Li States Research Articles

A microscopic three-cluster model in the hyperspherical formalism

The hyperspherical formalism is applied in the framework of the microscopic generator-coordinate method. Three-cluster systems are described with a single generator coordinate, the hyperradius. This model is a natural extension of previous microscopic theories, and can be applied to bound states and resonances in a range of nuclei exhibiting a cluster structure. As an illustrative example, the model is applied to 6He and 6Li. Density distributions and electric dipole transition strengths are calculated, including B( E2;0 +→2 +) in 6He. The results are consistent with a neutron halo structure for the ground state of 6He, and with a proton–neutron halo structure of its isobaric analog state in 6Li. No 1 − “soft dipole” resonance is found in 6He.

Probing chiral interactions in light nuclei

Chiral two- and three-nucleon interactions are studied in few-nucleon systems. We investigate the cut-off dependence and convergence with respect to the chiral expansion. It is pointed out that the spectra of light nuclei are sensitive to the three-nucleon force structure. As an example, we present calculations of the 1+ and 3+ states of 6Li using the no-core shell model approach. The results show contributions of the next-to-next-to leading order terms to the spectra, which are not correlated to the three-nucleon binding energy prediction.

Probing chiral interactions in light nuclei

Chiral two- and three-nucleon interactions are studied in few-nucleon systems. We investigate the cut-off dependence and convergence with respect to the chiral expansion. It is pointed out that the spectra of light nuclei are sensitive to the three-nucleon force structure. As an example, we present calculations of the 1 + and 3 + states of 6Li using the no-core shell model approach. The results show contributions of the next-to-next-to leading order terms to the spectra, which are not correlated to the three-nucleon binding energy prediction.

Optical model analysis of K+ scattering

Local optical model calculations using the Woods–Saxon form have been carried out for the elastic scattering of K+ from 6Li, 12C and 40Ca at kaon laboratory momenta 635, 715 and 800 MeV/c. These studies resulted in considerable ambiguities in the parameter values. Using the collective model and best-fit parameters, zero-range DWBA calculations have been carried out for inelastic scattering of K+ leading to the lowest 3+ state in 6Li and 2+ and 3− states in 12C. Transition strengths extracted from inelastic scattering, compared with known values, remove the ambiguity found for elastic scattering, making this simple model useful for other reaction studies with kaons. Further measurements for K+ scattering from 6Li are stressed.

Nano-beam production by RFQ with ion-trap injector

Nano-beam production by RFQ with ion-trap injector

Molar volume of 6Li, 7Li and natural Li in the liquid state

The densities of natural Li, concentrated 6Li and concentrated 7Li in the liquid state were each measured by the two-sphere-Archimedes method. The measured density of natural Li in the liquid state was in good agreement with previous data. From these data the molar volumes of natural Li, concentrated 6Li and concentrated 7Li in the liquid state were calculated. Among those three kinds of liquid Li, the molar volume changes in an almost common straight line with the variation of the temperature. This indicates that no isotope effect can be seen between the molar volume in the liquid state of 6Li and that of 7Li within the present experimental error. The simple model calculation of the molar volume of liquid Li was performed based on the hard sphere model immersed in the uniform electron gas. Calculated molar volumes agree with experimental ones.

Near threshold three-body final states in7Li+7Lireactions atElab=34 MeV

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First observation of neutron–proton halo structure for the 3.563 MeV 0+ state in 6Li via 1H(6He, 6Li)n reaction

The angular distributions of the charge exchange reaction 1 H( 6 He, 6 Li) n were measured in reverse kinematics with a secondary 6 He beam at the energy of 4.17 A MeV. The data were analyzed in the context of a microscopic calculation. It is shown that both the ground state of 6 He and the second excited state of 6 Li (3.563 MeV, 0 + ) have halo structure.

Microscopic Multicluster Description of Light Exotic Nuclei with Stochastic Variational Method on Correlated Gaussians

A microscopic theory taking account of the correlated motion between particles is extended to a multicluster model comprising 0s clusters and single nucleons. The relative motion between the constituents is described by a combination of the correlatedGaussian basis. A stochastic selection of nonlinear parameters of the basis functions is introduced and it has proven to be a very powerful and economical method. It is confirmed that virtually exact solutions are obtainable for bound state problems. By the extension to the multicluster model, a sophisticated description of light nuclei becomes possible with fairly good asymptotics of the wave functions. The halo structure in the ground states of 6,8He and the 3.563 MeV 0+ state of 6Li is examined. The core breakup effect in 6He is found to give appreciable contribution to the binding energy. 9Be-9B, 9Li-9C and 10Be-10C mirror nuclei are described by microscopic three- and four-cluster model. The quadrupole moments of 9C and 7Be are predicted to be -5.04 and -6.11 efm2. As a useful application of the microscopic multicluster model, it is shown that the resonance parameters of unbound states can be obtained simply by analytic continuation of the square root of the bound state energy as a function of the potential strength.

