208Pb Reaction Research Articles

Classical simulations of heavy-ion fusion reactions and weakly-bound projectile breakup reactions

Heavy-ion collision simulations in various classical models are discussed. Heavy-ion reactions with spherical and deformed nuclei are simulated in a classical rigid-body dynamics (CRBD) model which takes into account the reorientation of the deformed projectile. It is found that the barrier parameters not only on the initial orientations of the deformed nucleus, but also on the collision energy and the moment of inertia of the deformed nucleus. Maximum reorientation effect occurs at near- and below-barrier energies for light deformed nuclei. Calculated fusion cross-sections for 24Mg + 208Pb reaction are compared with a static-barrier-penetration model (SBPM) calculation to see the effect of reorientation. Heavy-ion reactions are also simulated in a 3-stage classical molecular dynamics (3S-CMD) model in which the rigid-body constraints are relaxed when the two nuclei are close to the barrier thus, taking into account all the rotational and vibrational degrees of freedom in the same calculation. This model is extended to simulate heavy-ion reactions such as6Li + 209Bi involving the weakly-bound projectile considered as a weakly-bound cluster of deuteron and 4He nuclei, thus, simulating a 3-body system in 3S-CMD model. All the essential features of breakup reactions, such as complete fusion, incomplete fusion, no-capture breakup and scattering are demonstrated.

Observation of superheavy nuclide 271Ds based on the Gas-filled separator at IMP

A gas-filled recoil separator was designed and built newly at Institute of Modern Physics (IMP) in Lanzhou. With the recent commissioning of the separator, the decay properties of 271Ds were studied via the 208Pb (64Ni, n) reaction at a beam energy of 313.3 MeV. One α-decay chain for 271Ds was established in total. The experimental result is consistent with the values reported in the literature.

Four-Body CDCC Analysis for Breakup Reactions of Three-Body Projectiles

We present a new method of smoothing discrete breakup cross sections calculated by the continuum-discretized coupled-channels method based on the complex-scaling method. One of the advantages of this approach is to be applicable to many-body breakup reaction systems. In this work, we apply the new smoothing method to analyses of 12C(6He, nn4He) and 208Pb(6He, nn4He) reactions at 240 MeV/A.

Projectile structure effects in multi-nucleon and cluster transfers in 16,18O+164Dy, 208Pb reactions

The yield of projectile like fragments (PLFs) has been measured in the reactions of 16,18O with 164Dy and 208Pb targets at energies above the Coulomb barrier. The experimental data are analyzed to ascertain the role of projectile structure on the multi-nucleon and cluster correlations in transfer reactions. The variation of transfer probability with the number of nucleons transferred from the projectile to the target nucleus at the grazing angle has been investigated for both the systems. In the case of 18O induced reactions the cross sections of the two neutron (2n) stripping as well as the 2n- correlated cluster transfer are strongly enhanced as compared to the 16O induced reactions. These results have been discussed on the basis of the possible influence of the valence di-neutrons in the 18O nucleus on multi-particle transfer reactions.

Quasi elastic cross sections for the 209Bi(e,e'p)208Pb reaction: Jefferson Lab experiment E06007

Quasi elastic cross sections were measured for the first time for both negative and positive missing momenta for the 209Bi(e,e'p)208Pb reaction leading to the ground state and hole states of 208Pb. Experimental cross sections obtained between -0.3 GeV/c to 0.3 GeV/c agree with theoretical calculations using RDWIA techniques both in shape and magnitude for the ground state. The data for the ground state production of 208Pb are consistent with a theoretical model assuming a single proton(1.06 ± 0.10) in the 1h9/2 orbit in 209Bi.

