Isoscalar and isovector nuclear matter properties and neutron skin thickness

Isoscalar and isovector nuclear matter properties are investigated in the Skyrme Hartree-Fock (SHF) and the relativistic mean field (RMF) models. The Skyrme parameters are related analytically to the isoscalar and the isovector nuclear matter properties of the Hamiltonian density. Linear correlations are found among the isovector nuclear matter properties of the Hamiltonian density in both the SHF and the RMF models. We also discovered that the correlations between the isovector properties and the incompressibility K show a singularity at the critical incompressibility K_c=306 MeV. It is shown that the neutron skin thickness gives crucial information about not only for the neutron EOS but also about the isovector nuclear matter properties and about the parameterization of Skyrme interaction. Charge exchange spin-dipole (SD) excitations are proposed to determine the neutron skin thickness model independently.     

Isovector deformation and its link to the neutron shell closure

DWBA analysis of the inelastic ^30-40S(p,p') and ^18-22O(p,p') scattering data measured in the inverse kinematics has been performed to determine the isoscalar (δ₀) and isovector (δ₁) deformation lengths of the 2⁺₁ excitations in the Sulfur and Oxygen isotopes using a compact folding approach. A systematic N-dependence of δ₀ and δ₁ has been established which shows a link between δ₁ and the neutron-shell closure. Strong isovector deformations were found in several cases, e.g., the 2⁺₁ state in ²⁰O where δ₁ is nearly three times larger than δ₀. These results confirm the relation δ₁>δ₀ anticipated from the core polarization by the valence neutrons in the open-shell (neutron rich) nuclei. The effect of neutron shell closure at N=14 or 16 has been discussed based on the folding model analysis of the inelastic ²²O+p scattering data at 46.6 MeV/u measured recently at GANIL.     

Study of N = 16 for Ne isotopes

²⁷Ne has been investigated through the one neutron transfer reaction ²⁶Ne(d,p)²⁷Ne in inverse kinematics at 9.7 MeV/nucleon. The results support the existence of a low lying negative parity state in ²⁷Ne which is a signature of a reduced sd-fp shell gap in the N = 16 neutron rich region, at variance with stable nuclei.     

Count fluctuation analysis in the triaxial strongly deformed potential well of 163Lu

High-spin properties of the triaxial strongly deformed potential well in ¹⁶³Lu at excitation energies above resolvable bands have been investigated. Gated E _γ-E _γ spectra display clear ridges with moments of inertia corresponding to those observed for the discrete strongly deformed bands. A fluctuation analysis of the ridges yields a number of two-step paths of ≈ 40 and ≈ 20, when gating on triaxial strongly or normally deformed bands, respectively. These results show that a potential well at large deformation coexists with the normally deformed well, and indicate a mixing between states in the two wells at higher excitation energy. Received: 4 January 2002 / Accepted: 6 May 2002     

The “island of inversion" from a nuclear moments perspective and the g factor of 35Si

This paper reviews recent results from electromagnetic moment measurements on isotopes in the island of inversion around N=20. The obtained moments on neutron rich Na, Mg, Al and Si isotopes allow to draw conclusions on the amount of intruder components in their ground state wave function, demonstrating a gradual transition from the normal sd-shell region into the island of inversion, starting at N=18 for Na, N=19 for Mg and N=20 for Al isotopes. A measurement of the ground state g factor of ³⁵Si (N=21), using a polarized fragment beam at GANIL, is discussed in more detail. The magnetic moment μ(³⁵Si, I^π= 7/2^-) = (-)1.638(4) μ_N is consistent with a normal ground state structure, dominated by a νf_7/2 neutron.     