Excitation of continuum states in 7Li and their decay by quantum tunneling

Strong forward-backward asymmetries were found in the recent α — t coincidence measurement of Coulomb breakup of 42 MeV- 7 Li via continuum states. The competition between E1 and E2 multipolarities and higher order effects in Coulomb excitation were investigated by solving the time-dependent Schrodinger equation. It is shown that higher order effects are an essential ingredient to the observed large asymmetries. The relevant reaction mechanism is discussed.

Charge-exchange reaction induced by 6He and nuclear densities

The charge-exchange reaction p( 6He, 6Li)n is studied in reverse kinematics with a secondary 6He beam at 41.6 MeV/nucleon. The transition connecting the ground state of 6He to its isobaric analog state in 6Li has been measured and analyzed in the context of a microscopic calculation. The angular distribution is shown to be sensitive to the nuclear interaction potential and to the radii of the density distribution of 6He. It is shown that only a coherent microscopic analysis may disentangle the different contributions.

Preparation of relativistic 7 Li + ion beams for precision experiments at storage rings

Heavy ion storage rings allow for tests of the structure of local space time via the Doppler effect. At the TSR/Heidelberg an experiment with high resolution laser spectroscopy at 7Li+ is performed. To gain the maximum resolution for saturation spectroscopy new methods of relativistic ion beam preparation and diagnostics have been developed. The laser cooling of the beam allows for precision determination of the mean velocity of the ions. A novel phase synchronous detection scheme, ultimately sensitive to single ions, gives insights into the cooling mechanism and dynamics. With an additional synchronous excitation scheme systematic uncertainties of the test experiment can be drastically reduced. After separation of the ground state ions from the triplet states of 7Li+ by the combination of laser and electron cooling, a bunched and cooled ensemble of fast moving high precision clocks with minimized perturbations by space charge effects and intra beam scattering is available.

Lifetime measurements of the [formula omitted] state of the 7Li 2

Rovibrational levels of the D 1Π u state of the 7Li 2 molecule were selectively excited by a tunable frequency-doubled mode-locked cavity-dumped cw dye laser. Their radiative lifetimes were measured by the delayed-coincidence single-photon counting technique. The comparison of the experimental results with the predictions of ab initio calculations yields a more comprehensive and quantitative understanding of the non-adiabatic coupling mechanisms between the D 1Π u state and the 3 1Π u state which determine the structure and dynamics of the rovibrational levels of the D 1Π u state of 7Li 2.

Parity-violating α-decay of the 3.56 MeV Jπ, T = 0+, 1 state of 6Li

We study the parity-violating alpha+d decay of the lowest 0+ state of Li-6 in a microscopic three-cluster model. The initial bound and the final scattering states are described consistently within the same model. The parity-violating decay width is calculated in perturbation theory using the parity-nonconserving nucleon-nucleon interaction of Desplanques, Donoghue, and Holstein (DDH). We find that the decay width is sensitive to dynamical degrees of freedom which are beyond the alpha+p+n model, for example, alpha excitation and breakup. In our full model, which contains breathing excitations of the alpha particle and H-3 + He-3 rearrangement, the parity-nonconserving decay width is Gamma=3.97x10^{-9} eV, using the DDH coupling constants.

Coupling effects of resonant and discretized non-resonant continuum states in 4He + 6Li scattering at 10 MeV/A

Alpha-particle scattering from the resonant (3 1 + ) and non-resonant continuum states of 6Li is studied at incident energy 10 MeV/A. The α + d breakup continuum part within the excitation energy E ex = 1.475–2.475 MeV is discretized in two energy bins. Unlike the results at higher incident energies, here the coupled-channel calculations show significant breakup continuum coupling effects on the elastic and inelastic scattering. It is shown that even when the continuum-continuum coupling effects are strong, the experimental data of the ground state and the resonant as well as discretized non-resonant continuum states impose stringent constraint on the coupling strengths of the non-resonant continuum states.

