Li States Research Articles

Quantum beats measurements of the hyperfine structure of 4f1F state in7Li II

The hyperfine structure of the 4f 1F state in 7Li II has been measured using the zero-field quantum beat technique. The three dominant frequencies are found to be 630+or-30, 980+or-50 and 1250+or-100 MHz. These values disagree slightly with the theoretical calculations of Ambry et al. (1968).

The decay of 8He

Delayed neutron and gamma emission following the beta decay of 8He have been studied. The P n value was determined to be (16 ± 1)%, indicating that (84 ± 1)% of the β-decays feed the 981 keV first excited state in 8Li. It was found that (32 ± 3)% of the emitted neutrons populate the first excited state in 7Li at 478 keV. The measured energy spectrum of delayed neutrons can be interpreted as mainly arising from the 3.21 MeV and 5.4 MeV states in 8Li. Limits were set on charged particle emission. The half-life was determined to be 117.5 ± 1.5 ms.

Laser-microwave spectroscopy of lithium ions: 23 S 1 hyperfine structure of7Li+

The hyperfine structure (hfs) splittings of the metastable 1s2s3S1 state of7Li+ have been measured with combined laser optical pumping and microwave resonance. A lowenergy Li+ ion beam, optically excited by an intersecting laser beam, passed a waveguide where radio frequency transitions were induced. The resulting population transfer among the hfs levels of the3S1 was detected via the change in intensity of the fluorescence light from a second crossing region of laser light and ion beam located past the waveguide. The magnetic hfs constantA(7Li+, 1s2s3S1) was measured and compared with theory. A deviation of the two transition frequenciesν(F=3/2−F=5/2) andν(F=1/2−F=3/2) from the interval rule is due to a depression of theF=3/2 hfs sublevel, caused by mixing of the 23S1 and 21S0 states via hyperfine interaction. This shift was never observed so far in a two-electron spectrum, because of absence ofI>1/2 isotopes in He, the only two-electron atom investigated spectroscopically with high precision. The size of the shift is in fair agreement with a theoretical estimate.

Pulsed optical-optical double resonnance spectroscopy of A 1Σ +g state of 7Li 2

Optical-optical double resonance using two nitrogen-laser pumped dye lasers has been used to make the first study of an excited 1σ + g state of 7Li 2. Preliminary molecular constants in cm −1 are Te = 29972, Y 10 = 227, Y 20 = −1.92, Y 01 = 0390, Y 02 = −4.3 × 10 −6and Y 11 = −1.4 × 10 −3,with r e = 0. 351 nm.

Electron scattering from the 3.65 MeV (0 +, T = 1) state in 6Li at high momentum transfer

The form factor ofthe 3.56MeV(0 +, T = 1) state of 6Li has been measured for momentum transfers q = 1.0–3.0 fm −1, and the 2.18 MeV (3 +, T = 0) and 5.37 MeV (2 +, T = 1) states have been measured up to q = 2.5 fm −1. The 3.56 MeV form factor is analysed in terms of a phenomenological shell model with l = i valence nucleons. The radial wave functions are found to have a greater radial distribution than given by the harmonic oscillator, more closely resembling Woods-Saxon functions. The M1 form factor is found to decrease at high momentum transfer somewhat more slowly than the models predict. A technique for determining the Mλ transition current density based on the Fourier-Bessel analysis is developed and applied to the M1 transition. The M1 transition current density is obtained within a moderate error band and compared with the harmonic oscillator and Woods-Saxon densities. The M1 radiative width is 8.18 ± 0.25 eV, in agreement with previous measurements.

6Li electromagnetic form factors and phenomenological cluster models

The longitudinal form factors of the ground and 2.18 MeV (3 +, T = 0) states, and the transverse form factors of the 3.56 MeV (0 +, T = 1) and 5.37 MeV (2 +, T = 1) states of 6Li are compared with the predictions based on fully antisymmetrized α-d and t-τ cluster models. The longitudinal form factors are adequately described by the α-d model, but the transverse form factors seem to be more consistent with a t-τ model which is close to the shell-model limit. Estimates are made for the ground state t and α spectroscopic factors. The 3.56 MeV M1 transition current density is calculated for both models and compared with experiment.