Neutron scattering from208Pb at 30.4 and 40.0 MeV and isospin dependence of the nucleon optical potential

Analysis of data involving nuclei far from stability often requires optical potential (OP) for neutron scattering. Since neutron data is seldom available, while proton scattering data is more abundant, it is useful to have estimates of the difference of the neutron and proton optical potentials. This information is contained in the isospin dependence of the nucleon OP. Here we attempt to provide it for the nucleon-208Pb system. The goal of this paper is to obtain accurate n+208Pb scattering data, and use it, together with existing p+208Pb and 208Pb(p,n)208$Bi*_{IAS} data, to obtain an accurate estimate of the isospin dependence of the nucleon OP at energies in the 30-60 MeV range. Cross sections for n+208Pb scattering were measured at 30.4 and 40.0 MeV, with a typical relative (normalization) accuracy of 2-4% (3%). An angular range of 15 to 130 degrees was covered using the beam-swinger time of flight system at Michigan State University. These data were analyzed by a consistent optical model study of the neutron data and of elastic p+208Pb scattering at 45 MeV and 54 MeV. These results were combined with a coupled-channel analysis of the 208Pb(p,n) reaction at 45 MeV, exciting the 0^+ isobaric analog state in 208Bi. The new data and analysis give an accurate estimate the isospin impurity of the nucleon-208Pb OP at 30.4 MeV, caused by the Coulomb correction to the proton OP. The corrections to the real proton OP given by the CH89 global systematics was found to be only few percent, while for the imaginary potential it was over 20% at the nuclear surface. Based on the analysis of the measured elastic p+208Pb data at 40 MeV, a Coulomb correction of similar strength and shape was also predicted for the p+208Pb OP at energy around 54 MeV.

Observation of the Superheavy Nuclide 271Ds

With the recent commissioning of a gas-filled recoil separator at Institute of Modern Physics (IMP) in Lanzhou, the decay properties of 271Ds (Z = 110) were studied via the 208Pb(64Ni, n) reaction at a beam energy of 313.3 MeV. Based on the separator coupled with a position sensitive silicon strip detector, we carried out the energy-position-time correlation measurements for the implanted nucleus and its subsequent decay α's. One α-decay chain for 271Ds was established. The α energy and decay time of the 271Ds nucleus were measured to be 10.644 MeV and 96.8 ms, which are consistent with the values reported in the literature.

Measurement of the Isovector Spin Monopole Resonance via the 208Pb,90Zr(t,3He) Reactions at 300 MeV/u

The isovector spin monopole resonances (IVSMR) of the β+ type were observed via the 208Pb and 90Zr(t, 3He) reactions at 300 MeV/u. The experiment was performed at the RI Beam Factory (RIBF) at RIKEN by using the newly-constructed magnetic spectrometer SHARAQ. The double differential cross sections for the 208Pb and 90Zr(t, 3He) reactions were obtained at the excitation energy of 0 ≤ Ex ≤ 70 MeV and the scattering angles of 0° ≤ 9 ≤ 3°. The monopole component was obtained as a difference between the zero-degree and backward-angle spectra. The signatures for the IVSMR were discovered at excitation energies of 12 MeV and 20 MeV in 208Pb and 90Zr, respectively, and they show good correspondences to the theoretical predictions.

A novel manifestation of α clustering in 212Po: Pure α-208Pb states revealed by their enhanced E1 decays

Excited states in 212Po have been populated by α transfer using the 208Pb(18O, 14C) reaction and studied with the Euroball γ multidetector array. The level scheme of 212Po has been extended up to ∼ 3.2 MeV excitation energy. Spin and parity values have been assigned thanks to the measurements of γ angular distributions and γ-γ angular correlations. Several γ lines have been found to be shifted by the Doppler effect, allowing for the measurements of the associated lifetimes by the DSAM method. The values, found in the [0.1-1.4] ps range, lead to very enhanced E1 transitions. The emitting states, which have non-natural parity values, are the fingerprints of the "α-208Pb" structure present in 212Po. They are in the same excitation-energy range as the states issued from shell-model configurations.

Structure of states in with major components of the neutron particle–hole configuration

Using the 208Pb(p, p′) reaction via isobaric analog resonances in 209Bi, a dozen states in the doubly magic nucleus 208Pb with major strength of the neutron particle–hole configuration are investigated. Among them, two doublets at Ex = 5.81, 6.01 MeV are resolved by comparing the mean cross sections on the d5/2 resonance. The 5.81 MeV doublet consists of two states with major configuration and spins 2− and 3−; the 2− state is newly identified. The distance between the two states is 0.4 ± 0.2 keV. The 6.01 MeV doublet consists of a 4− state with dominant configuration and a 3− state with a small admixture of . The distance between the two states is 1.8 ± 0.3 keV. The 5886 state has been newly assigned the spin of 4−. The spins of seven more states are verified. Almost the complete strength is identified in these 12 states. The center of gravity for the configuration and is in line with the predictions by the shell model without residual interaction to within 10 keV.

