Neutron scattering from 208Pb

Elastic scattering of 7, 9, 11, 20 and 26 MeV neutrons from ²⁰⁸Pb has been measured with the Ohio University Tandem Van de Graaff accelerator. Standard pulsed beam time-of-flight techniques were employed. Measurements of the incident flux at 0° were used to normalize the differential cross sections. The measured cross sections were corrected for dead time, detector efficiency, flux attenuation, multiple scattering, finite geometry, neutron source anisotropy and compound elastic contribution. Relative uncertainties are estimated to be between 5%–10% and the uncertainty in the normalization is estimated to be less than 5 %. The data were used to obtain neutron optical potential parameters. A comparison with proton optical parameters is presented, and the (p, n) quasi-elastic cross section is calculated and compared with available data. Deformation parameters for the 3^? state (Q = ?2.615 MeV) and 5ā (Q = ?3.198 MeV) in ²⁰⁸Pb were obtained at incident energies of 11 and 26 MeV.     

Compton scattering on208Pb

In this paper we briefly review the formalism of the nuclear Compton scattering in the frame of the low-energy theorems (LET). We treat the resonant terms of the amplitude, having collective intermediate nuclear states, as a superposition of Lorentz lines with energy, width and strength fixed by the photo-absorption experiments. The gauge terms are evaluated starting from a simple, but realistic, nuclear Hamiltonian. Dynamical nucleon-nucleon correlations are consistently taken into account, beyond those imposed by the Pauli principle. The comparison of the theoretical predictions with the data of elastic diffusion of photons from²⁰⁸Pb shows that LET are insufficient to account for the experiment.     

Energy dependence of tensor analyzing powers in deuteron elastic scattering from 208Pb

Differential cross section and all four analyzing powers iT₁₁, T₂₀, T₃₁ and T₂₂ have been measured for deuteron elastic scattering from ²⁰⁸Pb at incident energies from 10 to 23 MeV. The energy dependence of the tensor analyzing powers was investigated. A considerable deviation of the data from optical-model calculations including a T_r-type tensor potential is found near the Coulomb barrier.     

Cluster exchange and elastic scattering of heavy ions

The effect of cluster exchange on the nuclear interaction potential is examined. Cluster exchange results in nonlocal interactions that must be added to the direct local potential of the optical model. The dependence of exchange interactions on the parity of relative motion is analyzed. Scattering amplitudes corresponding to nonlocal exchange interactions are calculated in the Born approximation for ¹⁶O + ¹²C scattering.     

Elastic and inelastic scattering of identical heavy ions

The elastic and inelastic scattering of ¹²C, ¹²C, ²⁴Mg, ²⁴Mg, ²⁸Si, ²⁸Si was studied at bombarding energies E_c.m. = 5–18.8, 20–33, 29–36 MeV, respectively. The experimental arrangement includes two position sensitive detectors covering the angular ranges from O₁₂₄ = 20°–70°. The results are compared to calculations within the framework of the diffraction and optical model. A phase shift analysis is performed for the elastic scattering data of the ¹²C+¹²C system. It is shown that a shallow nuclei potential in needed to reproduce the ²⁸Si+²⁸Si results and that the ¹²C, ¹²C scattering shows evidence of resonance formation in the elastic channel.     

Inelastic scattering of heavy ions from deformed light nuclei at near-barrier energies

Angular distributions have been measured at energies near the Coulomb barrier for the systems ¹⁸O + ²⁴Mg, ¹²C + ²⁸Si and ^{16, 18}O + ²⁸Si for elastic scattering and inelastic scattering to the first 2⁺ states in ¹⁸O, ²⁴Mg and ²⁸Si. Coupled-channel calculations were required in order to reproduce the details of the strong Coulomb-nuclear interference minima. However, a satisfactory account of the main features of the data was obtained in a first-order DWBA analysis and with the closed formalism of Frahn. With the exception of ¹⁸O, it was sufficient to assume equal charge and optical-potential deformation lengths.     

