M. Meister

Measurements of the interaction cross sections for Ar and Cl isotopes

We have measured the interaction cross sections (σI) of 31–40Ar and 31–37Cl on carbon targets at energies of around 950 A MeV. The effective matter radii for these nuclei were deduced by a Glauber-model analysis. Combining our matter radii with measured charge radii for Ar isotopes, we could deduce the proton-skin thicknesses for the 32–40Ar isotopes, which were found to increase monotonically with decreasing neutron number. The larger radius of the proton drip-line nucleus 31Ar suggests an anomalous structure for this nucleus. In addition, using NaI(Tl) arrays surrounding the carbon target, we measured γ-rays emitted from excited states in these isotopes. In this way we could deduce the upper limits for the inelastic cross sections (σinela) on carbon targets at energies of around 950 A MeV.

8He-6He: a comparative study of nuclear fragmentation reactions

Dissociation of 227 MeV/u He-8 in a carbon target has been studied in kinematically complete experiments. The data include the relative energy spectrum, angular distributions in the neutron knock-out channel (He-6 + n) as well as diffractive dissociation and inelastic scattering into the (He-6 + 2n) channel. The data are compared with corresponding results from the well-known halo nucleus He-6. In both cases it is found that neutron knock-out is the: dominating reaction channel. The relative energy spectrum (He-6 + n) shows a structure, which is interpreted as being due to the I-pi = 3/2(-) resonance in the He-7 ground state with about equal contribution from its I-pi = 1/2(-) spin-orbit partner. The He-7 resonance shows a spin alignment similar to that observed in He-5, but with a smaller anisotropy indicating that the structure of the He-8 ground state is more complicated than that of He-6. The data in the (He-6 + 2n) channel were used to identify resonances in the excitation energy spectrum of He-8. If the spectrum is interpreted as two overlapping resonances, the spin-parity assignment for these is found to be 2(+) and 1(-), respectively.

Searching for the 5H resonance in the t+n+n system

19 pages, 7 figures, 2 tables, 2 appendices.-- PACS nrs.: 27.10.+h; 25.60.Gc.-- Printed version published Jul 28, 2003.

Halo excitations in fragmentation of 6He at 240 MeV/u on carbon and lead targets

Dissociation of a 240 MeV/u beam of He-6, incident on carbon and lead targets, has been studied in kinematically complete experiments to investigate low-lying excitation modes in the halo nucleus He-6. It is shown that alignment effects characterize the inelastic scattering and allow an unambiguous assignment of the spin of a narrow resonance observed in the excitation energy spectrum. The differential cross sections for the He-6 inelastic scattering on carbon and lead targets were deduced from the measured moments of the two neutrons and the a-particle. An analysis of these distributions shows that quadrupole and, possibly, monopole excitations characterize the hadronic interaction, while the dipole mode is dominating in Coulomb dissociation. Neither theoretically predicted new resonance states in He-6 nor nuclear excitation of a dipole mode were found. Direct evidence has been obtained for strong suppression of Coulornb post-acceleration in direct Coulomb breakup in a lead target.

8He-6He: a comparative study of electromagnetic fragmentation reactions

Dissociation of a 227 MeV/u 8He beam in a lead target has been studied in a kinematically complete experiment. The results are compared with similar data from the well-known halo nucleus 6He. Coulomb–nuclear interference in the inelastic channel gives evidence for a Iπ=1− resonance in 8He at Ex=4.15 MeV. The deduced B(E1) value of 0.38±0.07 e2 fm2 indicates a uniform distribution of the four neutrons around the greek small letter alpha-particle core.

New information on 12C states from the decays of 12N and 12B

Abstract The properties of states in 12 C above the 3α-threshold are of high current interest for nuclear astrophysics and for the nuclear many-body problem in general. We have initiated a series of experiments aimed at elucidating this region by using the β-decays of 12 N and 12 B. By applying the ISOL method for producing these radioactive isotopes, in combination with modern segmented charged particle detectors, our approach has significant advantages over previous studies of this region.

Structure of neutron-rich oxygen isotopes

One-nucleon removal reactions at relativistic energies have been used as a spectroscopic tool for obtaining information about the ground state of neutron-rich oxygen isotopes. Using the FRS at GSI, the longitudinal momentum distributions of the emerging fragments after breakup were measured along with the one-nucleon removal cross-sections. The relative contributions of the remaining fragments in their ground and excited states have been determined for a few cases from measurements of γ rays in coincidence with the longitudinal momentum distribution. The controversial case of 23O has been directly addressed. Comparison of our exclusive momentum distributions for the one-neutron removal channel to theoretical momentum distributions calculated with an Eikonal model for the knockout process rendered a spin and parity of Iπ = 1/2+ for the 23O ground state. This result resolves the existent experimental discrepancy and supports sub-shell closure at N = 14 with a fairly pure 2s1/2 neutron in 23O.

Spectroscopy of light exotic nuclei using nuclear break‐up

One‐nucleon removal reactions at relativistic energies have been used as spectroscopic tool to obtain information about the ground state properties of several neutron‐rich isotopes in the sd‐shell. Using the FRS at GSI, the longitudinal momentum distributions of the emerging fragments after one‐nucleon removal cross‐sections were measured. The relative contributions of the remaining fragments in their ground and excited states have been determined from measurements of γ rays in coincidence with the longitudinal momentum distributions. The interesting case of 23O will be directly address. The interpretation of our experimental results in the framework of a simple theoretical model will favours a spin and parity assignment of 1/2+ for the 23O ground state in agreement with shell model predictions.

Low-lying resonance states in 7He

The cross section for the 6He+n channel as a function of the relative energy shows a structure, which is interpreted as being due to the 7He I π = 3/2− ground state resonance with about equal contribution from its I π = 1/2− spin-orbit partner at 1.2(2) MeV with a width Г = 1.0(2) MeV.

6,8He: a comparative study of fragmentation reactions

Dissociation of 227 MeV/u 8He in lead and carbon targets has been studied in kinematically complete experiments, the data are compared with corresponding results for 6He.