arXiv subjects

H. Lenske

Publications and source records attributed to H. Lenske.

At least 91 records · Page 5Linked to original sources

arXiv2004

Probing the Nuclear Neutron Skin by Low-Energy Dipole Modes

Dipole excitations below the neutron threshold in neutron rich Sn isotopes are studied theoretically in the Quasiparticle-Phonon Model with HFB single particle input. Of special interest are the low-lying two-phonon 1- states and the Pygmy Dipole Resonance (PDR). The evolution of low-energy dipole excitations with neutron excess is investigated over the Sn isotopic chain including the experimentally unknown regions close to 132Sn. A dependence of the PDR strengths and centroid energies on the neutron skin thickness is found. Despite significant multi-phonon contributions to mean energies and transition strengths, the PDR states retain their one-phonon character. The fragmentation pattern is reduced with increasing neutron excess towards the N=82 shell closure which will be of advantage for future experimental work.

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arXiv2003

Baryon Interactions in Nuclear and Hypernuclear Matter

The production and structure of Λ hypernuclei are investigated in field theoretical models. The production in coherent p+A reactions is investigated by means of a resonance model. Results of exploratory calculations for associated strangeness production are presented for proton reactions on 40Ca. The target single particle wave functions are obtained from DDRH theory with density dependent Dirac-Brueckner meson-baryon vertices. The dependence of the in-medium vertices on density is discussed.

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arXiv2003

Pygmy Dipol Resonances as a Manifestation of the Structure of the Neutron-Rich Nuclei

Dipole excitations in neutron-rich nuclei below the neutron threshold are investigated. The method is based on Hartree-Fock-Bogoliubov (HFB) and Quasiparticle-Phonon Model (QPM) theory. Of our special interest are the properties of the low-lying 1- Pygmy Resonance and the two-phonon quadrupole-octupole 1- states in Sn-isotopes including exploratory investigations for the experimentally unknown mass regions. In particular we investigate the evolution of the dipole strength function with the neutron excess. The use of HFB mean-field potentials and s.p. energies is found to provide a reliable extrapolation into the region off stability.

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arXiv2003

Baryon-baryon interactions in free space and baryonic medium

We study the interactions of the octet baryons in a relativistic meson exchange approach connecting free-space and in-medium interactions with those in finite (hyper)nuclei. A short sketch of the model including the relativistic T- and G-matrix and the mapping of the G-matrix onto density dependent vertex functionals in the density dependent relativistic hadron field theory (DDRH) is given. From new spectroscopic data on medium mass hypernuclei we extract the single particle spectrum including spin-orbit splitting. An extraction of the $Λω$ vector- and tensor vertex as well as the $Λσ$-vertex shows a strong breaking of SU(3) symmetry.

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arXiv2002

The hypernuclear Auger effect within the density dependent relativistic hadron field theory

The hypernuclear Auger effect, given by the de-excitation of a Lambda hypernucleus by means of the transition of a Lambda hyperon from an initial to a lower lying final single particle state in conjunction with neutron emission from the core nucleus is studied in relativistic DDRH field theory. Baryonic interactions are obtained from the Bonn NN potential by Dirac-Brueckner theory and theoretically derived scaling laws for the meson-hyperon vertices. The model is applied to the ^{209}_Lambda Pb hypernucleus. Lambda and nucleon bound states as well as scattering states are calculated self-consistently in mean-field approximation. The Auger spectra of the emitted neutrons are of complex structure due to a huge combinatorial number of possible hypernuclear transitions. The sensitivity of the neutron Auger spectra to changes in the Lambda sigma and Lambda omega vertices are investigated. The theoretical results show that experimental applications of the hypernuclear Auger effect will require special efforts, e.g. by tagging on the energy of the initially created hyperon.

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arXiv2001

In-Medium Baryon Interactions and Hypernuclear Structure

We introduce a microscopic and relativistic theory describing free scattering and finite nuclei for the octet baryons in a consistent quantum field theoretical framework based on Dirac-Brueckner theory and nuclear mean-field. In a first quantitative approach - yet still heuristic for dealing with the $Λ$ hyperon - the quality of the description of finite (hyper)nuclei based on free meson exchange potentials is competitive with those from purely phenomenological relativistic mean-field calculations. In contrast to the latter our approach has the advantage of having a microscopic link to free interactions and complete control on which classes of diagrams are included. As a complementary way for determining hyperon-hyperon and hyperon-nucleon interactions the use of Hanbury-Brown-Twiss interferometry is discussed.

