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Dipole strength distributions measured in unstable neutron-rich nuclei in the Sn-132 mass region by utilizing relativistic Coulomb excitation of radioactive beams reveal a sizable fraction of "pygmy" strength below the giant dipole resonance. On the grounds of the RQRPA model, a strong linear correlation between the pygmy strength, parameters of the nuclear symmetry energy and the neutron skin thickness has been found. From the experimentally observed low-lying strength, the symmetry energy pressure p(0) = 2.3 +/- 0.8 MeV/fm(3) and neutron skin thicknesses of 0.23 +/- 0.04 fm Sn-130 and 0.24 +/- 0.04 fm Sn-130 have been extracted.

A concentration of dipole strength at energies below the giant dipole resonance was observed in neutron-rich nuclei around 132Sn in an experiment using the FRS-LAND setup. This so-called “pygmy” dipole strength can be related to the parameters of the symmetry energy and to the neutron skin thickness on the grounds of a relativistic quasiparticle random-phase approximation. Using this ansatz and the experimental findings for 130Sn and 132Sn, we derive a value of the symmetry energy pressure of p-bar0 = 2.2±0.5 MeV/fm3. Neutron skin thicknesses of Rn−Rp = 0.23±0.03 fm and 0.24±0.03 fm for 130Sn and 132Sn, respectively, have been determined. Preliminary results on 68Ni from a similar experiment using an improved setup indicate an enhanced cross section at low energies, while the results for 58Ni are in accordance with results from photoabsorption measurements.

This work contains overview of recent progress in studies of pygmy dipole strength and neutron skins in exotic nuclei.

This paper summarizes the yields and the emission patterns of K+ and of K− mesons measured in inclusive C+C, Ni+Ni and Au+Au collisions at incident energies from 0.6 A GeV to 2.0 A GeV using the Kaon Spectrometer KaoS at GSI. For Ni+Ni collisions at 1.5 and at 1.93 A GeV as well as for Au+Au at 1.5 A GeV detailed results of the multiplicities, of the inverse slope parameters of the energy distributions and of the anisotropies in the angular emission patterns as a function of the collision centrality are presented. When comparing transport-model calculations to the measured K+ production yields an agreement is only obtained for a soft nuclear equation of state (compression modulus KN ~ 200 MeV). The production of K− mesons at energies around 1 to 2 A GeV is dominated by the strangeness-exchange reaction K−N pi Y (Y = Lambda, Sigma) which leads to a coupling between the K− and the K+ yields. However, both particle species show distinct differences in their emission patterns suggesting different freeze-out conditions for K+ and for K− mesons.

An exclusive measurement of the Coulomb breakup of 8B into 7Be+p at 254A MeV was used to infer the low-energy 7Be(p,gamma)8B cross section. The radioactive 8B beam was produced by projectile fragmentation of 350A MeV 12C and separated with the FRagment Separator (FRS) at Gesellschaft für Schwerionenforschung in Darmstadt, Germany. The Coulomb-breakup products were momentum-analyzed in the KaoS magnetic spectrometer; particular emphasis was placed on the angular correlations of the breakup particles. These correlations demonstrate clearly that E1 multipolarity dominates within the angular cuts selected for the analysis. The deduced astrophysical S17 factors exhibit good agreement with the most recent direct 7Be(p,gamma)8B measurements. By using the energy dependence of S17 according to the recently refined cluster model for 8B of P. Descouvemont [Phys. Rev. C 70, 065802 (2004)], we extract a zero-energy S factor of S17(0)=20.6±0.8(stat)±1.2(syst) eV b. These errors do not include the uncertainty of the theoretical model to extrapolate to zero relative energy, estimated by Descouvemont to be about 5%.

An exclusive measurement of the Coulomb breakup of 8B into 7Be + p at 254A MeV was used to infer the low-energy 7Be(p , γ)8B cross section. The radioactive 8B beam was produced by projectile fragmentation of 350A MeV 12C and separated with the FRagment Separator (FRS) at Gesellschaft für Schwerionenforschung in Darmstadt, Germany. The Coulomb-breakup products were momentum-analyzed in the KaoS magnetic spectrometer; particular emphasis was placed on the angular correlations of the breakup particles. These correlations demonstrate clearly that E1 multipolarity dominates within the angular cuts selected for the analysis. The deduced astrophysical S17 factors exhibit good agreement with the most recent direct 7Be(p , γ)8B measurements. By using the energy dependence of S17 according to the recently refined cluster model for 8B of P. Descouvemont [Phys. Rev. C 70, 065802 (2004)], we extract a zero-energy S factor of S17(0)= 20.6 ± 0.8(stat) ± 1.2(syst) eV b. These errors do not include the uncertainty of the theoretical model to extrapolate to zero relative energy, estimated by Descouvemont to be about 5%.

