Bulletin of the American Physical Society
2005 2nd Joint Meeting of the Nuclear Physics Divisions of the APS and The Physical Society of Japan
Sunday–Thursday, September 18–22, 2005; Maui, Hawaii
Session 1WH: Workshop 8A: Neutron-rich Nuclei in Nuclear Astrophysics |
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Sponsoring Units: DNP JPS Chair: Jorge Piekarewicz Room: Ritz-Carlton Hotel Plantation 1 |
Sunday, September 18, 2005 9:00AM - 9:30AM |
1WH.00001: The Parity Radius Experiment at the Jefferson Laboratory Invited Speaker: The raduis of the distribution of neutrons in lead can be determined by measuring the parity-violating electroweak asymmetry in the scattering of polarized electrons. This data can, in turn, be used to obtain information about the symmetry energy of nuclear matter, with a minmimum of theoretical input, and then applied to descriptions of neutron stars. However, the experiment is challenging, requiring the measurement of a tiny asymmetry to a few percent of itself. Details about the theoretical and experimental issues will be presented. [Preview Abstract] |
Sunday, September 18, 2005 9:30AM - 10:00AM |
1WH.00002: Mean field theory for neutron rich nuclei and EOS Invited Speaker: We have already shown that there was a clear linear correlation between the neutron skin thickness of stable nuclei, such as $^ {208}$Pb, the pressure of neutron matter in the Skyrme Hartree Fock (SHF) and relativistic mean field (RMF) models. Here the pressure $P_{n}$ of neutron matter is defined by the first derivative of Hamiltonian density in the neutron matter with respect to the neutron density. There are some nuclear properties of infinite nuclear matter, such as the saturation density $\rho_{nm}$, the saturation energy per nucleon $E_{0}$, the incompressibility $K$, the symmetry energy $J$, the 1st derivative $L$ of $\varepsilon_ {\delta}$ and 2nd derivative $K_{sym}$ of $\varepsilon_{\delta} $. The isoscalar part $h$ are characterized by $\rho_{nm}$, $E_{0} $ and $K$, and the isovector part $\varepsilon_{\delta}$ is characterized by $J$, $L$ and $K_{sym}$. Furthermore these isoscalar part and the isovector part of the Hamiltonian density for infinite nuclear matter play an important role to characterize the equation of state (EOS). In Skyrme Hartree Fock model (SHF) including some parameters in the Skyrme force, there are many versions of Skyrme parameter sets parameterized by the experimental results of finite nuclei such as nuclear mass, charge radii and so on. All Skyrme parameter sets which we use recently can reproduce the empirical $\rho_{nm}$ and $E_{0}$, whereas the values of $K$, $J$, $L$ and $K_{sym}$ depend on the parameter sets entirely. However we found there were correlations among $K$, $J$, $L$ and $K_{sym}$ in SHF model. Furthermore if we fix the values of $\rho_{nm}$ and $E_{0}$ to 0.16~fm$^{-3}$ and 16 MeV, respectively, the nuclear matter properties $J$, $L$ and $K_{sym}$ are represented as a function of the incompressibility $K$, the neutron skin thickness of $^ {208}$Pb and the power of the total density in density- dependent term of Skyrme force. In addition to the content above, I will discuss the relation between the EOS of the neutron matter and the neutron skin thickness of the finite nuclei. [Preview Abstract] |
Sunday, September 18, 2005 10:00AM - 10:30AM |
1WH.00003: Experimental studies of giant resonances and nuclear incompressibility Invited Speaker: Isoscalar giant monopole resonances and isoscalar giant dipole resonances attract many nuclear physicists because they are related to the nuclear incompressibility and then to the equation of state(EOS) of nuclear matter, which plays an important role not only in nuclear physics but also in supernova explosion and neutron star formation. Due to the experimental difficulties to observe cleanly these giant resonances it is only recent years that we have obtained reliable data for peak positions of them and deduced a consistent value of nuclear incompressibility from them. The angular momentum transfers needed to excite them from the ordinary even-even nuclei are zero or one. Thus if we want to observe them by alpha particle scattering, we need to measure inelastic alpha particles at zero degrees and the extremely forward angles, where the angular distributions to excite them take their maximum values. In this talk we report recent data on isoscalar giant monopole and dipole resonances measured at RCNP ring cyclotron, which provides us a very clean and stable beam, the positions of which were monitored continuously during our measurements. We have measured inelastic alpha scattering of 400 MeV at extremely forward angles including zero degrees with the Grand Raiden magnetic spectrometer. In order to estimate and subtract the instrumental backgrounds we have utilized the double focusing property of the spectrometer and the ray-tracing type focal plane position detector. Angular distributions of 1 MeV energy bin above the excitation energy of 10 MeV have been multipole decomposed to give the excitation strength spectra separately for each giant resonance. From our giant resonance data of 208Pb we have deduced the nuclear incompressibility of 215 MeV by comparing with nonrelativistic RPA calculations. If time allows, we would like to mention our new results on the neutron skin thicknesses in 204,206,208Pb obtained by intermediate energy proton elastic scattering at RCNP. [Preview Abstract] |
