Neutron total and scattering cross sections of 6Li in the low-MeV range

Neutron total cross sections of ⁶Li are measured at intervals of ? 10 keV from ≈ 0.1 to 4.8 MeV with precisions of ≈ 1 to 3 %. Differential elastic scattering cross sections are measured at intervals of ? 100 keV from 1.5 to 4.0 MeV at 10 or more scattering angles distributed between ≈ 20 and 160 deg. Differential inelastic scattering cross sections are measured at selected angles in the energy range 3.5 to 4.0 MeV. The experimental results are analyzed in terms of R-matrix theory and the model parameters used to deduce the ${}^6\text{Li(n}\text{,α)}$ cross sections. The implications of the measurements and their interpretation on the level structure of ⁷Li and the reaction mechanisms are discussed.     

Alpha-deuteron structure of 6Li as predicted by a separable-potential three-body model

Schroedinger's equation with separable n-p (³S₁) and $\text{n}\text{-α (}{}^2\text{P}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{)}$ potentials solved to obtain a three-body model of the ⁶Li ground-state wave function. This model predicts the α-n-p binding energy of ⁶Li to be 4.67 MeV [Exp.: 4.53 MeV = 3.697 + 0.834 (Coulomb)], the asymptotic normalization constant of the d-α tail to be 2.39, and the amount of d+α component to be 65%. The ⁶Li→α+d vertex function is slightly more momentum dependent than present experiments suggest.     

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

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 $\text{1}\text{2}{}^{\mathrm{?}}\text{(0.48}\text{MeV}\text{)}$ 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.     

Elastic scattering of vector polarized 6Li

Vector polarized ⁶Li nuclei have been elastically scattered on ${}^{12}\text{C(E = 20, 22.8}\text{MeV}\text{)}$ and on ¹⁶O, ²⁸Si, ⁵⁸Ni (E = 22.8 MeV). For the lighter nuclei surprisingly large asymmetries have been observed. The order of magnitude of the polarization effects observed can be understood within a folding model for the ⁶Li spin-orbit potential.     

Very light neutron-rich nuclei studied via the (6Li, 8B) reaction

Results from investigations of the $\text{T}{}_{\mathrm{z}}\text{=}\text{3}\text{2}$ nuclei ⁵H, ⁷He and ⁹Li and of the T = 1 nuclide ⁴H employing the (⁶Li, ⁸B) reaction at 80 and 93 MeV are presented. In ⁹Li the locations of several low-lying Ip shell levels are indicated; in ⁷He and ⁴H the ground states are clearly observed. No evidence is found for the formation of a narrow ⁵H ground state.     

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.     

A microscopic DWBA analysis of (6Li, 6He) reactions

The spin-isospin flip (⁶Li, ⁶He) reaction has been studied. The experiment was performed with a 34 MeV ⁶Li beam incident upon ^{25, 26}Mg and ²⁷Al targets. Angular distributions associated with the lowest $\text{5}\text{2}{}^{\mathrm{+}}\text{,}\text{1}\text{2}{}^{\mathrm{+}}\text{,}\text{7}\text{2}{}^{\mathrm{+}}{}^{25}\text{Al}\text{, 5}{}^{\mathrm{+}}\text{, 3}{}^{\mathrm{+}}\text{, 1}{}^{\mathrm{+}}{}^{26}\text{Al and}\text{5}\text{2}{}^{\mathrm{+}}{}^{27}\text{Si}$ states were measured. Microscopic DWBA calculations were performed using Yukawa central and r². Yukawa (r² · Y) tensor (μ = 1.0 fm^?1) potentials averaged over the ⁶Li(⁶He) cluster model wave functions. Knock-on (KO) exchange corrections corresponding to the central interaction are included using the local-energy approximation (LEA) method. The conclusion is that the quasi-elastic process is of major importance in the (⁶Li, ⁶He) transition studied in this work.     

Spin assignments from the 142Nd(7Li, 6He)143Pm and 144Sm(7Li, 6He)145Eu reactions at 52 MeV

Angular distributions have been measured for transitions to low-lying states in ¹⁴³Pm and ¹⁴⁵Eu populated by the ¹⁴²Nd(⁷Li, ⁶He)¹⁴³ and the ¹⁴⁴Sm(⁷Li, ⁶He)¹⁴⁵Eu reactions at $\text{E(}{}^7\text{Li}\text{) = 52}$ MeV. Elastic scattering of ⁷Li at 52 MeV on ¹⁴²Nd and ¹⁴⁴Sm, and ⁶Li at 46 MeV on ¹⁴²Nd and at 45 MeV on ¹⁴⁴Sm, were measured. Optical-model parameters extracted from fits to the scattering data were used in a finite-range DWBA analysis of the angular distributions for levels below 1.40 MeV excitation energy in ¹⁴³Pm and 1.84 MeV in ¹⁴⁵Eu. The reaction cross sections forward of 6° c.m. allow unambiguous distinction to be made between 2d_{$\text{5}\text{2}$} and 2d_{$\text{3}\text{2}$} final states. Final-state spins have been assigned to d-states in ¹⁴³Pm at 1.40 MeV($\text{3}\text{2}$⁺)and in ¹⁴⁵Eu at 1.042 MeV ($\text{3}\text{2}$⁺). Existing assignments to other levels in both residual nuclei have been confirmed.     