The production and propagation of kaons and antikaons in relativistic heavy-ion collisions have been systematically investigated with the Kaon spectrometer at SIS/GSI. Experimental results on the K+ and K- yiels and on the azimuthal emission pattern of K+ mesons are presented. Within the framework of transport models the data can be explained assuming in-medium kaon-nucleon potentials. The comparison of K+production excitation functions obtained for Au+Au and C+C collisions with results of transport model calculations favours a soft nuclear equation-of-state.

The precise knowledge of the solar thermonuclear fusion reaction of 7Be(p, γ)8B is crucial for estimating the 8B solar neutrino flux and the predicted neutrino rates in terrestrial neutrino detectors. The flux of 8B solar neutrinos is particularly important for the results of the Homestake, Super-Kamiokande, and SNO experiments, because they measure high-energy solar neutrinos mainly or solely from the 8B decay. We have measured the Coulomb dissociation of 8B into 7Be and proton on a 208Pb target at 254 MeV/nucleon using the large acceptance focusing spectrometer KaoS at GSI. Since the dissociation process can be regarded as absorption of a virtual photon, the Coulomb dissociation cross section is converted to the photo-absorption 8B( γ,p)7Be cross section which is directly related to the 7Be(p, γ)8B cross section. In a first experiment the astrophysical S factor for the 7Be(p,γ)8B reaction is extracted to be S17(0)=20.6 ±1.2(exp) ±1.0(theo) eV-b for 0.25-2.78 MeV (1). This result agrees with the presently adopted zero-energy S17 factor obtained in direct-reaction measurements and with the results of other Coulomb dissociation studies performed at 46.5 and 51.2 A MeV. A second run allowed to also obtain information on the E1/E2 ratio by analysing the angular correlation of 7Be and the proton.

The production of kaons and antikaons in nucleus-nucleus collisions has been studied with the kaon spectrometer at SIS/GSI. The K-/K+ ration was measured in C+C, Ni+Ni, Au+Au collisions at 1.5 AGeV and was found to be almost independent of the system size. In Ni+Ni collisions at 1.93 AGeV we measured the phase-space distributions of K- and K+ mesons. Within the framework of transport models, the measured K+ and K- yields can only be explained if in-medium modifications of the K meson masses are assumed. In Au-Au collisions at 1 AGeV, we observe non-vanishing elliptic flow near mid-rapidity and in-plane flow at target rapidity for K+ mesons. This effect can be explained by transport models when a repulsive in-medium K+ N potential is taken into account. The ratio of the K+ meson excitation functions for Au+Au and C+C collisions increases with decreasing beam energies. This effect is expected for a soft nuclear equation of state.

Production cross sections of charged pions, kaons and antikaons have been measured in C+C and C+Au collisions at beam energies of 1.0 and 1.8 AGeV for different polar emission angles. The kaon and antikaon energy spectra can be described by Boltzmann distributions whereas the pion spectra exhibit an additional enhancement at low energies. The pion multiplicity per participating nucleon M(pi+)/A_part is a factor of about 3 smaller in C+Au than in C+C collisions at 1.0 AGeV whereas it differs only little for the C and the Au target at a beam energy of 1.8 AGeV. The K+ multiplicities per participating nucleon M(K+)/A_part are independent of the target size at 1 AGeV and at 1.8 AGeV. The K- multiplicity per participating nucleon M(K-)/A_part is reduced by a factor of about 2 in C+Au as compared to C+C collisions at 1.8 AGeV. This effect might be caused by the absorption of antikaons in the heavy target nucleus. Transport model calculations underestimate the K-/K+ ratio for C+C collisions at 1.8 AGeV by a factor of about 4 if in-medium modifications of K mesons are neglected.

We have measured the Coulomb dissociation of 8B into 7Be and proton at 254 MeV/nucleon using a large-acceptance focusing spectrometer. The astrophysical S17 factor for the 7Be(p,gamma)8B reaction at E{c.m.} = 0.25-2.78 MeV is deduced yielding S17(0)=20.6 \pm 1.2 (exp.) \pm 1.0 (theo.) eV-b. This result agrees with the presently adopted zero-energy S17 factor obtained in direct-reaction measurements and with the results of other Coulomb-dissociation studies performed at 46.5 and 51.2 MeV/nucleon., Comment: paper to be published in Phys. Rev. Lett. 3 figures. New Version fixes formatting problems with the figures only. There are no other changes