Hans Feldmeier

Pair decay width of the Hoyle state and carbon production in stars

The pair decay width of the first excited 0+ state in 12C (the Hoyle state) is deduced from a novel analysis of the world data on inelastic electron scattering covering a wide momentum transfer range, thereby resolving previous discrepancies. The extracted value ?? = (62.3 ? 2.0) ?eV is independently confirmed by new data at low momentum transfers measured at the S-DALINAC and reduces the uncertainty of the literature values by more than a factor of three. A precise knowledge of ?? is mandatory for quantitative studies of some key issues in the modeling of supernovae and of asymptotic giant branch stars, the most likely site of the slow-neutron nucleosynthesis process.

Clusters, Halos, And S-Factors In Fermionic Molecular Dynamics

In Fermionic Molecular Dynamics antisymmetrized products of Gaussian wave packets are projected on angular momentum, linear momentum, and parity. An appropriately chosen set of these states span the many-body Hilbert space in which the Hamiltonian is diagonalized. The wave packet parameters – position, momentum, width and spin – are obtained by variation under constraints. The great flexibility of this basis allows to describe not only shell-model like states but also exotic states like halos, e.g. the two-proton halo in 17 Ne, or cluster states as they appear for example in 12 C close to the α breakup threshold where the Hoyle state is located. Even a fully microscopic calculation of the 3 He( α,γ ) 7 Be capture reaction is possible and yields an astrophysical S-factor that compares very well with newer data. As representatives of numerous results these cases will be discussed in this contribution, some of them not published so far. The Hamiltonian is based on the realistic Argonne V18 nucleon-nucleon interaction.

Electron scattering on the Hoyle state and carbon production in stars

High‐resolution inelastic electron scattering experiments were performed at the S‐DALINAC for a precise determination of the partial pair width Γπ of the second Jπu2009=u20090+ state, the so‐called Hoyle state, in 12C. Results for the monopole matrix element (directly related to Γπ) from a nearly model‐independent analysis based on an extrapolation of low‐q data to zero momentum transfer are presented. Additionally, a Fourier‐Bessel analysis of the transition form factor is discussed. The combined result of both methods leads to a pair width Γπ62.2(10)u2009μeV.

Microscopic calculation of the 3He(α,γ)7Be reaction rate using realistic interactions

The cross sections for the 3He(α,γ)7Be and the 3H(α,γ)7Li radiative capture reactions are calculated in the fully microscopic Fermionic Molecular Dynamics approach using a realistic effective interaction obtained in the Unitary Correlation Operator Method. The model space is divided in an external region where bound and scattering states are described by antisymmetrized products of 4He and 3He/3H ground states and an internal region where additional many-body wave functions obtained by variation after parity and angular momentum projection enlarge the Hilbert space. These additional configurations, representing polarized cluster configurations, are necessary for a successful description of the bound and scattering states. The calculated S-factor for the 3He(α,γ)7Be reaction is in good agreement with recent experimental data both in absolute normalization and energy dependence. In case of the isospin mirror reaction 3H(α,γ)7Li the calculated S-factor is larger than the experimental data by about 15%. Dipole matrix elements are analyzed in terms of overlap functions calculated from the A-body wave functions.

Clusters And Shell‐Structure In Light Nuclei

Light nuclei in the p‐shell are studied in the Fermionic Molecular Dynamics model. No a priori assumptions are made with respect to cluster structure or single‐particle energies. The same effective interaction based on the Argonne V18 interaction is used for all nuclei. Short‐range central and tensor correlations are treated explicitly using a unitary correlation operator. Calculations of binding energies and radii for Helium and Carbon isotopes are presented. The evolution of cluster and single‐particle structures with increasing neutron number are discussed. The spectrum of 12C is calculated in a multiconfiguration calculation. The molecular structure of the excited states is investigated.