Excitation function of quasi-free processes in the6Li+6Li → 3α reaction

The⁶Li+⁶Li → 3α reaction was studied at energies around the Coulomb barrier. Excitation functions of the quasi-free processes⁶Li(d,α)⁴ He with an α-particle spectator either in the target or in the beam were measured.     

A useful spectroscopic tool: The (7Li, 6He) reaction

Angular distribution measurements for the reactions ²⁴Mg(⁷Li, ⁶He)²⁵ Al and ⁵²Cr(⁷Li, ⁶He)⁵³ Mn at$\text{E(}{}^7\text{Li) = 34}$ MeV show that for angles less than 15°, the shapes of the measured angular distributions allow $\text{d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$ or $\text{d}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$, and $\text{f}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}$ or $\text{f}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$ final-state configurations to be distinguished when compared with finite-range DWBA calculations.     

The tensor polarization of 5Li (32−) in the reaction α + d → α + p + n

At the Hamburger Isochronous Cyclotron the formation of the particle unstable ground state of ${}^5\text{Li(}\text{3}\text{2}{}^{\mathrm{?}}$ has been investigated in the reaction α + d → α + p + n at E_α = 28.3 MeV. The neutrons were detected in two liquid scintillation counters, while a large Si (Li) detector was used to observe the α-particles. In a first analysis, the tensor polarization parameters of the five-nucleon system ⁵Li are determined from the experimental data. The results are in quite good agreement with theoretical predictions of Hackenbroich et al.     

Three-body calculation of the 2H(α, αp)n reaction

The ²H(α, αp)n reaction at 15–42 MeV is analysed by solving the Amado-Lovelace equations. Rank-one separable potentials with the Yamaguchi-type form factor are assumed in ${}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$s, ${}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$$\text{p and}{}^{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{p}$ waves for the N-α subsystem and in the triplet s-wave for the n-p subsystem. The agreement with experiment is quite good in this energy region. The role of multiple-scattering terms is discussed. Interesting features of the reaction mechanism, such as the decay process through the 1⁺ resonance of ⁶Li, are found in this energy region. The triton transfer process at 15 MeV predicted by the modified impulse approximation is not found. The numerical method is discussed. It is shown that the rotated contour method causes no difficulties even when it is applied to breakup amplitudes.     

Mechanism of the 6Li(6Li, 2α)α reaction from 36 to 46 MeV

The principal mechanism of the ⁶Li(⁶Li, 2α)α reaction for E₀ = 36 to 46 MeV is the formation and sequential decay of ⁸Be levels near E_x = 11, 17, and 20 MeV. In contrast to previous results obtained at lower bombarding energies, the cross section for the direct reaction involving a spectator α-particle is, under the most favorable conditions, only one third of that for excitation of these ⁸Be levels.     

Study of the (p,pn) reaction in light nuclei

The (p, pn) reaction on ²H, ⁶Li, ⁷Li, ⁹Be has been studied at 47 MeV bombarding energy. Excitation energy spectra and energy sharing spectra are presented. Fragmentary information on ¹⁰B, ¹¹B and ¹²C was also obtained. Sequential decay contributions to the ⁶Li(p, pn)⁵Li¹ reaction suggest an admixture of parentage $\text{(α)π(}\text{p}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{)π(}\text{d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}\text{)}$ for the (3^?), 23 MeV excited state of ⁶Be. A possible 15.5 MeV excited state in ⁶Be is reported. An extensive study of the ⁹Be (p, pn)⁸Be reaction for 45 MeV protons was made. Results for θ_p = θ_n = 25°, 30°, 35°, 40°, 45°, 55°, 65°; θ_n = 40°, θ_p = 25°, 30°, 35°, 40°, 45°, 55°, 65°; θ_p/θ_n = 40°/45°, 38.5° are reported and compared to DWIA calculations. Agreement is good when the lower momentum components of the wave function are sampled, and deteriorates as higher momenta are required. Results are in agreement with the Cohen and Kurath spectroscopic factors for ⁹Be.     