Radiative lifetimes, quenching cross sections and population mechanisms of the C 1Π u, 2 3Π g and 1 3Δ g states of 7Li 2

The laser- and collision-induced fluorescence of Li 2 molecules in a heat-pipe, excited by a tunable frequency-doubled mode-locked cavity-dumped cw dye laser into selected rovibrational levels of the C 1Π u state, was observed with spectral resolution using a monochromator and with temporal resolution using the single-photon counting technique. The effective lifetimes of the directly excited C 1Π u state and of the collisionally excited 2 3Π g and 1 3Δ g states were measured as a function of the lithium vapor pressure and the argon buffer gas pressure. This yields the radiative lifetimes and collisional quenching cross sections of the three states. It is shown that the population mechanism of the 2 3Π g and 1 3Δ g states is the collisional energy transfer from the C 1Π u state. Measurement of the total fluorescence intensities emitted from the excited states under cw laser excitation provides the cross sections for the collisoinal population transfer.

Padé-approximant techniques for processing scattering data II. Energy-dependent phase-shift analysis of low-energy4He +2H scattering

The general method for Pade-parametrization of experimental scattering data proposed by the authors recently has been applied to analytic parametrizations of theS-matrix and of partialwave amplitudes with the aim of carrying out an energy-dependent phase-shift analysis. It has been shown that the high universality and flexibility of the Pade-approximants make it possible to describe the partial-wave amplitudes by a very small number of parameters (say 2–3 in each partial wave) and to create a universal scheme for parametrization of the full reaction amplitude based on the following hybrid approach: the energy dependence of the inner partial amplitudes is represented by Pade approximants whereas the peripheral part of the full amplitude is taken from the low-order iterations of the exact many-particle Faddeev —Yakubovsky equations. As an example of the technique an energy-dependent phase-shift analysis of low-energy2H +4He scattering has been carried out and the parameters of the 3+ 0,2+ 0 and 1+ 0 resonant states of6Li have been obtained together with the value of the vertex constant of the virtual decay6Lig.s. → →4He +2H. The values of the energy and width obtained from experimental data of the 2+ 0 and particularly of the 1+ 0 level differ significantly from the values published in the literature.

Widths of the positive parity states in 7Li for the (α+t) channel

Formulae for calculating the spectroscopic amplitudes of the positive parity states in 7Li for the ( alpha +t) channel are derived. The wavefunctions of the simple harmonic oscillator shell model with an LS-scheme for the initial and final nuclei are used. The formulae are applied for calculating the spectroscopic amplitudes for transitions from the dipole states of 7Li into ( alpha +t). Transitions to the alpha excited states are also included. Widths of the positive parity states for the ( alpha +t) channel are computed and the results are compared with experimental data.

Continuous distribution of αt relative kinetic energies in 7Li breakup reactions

A newly developed technique has been used to probe a previously inaccessible region of the continuum in the at channels of 7Li breakup reactions. This was used to investigate reactions of 63 MeV 7Li with 58Ni, 120Sn, 144Sm, and 208Pb targets. Breakup pairs of α-particles and tritons emitted within a finite opening angle were analyzed according to their magnetic rigidity using a K = 150 Enge split-pole spectrograph. The αt coincidences were taken with two single-wire proportional counters mounted at different positions along the focal plane of the spectrograph. The large energy acceptance (Emax/Emin ≈ 8) of the spectrograph is crucial to the present coincidence measurements involving two particles with different q/m values. Very small relative energy components were studied without any detection threshold. A pronounced bump was observed immediately above the αt particle threshold. No strong indication of the post Coulomb acceleration was observed. The excitation of the continuum states as well as the 7−2 state in 7Li is discussed in terms of the distorted-wave Born approximation. The astrophysical implications of the data are also discussed regarding the Coulomb excitation of the continuum. The present technique offers the promise of very high sensitivity at small relative energies if it were applied in conjunction with ray-tracing, a thin target, and a dispersion-matched beam.

Symmetry properties of nearside and farside amplitudes in polarised heavy-ion scattering

The elastic and inelastic scattering amplitudes in the collision of 44 MeV polarised 7Li by 26Mg are analysed in terms of their nearside and farside components. It is shown that in the case of kinematic matching (elastic scattering and inelastic scattering to the first excited 1/2- state of 7Li), the nearside amplitude has a symmetry around the recoil direction and the tidal spin is conserved, and thus the corresponding analysing powers satisfy shape-effect relations. The symmetry is violated in the nearside amplitude in the case of kinematic mismatch and the farside amplitude is not symmetric around the recoil direction irrespective of kinematic matching conditions.