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تسجيل مستخدم جديدAccessing the Single-Particle Structure of the Pygmy Dipole Resonance in $^{208}$Pb
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New experimental data on the neutron single-particle character of the Pygmy Dipole Resonance (PDR) in $^{208}$Pb are presented. They were obtained from $(d,p)$ and resonant proton scattering experiments performed at the Q3D spectrograph of the Maier-Leibnitz Laboratory in Garching, Germany. The new data are compared to the large suite of complementary, experimental data available for $^{208}$Pb and establish $(d,p)$ as an additional, valuable, experimental probe to study the PDR and its collectivity. Besides the single-particle character of the states, different features of the strength distributions are discussed and compared to Large-Scale-Shell-Model (LSSM) and energy-density functional (EDF) plus Quasiparticle-Phonon Model (QPM) theoretical approaches to elucidate the microscopic structure of the PDR in $^{208}$Pb.
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Scattering of protons of several hundred MeV is a promising new spectroscopic tool for the study of electric dipole strength in nuclei. A case study of 208Pb shows that at very forward angles J^pi = 1- states are strongly populated via Coulomb excita
tion. A separation from nuclear excitation of other modes is achieved by a multipole decomposition analysis of the experimental cross sections based on theoretical angular distributions calculated within the quasiparticle-phonon model. The B(E1) transition strength distribution is extracted for excitation energies up to 9 MeV, i.e., in the region of the so-called pygmy dipole resonance (PDR). The Coulomb-nuclear interference shows sensitivity to the underlying structure of the E1 transitions, which allows for the first time an experimental extraction of the electromagnetic transition strength and the energy centroid of the PDR.
The pygmy dipole resonance has been studied in the proton-magic nucleus 124Sn with the (a,ag) coincidence method at E=136 MeV. The comparison with results of photon-scattering experiments reveals a splitting into two components with different structu
re: one group of states which is excited in (a,ag) as well as in (g,g) reactions and a group of states at higher energies which is only excited in (g,g) reactions. Calculations with the self-consistent relativistic quasiparticle time-blocking approximation and the quasiparticle phonon model are in qualitative agreement with the experimental results and predict a low-lying isoscalar component dominated by neutron-skin oscillations and a higher-lying more isovector component on the tail of the giant dipole resonance.
A high-resolution study of the electromagnetic response of $^{206}$Pb below the neutron separation energy is performed using a ($vec{gamma}$,$gamma$) experiment at the HI$vec{gamma}$S facility. Nuclear resonance fluorescence with 100% linearly polari
zed photon beams is used to measure spins, parities, branching ratios, and decay widths of excited states in $^{206}$Pb from $4.9$ to $8.1$MeV. The extracted $Sigma$$B$(E1)$uparrow$ and $Sigma$$B$(M1)$uparrow$ values for the total electric and magnetic dipole strength below the neutron separation energy are 0.9$pm$0.2e$^2$fm$^2$ and 8.3$pm$2.0$mu_{N}^2$, respectively. These measurements are found to be in very good agreement with the predictions from an energy-density functional (EDF) plus quasiparticle phonon model (QPM). Such a detailed theoretical analysis allows to separate the pygmy dipole resonance from both the tail of the giant dipole resonance and multi-phonon excitations. Combined with earlier photonuclear experiments above the neutron separation energy, one extracts a value for the electric dipole polarizability of $^{206}$Pb of $alpha_{D}!=!122pm10$mb/MeV. When compared to predictions from both the EDF+QPM and accurately calibrated relativistic EDFs, one deduces a range for the neutron-skin thickness of $R_{rm skin}^{206}!=!0.12$-$0.19$fm and a corresponding range for the slope of the symmetry energy of $L!=!48$-$60$MeV. This newly obtained information is also used to estimate the Maxwellian-averaged radiative cross section $^{205}$Pb(n,$gamma$)$^{206}$Pb at 30keV to be $sigma!=!130!pm!25$mb. The astrophysical impact of this measurement--on both the s-process in stellar nucleosynthesis and on the equation of state of neutron-rich matter--is discussed.
The $^{208}$Pb($p$,$ngammabar p$) $^{207}$Pb reaction at a beam energy of 30 MeV has been used to excite the anti-analog of the giant dipole resonance (AGDR) and to measure its $gamma$-decay to the isobaric analog state in coincidence with proton dec
ay of IAS. The energy of the transition has also been calculated with the self-consistent relativistic random-phase approximation (RRPA), and found to be linearly correlated to the predicted value of the neutron-skin thickness ($Delta R_{pn}$). By comparing the theoretical results with the measured transition energy, the value of 0.190 $pm$ 0.028 fm has been determined for $Delta R_{pn}$ of $^{208}$Pb, in agreement with previous experimental results. The AGDR excitation energy has also been used to calculate the symmetry energy at saturation ($J=32.7 pm 0.6$ MeV) and the slope of the symmetry energy ($L=49.7 pm 4.4$ MeV), resulting in more stringent constraints than most of the previous studies.
The validity of the Brink-Axel hypothesis, which is especially important for numerous astrophysical calculations, is addressed for 116,120,124Sn below the neutron separation energy by means of three independent experimental methods. The $gamma$-ray s
trength functions (GSFs) extracted from primary $gamma$-decay spectra following charged-particle reactions with the Oslo method and with the Shape method demonstrate excellent agreement with those deduced from forward-angle inelastic proton scattering at relativistic beam energies. In addition, the GSFs are shown to be independent of excitation energies and spins of the initial and final states. The results provide the most comprehensive test of the generalized Brink-Axel hypothesis in heavy nuclei so far, demonstrating its applicability in the energy region of the pygmy dipole resonance.
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