Direct and resonant capture observed in the 3H(τ,γ)6 Li reaction

A direct capture model is used to calculate the ³H(τ, γ)⁶ Li cross section. The result for the ground-state cross section is in very good agreement with the data. The calculation for the cross section for the first excited state is in good agreement with the shape and angular dependence of the measured cross section at lower energies. Above E_τ=10 MeV a large discrepancy indicates a non-direct contribution. This is interpreted as a resonance in the compound nucleus ⁶Li with a configuration other than ${}^3\text{He}\text{+}{}^3\text{H}$.     

Experimental investigation of highly excited states of the 5,6He and 5,6Li nuclei in the (6Li, 7Be) and (6Li, 7Li) one-nucleon pickup reactions

The (⁶Li, ⁷Be) and (⁶Li, ⁷Li) reactions on ⁶Li and ⁷Li nuclei were investigated in the angular interval 0°–20° in the laboratory system at a ⁶Li energy of 93 MeV. In addition to low-lying states of the ^5,6He and ^5,6Li nuclei, broad structures were observed near the t(³He)+d and t(³He)+t thresholds at the excitation energies of 16.75 (3/2⁺) and ～20 MeV (for ⁵He), 16.66 (3/2⁺) and ～20 MeV (⁵Li), 14.0 and 25 MeV (⁶He), and ～20 MeV (⁶Li). The angular distributions measured in the ⁷Li(⁶Li, ⁷Be)⁶He reaction for transitions to the ground state (0⁺) and excited states at E _x=1.8 MeV (2⁺) and 14.0 MeV of the ⁶He nucleus were analyzed by the finite-range distorted-wave method assuming the 1p-and 1s-proton pickup mechanism. The (⁶Li, ⁷Be) and (⁶Li, ⁷Li) reactions were shown to proceed predominantly through the one-step pickup mechanism, and the broad structures observed at high excitation energies are considered as quasimolecular states of the t(³He)+d and t(³He)+t types.     

Direct transfer and two-step processes in the 42Ca(α, 3He)43Ca reaction

The ⁴²Ca(α, ³He)⁴³Ca reaction has been studied at 36 MeV incident energy. Angular distributions have been measured from 4° to 42° using a split-pole spectrometer and position sensitive Si detectors, for about 40 levels located up to 6 MeV excitation energy. A local zero-range DWBA analysis has been carried out; l = 3 and 4 assignments are tentatively proposed for levels located above 4 MeV excitation energy, indicating a strong fragmentation of the $\text{1}\text{f}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$ strength between 4 and 6 MeV and the location of the main component of the $\text{1}\text{g}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}$ strength above 6 MeV. A number of weakly excited levels cannot be reproduced by DWBA analysis. Their angular distributions have been compared with the results of coupled-reaction-channel calculations assuming two-step excitation of weak coupling states with a [⁴²Ca¹ ? $\text{f}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}$ structure. A reasonable agreement has been obtained, confirming that the two-step process cannot be neglected in the analysis of the (α, ³he) reaction.     

A particle-gamma correlation study in the 52Cr(d,pγ) reaction

Particle-gamma angular correlations for states populated in the ⁵²Cr(d, p) reaction have been measured. States in ⁵³Cr for which data are presented are the $\text{5}\text{2}$^?, 1.01 MeV, the $\text{7}\text{2}$^?, 1.29 MeV, the $\text{7}\text{2}$^?, 1.54 MeV, and the $\text{3}\text{2}$^?, 2.32 MeV levels. The DWBA is found to give a good representation of the data for strongly populated levels when the particle detector is located near the stripping peak. The data for the 1.29 MeV level confirm the previously suggested particle-excited core wave function.     

Comparison of the noncoplanar 6Li(p,pd)4He reactions at 120 and 200 MeV

The ⁶Li(p, pd)⁴He reaction was studied at 200.2 MeV, at the quasi-free angle pair (θ_p, θ_d) = (54°, ?48.9°), for noncoplanarity angles φ from 0° to 28°. ⁶Li αd spectroscopic factors of 0.84 and 0.76 are deduced from our coplanar data at this energy and 120 MeV, respectively, for ground-state 2S Woods-Saxon wave functions. A recent microscopic three-body calculation predicts spectroscopic factors from 0.70 to 0.75; using the ground-state wave functions from this study, we deduce a factor of 0.76 from the 200 MeV data. DWIA calculations fit the measured integrated cross sections versus φ for spectator momenta P_α ? 100 MeV/c at both bombarding energies, but underpredict them for larger P_α. Momentum form factors were better reproduced with 1S αd cluster wave functions for a soft-core bound-state potential than with the 2S Woods-Saxon wave functions, but the former wave functions generate unphysically large (～1.25) spectroscopic factors.