Erratum to: Neutron particle-hole structure of states in 208Pb determined from the proton decay of the d5/2 isobaric analog resonance in 209Bi

Proton scattering on 208Pb via isobaric analog resonances (IARs) in 209Bi generates neutron particle-hole configurations in excited states of 208Pb . The spin and parity of the neutron particle correspond to the spin and parity of the resonance. We performed experiments on the 208Pb (p, p’) reaction via all known isobaric analog resonances in 209Bi with the Q3D magnetic spectrograph in Munchen. We observed more than 130 levels in the range of excitation energies E x < 6.4 MeV. In the proton decay of the d 5/2 resonance, 29 levels have cross-sections larger than 5% of the maximum cross-section of any level. Six levels in the range 4.6 < E x < 5.2 MeV correspond to states with known spins and major $\ensuremath \nu(d_{5/2}^{+1} p_{1/2}^{-1})$ strength. Six levels in the range 5.7 < E x < 6.4 MeV contain most of the $\ensuremath \nu(d_{5/2}^{+1} p_{3/2}^{-1})$ strength. At least one level originates from an unresolved doublet. Among the states with a major $\ensuremath \nu(d_{5/2}^{+1} p_{3/2}^{-1})$ component, only states with spins 1- and 3- are firmly identified. The comparison to previous data taken with semiconductor detectors reveals details of the structure of at least 18 states in the range 4.6 < E x < 6.4 MeV.

Coexistence of “ $ \alpha$ + 208Pb ” cluster structures and single-particle excitations in 212 84Po128

Excited states in 212Po have been populated by $ \alpha$ transfer using the 208Pb(18O,14C) reaction at 85MeV beam energy and studied with the EUROBALL IV $ \gamma$ multi-detector array. The level scheme has been extended up to ∼ 3.2 MeV excitation energy from the triple- $ \gamma$ coincidence data. Spin and parity values of most of the observed states have been assigned from the $ \gamma$ angular distributions and $ \gamma$ - $ \gamma$ angular correlations. Several $ \gamma$ -lines with E γ < 1 MeV have been found to be shifted by the Doppler effect, allowing for the measurements of the associated lifetimes by the DSAM method. The values, found in the range [0.1-0.6]ps, lead to very enhanced E1 transitions. All the emitting states, which have non-natural parity values, are discussed in terms of $ \alpha$ - 208Pb structure. They are in the same excitation-energy range as the states issued from shell-model configurations.

Simultaneous analysis of elastic scattering and transfer/breakup channels for the 6He + 208Pb reaction at energies near the Coulomb barrier

The elastic and α-production channels for the He 6 + Pb 208 reaction are investigated at energies around the Coulomb barrier ( E lab = 14 , 16, 18, 22, and 27 MeV). The effect of the two-neutron transfer channels on the elastic scattering has been studied within the Coupled-Reaction-Channels (CRC) method. We find that the explicit inclusion of these channels allows a simultaneous description of the elastic data and the inclusive α cross sections at backward angles. Three-body Continuum-Discretized Coupled-Channels (CDCC) calculations are found to reproduce the elastic data, but not the transfer/breakup data. The trivial local polarization potential (TLP) derived from the CRC and CDCC calculations are found to explain the features found in previous phenomenological optical model calculations for this system.

Coulomb reorientation effect on near-barrier heavy-ion fusion reactions in rigid-body rotational dynamics calculation

A classical rigid-body dynamics model is used for study of Coulomb reorientation effect on the fusion dynamics. Model calculations show that barrier parameters depend on the initial orientations and also on the collision energy. Calculated fusion cross sections for 24Mg+ 208Pb and 16O+ 154Sm reactions show that reorientation effect is more pronounced in the case of 24Mg+ 208Pb reaction which involves a lighter deformed nucleus.

Dynamic approach to the analysis of angular distributions of fission and quasi-fission fragments

A dynamic approach to the calculation of angular distributions of fission and quasi-fission fragments is proposed. The approach is tested in the analysis of the experimental data for the 28Si, 32S + 208Pb reactions at Elab = 160–280 MeV. Dependence of the relaxation time for the degree of freedom related to the projection of the angular momentum onto the symmetry axis of the decaying system on the deformation and the angular momentum is discussed.

