Asymptotic three-particle approach to the Coulomb breakup process <Superscript>6</Superscript>Li + <Superscript>208</Superscript>Pb → <Superscript>208</Superscript>Pb + α + <Emphasis Type="Italic">d</Emphasis>

On the basis of the distorted-wave method, experimental data on the triple-differential cross section for the Coulomb breakup reaction ²⁰⁸Pb(⁶Li, αd)²⁰⁸Pb are analyzed by employing a correct expression for the final-state ²⁰⁸Pb–α–d three-particle Coulomb wave function. It is shown that the effect of final-state three-particle Coulomb dynamics can be used to assess the kinematical condition of clean Coulomb breakup processes. New values of the astrophysical S factor for the direct-radiative-capture reaction d(α, γ)⁶Li at ultralow energies in the range of 70 ≤ E _dα ≤ 600 keV were extracted from experimental data. The value of S(0) = 1.60 ± 0.17 MeV nb was obtained.     

Axially symmetric time-dependent Hartree-Fock study of the 208Pb + 208Pb reaction

A time-dependent Hartree-Fock calculation assuming axial and reflection symmetry constraints is performed for the head-on collision of ²⁰⁸Pb + ²⁰⁸Pb systems at E_lab≈1600 MeV. The calculation suggests the excitation of surface modes in the reaction and the energy loss characteristic of a deep inelastic process. An estimate of neck-radius and neck-formation time is given.     

12C1 and 8Be production in 12C + 208Pb collisions

The mechanisms involved in the production of fast α-particles in ¹²C-induced reactions have been studied for the ¹²C + ²⁰⁸Pb system at the bombarding energies of E_12c = 132, 187 and 230 MeV. Absolute cross sections for the reactions ²⁰⁸Pb(¹²C. ¹²C¹→α + ⁸Be), ²⁰⁸Pb(¹²C, ⁸Be(g.s.)) and ²⁰⁸Pb(¹²C, ⁸Be(2.94 MeV)) have been determined by coincidence measurement of two or three correlated α-particles. Inclusive α-particle production cross sections were also measured at E_12c = 187 MeV. It is found that the inelastic process (¹²C, ¹²C¹→α + ⁸Be) does not contribute significantly to fast α-particle production but that the production of ⁸Be by projectile fragmentation is an important source of α-particles. At the highest bombarding energy (230 MeV) it appears that the ¹²C → 3α fragmentation reaction becomes more prominent at the expense of the ¹²C→α + ⁸Be fragmentation channel.     

On a proposed phenomenological determination of the strength of the dissipative forces in heavy ion reactions

We investigate a mechanism which shows that the experimental determination of the direction of the final spins of the nuclei in deep inelastic collisions yields distinct information on the strength of the radial dissipative forces since, in certain systems, the classical trajectories are accordingly scattered to positive or to negative deflection angles. The radial friction constant marking off both regions from each other amounts to c_r ≈ 2 × 10^?21 MeV fm sec for Ar + Th at E_lab^inc = 388 MeV and to c_r ≈ 1 × 10^?21 MeV fm sec for O + Pb at E_lab^inc = 315 MeV.     

Strong reaction channels at barrier energies in the system 6Li + 208Pb

Large cross-section reaction channels were measured in the systems ⁶Li( ⁷Li) + ²⁰⁸Pb with high statistical accuracy at 5(3) energies around the Coulomb barrier from 29 to 39 MeV. These channels were assigned (mainly) to the breakup of ⁶Li, very loosely bound, into α + d and to the breakup of ⁵Li, produced by n-transfer to the target, into α + p and to similar processes with ⁷Li beam. The cross-sections with ⁶Li, S _α = 1.475 MeV, are systematically larger than the ⁷Li ones. This reflects, most likely, the higher binding energy of ⁷Li, S _α = 2.468 MeV. Theoretical predictions for the ⁶Li + ²⁰⁸Pb system which include for ⁶Li breakup to continuum states within a continuum discretized coupled-channels approach (CDCC) and resonant breakup plus n-transfer with DWBA reproduce the angular distribution shapes but still underestimate the cross-sections by a factor ∼ 3. Received: 15 January 2001 / Accepted: 3 March 2001     

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.     