Analysis of 185 MeV proton scattering from6Li

Families of optical model parameters are found characterized by differences in volume integral per nucleon pair for the real potential of the order of 100 MeV·fm3. The distorted wave Born approximation (DWBA) analysis is carried our for inelastic scattering of protons to the first excited state (3+,T=0, 2.184 MeV) in6Li. The deformation parameters ¦β2¦ and ¦β4¦ are extracted. The need for further differential cross section measurements as well as polarization measurements are stressed.

Scattering and transfer reactions for 6, 7Li + 90, 91Zr

Abstract Elastic scattering data have been measured for the 7Li + 90Zr, 6Li + 90Zr, and 6Li + 91Zr systems at E(Li) = 34 MeV. Inelastic scattering data for the 7Li + 90Zr and 6Li + 90Zr systems were also measured for the 2+(2.18 MeV) and 3−(2.75 MeV) states in 90Zr and the 1 2 − (0.48 MeV ) state in 7Li. Optical model analyses of the elastic scattering data and DWBA analyses for the states in 90Zr were performed. The deduced deformation lengths for the 2+ state agreed with those extracted in other studies but the deformation length for the 3− state was smaller. The 90Zr(7Li, 6Li)91Zr angular distributions were measured for the 1.21 and 2.03 MeV states and the 2.19 MeV doublet in 91Zr. Also, 90Zr(7Li, 6He)91Nb angular distributions were measured for the ground states, 0.10, 3.41 and 4.82 MeV states in 91Nb. The transitions well matched in angular momentum were described by finite-range DWBA calculations, while other transitions displayed the same phase problems seen with heavier ions. The extracted spectroscopic information was consistent with the results of other reaction studies. At the present energy, it was not possible to determine whether the l = 1 phase problem that occurs for heavy-ion single-nucleon transfer reactions on 2s-1d shell nuclei occurs in 91Zr also.

Faddeev equations for the6Li nucleus

The6Li nucleus is considered as composed of an alphaparticle, a proton and a neutron. With this construction the6Li nucleus is studied as a three-body problem. The Faddeev equations are given, by taking separable potentials for the interaction between the alpha, the proton and the neutron particles in pairs. The nucleon-nucleon potential is taken as a central term of the Yamaguchi type and also including a tensor force. To investigate the short-range correlations in this three-body system, the nucleon-nucleon potential is taken to include a repulsive soft-core part in addition to the attractive part considered. The nucleon-alpha potential is taken as central and spin-orbit terms which fit the corresponding experimental nucleon-alpha phase shifts. The effect of Coulomb forces between the proton and the alpha-particle is taken into account. By solving the coupled Faddeev equations numerically, the binding energies for the ground and excited states of6Li are calculated. Inclusion of repulsive-core effects is found to change the results by 19%.

The four-body reaction6Li(p,pα)np as a probe of the cluster structure of6Li

The ground state of 6Li is examined through a comparison of the reactions 6Li(p,p alpha )d and 6Li(p,p alpha )np at a bombarding energy of 39.8 MeV. Evidence is presented for the existence of a cluster structure of alpha particle plus neutron-proton pair within the ground state of 6Li.

Deuteron breakup by low-energy α-particles

An investigation of the 2H(α, αp)n reaction at a lab energy of 15 MeV has been performed. Results from kinematically complete measurements at eleven angular combinations are presented and analyzed in terms of a semiphenomenological impulse approximation. It is found that if contributions from three-nucleon transfer reactions and n-p final state interactions as well as sequential reactions via states in 6Li are included, it is possible to reach qualitative agreement with the data.

Precision measurement of the energy of the second excited state in 6Li

The excitation energy Eexc of the second excited state in 6Li has been measured by recording the de-excitation γ-rays from the 6Li(p, p′) 6Li ∗ (3.56 MeV) and 9Be(p, α) 6Li ∗ (3.56 MeV) reactions. The γ-ray spectra have been recorded at supplementary angles with respect to the direction of emission of the recoil 6Li ∗ nuclei and a method has been developed to determine E exc with an error of ≲ 1 keV. The weighted average of the two measurements is E exc = (3562.9±0.6) keV.