Resonant and weakly-bound one-particle levels in quadrupole-deformed potentials

Both positive-energy and weakly-bound one-particle levels for neutrons in Y₂₀ deformed Woods-Saxon potentials are examined in comparison with those in spherical Woods-Saxon potentials. While s_1/2 levels play a unique role in spherical drip-line nuclei, the Ω^π = 1/2⁺ levels in Y₂₀ deformed potentials, which always contain some amount of s_1/2 component, exhibit an important role in deformed drip-line nuclei. As the potential strength becomes weaker, some weakly-bound Ω^π = 1/2⁺ levels continue to the positive-energy region as one-particle resonant levels, while others have no such continuation. Among an infinite number of one-particle levels at a given positive-energy and in a given deformed potential, only some selected levels expressed in terms of eigenphase are found to be important in the pair-correlated ground state of neutron-drip-line nuclei.     

Reliability of the pseudospin symmetry in atomic nuclei

The reliability of the pseudospin symmetry (PSS) in atomic nuclei is analyzed in the framework of the relativistic Hartree approach. We find that the nuclear surface strongly increases the effect of the pseudospin-orbit potential (PSOP), spoiling the possibility of the exact realization of the PSS even in the limit of a vanishing PSOP. It is also shown that the PSS cannot be explained by the fact that Σ_S ≃ - Σ. New arguments to explain the PSS in finite nuclei are given. The important role the spin-orbit interaction plays in the achievement of the PSS is also discussed. Received: 22 July 2002 / Accepted: 18 February 2003 / Published online: 20 May 2003     

Coulomb excitation of 74Ge beam

⁷⁴Ge beam was Coulomb-excited on a ^natPb target. Ten E2 matrix elements including diagonal matrix elements for 5 low-lying states have been determined using the least-squares search code GOSIA. The expectation values of the rotational invariants 〈Q ²〉 and 〈cos3δ〉 show the small and triaxial deformation of the two lowest members of the ground-state band , while the 0₂ ⁺ and 2₂ ⁺ states are found to be almost spherical. Received: 31 August 2000 / Accepted: 4 December 2000     

Stability of Strutinsky Shell Correction Energy in Relativistic Mean Field Theory

The single-particle spectrum obtained from the relativistic mean field （RMF） theory is used to extract the shell correction energy with the Strutinsky method. Considering the delicate balance between the plateau condition in the Strutinsky smoothing procedure and the convergence for the total binding energy, the proper space sizes used in solving the RMF equations are investigated in detail by taking ^208 Pb as an example. With the proper space sizes, almost the same shell correction energies are obtained by solving the RMF equations either on basis space or in coordinate space.     

ISAC-I and ISAC-II: Present status and future perspectives

The ISAC facility at TRIUMF utilizes up to 100 μA from the 500 MeV H^- cyclotron to produce the RIB using the Isotopic Separation On Line (ISOL) method. The ISAC-I facility comprised the RNB production target stations, the mass separator and the beam delivery to low energy area and to a room temperature linear accelerator composed of a 4-rod RFQ and an inter-digital H type structure Drift Tube LINAC. ISAC-I linear accelerator can provide beam from A = 3 to 30 amu with an energy range from 0.15 to 1.5 A MeV. Since the beginning of operations target development program has been to increase proton beam currents on targets. Now we routinely operate our target at 50 to 85 μA and recently we have operated our target at 100 μA. Other developments are in place to add other ion sources, laser, FEBIAD and ECRIS to the actual surface ion source. The last two five year plans were mainly devoted to the construction of a heavy ion superconducting LINAC (ISAC-II), that will upgrade the mass and the energy range from 30 to 150 and 1.5 to 6.5 A MeV, respectively. We are now commissioning the medium β section and first experiment is scheduled for the fall 2006.     

In-beam γ spectroscopy using DIC with a radioactive Ne beam

A study of deep-inelastic and multi-nucleon transfer reactions to populate neutron-rich O, Ne and F nuclei is here presented. The reaction under analysis employed a beam of radioactive ²⁴Ne at 7.9 AMeV, provided by the SPIRAL facility at Ganil, impingin on a ²⁰⁸Pb target. The reaction products have been detected in the VAMOS spectrometer in coincidence with gamma rays measured by the EXOGAM array. Preliminary results here presented show a selectivity in the population of states of different nature.     