Isoscalar quadrupole giant resonance in208Pb: A statistical analysis of high-resolution inelastic electron scattering spectra

The fine structure observed in high resolution inelastic electron scattering spectra for²⁰⁸Pb in the excitation energy range of the isoscalar giant quadrupole resonance has been analysed with a fluctuation analysis technique. The obtained density of levels as a function of excitation energyE _x has then been used as a constraint for the shape and magnitude of the (radiative) background subtracted spectra subjected to a multipole decomposition. The derivedE2 strength in the region 8.0≦E _x≦11.5 MeV exhausts (50 _?7 ⁺¹⁵ % of the energy weighted sum rule. TheE2 strength found is considerably larger than previously estimated from the same spectra and it is now both in shape and magnitude in good agreement with results from a recent²⁰⁸Pb(e, e′n) coincidence experiment.     

Comparison of the yields of high-energy gamma-rays withE γ≧10 MeV measured in the reactionsnatSn+40Ar and68Zn+64Zn

The spectra of theKX-rays of reaction products were observed in coincidence with high energy gamma-rays (E _γ≧10 MeV) in the^natSn+⁴⁰Ar (E _Ar=300 MeV) and⁶⁸Zn +⁶⁴Zn (E _Zn=290 MeV) reactions. Two groups ofKX-rays can be clearly identified in the spectra. The first one corresponds to the evaporation residue of the compound nucleus and the second one to the target-like products of the reaction. The spectra of gamma transitions gated by theKX-rays of the evaporation residues were measured in both reactions. The relative yields and the positions of the broad shoulders observed at an energyE _γ≧10 MeV in these spectra, differ drastically in the asymmetric and the symmetric systems. The ratio between the yields of gamma-rays emitted at angles of 0° and 90° to the beam direction was determined in the^natSn+⁴⁰Ar reaction. It was found to be greater than unity only atE _γ=(10–14) MeV. This experimental evidence may indicate the emission of high energy gamma-rays from the dinuclear system prior to the formation of the compound nucleus.     

Inclusive break-up reactions of 6Li at an incident energy of 26 MeV/nucleon

Inclusive charged particle spectra were measured from nuclear reactions induced by 156 MeV ⁶Li on ⁴⁰Ca. At forward angles the spectra exhibit broad break-up distributions centered around the energy corresponding to the beam velocity. The double differential cross sections together with previous results for a ²⁰⁸Pb target were analyzed in the framework of the DWBA approach to projectile break-up taking into account elastic and inelastic reactions of the break-up fragments. The high energy tails of the background due to preequilibrium emission of complex charged particles were estimated on the basis of the coalescence model.     

The 207Pb(d,p)208Pb reaction with high resolution

The reaction ²⁰⁷Pb(d, p)²⁰⁸Pb has been studied at E_d = 18.0 MeV using the MPI Heidelberg Emperor tandem accelerator and multiple-gap spectrograph. The average resolution of 18 keV was better than previous measurements, and this, combined with the long beam exposure, allowed many finer details of the neutron particle-hole structure of ²⁰⁸Pb to be examined. In all, 45 levels were seen up to E_x = 6.3 MeV, many of which had never been seen before. A DWBA analysis of the stripping transitions angular distributions is made, and values of (2J + 1)S_ij, energy centroids, and summed strengths are given. An interesting cluster of 1i_{$\text{11}\text{2}$} and 2g_{$\text{9}\text{2}$} levels at E_x ≈ 4.2 MeV are resolved and are compared to recent isobaric analog state data. A search for weak stripping strength to known pairing vibrations in ²⁰⁸Pb gave essentially negative results and points to the need for improved ultra-resolution measurements.     

A comparative analysis of the density distributions and the structure models of <Superscript>9</Superscript>Li

In the present study, we have analysed the elastic scattering cross-section data of ⁹Li + ¹²C system at E _lab = 540 MeV and ⁹Li + ²⁰⁸Pb system at E _c.m. = 28.3 MeV for some cluster models and various density distributions of the ⁹Li nucleus. First, we have obtained five different density distributions of the ⁹Li nucleus to generate real potentials with the help of double-folding model. For these densities, we have calculated the elastic scattering angular distributions. Secondly, using a simple approach, we have investigated some cluster models of the ⁹Li nucleus consisting of ⁶He + ³H and ⁸Li + n systems. We have presented the comparison of elastic scattering angular distributions for each system with each other as well as with the experimental data. Finally, we have given the cross-section values obtained from the theoretical calculations for all the systems studied in this paper.     