Experiments on searching for the heaviest elements

At the Institute of Physical and Chemical Research (RIKEN) a series of experiments studying the productions and their decays of the heaviest elements have been performed by using a gas-filled recoil ion separator GARIS. Results on the isotope of the 112th element, 277112, and on that of the 113th element, 278113, are reviewed. Tow decay chains which are assigned to be ones originating from the isotope 277112 were observed in the 208Pb( 70Zn, n) reaction. Both chains consisted of four consecutive alpha decays followed by a spontaneous fission. The results provide a confirmation of the production and decay of the isotope 277112 reported by a research group at Gesellschaft für Schwerionenforschung (GSI), Germany, produced via the same reaction by using a velocity filter. Tow decay chains, both consisted of four consecutive alpha decays followed by a spontaneous fission, were observed also in the reaction 209Bi( 70Zn, n). Those are assigned to be the convincing candidate events of the decays of the isotope of the 113th element, 278113, and its daughter nuclei, 274Rg, 270Mt, 266Bh, and 262Db.

Angular anisotropy of the fusion-fission and quasifission fragments

The anisotropy in the angular distribution of the fusion-fission and quasifission fragments for the 16O + 238U , 19F + 208Pb and 32S + 208Pb reactions is studied by analyzing the angular-momentum distributions of the dinuclear system and compound nucleus which are formed after capture and complete fusion, respectively. The orientation angles of the axial symmetry axes of the colliding nuclei relative to the beam direction are taken into account for the calculation of the variance of the projection of the total spin onto the fission axis. It is shown that there is a large contribution of the quasifission fragments in the 32S + 208Pb reaction to the deviation of the experimental angular anisotropy from the statistical model results. Enhancement of anisotropy at low energies in the 16O + 238U reaction is connected with the quasifission of the dinuclear system having low temperature and relatively small effective moment of inertia.

Double differential cross sections of light-charged particle emission for n + 208Pb reaction

The model for the pre-equilibrium emission of light composite particles in the framework of the exciton model is used at intermediate energies. The main objective of this work is to provide a comprehensive and improved description of the energy spectra and double differential cross sections of nucleon and the light composite particle (deuteron, triton, helium and alpha-particle) emissions in the intermediate-energy range. The energy spectra and double differential cross sections for n + 208Pb reaction are calculated and analyzed by using nuclear theoretical models. Theoretical calculated results are compared with existing experimental data.

Study of fusion cross-sections of 16O + 208Pb and 28Si + 208Pb reactions by effective soft-core nucleon-nucleon interaction

In this paper, the cross-sections of fusion reactions 16O + 208Pb, 28Si + 208Pb, 40C + + 40Ca, 40Ca + 48Ca, 58Ni + 58Ni, and 16O + 154Sm at bombarding energies above and near the fusion barrier have been investigated. The fusion cross-sections have been studied by means of the Monte Carlo method and effective soft-core nucleon-nucleon interaction. One adjustable parameter was used in these calculations. This parameter can change the strength and repulsive parts of soft-core potential values. It has to be adjusted, so that the analytical results are in acceptable agreement with the experimental data. In our calculations, we have taken the range of the nucleon-nucleon soft-core interaction to be constant and equal to that of the M3Y-Raid potential. Results show that the higher values for the diffusion parameter in the Woods-Saxon potential obtained from a careful analysis of 16O + 208Pb and 28Si + 208Pb reactions are due to the many particle effects on the nucleon-nucleon potential.

Conversion-electron study of 0+ excitations in 208Pb

The 208Pb(p,p′e−) reaction has been studied at a proton energy of 17.3 MeV. Singles data show E0 transitions from the 0+2 and 0+3 states at 4868 and 5241 keV, respectively, to the ground state. Proton–electron gated spectra indicate that few electrons from the 0+2 state are in coincidence with backscattered protons, while those from the 0+3 state are somewhat more abundant. From the singles spectra, an upper limit for the E3(0+3 → 3−1)/E0(0+3 → 0+1) branching ratio of less than four has been measured. This observation is in agreement with previous predictions for a two-phonon octupole excitation.