Particle octupole-vibration coupling near 208Pb

High-spin states in ²⁰²Pb and ²⁰³Pb have been investigated by in-beam γ-ray spectroscopy following the reaction ¹⁹⁸Pt(⁹Be,xn). A search for magnetic rotational bands in these isotopes confirmed one of the two bands previously assigned to ²⁰²Pb and revealed a new band in this isotope. No evidence for magnetic rotation has been found in ²⁰³Pb. Received: 24 July 2000 / Accepted: 28 September 2000     

Angular dependence of the207Pb NMR linewidth in Pb(NO3)2 due to14N—207Pb direct dipole-dipole interaction

NMR signals of²⁰⁷Pb²⁺ have been measured in a single crystal of Pb(NO₃)₂. For rotation axes perpendicular to the static magnetic field and perpendicular to the (100) or (011) plane the linewidths of these signals vary by a factor 2 for different rotation angles of the single crystal. This dependence of the linewidth on the orientation has been explained by a¹⁴N —²⁰⁷Pb direct dipole-dipole coupling and it has been used to assign the different Pb²⁺ NMR signals to the different Pb²⁺ sites in the cubic unit cell of Pb(NO₃)₂.     

Gross structure in excitation functions for207Pb(p,p′)207Pb in the region of overlapping isobaric analogue resonances

Prominent resonances in the reaction²⁰⁷Pb(p, p′)²⁰⁷Pb leading to the low-lying states in²⁰⁷Pb(1/2^? g.s., 5/2^? 0.57 MeV, 3/2^? 0.89 MeV, 7/2^? 2.33 MeV) are observed betweenE _p =16 and 18 MeV. The structures in the excitation functions are interpreted as being due to analogues of states in²⁰⁸Pb which are mixtures of neutron particle-hole configurations.     

Isomer shifts of the octupole doublet in muonic 207Pb

Isomer shifts in the doublet $\text{5}\text{2}{}^{\mathrm{+}}$, $\text{7}\text{2}{}^{\mathrm{+}}$ levels in muonic ²⁰⁷Pb have been measured. The shifts are essentially the same as in the core 3^? collective state. They support the hypothesis that the octupole vibration does not conserve volume.     

Delayed gamma rays from muon capture on 207Pb

We have measured the energies and intensities of delayed γ-rays from ^207,201Tl following muon capture on ²⁰⁷Pb. Probabilities for the emission of 0–6 neutrons have been deduced. Measured and predicted neutron multiplicities (based on an impulse approximation model) are in reasonable agreement for multiplicities of 2, 3, 4 and 5. Experimental values for high and low multiplicities are not accounted for by the theory. High angular momentum states involving the $\text{h}{}_{\mathrm{<mtext>11</mtext><mtext>2</mtext>}}$ proton hole are observed in several of the thallium daughters. Using a simple shell-model approach, we have correlated μ-capture on the $\text{h}{}_{\mathrm{<mtext>11</mtext><mtext>2</mtext>}}$ proton shell with the energy spectrum of directly emitted neutrons.     

Particle production in Pb+Pb collisions at the SPS

Experiment NA49 at the CERN SPS employs a large acceptance detector to study particle spectra, yields and correlations in nucleus-nucleus, nucleon-nucleus and nucleon-nucleon collisions. Preliminary results on pion, kaon, A and ā production in central Pb+Pb collisions at 40, 80 and 158 A·GeV beam energy are shown and compared to measurements at lower and higher energies.     

Nuclear magnetic shielding tensors of207Pb2+ in Pb(NO3)2

The NMR signals of²⁰⁷Pb were observed in a single crystal of Pb(NO₃)₂ and could be assigned to the four different Pb²⁺ sites by the dependence of the linewidths on the orientation. Four different nuclear magnetic shielding tensors with equal principal values but with different characteristic vectors could be determined. The symmetry of the shielding tensors is in agreement with the symmetry at the Pb²⁺ sites. It is shown, that intermolecular contributions can not account for the anisotropy of the nuclear magnetic shielding, which is 3 of the isotropic absolute magnetic shielding.     