Orbital dispersion and wavefunction mapping in inclusive break-up experiments

Inclusive break-up cross sections of the reactions¹²C (⁶Li,a)X and²⁰⁸Pb (⁶Li,a)X at 26 MeV/amu, observed for the emission of break-up fragment into the forward hemisphere, are analysed in view of the information about the internal momentum distribution of the projectile. The longitudinal momentum distribution of the fragments in the projectile rest-frame is found to reflect the internal momentum distribution of the clusters whereas the transverse distribution is affected by the projectile-target interaction.     

Light particle accompanied quasifission in superheavy composite systems

The dynamics of the excited superheavy system with Z = 118 in the reaction ⁸⁶Kr + ²⁰⁸Pb at E _Kr = 460, 500, and 600 MeV has been investigated. The mass and kinetic energy of binary fragments were measured by the time-of-flight method. Double-differential distributions of protons and α particles were measured in coincidence with fragments. The proton spectra can be described considering only evaporation from fragments. Evidence of the neck fragmentation was obtained from analysis of double-differential α spectra. Properties of the α-particle neck fragmentation component are close to those known from the ternary fission of actinide nuclei, but the multiplicity is much larger than can be expected from extrapolation of the ternary fission data. The text was submitted by the authors in English.     

The 6Li(e,e′α) and 6Li(e,e′d) reactions at 520 MeV

The reactions ⁶Li(e,e′α) and ⁶Li(e,e′d) have been studied at T₀ ≈ 520 MeV and q ≈ 1.4 fm^?1, by coincidence measurements. The angular dependence of the cross sections is interpreted in the plane wave impulse approximation to obtain the momentum distribution of α-particle and deuteron clusters in ⁶Li. The amount of clustering of ⁶Li is also discussed.     

A search for resonant two-step α-exchange in 6Li + 12C scattering

Excitation functions for ¹²C(⁶Li, ⁶Li)¹²C (gs, 4.43 MeV) have been measured at 10 angles (40° ? θ_cm ? 160°) over the energy range 20 MeV < E_lab < 36 MeV. A single anomaly of width Γ ≈ 800 keV is observed at E_lab = 22.8 MeV. The results casts doubts on the resonant two-step α-exchange mechanism suggested to occur in this system.     

Trinucleon cluster structure at high-excitation energies in A=6 nuclei

Trinucleon molecular structures in ⁶He and ⁶Be were investigated by using the ⁶Li(⁷Li, ⁷Be)⁶He reaction at 455 MeV and ⁶Li(³He, t)⁶Be reaction at 450 MeV, respectively. Binary decays into t + t from a broad state at E _x =18.0±1.0 MeV in ⁶He and into ³He + ³He from one at E _x =18.0±1.2 MeV in ⁶Be, respectively, were observed by measuring trinucleon cluster decays in coincidence with reaction particles. The branching ratios for binary decay were estimated to be about 0.7 for ⁶He and ⁶Be. These large branching ratios show that a trinucleon cluster state exists as an isobaric partner around E _x =18 MeV in ⁶He and ⁶Be.     

Temperature dependence of quantal and thermal dampings of the hot giant dipole resonance

A systematic study of the damping of the giant dipole resonance (GDR) in ⁹⁰Zr, ¹²⁰Sn and ²⁰⁸Pb as a function of temperature T is performed. The double-time Green function technique is employed to determine the single-particle and GDR dampings. The single-particle energies, obtained in the Woods-Saxon potential for these nuclei, are used in the calculations. The results show that the coupling of collective vibration to the pp and hh excitations, which causes the thermal damping width, is responsible for the enlargement of the total width with increasing temperature up to T ≈ 3MeV and its saturation at higher temperatures. The quantal width, which arises from the coupling of the collective mode to the ph excitations decreases slowly with increasing temperature. The effect of single-particle damping on the GDR width is small. The results are found in an overall agreement with the experimental data for the GDR width, obtained in the inelastic α scattering and heavy-ion fusion reactions at excitation energies E^* ⩽ 450 MeV. At high excitation energies (E^* > 400 MeV) a behavior similar to the transition from zero to ordinary sounds is observed.     

Excitation of inner hole states in heavy nuclei with the (τ, α) reaction at 205 MeV

Energy spectra have been measured for the (τ, α) reaction at 205 MeV on⁹⁰Zr, ^118,124Sn and ²⁰⁸Pb. Results on the valence neutron shells and on the inner 1f_7/2 shell in ⁹⁰Zr, 1g_9/2 subshell in ^118,124Sn (T_< and T_> components) and 1h_11/2 subshell in ²⁰⁸Pb are presented.