Rotational bands in 181Ta

High-spin states in ¹⁸¹Ta have been studied via the ¹⁷⁶Yb(¹¹B,α2n) reaction at 52 MeV using the PEX array and at 57 MeV using the NORDBALL array, with α-particle detection. The previously known, K^π=(7/2)⁺ ground state band and K^π=(9/2)⁻ band have been extended to spins (29/2)⁺ and (31/2)⁻, respectively. Two new one-quasiparticle bands, the K^π=(5/2)⁺ band built on the known (5/2)⁺ isomer and a K^π=((1/2)⁻) band have been observed. Two other rotational bands with three-quasiparticle structure, K^π=(15/2)⁻ and ((19/2)⁺ with π(7/2)[404]ν²(1/2)[510](9/2)[624] and π(9/2)[514]ν²(1/2)[510](9/2)[624] configurations, respectively, have been newly observed. The half-life of the K^π=((19/2)⁺) bandhead which decays to the head of the (15/2)⁻ band has been measured to be 140(36) ns. However, transitions from the ((19/2)⁺) state to the (15/2)⁻ band have not been observed. Received: 26 August 1998     

Neutrino-nucleon scattering rate in proto-neutron star matter

We present a calculation of the neutrino-nucleon scattering cross-section which takes into account the nuclear correlations in the relativistic random phase approximation (RPA). Our approach is based on a quantum-hadrodynamics model with exchange of σ, ω, π, ρ and δ mesons. In view of applications to neutrino transport in the final stages of supernova explosion and proto-neutron star cooling, we study the evolution of the neutrino mean free path as a function of density, proton-neutron asymmetry and temperature. Special attention was paid to the issues of renormalization of the Dirac sea, residual interactions in the tensor channel, coupling to the delta-meson and meson mixing. In contrast with the results of other authors, we find that the neutral-current process is not sensitive to the strength g' of the residual contact interaction. As a consequence, it is found that RPA corrections with respect to the mean-field approximation amount to only 10% to 15% at high density. Received: 27 June 2001 / Accepted: 14 January 2002     

A relativistic quark model for mesons with an instanton-induced interaction

We present new results of a relativistic quark model based on the Bethe-Salpeter equation in its instantaneous approximation. Assuming a linearly rising confinement potential with an appropriate spinorial structure in Dirac space and adopting a residual interaction based on instanton effects, we can compute masses of the light mesons up to highest observed angular momenta with a natural solution of the U _A(1) problem. The calculated ground states masses and the radial excitations describe the experimental results well. In this paper, we will also discuss our results concerning numerous meson decay properties. For processes like π⁺/K ⁺↦e ⁺υ_eγ and 0^-↦γγ at various photon virtualities, we find a good agreement with experimental data. We will also comment on the form factors of the K _?3 decay and on the decay constants of the π, K and η mesons. For the sake of completeness, we will furthermore present the electromagnetic form factors of the charged π and K mesons as well as a comparison of the radiative meson decay widths with the most recent experimental data. Received: 28 August 2000 / Accepted: 12 September 2000     

Present and future experiments with stored exotic nuclei at relativistic energies

Pioneering experiments with stored and cooled exotic nuclei at relativistic energies have been performed using the combination of the fragment separator FRS with the storage-cooler ring ESR. Exotic nuclei created in peripheral collisions are spatially separated in-flight and injected into the storage ring for high-precision mass and unique lifetime measurements. Lifetimes of stored bare and few-electron nuclei have been measured to study the influence of the electron density on the β decay. This condition, relevant for stellar plasma, can now be systematically investigated in the laboratory for the first time. Characteristic experiments of the present FRS-ESR system are presented and perspectives for a next-generation facility are briefly outlined. Received: 1 May 2001 / Accepted: 4 December 2001     

Exciting isomers from the first stopped-beam RISING campaign

First results are reported from a major new initiative of experiments, which focus on nuclear structure studies at extreme isospin values by means of isomer spectroscopy. The experiments represent the first part of the so-called stopped-beam campaign within the Rare ISotope INvestigations at GSI (Rising) project. Time-correlated γ decays from individually identified nuclear species have been measured, allowing the clean identification of isomeric decays in a wide range of exotic nuclei both at the proton drip-line and in heavy, neutron-rich systems. An overview of the experimental technique will be given, together with the performance of the new germanium detector array and future research plans for the collaboration.     

