H. G. Bohlen

Elastic transfer processes in heavy ion scattering

Abstract A survey of the scattering of nuclei on nuclei of adjacent masses in which elastic transfer can occur is given. Scattering systems with mass differences ranging from 1 nucleon up to 5 nucleons are discussed. The interpretation of the available data in terms of the (i) optical model, (ii) the coherent addition of elastic scattering and transfer using the DWBA and (iii) in terms of the molecular two state approximation and coupled channels is discussed. Examples of calculations are given and the extraction of spectroscopic information is illustrated. The occurrence of multiple exchange of nucleons — higher order effects compared to DWBA — is discussed. It is shown that these effects are possible for weakly absorbing scattering systems. Future applications and extensions to larger masses are outlined.

Study of diffractive and refractive structure in the elastic 16O+16O scattering at incident energies ranging from 124 to 1120 MeV

Abstract The experimental data on elastic 16 O + 16 O scattering at incident energies ranging from 124 to 1120 MeV have been analyzed within the standard optical model (OM), using either the phenomenological (Woods-Saxon squared) potential or that calculated within the double-folding model for the real part of the optical potential. Structure of the elastic cross sections at smallest scattering angles was found to be of a pure diffractive nature, which enabled a consistent check of the absolute normalization of the elastic data under study. The OM analysis shows unambiguously the evolution of the refractive scattering pattern in the 16 O + 16 O system over this energy range. The large angle region of the data is dominated by the refractive far-side scattering. The oscillating Airy structure can be consistently described by a set of optical potentials with the real part given by the folding model and a weak absorptive imaginary potential.

Spectroscopy of exotic nuclei with multi-nucleon transfer reactions

Structure investigations of neutronrich helium isotopes and of10Li and11Be have been performed using multi-nucleon transfer reactions. New results have been obtained for7He He, where we have evidence for the observation of the first excited state, i.e. the 1/2-resonance, which is of interest for the spin-orbit splitting in these light neutronrich nuclei. Our results on8He,9He and10He are briefly summarized. Reactions of distinct selectivity have been chosen to study the states of10Li. The structure of the eight states found in these reactions is discussed, as well as their relevance for the decay of11Li resonances. The isotopes10Be and11Be have been investigated to study the a-cluster structure and high-lying states. Nine states of the observed11Be-resonances can be characterized as a molecular rotational band extending up to 25 MeV excitation energy, with a probable maximum spin of 19/2. The reaction mechanism is studied in terms of two-step calculations, where protons are picked-up from the target and neutrons are transferred to the target. Using measured strength amplitudes for the single steps results in a quantitative description of the observed two-step cross sections.

Nuclear rainbow structures in the elastic scattering of 16O on 16O at EL=350 Me

Abstract The elastic scattering of 16O on 16O has been measured at 22MeV/u in a large angular range up to θcm=61° with high accuracy. Besides the Fraunhofer diffractive patterns at forward angles a clear oscillatory structure is observed at large angles. It is shown in a semi-classical analysis that this structure is due to a rainbow interference of two refractive amplitudes (the second Airy maximum is identified). These data show a nuclear rainbow structure for the first time in heavy-ion scattering (A>6) with unambiguous clarity.

Refractive scattering and the nuclear rainbow in the interaction of12, 13C with12C at 20MeV/N

The elastic and inelastic scattering and the neutron transfer have been measured for the systems12C +12C and13C +12C at 20MeV/N up to θcm= 60° with theQ3D -spectrometer. The angular distributions of the elastic scattering show an enhanced cross section at angles larger than 40°. It can be identified as refractive scattering with the clear signature of a nuclear rainbow.L-cut-off calculations show that these contributions come fromL-values which are significantly lower than the grazingL-value. The deflection function has a broad minimum in thisL-range which is typical for rainbow scattering. TheS-matrix is decomposed by a phenomenological parametrization into a refractive and a diffractive part. The interference of these amplitudes plays an important role in the rainbow enhancement. The spatial localization of the refractive scattering is deduced from the turning points of the corresponding trajectories; a localization between 2.5 fm and 4 fm is found. Semi-classical calculations with complex trajectories in the single-turning-point approximation show good agreement with the quantummechanical calculations. Refractive contributions are not observed in the inelastic scattering. This can be explained by reducing the strength of the conventional collective form factor in the internal region. In contrast to this the enhancement at large angles is seen in the one-neutron transfer channels where the refractive scattering is dominant. This is the first observation of such contributions to heavy-ion transfer reactions.

