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.     

Evidence for a two-phonon octupole vibrational state in208Pb

Collective properties of^206,208Pb have been investigated with gamma-ray spectroscopy using nuclear and electromagnetic excitation by²⁰⁸Pb projectiles. From the measurement of particle-particle- and particle-particle- coincidences a new energy level in²⁰⁸Pb at 5.683 MeV was observed and the data suggest an interpretation as a 2-phonon octupole vibrational state with spin 4⁺ or 6⁺. For this state one finds the same dynamical deformation parameter ₃ as for the first-excited collective 3^– state. The results are compared to predictions of models which consider, in particular, the aspects of the quadrupole interaction and the interaction of the 2-particle pairing vibration with the 2-phonon octupole vibration. For the²⁰⁶Pb+²⁰⁸Pb system at 6.2 MeV/u the nuclear potential could be determined from the measured inelastic cross sections.Dedicated to Prof. Dr. P. Kienle on the occasion of his 60th birthday     

Cross sections of spallation residues produced in 1A GeV 208Pb on proton reactions

Spallation residues produced in 1 GeV per nucleon 208Pb on proton reactions have been studied using the Fragment Separator facility at GSI. Isotopic production cross sections of elements from 61Pm to 82Pb have been measured down to 0.1 mb with a high accuracy. The recoil kinetic energies of the produced fragments were also determined. The obtained cross sections agree with most of the few existing gamma-spectroscopic data. The data are compared with different intranuclear-cascade and evaporation-fission models. Drastic deviations were found for a standard code used in technical applications.     

Evidence for quasi-fission in40Ar+208Pb collisions near the coulomb-barrier

Fission-fragment angular distributions were measured in the reaction of⁴⁰Ar with²⁰⁸Pb near the fusion barrier. For nearly symmetric mass-/charge splits we find angular distributions symmetric around θ=90 degrees, however, with unusually large anisotropies. These develop gradually into forward-backward asymmetric distributions as one moves away from mass-/charge symmetry. This indicates that non-compound fission (‘quasi-fission’) competes with true fusion-fission. The relative contribution of quasi-fission to the total fission cross section is somewhere between 51 and 85%. In the framework of the extra-push model this is equivalent to an extra-extra push energy for compound-nucleus formation inside the true fission saddle point of 4<E _xx <9 MeV in agreement with a recent empirical parametrization of fusion-barrier shifts based on fusion-fission cross sections. On the basis of cross sections for fusion-evaporation residues it had previously been concluded that fusion of⁴⁰Ar with Pb isotopes occurs unhindered. Possible reasons for this apparent discrepancy are discussed.     

Evidence for partial rotation alignment in proton emitting Pr

Using nonadiabatic quasiparticle calculations we reproduce the experimental half-life for proton radioactivity in ¹²¹Pr assuming that the decaying state has angular momentum Jπ=7/²⁻$J^{\pi }=7/2^{-}$, thus showing for the first time clear evidence for partial rotation alignment in a proton emitting nucleus. The treatment of the pairing interaction in the BCS approach produces profound changes in the ordering of energy levels, and at high deformation, the state 7/²⁻$7/2^{-}$ coming from the h_11/2${\mathrm{h}}_{11/2}$ spherical shell becomes the bandhead.     

Proton orbit sizes in 208Pb

The sub-Coulomb (t,α) reaction has been employed in the determination of the rms radii of the $\text{3s}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{, 2}\text{d}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{, 1}\text{h}{}_{\mathrm{<mtext>11</mtext><mtext>2</mtext>}}$, and $\text{2}\text{d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$ proton orbits in ²⁰⁸Pb. The experimental values are compared to the results of Hartree-Fock calculations.     

Experimental constraints on size and phase of proton and neutron contributions to the isoscalar 1+ state in208Pb

The 1⁺ state at 5.846 MeV in²⁰⁸Pb was observed in a high-resolution²⁰⁹Bi(d³He)²⁰⁸Pb experiment with a spectroscopic factor of C²S=0.17. From this, together with the transition probability BM1↑=1.6μ _N ² from a ( \((\vec \gamma ,\gamma ')\) ) experiment and the normalization condition we derive size and relative phase of proton and neutron spinflip amplitudes of the 1⁺ state wave function; these are in good agreement with predictions of TDA and RPA calculations. Presumably only few nuclear states have ever been subject to so much interest from both theoretical and experimental sides as the lowest 1⁺ state in²⁰⁸Pb [1]. The reasons are the expected simplicity of its configuration and that its energy and particle-hole structure allow one to expect information on the spin-dependent parts of the residual interaction. It is the aim of the present work to determine the leading terms of the wave function of the 1 state directly from experimental results without making essential assumptions, in contrast to previous analyses.     

