Li States Research Articles

E1 polarization in coulomb excitation of 7Li

Coulomb excitation of the 0.478 MeV J π = 1 2 − state of 7Li has been studied by a partiele-γ coincidence technique. From the dependence of the excitation probabilities on bombarding energy and scattering angle a sizeable interference contribution from E1 excitation of continuum states has been determined. General expressions for the size of the E1 polarization effect in Coulomb excitation are given and the observed magnitudes in 6,7Li are compared with schematic model calculations.

The reaction 3H( 3He, d) 4He at very low energies: Energy dependent violations of the Barshay-Temmerisospin theorem and highly excited states in 6Li

Angular distributions of the reaction 3H( 3He, d) 4He have been measured at E( 3 He) = 291, 369, 600 and 800 keV and have been found on the average to be symmetric about 90° in the c.m. system, in accord with the Barshay-Temmer isospin theorem. The slight asymmetries (≦ 1%) together with the energy dependence of larger asymmetries from other experiments suggests strongly isospin mixing in the compound nucleus 6Li.

Isobaric analogue states in6Li and the solar neutrino anomaly

A search is reported for the isobaric analogue state in 6Li to the state in 6Be proposed recently as a possible reason for the solar neutrino anomaly. Upper limits to the radiative probability of transition to such a state are obtained.

Coulomb excitation of the [formula omitted] state in 7Li

Coulomb excitation of the 1 2 − state in 7Li was induced by scattering 7Li on 208Pb and 209Bi targets. The excitation probabilities were measured for a wide range of scattering angles and bombarding energies. The deviations of the measured excitation probabilities from the predictions of first-order Coulomb excitation theory were analysed in terms of the reorientation effect. With the inclusion of contributions from virtual E1 excitations of the α + t channel it was possible to give an estimate of the ground state quadrupole moment and the B( E2, 3 2 → 1 2 ) value which are consistent with the results of previous independent measurements.

M1 transitions and the validity of SU(4) symmetry in light nuclei

A sum rule for M1 transitions is derived and is found to be saturated with low11ing states in 6Li and 10B. This is consistent with a very small amount of supermultiplet impurity present in the ground state. For self-conjugate 4 N and conjugate 4 N ± 1 nuclei, using experimental data on ΔT = 1 M1 transitions from the ground to excited states, we are able to estimate the super-multiplet impurity present in the ground state. It is found that the amount of supermultiplet impurity reaches a maximum at the middle of the shell and decreases to zero at the beginning and at the end of the shell. Under the assumption that the giant magnetic dipole resonance dominates the sum rules we find a relation which predicts the excitation energy of the T = 0, J π = 1 + state in 12C to be 12 MeV as compared with the experimental value of 12.71 MeV.

The role of the α+t channel in the Coulomb excitation of the [formula omitted] state in 7Li

The role of the virtual excitation of the α+t channel in exciting the 1 2 − state in 7Li is discussed in terms of the known properties of the α+t channel. It is shown that this effect is larger than the reorientation caused by the ground state quadrupole moment. Existing data on the Coulomb excitation of the 1 2 − state is analysed in terms of both the virtual breakup and the reorientation effects. The resulting Q 3 2 and B( E2; 3 2 → 1 2 ) are consistent with values obtained in former measurements.

Nuclear polarizability of 7Li from Coulomb excitation

Excitation of the first excited state in 7Li in the Coulomb and nuclei fields of 208Pb was studied. Below E( 7Li) = 24 MeV the polarizability of 7Li was deduced to be α = 0.32 ± 0.07 fm 3. At higher energies a large backward cross section for α-particle production was observed reflecting the prominent α-t cluster structure of 7Li.