Neutron particle-hole electric dipole states in 206, 207, 208Pb

Inelastic proton scattering on ²⁰⁶Pb, ²⁰⁷Pb and ²⁰⁸Pb through isobaric analog resonances has been used to study neutron particle-hole excitations with large ground-state γ-branches in these Pb isotopes. Relative (p, p′) cross sections at 90° are extracted for structures selectively excited on the $\text{d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}\text{,}\text{s}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{and d}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{?}\text{g}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}$ resonances. Interpretation of excitations is ²⁰⁶Pb and ²⁰⁷ Pb in terms of coupling to states in ²⁰⁸Pb is discussed. Branching ratios for 1^?states in ²⁰⁸Pb at 4.84, 5.29, 5.94 and 6.31 MeV and the $\text{1}\text{2}{}^{\mathrm{+}}$ state in ²⁰⁷Pb at 4.63 MeV are deduced.     

Properties of the quasiparticle excitations in 207Pb and 209Pb from an extrapolation of the optical-model potential

A previously developed dispersion relation approach is used to calculate the shell-model potential in the case of neutrons in ²⁰⁸Pb, in the energy domain (-50 MeV, 0). This potential contains a dispersive contribution besides a Hartree-Fock type component, and thereby includes correlation and polarization effects. The shell-model and the Hartree-Fock type potentials are assumed to have Woods-Saxon shapes with diffuseness a_v = 0.70 fm; the energy dependence of their depths and radii is calculated. The energy dependence of the shell-model potential is characterized by the effective mass, whose dependence upon radial distance and neutron energy is determined. The effective mass is a sensitive function of energy, in contrast to its Hartree-Fock type component which is nearly independent of energy. Attention is drawn to the fact that the effective mass in nuclear matter cannot be straightforwardly identified with the effective mass at the nuclear centre. The effective mass presents a sharp peak at the nuclear surface near the Fermi energy and a dip at the surface for energies 10 to 20 MeV away from the Fermi energy. The spectroscopic factors of single-particle excitations in ²⁰⁷Pb and ²⁰⁹Pb are calculated from the difference between the effective mass and its Hartree-Fock type component. The predicted values of the valence single-particle wave functions at large radial distances are in fair agreement with experimental values deduced from analyses of sub-Coulomb pickup reactions. It is shown that the dispersive contribution increases the level density parameter by about 25%, in agreement with previous microscopic or semi-phenomenological models; the calculated level density parameter is in good agreement with the empirical value.     

Damping of the high-spin neutron hole orbitals of the207Pb

The neutron strength functions of the high-spin 1g _7/2 and 1h _11/2 hole states of the²⁰⁷Pb have been deduced within the formalism of the hole-core vibrational coupling scheme. The attenuation of the shell-model strengths of these two high-spin hole states have been compared with the distribution of the neutron strength functions of the discrete 1h _9/2 and 1i _13/2 states. The theoretical results have been discussed in the light of the recent experimental findings on the neutron hole orbitals of the²⁰⁷Pb nucleus. Side by the side the implication of the present research works has been reviewed on the basis of the other existing theoretical calculations on the high-spin neutron hole states of the²⁰⁷Pb.     

Proton decay of the isobaric analogs of the ground states of207Pb and208Pb

The proton decays \(\tilde p\) of isobaric analog states of^{207, 208}Pb in the reactions^{207, 208}Pb(p, n) have been studied using a neutron-proton coincidence technique. The width anomaly observed in (p, n) [4] and (p, n \(\tilde p\) ) singles spectra [11] is resolved in the coincident spectra.     

The {(208Pb)3-⊗ (p 1/2)−1} doublet in207Pb

Excitation curves measured to the 7/2⁺ and 5/2⁺ doublet in²⁰⁷Pb resonate selectively on the 5^? and 4^? analog resonances respectively. This behavior is explained from the structure of the known wave functions involved.     

207Pb chemical shift thermometer at high temperature for magic angle spinning experiments

The temperature dependence of 207Pb chemical shift in magic angle spinning (MAS) NMR spectrum of Pb(NO3)2 provides a sensitive method to calibrate sample temperatures in MAS NMR. The temperature dependence is uniform in the temperature range between 30 degrees C and 400 degrees C. The NMR sensitivity and the line width are also favorable.     

Field cooled and zero field cooled 207Pb NMR of the relaxor ferroelectric PMN

A 2.8 kV/cm electric field has been applied parallel to the external magnetic field along the [111] direction of a PMN single crystal and the 207Pb NMR spectra were measured at 9.1 T. Whereas the zero field cooled (ZFC) spectrum exhibits a Gaussian-like line shape, the FC spectrum clearly shows a two peak structure. One of the two peaks coincides with the ZFC spectrum. The other peak is shifted by about 100 kHz towards lower frequencies with respect to the ZFC peak and seems to be characteristic for the ferroelectric state. The ferroelectric shift agrees with the predictions of the spherical random bond-random field model.     