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.     

201 MeV proton excitation of giant resonances in 208Pb: Macroscopic and microscopic analysis

The region of the giant resonances in ²⁰⁸Pb has been investigated by inelastic scattering of 201 MeV protons. To test the analysis, angular distributions were measured for the low-lying 3^?, 5^?, 2⁺ and 4⁺ collective states. The giant isoscalar quadrupole resonance (ISGQR) is split into two structures, one at 9.0 MeV with a full width at half-maximum Γ = 1.0 MeV, the other one at 10.6 MeV (Γ = 2.0 MeV), with fine structures at 8.9, 9.3, 10.1, 10.6 and 11 MeV. A macroscopic analysis using the distorted-wave Born approximation (DWBA) leads for the low-lying collective levels, as well as for the ISGQR, to transition probabilities too small by a factor of two, compared with those obtained in other reactions. Microscopic analysis using the distorted-wave impulse approximation (DWIA), with three different sets of random phase approximation (RPA) transition densities, is in very good agreement with the data. At forward angles, in the 12 to 16 MeV excitation energy region, a strong resonance at 13.5 MeV (Γ = 3.6 MeV) is accounted for by the Coulomb excitation of the isovector giant dipole resonance (IVGDR); at larger angles the results are compatible with the excitation of the isoscalar monopole resonance (ISGMR) located at 13.9 MeV (Γ = 2.6 MeV).A resonance located at 21.5 MeV (Γ = 5.7 MeV) appears as the superposition of an isovector quadrupole resonance (IVGQR) excited by Coulomb interaction and a resonance of multipolarity L = 1 ΔT = 0 (ISGDR “squeezing mode”).     

Analysis of proton transfer in polarized 7Li scattering by 208Pb at 33 MeV

Angular distributions of the differential cross section and the second-rank tensor analysing power T₂₀ have been measured for the one-proton transfer reaction induced by polarized ⁷Li on ²⁰⁸Pb at 33 MeV leading to several states in ²⁰⁹Bi. The experimental data were analysed simultaneously with data for elastic and inelastic scattering of ⁷Li by means of CCBA and CRC calculations. The spectroscopic factors for ²⁰⁹Bi states are obtained.     

The impact parameter dependence of the K MO X-ray emission in208Pb +208Pb collisions

The emission probabilities for quasimolecular K-x-radiation (K-MOR) have been measured in²⁰⁸Pb +²⁰⁸Pb collisions at 4.3 MeV/u and 4.8 MeV/u as a function of the scattering angle using the particle-photon-coincidence technique. The probabilities exhibit a superposition of two exponential slopes with different fall-off constants. We identify the sharp fall off at very small impact parameters (<60 fm) with the K-MOR contribution from the decay of 1sσ-vacancies. The flatter slope for impact parameters larger than 60 fm is attributed to the radiative decay of holes in 2p _1/2 σ molecular states. In this way, experimental “1sσ-MOR-emission probabilities” could be extracted and compared with theory. For 4.3 MeV/u the1sσ- and 2p _1/2 σ-excitation probabilities could also be determined and have been compared to coupled-channel calculations and to a scaling law for 1sσ excitation.     

Measurement of proton polarization in the 208Pb(d,p)209Pb reaction at an incident energy of 20 MeV

Angular distribution measurements of proton polarization in the ²⁰⁸Pb(d, p)²⁰⁹Pb reaction to the ground g_9/2 state were made at an incident deuteron energy of 20 MeV using a high efficiency proton polarimeter. Distorted wave Born approximation analysis and continuum discretized coupled channel analysis were performed on the data and compared with measurements of the angular distribution of the vector analyzing power for the same reaction. The result of the calculations predict that the angular distribution of the vector analyzing power should be similar to that of the polarization. However, the experimental data gives different angular distributions. The measured vector analyzing power is well reproduced by the calculations, while the polarization is not well reproduced.     

Evidence of projectile polarisation effects on the reaction 208Pb(17O, 16O)209Pb

Coupled reaction channels calculations (CRC) of the reactions ²⁰⁸Pb(¹⁷O, ¹⁶O)²⁰⁹Pb leading to different states of ²⁰⁹Pb are compared with DWBA predictions at projectile energies of 78, 86 and 102 MeV. The calculations exhibit strong effects of multistep processes on Q-value and angular-momentum-mismatched transfer reactions. It is shown that the contribution to the transfer through the inelastic excitation of ¹⁷O contains a major part of the multistep effect. A simple three-channel model comprising the elastic, inelastic and transfer channels is constructed which simulates the CRC effects on the transfer cross sections. The polarization effects of the eliminated channels give rise to effective potentials which are mainly imaginary.