Density-dependent forces in inelastic proton scattering from 208Pb and the shape of the imaginary transition potential

An effective energy- and density-dependent force is used to analyse microscopically inelastic nucleon scattering to low-lying collective states in ²⁰⁸Pb (3⁻, 5₁⁻, 5₂⁻ and 2⁺). In the energy range from 10 to 200 MeV, quantitative agreement with experiment is obtained without need for a renormalization factor in the cross section. Polarization data are also included in the analysis. The shape of the imaginary transition potential is studied in detail. The density dependence of the force is shown to lead to changes in the geometry as predicted by the collective model.     

Three-particle correlations from208Pb+6Li

Three-particle correlations in the reaction²⁰⁸Pb+⁶Li were studied near the Coulomb threshold between \(E_{6_{Li} } = 24\) and 30 MeV. Three reaction mechanisms contribute predominantly to the observed coincidences of the charged particles: 1. Coulomb excitation of the 2.184 MeV,J=3⁺ state of⁶Li, followed by the decay intoα+d, 2. Deuteron pick-up of the⁶Li to the ground state of⁸Be and sequential decay into twoα-particles and 3. Neutron-transfer to the ground state and the first excited states of²⁰⁹Pb:²⁰⁸Pb(⁶Li,αp). The last two reaction mechanisms explain the previously measured large contributions to theα-channel relative to thed-channel.     

MO X-ray angular distributions from 1 GeV208Pb+208Pb and209Bi+209Bi collisions

The Doppler shift and the anisotropy of the MO X-rays above the separated-atomK-lines have been measured in²⁰⁸Pb+²⁰⁸Pb and²⁰⁹Bi+²⁰⁹Bi collisions at beam energies between 4.2 and 4.8 MeV/u. The Doppler shift is found to be consistent with a source moving with 90% of the center of mass velocity of the colliding system. One peak is observed in the anisotropy spectrum. This peak might be attributed to transitions from the continuum into the minimum of the 2p _1/2 σ MO or from the 3p _1/2 σ and 2p _3/2 σ MO's into the 2 _sσ MO near the united atom limit. Radiative electron capture and secondary electron bremsstrahlung could be excluded as major sources of the observed radiation. Contrary to lighter systems no anisotropy corresponding to transitions into the 1 _sσ orbital could be observed.     

Measurement of the neutron radius of 208Pb through parity violation in electron scattering

We report the first measurement of the parity-violating asymmetry A(PV) in the elastic scattering of polarized electrons from 208Pb. A(PV) is sensitive to the radius of the neutron distribution (R(n)). The result A(PV)=0.656±0.060(stat)±0.014(syst) ppm corresponds to a difference between the radii of the neutron and proton distributions R(n)-R(p)=0.33(-0.18)(+0.16) fm and provides the first electroweak observation of the neutron skin which is expected in a heavy, neutron-rich nucleus.     

Multistep scattering in the 160 MeV208Pb(α, α′) reaction

New experimental data has been obtained for the²⁰⁸Pb(α, α′) reaction induced by 160 MeV alpha particles, for inelastic scattering to forward angles. We use these data to investigate the applicability of the multistep scattering theory of Feshbach, Kerman, and Koonin for describing this reaction. The mechanism we study, following the work of Gadioli et al. [1], is of the incident alpha particle remaining intact throughout the scattering process, exciting nucleon particle-hole pairs through multistep process. We conclude that this mechanism, combined with compound nucleus decay at low emission energies, can account for much of the observed data. However, there are indications that other processes also contribute at energies above the compound nucleus emission regime, and we outline future theoretical analyses that are needed.     

Determination of the neutron scattering length of208Pb by the neutron interferometry method

The coherent scattering length of²⁰⁸Pb was measured by means of the neutron interferometry method and the value ofb _c =9.494(29) fm was obtained.     

High-spin states in 208Pb

High-spin states in ²⁰⁸Pb have been studied by γ-ray-spectroscopy methods in deep inelastic reactions induced by beams of ²⁰⁸Pb, ¹³⁶Xe and ⁷⁶Ge beams on a thick ²⁰⁸Pb target. The 11⁺ ₂ state and new γ-transitions between the one-particle one-hole states of highest spin have been found and electromagnetic matrix elements verified. High-spin states of two-particle two-hole structure have been detected for the first time. The results are compared to shell model calculations with realistic interactions in the complete Kuo-Herling space. Received: 31 October 2000 / Accepted: 26 March 2001