Spectroscopy of 10He

Abstract The mass of 10 He has been measured with the double charge exchange reaction 10 Be( 14 C, 14 O) 10 He at E Lab = 334.4 MeV and a mass excess M.E. = 48.81 (7) MeV is obtained. It follows that 10 He is particle unstable against 2 n - emission by S 2n = −1.07(7) MeV . Two excited states are observed: a broad resonance at an excitation energy of E x = 3.24(20) MeV , and a strong resonance at E x = 6.80(7) MeV .

Observation of the nuclear rainbow scattering for12C+12C atE Lab =300 MeV

The elastic and inelastic scattering of12C on12C has been measured in the angular range between 2.8° and 70.4° in the c.m. system atELab=300 MeV. Optical model calculations have been performed with Woods-Saxon and folded potentials, the ground state and the first 2+-state were coupled in the calculations. The large cross sections of the elastic scattering at large angles is related to the nuclear rainbow scattering, which is centered at about 56°. This requires a potential depth of 100 MeV at a distance of 3 fm, the fit to the data is sensitive down to this region. The calculations with the folded potential show a better agreement with the data than those with the Woods-Saxon shape. The total reaction cross section of 1,420 mb, obtained from the optical model analysis, corresponds to the geometrical value; no transparency is observed.

Nuclear rainbow scattering and nucleus–nucleus potential

Elastic scattering of α-particles and some tightly bound light nuclei has shown the pattern of rainbow scattering at medium energies, which is due to the refraction of the incident wave by a strongly attractive nucleus–nucleus potential. This review gives an introduction to the physics of the nuclear rainbow based essentially on the optical model description of the elastic scattering. Since the realistic nucleus–nucleus optical potential (OP) is the key to explore this interesting process, an overview of the main methods used to determine the nucleus–nucleus OP is presented. Given the fact that the absorption in a rainbow system is much weaker than that usually observed in elastic heavy-ion scattering, the observed rainbow patterns were shown to be linked directly to the density overlap of the two nuclei penetrating each other in the elastic channel, with a total density reaching up to twice the nuclear matter saturation density ρ0. For the calculation of the nucleus–nucleus OP in the double-folding model, a realistic density dependence has been introduced into the effective M3Y interaction which is based originally on the G-matrix elements of the Reid and Paris nucleon–nucleon (NN) potentials. Most of the elastic rainbow scattering data were found to be best described by a deep real OP like the folded potential given by this density-dependent M3Y interaction. Within the Hartree–Fock formalism, the same NN interaction gives consistently a soft equation of state of cold nuclear matter which has an incompressibility constant K≈ 230–260 MeV. Our folding analysis of numerous rainbow systems has shown that the elastic α-nucleus and nucleus–nucleus refractive rainbow scattering is indeed a very helpful experiment for the determination of the realistic K value. The refractive rainbow-like structures observed in other quasi-elastic scattering reactions have also been discussed. Some evidences for the refractive effect in the elastic scattering of unstable nuclei are presented and perspectives for future studies are discussed.

Hot nuclei in reactions induced by 475 MeV, 2 GeV 1H and 2 GeV 3He

Abstract Inclusive neutron multiplicity distributions have been measured for 475 MeV, 2 GeV proton- and 2 GeV 3 He-induced reactions on Ag, Au, Bi, U targets. There is general agreement between these multiplicity data and results of intranuclear cascade calculations. The results indicate a broad distribution of excitation energies with 10% of the events exceeding 500 MeV. For a thermalized nucleus this would translate into temperatures exceeding 5 MeV

Spectroscopy of 13Be

Abstract Six quasi-stationary states of 13 Be populated in the 14 C( 11 B, 12 N) 13 Be reaction at E lab = 190 MeV are reported. A Q -value = −39.60(9) MeV and a mass excess, M.E.= 33.95(9) MeV, have been found for the lowest observed spectral line. The ground state is unstable with respect to one-neutron emission by 0.80(9) MeV. Excitation energies of 1.22(10), 2.10(16), 4.14(12), 5.09(14) and 7.0(2) MeV have been obtained for the observed spectral lines.