B. Gebauer

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 .

High energy ERDA with very heavy ions using mass and energy dispersive spectrometry

The capabilities of the elastic recoil detection analysis (ERDA) method for depth profiling are investigated using high energy very heavy ion beams with energies close to the Coulomb barrier from the VICKSI accelerator and mass and energy dispersive recoil spectrometry by means of a time-of-flight energy detector telescope. Due to the unusually high beam energies of the 129Xe and 127I beams of 260 to 420 MeV, which nevertheless remain safely within the Coulomb scattering regime, the high velocities even of heavy recoil ions allow us their detection with unambiguous mass and adequate energy resolution for depth profiling up to depths of e.g. 3–4 μm in Si and 2 μm in GaAs. The concentration profiles of all target elements are measured simultaneously, uniquely resolved, since their isotopic masses are separated at least up to the Ga/As mass range. Typical depth resolutions obtained are comparable with values for RBS. The kinematical shifts between the separate depth profile spectra of the different isotopes of a probed atomic element clearly demonstrate that the depth resolution is reduced if only charges but no masses are resolved unless one isotope dominates.

Stud of weakly bound and unbound states of exotic nuclei with binary reactions

Abstract Binary reactions using neutron-rich beams (and targets) give the best approach to study the level structure of exotic nuclei. Systematic cross section dependencies of multi-step transfer reactions are discussed. Some rections, for example the proton-neutron exchange ( 12 C, 12 N) and ( 13 C, 13 N) , have distinct selectivity for the population of unnatural or natural parity states, respectively. This selectivity has been applied to new spectroscopic studies of 10 Li using the 10 Be( 12 C, 12 N) and 9 Be)(P 13 C, 12 N) reactions. The lowest resonance of 10 Li observed at 0.24(6) MeV above the neutron threshold. The results are discussed together with other available data.

Novel large-area thermal neutron imaging detectors comprising 157GdCsI-convertors and micro-strip gas detectors with low-pressure, two-stage amplification and delay line readout

A novel large-area (0.5 × 0.5 m2) two-dimensional position-sensitive detector for thermal and sub-thermal neutrons is being developed comprising 157GdCsI multilayer convertor foils and two fourfold segmented micro-strip gas detector (MSGC) planes for electron multiplication on either side of the convertor. The MSGC operates in low-pressure, two-stage amplification mode; their novel design is optimized for that mode and for obtaining high signals on both coordinates. Fast signal rise times (5–6 ns) and very high gains (∼ 5 × 105) are expected according to calculations; thus, simple delay line readout, with one interpolating strip between readout nodes, can be used delivering expected position resolutions <0.5 mm (fwhm) and tolerable count rates of 105–106 events/s depending on delay line segmentation.

Mass spectroscopy of13Be

The mass of13Be has been measured with the reaction13C(14C,14O)13Be at ELab=337 MeV. A Q-value of Q0=−37.02(5) MeV was obtained and the mass excess is M.E.=35.16(5) MeV. If the observed line corresponds to the ground state,13 Be is particle unstable with respect to the oneneutron emission by 2.01 MeV. The observed line width of 0.3(2) MeV supports an assignment ofJπ=5/2+ or 1/2−, but excludesJπ=1/2+. An excited state is seen at 3.12(7) MeV; there are indications of a second excited state at 6.5(2) MeV.

Spectroscopy of 13B, 14B, 15B and 16B using multi-nucleon transfer reactions

Abstract: New information has been obtained on excited states of the neutron-rich boron isotopes 14B, 15B and 16B, using the reactions 12C(14C,12N)14B, 13C(14C,12N)15B and 14C(14C,12N)16B at about 24 MeV/A. The mass excess of 16B has been measured for the first time, it is 37.08(6) MeV. This means that 16B is unbound by only 0.04(6) MeV. Furthermore, the nucleus 13B has been investigated with the four reactions 16O(14C,17F), 12C(14C,13N), 12C(13C,12N) and 12C(15N,14O). Choosing different target-projectile combinations, it was possible to populate states with different selectivity. New states are observed in 13B at excitation energies above the threshold for two-neutron decay.

Model-unrestricted nucleus-nucleus scattering potentials from measurement and analysis of 16O + 16O scattering☆

Abstract The elastic scattering cross section for 16 O ions on 16 O targets has been measured with high accuracy over large angular ranges at incident energies from 250 to 704 MeV. From these data which sample both diffractive and refractive scattering processes, we extract the underlying scattering potentials using model-unrestricted analysis methods. The extracted potentials fit very well into the systematics found in light-ion scattering. The real potential parts also compare very favourably with microscopically calculated folding potentials based on a weak density-dependence of the underlying effective nucleon-nucleon interaction at large overlap densities.