = 1-. To admit discrimination between these two cases, the existing experimental inelastic data have to be extended to smaller angles. We also try the sensitivity of the elastic scattering of halo nuclei at energies about 100 MeV/u to details of the halo distribution.

Investigation of the nucleus-nucleus interaction at small distances in elastic scattering of 6li and the reaction (6Li, 6He) on carbon isotopes

Abstract The cross sections of elastic scattering of 6Li and of the charge-exchange reaction (6Li, 6He) on the nuclei 13C and 14C were measured at the projectile energy 93 MeV. A theoretical analysis of the reaction was carried out using microscopic DWBA with central and tensor components of the nucleon-nucleon forces and the decomposition of the reaction amplitude to near side and far side components. The far components were seen to dominate both the elastic scattering and the reaction for the angles above 30°, and a small shoulder in the differential cross section curves around 90° in elastic scattering and in 14C(6Li, 6He) 14N(3.95) may be interpreted as a rudimentary nuclear rainbow. The analysis of the scattering data lead to a new family of potentials for the systems 6Li + 12,13,14C. The reproduction of the differential cross sections for population of the 1+ states in the charge-exchange reaction at large angles called for a change of the form factors, compared to the theoretical ones, for radii which were considerably smaller than the strong absorption radius. It is possible that to reproduce the (6Li, 6He) reaction data, a 20% difference between the imaginary potentials of 6He and 6Li is needed.