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arXiv2001

Nuclear Matter Spectral Functions by Transport Theory

Quantum transport theory is used to calculate the nucleon spectral function in infinite nuclear matter. A self-consistent description is obtained by utilizing the relations between collision rates and correlation functions. Static and dynamical self-energies are taken into account in the single particle propagators. The real parts of the non-static self-energy contributions are calculated by dispersion theory thus conserving the analyticity of momentum distributions. The transport theoretical spectral functions, momentum distributions, occupation probabilities and responce functions are in close agreement with results of variational and other many-body theoretical calculations. The results indicate that the nucleon spectral functions are determined only by the average short-range correlation strength.

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arXiv2001

Single particle properties of Lambda hypernuclei in the density dependent relativistic hadron field theory

The density dependent relativistic hadron field theory is used to describe single particle properties of Lambda hypernuclei. The discussion focuses on the spin-orbit systematics in the relativistic mean-field formalism by discussing general effects of the Lambda continuum threshold and the delocalization of the Lambda wave function. Theoretical predictions for the hypernuclear Auger effect are presented.

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arXiv2001

Many-body Theory at Extreme Isospin

The structure of nuclei far off beta-stability is investigated by nuclear many-body theory. In-medium interactions for asymmetric nuclear matter are obtained by (Dirac-) Brueckner theory thus establishing the link of nuclear forces to free space interactions. HFB and RPA theory is used to describe ground and excited states of nuclei from light to heavy masses. In extreme dripline systems pairing and core polarization are found to be most important for the binding, especially of halo nuclei. The calculations show that far off stability mean-field dynamics is gradually replaced by dynamical correlations, giving rise to the dissolution of shell structures.

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arXiv2000

Probing Isospin Dynamics in Halo Nuclei

Nuclear many-body theory is used to study nuclear matter and finite nuclei at extreme isospin. In-medium interactions in asymmetric nuclear matter are obtained from (Dirac-) Brueckner theory. Neutron skin formation in Ni and Sn isotopes is investigated by relativistic mean-field calculations in DDRH theory with density dependent meson-nucleon vertices. Applications to light nuclei are discussed with special emphasis on pairing and core polarization in weakly bound nuclei. Approaches accounting for continuum coupling in dripline pairing and core polarization are presented. Calculations for the halo nuclei $^8$B, $^{11}$Be and $^{19}$C show that shell structures are dissolving when the driplines are approached. Relativistic breakup data are well described by eikonal calculations.

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arXiv2000

The structure of lambda hypernuclei

The density dependent relativistic hadron field theory is extended to also describe strange systems. It is seen that the application to hypernuclei works extremely well. Important spin orbit effects in lambda hypernuclei are studied and their experimental relevance is pointed out.

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arXiv2000

Potential and limitations of nucleon transfer experiments with radioactive beams at REX-ISOLDE

As a tool for studying the structure of nuclei far off stability the technique of gamma-ray spectroscopy after low-energy single-nucleon transfer reactions with radioactive nuclear beams in inverse kinematics was investigated. Modules of the MINIBALL germanium array and a thin position-sensitive parallel plate avalanche counter (PPAC) to be employed in future experiments at REX-ISOLDE were used in a test experiment performed with a stable 36S beam on deuteron and 9Be targets. It is demonstrated that the Doppler broadening of gamma lines detected by the MINIBALL modules is considerably reduced by exploiting their segmentation, and that for beam intensities up to 10^6 particles/s the PPAC positioned around zero degrees with respect to the beam axis allows not only to significantly reduce the gamma background by requiring coincidences with the transfer products but also to control the beam and its intensity by single particle counting. The predicted large neutron pickup cross sections of neutron-rich light nuclei on 2H and 9Be targets at REX-ISOLDE energies of 2.2 MeV A are confirmed.

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arXiv2000

Application of the density dependent hadron field theory to neutron star matter

The density dependent hadron field (DDRH) theory, previously applied to isospin nuclei and hypernuclei is used to describe $β$-stable matter and neutron stars under consideration of the complete baryon octet. The meson-hyperon vertices are derived from Dirac-Brueckner calculations of nuclear matter and extended to hyperons. We examine properties of density dependent interactions derived from the Bonn A and from the Groningen NN potential as well as phenomenological interactions. The consistent treatment of the density dependence introduces rearrangement terms in the expression for the baryon chemical potential. This leads to a more complex condition for the $β$-equilibrium compared to standard relativistic mean field (RMF) approaches. We find a strong dependence of the equation of state and the particle distribution on the choice of the vertex density dependence. Results for neutron star masses and radii are presented. We find a good agreement with other models for the maximum mass. Radii are smaller compared to RMF models and indicate a closer agreement with results of non-relativistic Brueckner calculations.