The 3H( 3He, γ) 6Li capture reaction and the structure of 6Li

The capture reaction 3H( 3He,γ) 6Li has been studied for 3He bombarding energies from 9 to 26 MeV. Transitions to the ground and first four excited states in 6Li are observed. The 90° differential cross section for the ground state transition decreases smoothly from 9 to 26 MeV. The 90° differential cross section for the first excited state transition shows a broad maximum centered at E 3 He = 20.6 ± 0.4 MeV, indicating a possible resonance in 6Li at E x = 26.2 ± 0.2 MeV. The excitation function for the transition to the third excited state also shows a broad maximum centered at E x = 26.6 ± 0.4 MeV. The excitation functions for the transitions to the second ( T = 1, 0 +) and fourth ( T = 1, 2 +) excited states have also been measured. The former does not show any resonance structure, while the latter shows a broad maximum indicating a possible resonance in 6Li at E x = 26.7 ± 0.4 MeV. These results are compared with theoretical predictions.

Isospin purity of the T = 1 states in 6Li

A study of the relative cross sections of the analogue reactions 7Li(d,tt)6Li and 7Li(d, τ)6He shows that the two lowest T = 1 levels in 6Li have high isospin purity (α2 < 0.008). Additional confirmation of high degree of isospin purity comes from an examination of the wave functions involved.

Investigation of the low-lying states in 6Li by inelastic electron scattering

The inelastic electron scattering cross sections for the excitation of the low-lying states of 6Li have been measured in the momentum transfer region q = 0.7–1.8 fm −, with emphasis on the transverse transitions to the 3.56 and 5.36 MeV T = 1 states. The intermediate coupling shell-model wave functions both in these states and in the ground state have been determined using radial wave functions generated in finite potentials. The form factors, especially the form factor for the M1 transition to the 3.56 MeV state, are sensitive to the radial distribution of the outer nucleons in 6Li. The form factor and nucleon distribution of the four 6Li core nucleons relative to the nuclear center of mass have been deduced from the well-known elastic charge form factor and fitted to a Gaussian shape.

The formation of the 6Li g.s. and the 6Li, 3.56 Mev, T = 1 state in the 19F( 3He, 6Li) 16O g.s. reaction

The 19F( 3He, 6Li) 16O g.s. angular distributions were measured for the 6Li, 1 +, T = O ground state and the 6Li, 0 +, T = 1, 3.56 MeV level at E 3He = 28 MeV . The angular distributions show pronounced diffraction patterns which are reasonably fitted by zero and finite-range DWBA calculations. The intensity ratio for both states in 6Li is very close to the expected value for coupling a 3He and a triton cluster to either a singlet or a triplet state. The absolute magnitudes of the ( 3He, 6Li) cross sections may be understood in terms of separating the shell-model configuration s 4p 2 into (s 3+sp 2).

Reactions in the even parity states of 6Li

Reactions in the even parity T = 0 states of 6Li were calculated starting from a realistic 6-nucleon Hamiltonian. Low energy results agree well with experiments. Large α + d 〈 3H + 3He transitions in the 1 + state due to compound state coupling were found at higher energies.

On the elementary-particle treatment of muon capture and radiative pion capture in 6Li

The elementary-particle approach to muon capture and radiative pion capture in nuclei is here discussed for the particular case of the transition between the ground states of 6Li and 6He. Its validity is proved by comparison with standard calculations in the impulse approximation. The calculated capture rates are found to be consistent with the experimental data in contra-distinction to previous estimates.

Coulombformfaktoren für7Li

Using refined clustermodel wave functions the Coulomb form factors for the elastic scattering of electrons as well as the formfactors for the inelastic scattering leading to the first and second excited states of7Li have been calculated.

The 6Li(p, n) 6Be and 7Li(p, n) 7Be reactions at intermediate proton energies

The 6,7Li(p, n) 6,7Be reactions were studied over the angular range 0° ≦ θ 1ab ≦ 60° at both E p = 30 and 50 MeV. In the case of 6Be, the ground state and two other states were observed at 1.63 ± 0.16 and 3.47 ± 0.30 MeV excitation, respectively. In 7Be, in addition to well-known states, two previously unobserved levels were observed at 11.7 ± 0.2 and 12.8 ± 0.2 MeV excitation. The highest and previously unobserved 6Be level is in good agreement with cluster-model calculations and some 6He data, therefore a further T = 1 state in 6Li is predicted. The angular distributions of the 6,7Be g.s. transitions were fairly well reproduced using a DWBA calculation with a microscopic interaction. A real macroscopic ( t · T) potential successfully predicted the shape of the angular distribution for the 7Be g.s. transition at both energies, but an additional imaginary ( t · T) component appeared to be required to give the correct cross-section magnitude at 30 MeV.

