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تسجيل مستخدم جديدElectric and magnetic dipole modes in high-resolution inelastic proton scattering at $0^circ$
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Inelastic proton scattering under extreme forward angles including $0^circ$ and at energies of a few hundred MeV has been established as a new spectroscopic tool for the study of complete dipole strength distributions in nuclei. Such data allow an extraction of the electric dipole polarizability which provides important constraints parameters of the symmetry energy, which determine the neutron skin thickness and the equation of state (EOS) of neutron-rich matter. Also new insight into the much-debated nature of the pygmy dipole resonance (PDR) is obtained. Additionally, the isovector spin-M1 resonance can be studied in heavy nuclei, where only limited experimental information exists so far. Together with much improved results on the isoscalar spin-M1 strength distributions in $N = Z$ nuclei, these data shed new light on the phenomenon of quenching of the nuclear spin response. Using dispersion matching techniques, high energy resolution ($Delta E/E leq 10^{-4} ,,$ full width at half maximum, FWHM) can be achieved in the experiments. In spherical-vibrational nuclei considerable fine structure is observed in the energy region of the isovector giant dipole resonance (IVGDR). A quantitative analysis of the fine structure with wavelet methods provides information on the role of different damping mechanisms contributing to the width of the IVGDR. Furthermore, level densities can be extracted from a fluctuation analysis at excitation energies well above neutron threshold, a region hardly accessible by other means. The combination of the gamma strength function (GSF) extracted from the E1 and M1 strength distributions with the independently derived level density permits novel tests of the Brink-Axel hypothesis underlying all calculations of statistical model reaction cross sections in astrophysical applications in the energy region of the PDR.
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A survey of the fine structure of the Isovector Giant Dipole Resonance (IVGDR) was performed, using the recently commissioned zero-degree facility of the K600 magnetic spectrometer at iThemba LABS. Inelastic proton scattering at an incident energy of
200 MeV was measured on $^{27}$Al, $^{40}$Ca, $^{56}$Fe, $^{58}$Ni and $^{208}$Pb. A high energy resolution ($rm{Delta}it{E} simeq$ 40 keV FWHM) could be achieved after utilising faint-beam and dispersion-matching techniques. Considerable fine structure is observed in the energy region of the IVGDR and characteristic energy scales are extracted from the experimental data by means of a wavelet analysis. The comparison with Quasiparticle-Phonon Model (QPM) calculations provides insight into the relevance of different giant resonance decay mechanisms. Photoabsorption cross sections derived from the data assuming dominance of relativistic Coulomb excitation are in fair agreement with previous work using real photons.
Inelastic proton scattering experiments were performed at the Research Center for Nuclear Physics, Osaka, with a 295 MeV beam covering laboratory angles 0{deg}-6{deg} and excitation energies 6-22 MeV. Cross sections due to E1 and M1 excitations were
extracted with a multipole decomposition analysis and then converted to reduced transition probabilities with the virtual photon method for E1 and the unit cross section method for M1 excitations, respectively. Including a theory-aided correction for the high excitation energy region not covered experimentally, the electric dipole polarizability was determined from the E1 strength distributions. Total photoabsorption cross sections derived from the E1 and M1 strength distributions show significant differences compared to those from previous ($gamma$,xn) experiments in the energy region of the isocvector giant dipole resonance (IVGDR). The widths of the IVGDR deduced from the present data with a Lorentz parameterization show an approximately constant value of about 4.5 MeV in contrast to the large variations between isotopes observed in previous work. The IVGDR centroid energies are in good correspondence to expectations from systematics of their mass dependence. Furthermore, a study of the dependence of the IVGDR energies on bulk matter properties is presented. The E1 strengths below neutron threshold show fair agreement with results from ($gamma$,$gamma$) experiments on 112,116,120,124Sn in the energy region between 6 and 7 MeV. At higher excitation energies large differences are observed pointing to a different nature of the excited states with small ground state branching ratios. The isovector spin-M1 strengths exhibit a broad distribution between 6 and 12 MeV in all studied nuclei.
The nucleus is one of the most multi-faceted many-body systems in the universe. It exhibits a multitude of responses depending on the way one probes it. With increasing technical advancements of beams at the various accelerators and of detection syst
ems the nucleus has, over and over again, surprised us by expressing always new ways of organized structures and layers of complexity. Nuclear magnetism is one of those fascinating faces of the atomic nucleus we discuss in the present review. We shall not just limit ourselves to presenting the by now very large data set that has been obtained in the last two decades using various probes, electromagnetic and hadronic alike and that presents ample evidence for a low-lying orbital scissors mode around 3 MeV, albeit fragmented over an energy interval of the order of 1.5 MeV, and higher-lying spin-flip strength in the energy region 5 - 9 MeV in deformed nuclei, nor to the presently discovered evidence for low-lying proton-neutron isovector quadrupole excitations in spherical nuclei. To the contrary, we put the experimental evidence in the perspectives of understanding the atomic nucleus and its various structures of well-organized modes of motion and thus enlarge our discussion to more general fermion and bosonic many-body systems.
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data are compared to calculations employing realistic nucleon-nucleon interactions, both with and without three-nucleon forces. Excellent agreement with the tensor analyzing powers and cross section is found, while the Ay and iT11 data are found to be underpredicted by the calculations.
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