Sunday, September 18, 2005 10:30AM - 11:00AM |
1WH.00004: Giant resonances in neutron-rich nuclei studied with TDHF simulation in the continuum Invited Speaker: In weakly bound systems such as neutron-rich nuclei, the continuum plays an important role in the ground and excited states. Many-body correlations in low-density nuclear matter is an interesting subject in nuclear structure physics and have a significant impact on nuclear reactions as well. We study excited and resonance states in the continuum with time-dependent Hartree-Fock theory in the linear regime. In order to treat the nucleonic continuum, we adopt the absorbing boundary condition which has been often utilized in calculations of atomic collision. The method is equivalent to the continuum RPA theory, but it is applicable to systems without spherical symmetry. Dynamical properties of nuclear vibrations and their damping is investigated in real-time simulation of the small-amplitude TDHF. We show giant resonances in stable and unstable (neutron-rich) nuclei and discuss possible effects on low-energy reactions. [Preview Abstract] |
Sunday, September 18, 2005 11:00AM - 11:30AM |
1WH.00005: Photonuclear Probe of Astrophysics Invited Speaker: Real photons are a good probe of E1 and M1 $\gamma$-strength functions of nuclei. In stellar conditions, both neutron capture and photodisintgration are sensitive to the $\gamma$- strength function in the low-energy tail of the electric giant dipole resonance (GDR) that are assumed to be built on individual excited states. The $\gamma$-strength function is a key ingredient to improve the reliability of the Hauser-Feshbach model description of neutron capture and photodisintegration. Recently precision determination of photoneutron cross sections of astrophysical importance has become possible with use of quasi-monochromatic $\gamma$ beams from laser Compton backscattering at AIST. These data provide a good insight into the $\gamma$-strength function in the low-energy tail of GDR. This talk covers precision measurements of photodisintegration cross sections in the context of the p-process and s-process nucleosyntheses. [Preview Abstract] |
Sunday, September 18, 2005 11:30AM - 12:00PM |
1WH.00006: $^{4}$He+$\gamma $ cross sections for astrophysical interest Invited Speaker: Roles of the neutrino-induced nuclear reactions of $^{4}$He in supernova explosions and rapid process (r-process) of stellar nucleosynthesis are current issues in nuclear astrophysics. Recent theoretical studies of the supernova explosions suggest a critical role of the neutrino-inelastic scattering on $^{4}$He in energy transport by outgoing shock wave. The neutrino-inelastic scattering on $^{4}$He is considered to be also important for productions of $^{7}$Li via r-process in neutrino-driven wind of supernovae. To study the roles of the neutrinos in supernova explosions, one needs information about response of $^{4}$He nucleus to neutrino-inelastic scattering in the excitation energy range of 20$\sim $40 MeV, which corresponds to the energies of the neutrinos emitted from supernovae. Photonuclear reactions of $^{4}$He in the giant dipole resonance (GDR) region is a unique tool to study the nuclear response of $^{4}$He, since the electromagnetic transitions are direct analogs of the neutrino transitions by weak neutral current. However, there has been a large discrepancy between the existing data of the photonuclear reactions of $^{4}$He in the GDR region. Therefore we have performed a new measurement of the photodisintegration cross sections of $^{4}$He in the energy range from 21.8 to 29.8 MeV using laser-Compton backscattered photons and a time projection chamber containing $^{4}$He gas as an active target. The obtained cross sections increase monotonically with energy up to 29.8 MeV, contrary to a recent calculation by the Lorentz integral transform method. Most recently, a coupled-channel calculation including the tensor force has been performed, and the calculated values are in excellent agreement with the present experimental ones. In this paper the present result will be compared to recent theoretical calculations, and its astrophysical implications will be discussed. [Preview Abstract] |
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