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arXiv2000

Dirac-Brueckner Approach to Hyperon Interactions and Hypernuclei

The density dependent relativistic hadron (DDRH) theory is introduced as an effective field theory for nuclei and hypernuclei. A Dirac-Brueckner approach to in-medium nucleon-hyperon interactions is presented. Density dependent meson-baryon vertices are determined from DBHF self-energies in infinite matter. Scaling laws for hyperon vertices are derived from a diagrammatic analysis of self-energies. In a local density approximation the DB vertices are applied in relativistic DDRH Hartree calculations to finite hypernuclei. Lambda single particle spectra and spin-orbit splittings are described reasonably well over the whole mass region.

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arXiv2000

Density dependent hadron field theory for asymmetric nuclear matter and exotic nuclei

The density dependent relativistic hadron field (DDRH) theory is applied to strongly asymmetric nuclear matter and finite nuclei far off stability. A new set of in-medium meson-nucleon vertices is derived from Dirac-Brueckner Hartree-Fock (DBHF) calculations in asymmetric matter, now accounting also for the density dependence of isovector coupling constants. The scalar-isovector $δ$ meson is included. Nuclear matter calculations show that it is necessary to introduce a momentum correction in the extraction of coupling constants from the DBHF self-energies in order to reproduce the DBHF equation of state by DDRH mean-field calculations. The properties of DDRH vertices derived from the Groningen and the Bonn A nucleon-nucleon (NN) potentials are compared in nuclear matter calculations and for finite nuclei. Relativistic Hartree results for binding energies, charge radii, separation energies and shell gaps for the Ni and Sn isotopic chains are presented. Using the momentum corrected vertices an overall agreement to data on a level of a few percent is obtained. In the accessible range of asymmetries the $δ$ meson contributions to the self-energies are found to be of minor importance but asymmetry dependent fluctuations may occur.

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arXiv2000

Transport Theoretical Approach to the Nucleon Spectral Function in Nuclear Matter

The nucleon spectral function in infinite nuclear matter is calculated in a quantum transport theoretical approach. Exploiting the known relation between collision rates and correlation functions the spectral function is derived self-consistently. By re-inserting the spectral functions into the collision integrals the description of hard processes from the high-momentum components of wave functions and interactions is improved iteratively until convergence is achieved. The momentum and energy distributions and the nuclear matter occupation probabilities are in very good agreement with the results obtained from many-body theory.

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arXiv1999

Density dependent hadron field theory for hypernuclei

The Density Dependent Relativistic Hadron Field (DDRH) theory, previously introduced and applied to isospin nuclei, is extended to hypernuclei by including the octet hyperons. Infinite matter Dirac-Brueckner theory for octet baryons and the derivation of in-medium DDRH baryon-meson vertices is discussed. From the properties of Dirac-Brueckner interactions it is found that hyperon and nucleon self-energies and vertices are related by the ratios of free space coupling constants. This leads to simple scaling laws for the in-medium hyperon and nucleon vertices. The model is applied in relativistic DDRH mean-field calculations to singl$Λnuclei. Free space N-Lambda T-matrix results are used for the scalar vertex. As the only free parameter the hyperon vector vertex scaling factor is adjusted to a selected set of hypernuclear data. Spectroscopic data of single Lambda hypernuclei over the full mass range are well described. The reduced Lambda spin-orbit splitting is reproduced and found to be related closely the medium dependence of scalar and vector interactions.

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arXiv1998

Longitudinal momentum distributions of {16,18}C fragments after one-neutron removal from {17,19}C

The fragment separator FRS at GSI was used as an energy-loss spectrometer to measure the longitudinal momentum distributions of {16,18}C fragments after one-neutron removal reactions in {17,19}C impinging on a carbon target at about 910 MeV/u. The distributions in the projectile frames are characterized by a FWHM of 141+-6 MeV/c for {16}C and 69+-3 MeV/c for {18}C. The results are compared with experimental data obtained at lower energies and discussed within existing theoretical models.

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