Multiple-rotor-cycle 2D PASS experiments with applications to (207)Pb NMR spectroscopy

Thetwo-dimensional phase-adjusted spinning sidebands (2D PASS) experiment is a useful technique for simplifying magic-angle spinning (MAS) NMR spectra that contain overlapping or complicated spinning sideband manifolds. The pulse sequence separates spinning sidebands by their order in a two-dimensional experiment. The result is an isotropic/anisotropic correlation experiment, in which a sheared projection of the 2D spectrum effectively yields an isotropic spectrum with no sidebands. The original 2D PASS experiment works best at lower MAS speeds (1-5 kHz). At higher spinning speeds (8-12 kHz) the experiment requires higher RF power levels so that the pulses do not overlap. In the case of nuclei such as (207)Pb, a large chemical shift anisotropy often yields too many spinning sidebands to be handled by a reasonable 2D PASS experiment unless higher spinning speeds are used. Performing the experiment at these speeds requires fewer 2D rows and a correspondingly shorter experimental time. Therefore, we have implemented PASS pulse sequences that occupy multiple MAS rotor cycles, thereby avoiding pulse overlap. These multiple-rotor-cycle 2D PASS sequences are intended for use in high-speed MAS situations such as those required by (207)Pb. A version of the multiple-rotor-cycle 2D PASS sequence that uses composite pulses to suppress spectral artifacts is also presented. These sequences are demonstrated on (207)Pb test samples, including lead zirconate, a perovskite-phase compound that is representative of a large class of interesting materials.     

CP-MAS 207Pb with 19F decoupling NMR spectroscopy: medium range investigation in fluoride materials

The isotropic chemical shift of 207Pb is used to perform structural investigations of crystalline fluoride compounds (PbF2, Pb2ZnF6, PbGaF5, Pb3Ga2F12 and Pb9Ga2F24) and transition metal fluoride glasses (TMFG) of the PZG family (PbF2-ZnF2-GaF3). Using 207Pb Cross Polarisation Magic Angle Spinning (CP-MAS) NMR with 19F decoupling, it is shown that the isotropic chemical shift of 207Pb varies on a large scale (1000 ppm) and that the main changes of its value are not due to the nearest neighbour fluorines but may be related to the number of next nearest neighbour (nnn) Pb2+ ions. In this way, it is demonstrated that 207Pb chemical shift is an interesting probe to investigate medium range order in either crystalline or glassy fluoride systems. The 207Pb delta(iso) parameter has been linearly correlated to the number of nnn Pb2+ ions.     

New experimental access to the two-phonon octupole vibration in 208Pb

The feasibility of a new experimental method to identify two-phonon octupole vibrations is investigated for the example of ²⁰⁸Pb. The 2⁺ member of the 3_I⁻ ⊗ 3_I⁻ multiplet can be excited with the (γ, γ′) reaction and a sizable E1 decay to the 3_I⁻ level serves as a signature of the two-phonon character. Numerical estimates within the quasiparticle-phonon model indicate that such an experiment should be possible with state-of-the-art large volume Ge detectors like the EUROBALL cluster module.     

207Pb NMR of minium, Pb3O4: evidence for the [Pb2]4+ ion and possible relativistic effects in the Pb-Pb bond

Solid Pb3O4 has been studied with 207Pb nuclear magnetic resonance (NMR) spectroscopy. The 207Pb NMR chemical-shift tensor of the Pb2+ site has principal values of delta11 = 1980 +/- 5 ppm, delta22 = 1540 +/- 5 ppm, and delta33 = -1108 +/- 10 ppm; delta(iso) = 804 +/- 10 ppm. The chemical-shift tensor of the Pb4+ site is axial, with principal values delta(parallel) = -1009 +/- 3 ppm and delta(perpendicular) = 1132 +/- 3 ppm; delta(iso) = -1091 +/- 3 ppm. The Pb4+-Pb2+ scalar coupling constant J(Pb-Pb) = 2.3 +/- 0.1 kHz. The main contribution to the Pb2- chemical-shift anisotropy is proposed to arise from an exchange interaction in the Pb2+-Pb2+ pairs, conventionally regarded as molecular [Pb2]4+ ions.