S. N. Ershov

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.

Direct Experimental Evidence for a Soft-Dipole Response in 6He

The differential cross-sections of the charge exchange reaction 6Li(7Li, 7Be) 6He were measured at beam energies 78 and 82 MeV. In addition to the 0+ bound state and sharp 2+ resonance, three wide bumps were identified in the experimental continuum spectrum of 7Be at small angles. The maximum at excitation energy of ~ 6 MeV (Γ ~ 5 MeV FWHM) is consistent with a soft-dipole response, expected in nuclei with neutron halo.

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.

Microscopic DWIA analysis of the (p, n) reactions

Abstract The charge-exchange nuclear excitations for the isobars 48 Ca → 48 Sc , 90 Zr → 90 Nb and 208 Pb → 208 Bi are considered in the framework of the theory of finite Fermi systems by exactly taking into account the particle-hole continuum. The distorted-wave impulse approximation (DWIA) is employed to calculate the angular distributions for the isobaric analogue states (IAS), Gamow-Teller resonances (GTR), dipole ( L = 1, S = 0), spin-dipole ( L = 1, S = 1), and so on, up to L = 7 excitations in the (p, n) reactions at bombarding proton energies E p between 100 and 200 MeV. The results obtained are compared with experimental data and the conclusions concerning both the effective nucleon-nucleon interaction in the chargeexchange channel and the local quasiparticle charge e q [ στ ] with respect to the στ fields are discussed. It occurred that just below the GTR at small angles there is essentially no background from transitions with L > 0. It is shown that the theory describes fairly well the experimental data with the Landau-Migdal spin-isospin strength parameter g ′ = 1.1 ( G ′ = 330 MeV · fm 3 ) and e q [ στ ] = 0.8. The latter characterizes the quenching of the low-energy spin-isospin-flip transitions, i.e. the renormalization of the axial-vector vertices in nuclear matter.

A study of the 14C(6Li, 6He)14N reaction at 93 MeV

Abstract The reaction 14 C( 6 Li, 6 He) 14 N was investigated with 93 MeV 6 Li ions in an angular interval of 7–26°. Angular distributions were analysed for the four most intense groups of 6 He nuclei, corresponding to transitions to the ground (1 1 + ) and the excited (1 2 + , 2 1 − , 4 1 − ) states of 14 N. In the theoretical analysis a mechanism of the spin-isospin excitation was suggested in the DWBA frame with the finite range of interaction and recoil in the light system ( 6 Li 6 He) taken into account. In the calculations both shell-model wave functions and transition densities obtained in the theory of finite Fermi systems (FFS) were used. From the comparison between theory and experiment the Landau-Migdal force constant g ′ is estimated in order to obtain some information on the degree of nuclear proximity to the threshold of pion condensation.

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.

Isovector soft dipole mode in 6Be

By using the H-1(Li-6,Be-6)n charge-exchange reaction, population of continuum states in Be-6 was observed up to E-T = 16 MeV, E-T being the Be-6 energy above its three-body decay threshold. In kinematically complete measurements performed by detecting alpha + p + p coincidences, an E-T spectrum of high statistics was obtained, containing approximately similar to 5 x 10(6) events. The spectrum provides detailed correlation information about the well-known 0(+) ground state of Be-6 at E-T = 1.37 MeV and its 2(+) state at E-T = 3.05 MeV. Moreover, a broad structure extending from 4 to 16 MeV was observed. It contains negative parity states populated by Delta L = 1 angular momentum transfer without other significant contributions. This structure can be interpreted as a novel phenomenon, i.e. the isovector soft dipole mode associated with the Li-6 ground state. The population of this mode in the charge-exchange reaction is a dominant phenomenon for this reaction, being responsible for about 60% of the cross-section obtained in the measured energy range.

New insight in halo fragmentation

A microscopic approach to fragment momentum distributions in Boromean two-neutron halo nuclei has been developed, which takes the correlated structure of the halo continuum into account. The crucial role of the low-lying three-body resonances and soft modes is demonstrated.

New soft mode in the 11Li nucleus excited through (p,p′) reaction

Abstract Inclusive proton energy spectra from the 11 Li(p,p′) reaction at E p =60 and 100 MeV are calculated using the distorted wave method and particle-hole nucleon transition densities in a self-consistent mean field. It is shown that the excitation of a cluster of resonances with various spin-parities just above continuum threshold, i.e. a new soft mode, causes a very distinct bump in these spectra with the main contribution, at small scattering angles θ cm , coming from the soft monopole transitions.

Discrete expansions of continuum wave functions

Abstract Different methods of expanding continuum wave functions in terms of discrete basis sets are discussed. The convergence properties of these expansions are investigated, both from a mathematical and a numerical point of view, for the case of potentials of Woods-Saxon and square well type.