The spin-isospin dependent interaction from inelastic proton scattering on 6Li

The differential cross section for the inelastic scattering of protons leading to the ( J π = 0 +, T = 1) state of 6Li at 3.56 MeV has been measured at E p = 24.4 MeV. A real spin-isospin dependent interaction with a Yukawa shape, a strength of 12.7 MeV and a range of 1.0 fm fits the data well at small angles.

Level structure in 9Be and 6Li from 7Li( 3He, p) 9Be and 7Li( 3He, α) 6Li

Proton and alpha-particle spectra from the 3He bombardment of 7Li have been taken over a variety of bombarding energies and observation angles with the intent of exploring the level structure in the residual nuclei 9Be and 6Li. In 9Be, previously unreported levels have been found at excitations of 13.78 ± 0.03 MeV and 16.671 ± 0.008 MeV with widths of 590 ± 60 keV and 41 ± 4 keV, respectively. Two overlapping levels have been found at 11.81 ± 0.02 MeV and 11.29 ± 0.03 MeV with widths of 400 ± 30 keV and 620 ± 70 keV, respectively. No evidence for previously reported levels at 7.94 and 9.1 MeV has been found. The alpha-particle spectra show states in 6Li at excitations of 0.0, 2.17 ± 0.02, 3.55 ± 0.02 and 5.34 ± 0.02 MeV. No evidence for states previously reported in this reaction at 4.3, 6.63, 7.40, 8.37 and 9.3 MeV was found. Alpha-particle groups from sequential decays through states in 8Be have been seen and identified.

Elastic and inelastic scattering of fast neutrons from 6Li and 7Li

The differential elastic and inelastic neutron scattering cross sections of 6Li and 7Li have been measured at incident neutron energies of 4.83, 5.74 and 7.5 MeV for 6Li and at energies of 3.35, 4.83, 5.74 and 7.5 MeV for 7Li. Scattered neutrons and gamma rays were observed independently. The cross sections were measured with a neutron time-of-flight spectrometer relative to the well-known cross section for neutron scattering from hydrogen. The gamma-ray spectra were measured with a NaI(Tl) spectrometer using time-of-flight techniques to eliminate the neutron background. The 2.184 MeV state in 6Li was excited by neutron inelastic scattering. The 3.56 MeV state in 6Li was not observed in either the neutron or gamma-ray studies. Scattered neutrons from the 0.478 MeV state in 7Li were resolved at both 3.35 and 4.83 MeV. Scattered neutrons from the 4.63 MeV state in 7Li were observed at incident neutron energies of 5.74 and 7.5 MeV. A continuum of neutrons due to three-particle break-up was observed for both 6Li and 7Li at 4.83, 5.74 and 7.5 MeV.

Elastic scattering of deuterons by alpha particles

Elastic scattering of deuterons by alpha particles for deuteron energies from 3 to 14 MeV has been investigated for 20 angles. Complex phase shift analysis has been performed on the data. There does not seem any indication of negative-parity states in 6Li as reported by Senhouse and Tombrello.

A study of the7Li(p,n) reaction in the impulse approximation

The (p,n) reaction in 7Li at 94 MeV is studied in the impulse approximation. The differential cross sections for exciting the isobaric analogue state and the 0.431 MeV state in 7Be are obtained using LS-coupled shell-model wave functions. Fairly good shape agreement with the available experimental results is obtained. These results are discussed, together with the inelastic proton scattering to the 0.478 MeV state in 7Li, as a mode of investigating the model wave functions used; where the small-angle results as predicted by the theory appear to be in conflict with the experimental data. Need for more experimental data at small angles for these reactions is stressed.