Absolute excitation energy measurements using an electrostatic analyzer

The excitation energies of several states in 6Li, 11B, 12C, 27Al and 56Fe have been measured in terms of the standard meter and the standard volt. The energy of the electron beam from a FN Tandem Accelerator was measured absolutely with a 61 cm radius electrostatic analyzer to an uncertainty of 0.037%. Thin target bremsstrahlung produced by this beam was used to excite the states in question. The excitation energies are as follows: (2126.5 ± 1.5) keV for the 2.12 MeV state in 11B; (2984.2 ± 1.5) keV for the 2.98 MeV state in 27Al; (3451.3 ± 1.7) keV for the 3.45 MeV state in 56Fe; (3569.0 ± 1.7) keV for the 3.56 MeV state in 6Li; (4439.4 ± 1.6) keV for the 4.44 MeV state in 12C; (4446.4 ± 1.8) keV for the 4.44 MeV state in 11B; and (5021.1 ± 1.8) keV for the 5.02 MeV state in 11B. These results are compared with particle-reaction and gamma ray spectroscopy measurements.

Cross Section for the 9Be(n, t1)7Li Reaction Between 13.3 and 15 MeV

The magnitude and energy dependence of the 9Be(n,t1)7Li reaction have been measured between 13.3.- and 15-MeV neutron energy by observing the gamma decay of the first excited state of 7Li with a Ge...

Isospin purity of the 5.37 MeV state in 6Li

The 6Li(p, pα)d, 6Li(p, pα)np and 6Li(p, pp) 5He (g.s.) reactions were measured at 11 and 12 MeV bombarding energy, θ 1 = 135° and θ 2 = 30°. In the coincidence spectra, final state interactions (FSI) corresponding to the J π = 2 + doublet in 6Li were clearly seen and their widths and energy positions determined. In addition, the p-α spectrum gives evidence for a level in 6Li at 5.7 MeV excitation with a width of approximately 1 MeV. No indication for decay of the 6Li(5.37 MeV, 2 +, T = 1) state in the d-α channel was found. By comparing the corresponding yields in the isospin allowed and forbidden channels, an upper limit of α 2 ≦ 8 × 10 −3 was deduced for a possible T = 0 admixture in the T = 1 state at 5.37 MeV.

The characteristics of the third excited state of6Li

A kinematically complete study of the reaction 7Li(3He, alpha )6Li(d) alpha was used to measure the excitation energy and width of the third excited state of 6Li. The results Ex=4.3+or-0.1 MeV and Gamma =0.6+or-0.1 MeV were obtained.

An experiment on isospin-changing parity violation involving hadrons in nuclei

An experiment is reported on the parity-violating reaction d(α, γ)6Li, leading to theJ P =0+,I=1,E x =3.562 MeV state, which, being a ΔI=1 transition, could be strongly enhanced by the presence of neutral currents in hadronic weak interactions in nuclei. The experiment has been carried out by exposing a deuterium target to a beam of α-particles reaching the resonant energy in the region in the middle of the tube containing the gas. Gamma-rays from this region are detected by a 90 cm3 Ge(Li) counter connected to a 4096 channel analyser through an amplifying and analysing chain. The various sources of the background of spurious counting in the Ge(Li) and the procedure adopted to reduce their effect are discussed. We find no evidence for a nonzero parity-violating width of the 3.562 MeV state of6Li and can set an upper limit for this width of 8·10−4 eV, which is an order of magnitude or more lower than that obtained in the previous experiments on this reaction.

The T = Tz + 2 GDR and the dipole states of the A = 6 and 14 nuclei

The isospin properties of the giant dipole resonance (GDR) built on states with isospin T = T z + 1 are discussed. Results of a shell-model calculation with realistic interactions for dipole transitions to various low-lying states in 6Li, 6He, 14N and 14C are presented. The T = T z + 2 component of the GDR in 14N decaying to the 0 + T = 1 state is predicted at the surprisingly low energy E x = 26.0 MeV.

Search for the parity-forbidden heavy-particle width Γ′ d α of the Ex = 3.562 MeV state in 6Li

Alpha-particle capture by deuterium has been investigated at excitation energies in 6Li close to the J π = 0 +, T = 1, E x, = 3.562 MeV state. A search for resonance γ-rays from this state has yielded a negative result and an upper limit for the heavy-particle width Γ′ d α ≦ 0.017 eV.

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.