Novel approach to pion and eta production in proton-proton collisions near threshold

We evaluate the threshold matrix–element for the reaction pp→ppπ⁰ in a fully relativistic Feynman diagrammatic approach. We employ a simple effective range approximation to take care of the S–wave pp final–state interaction. The experimental value for the threshold amplitude A = (2.7 −i0.3) fm⁴ can be reproduced by contributions from tree level chiral (long–range) pion exchange and short–range effects related to vector meson exchanges, with ω-exchange giving the largest individual contribution. Pion loop effects appear to be small. We stress that the commonly used heavy baryon formalism is not applicable in the NN–system above the pion production threshold due to the large external momentum, |p|≃ (Mm _π)^−1/2, with M and m _π the nucleon and the pion mass, respectively. We furthermore investigate the reaction pp→pnπ⁺ near threshold within the same approach. We extract from the data the triplet threshold amplitude as B = (2.8 −i1.5) fm⁴. Its real part can be well understood from (relativistic) tree level meson–exchange diagrams. In addition, we investigate the process pp→ppη near threshold. We use a simple factorization ansatz for the ppη final–state interaction and extract from the data the modulus of the threshold amplitude, |C|= 1.32 fm⁴. With g _ηN= 5.3, this value can be reproduced by (relativistic) tree level meson–exchange diagrams and η–rescattering, whose strength is fixed by the ηN scattering length. We also comment on the recent near threshold data for η^′–production. Received: 27 November 1998     

Nucleon form factors in a relativistic quark model

Electromagnetic form factors of protons and neutrons are investigated based on a relativistic quark model with the inclusion of a pion cloud. Pseudo-scalar π-quark interaction is employed to study the coupling between the nucleon and the π. The results show the important role of the pion cloud for the neutron charge form factor. Moreover, our numerical analysis indicates a difference between the relativistic and the nonrelativistic treatments. Received: 10 March 1999 / Revised version: 14 June 1999     

First triaxial superdeformed band in 170Hf

First evidence is presented for triaxial superdeformation in ¹⁷⁰Hf. High-spin states in this nucleus have been investigated in a γ-ray coincidence measurement using the EUROBALL spectrometer array. A new band was discovered which has moments of inertia that are very similar to the ones of triaxial superdeformed bands in neighbouring Hf and Lu nuclei. The intensities with which these bands are populated are different from what may be expected from calculated potential-energy minima. Received: 11 September 2002 / Accepted: 15 October 2002 / Published online: 10 December 2002 RID="a" ID="a"e-mail: neusser@iskp.uni-bonn.de Communicated by D. Schwalm     

Evidence for a proton halo in 27P through measurements of reaction cross-sections at intermediate energies

Measurements of reaction cross-sections ( σ_R's) for some proton-rich nuclei ( N = 11–15 isotones) on carbon target at intermediate energies have been performed on RIBLL of HIRFL. A larger enhancement of the σ_R for ²⁷P has been observed than for its neighboring nuclei. A large difference between the proton and neutron density distributions (proton halo) is necessary to explain the enhanced cross-section for ²⁷P within the framework of the Glauber model. Density distributions with HO-type core plus Yukawa-square tail and rms radii for ²⁷P have been deduced from the measured σ_R data for the first time, which conform the long tail in its densities as predicted by RMF calculations. Received: 23 May 2001 / Accepted: 7 November 2001