(See Energy Level Diagrams for 7Li)
GENERAL: See (1957HU1C, 1959BA1M, 1959BA1D, 1959BR1E, 1959FE1B, 1959MA1F, 1959MA1G, 1960KU1B, 1960PE11, 1960PH1A, 1960SH1A, 1960TA1C, 1961BA1D, 1961BA1E, 1961BL1C, 1961CL10, 1961KH03, 1961TA05, 1961TO04, 1962CL1F, 1962CR09, 1962IN02, 1963CH08, 1963CL1B, 1963KL1A, 1963SC30, 1964BE1N, 1964GR1J, 1964MA1G, 1964NE1E, 1964OL1A, 1964SA1F, 1965BE1R, 1965FA1B, 1965JA1L, 1965NE1C, 1965PR1F).
μ = +3.2564 nm (1965FU1G).
Excitation functions and angular distributions have been studied in the range Eα = 0.5 to 1.9 MeV by (1961GR27), Eα = 0.5 to 1.3 MeV by (1959HO03). The cross section rises smoothly, as expected for a direct capture process; at Eα = 1.32 MeV, σ = 3.58 ± 0.06 μb, and the corresponding reduced cross section factor S = 0.064 ± 0.016 keV · b (1961GR27) in good agreement with the value 0.05 calculated by (1961CH1C). Cross sections of (1961GR27) are 2 to 2.5 times higher than those of (1959HO03).
The branching ratio of γ0/γ1 (to 7Li(0) and 7Li*(0.48)) is 1/(0.4 ± 0.05), essentially independent of energy and angle. At Eα = 0.56 MeV, the angular distribution of γ's is isotropic (± 6%), while some preference for forward emission appears at Eα = 1.6 MeV. Assuming θ2α = 1.25 and 1.05 for the ground and 0.48 MeV states of 7Li, and taking into account only E1 capture from s- and d-waves, (1963TO06) have calculated the total cross section from 0 to 7 MeV and the γ-ray intensity ratios: the calculations are in excellent agreement with the data of (1961GR27). See also (1961TO04).
At Eα = 38.5 MeV, two groups of protons are observed leading to the ground and 0.48 MeV states of 7Li (1958BU38).
Two γ-rays with Eγ = 7.26 ± 0.03 and 6.78 ± 0.05 MeV, and relative intensities 10 and 7.5 ± 2.0, corresponding to transitions to the first two states of 7Li are observed (1957BA18). The total radiative capture cross section is 45 ± 10 mb (1964ST25). See also (1961JA19, 1961TO04).
The total cross section has been measured for En = 4 eV to 29 MeV (1958HU18, 1960HU08, 1960PE25, 1963BA50, 1964AR25, 1964ST25). A pronounced resonance occurs at En = 262 keV with a peak cross section of 11.2 b (1960HU08). The elastic contribution is 7.2 b (1961LA1A). No other clearly defined resonance is observed, although the cross section exhibits a broad maximum at En ≈ 5 MeV (1954JO17, 1960HU08). The coherent scattering length (thermal, bound) is 1.8 fm (1964ST25).
Angular distributions are tabulated by (1963GO1M): see also (1961LA1A, 1962BA1W, 1963BA50). All observations near the 0.262 MeV resonance are consistent with p-wave formation of a Jπ = 5/2- level (7Li*(7.48)). Table 7.2 (in PDF or PS) gives the resonance parameters compared with those for 7Be*(7.18). These states are believed to have a 4P5/2 character, in agreement with their large θ2n and θ2p (see 7Be and (1956ME1A, 1957MA57)).
The scattering of polarized neutrons on 6Li has been studied for En = 0.19 to 0.42 MeV (1961DA04), 0.14 to 0.66 MeV (1962EL01), and 0.2 to 2.0 MeV (1964LA19). The data agree reasonably well with polarizations calculated from the resonance parameters of Table 7.2 (in PDF or PS) with a background of s-wave potential scattering in which channel spin Jc = 1/2 dominates (1961DA04, 1962EL01, 1964LA19).
The cross section is 33 ± 15 mb at En = 10.2 MeV, 70 ± 6 mb at En = 14.1 MeV (1963AS01).
Excitation functions and angular distributions are summarized in (1958HU18, 1960HU08, 1963GO1M, 1964ST25). Recent measurements are reported for En < 30 keV (1961BE24) and for En = 0.19 MeV (1963BA2A), 1 to 600 keV (1965SC07), 0.1 to 0.3 MeV (1959PA02), 9 to 340 keV (1959BA46), 1.2 to 8 MeV (1959MU25), 2.0 to 2.65 MeV (1959PU75), 8 to 14 MeV (1963MA61), 14.4 MeV (1964VA19). See also (1960PE24, 1961BE24, 1962BE1P, 1962CA1F, 1963AL1J, 1963CH20, 1963WA1K).
The isotropic thermal cross section is 949 b: in the eV-keV range, the cross section falls off somewhat more slowly than 1/v (1958HU18); for En = 9 to 90 keV, σ = 3.96E-0.367 b (E in keV) (1959BA46). The failure to follow the 1/v law may reflect a broad s-wave resonance (1959BA46, 1959GA08, 1960LA1C, 1961BE24). Results of (1965SC07) are consistent with strict 1/v dependence for En < 20 keV. (1956MA83) have analyzed cross section data for En = 20 to 565 keV and find no need for an s-wave resonance. The s-wave background contribution is mainly (75%) in the Jc = 1/2+ channel.
A resonance occurs at En = 258 keV, with σmax = 2.75 b (1959BA46), 2.80 ± 0.22 b (1959GA08): see also (1965MA1Y). The resonance is formed by p-waves, Jπ = 5/2-, and has a large neutron width and a small α-width (Table 7.2 (in PDF or PS)) (1959GA08). Above the resonance the cross section decreases monotonically to En = 18 MeV, except for a slight bump near En = 1.6 to 2.1 MeV (1959GA08, 1959MU25). A careful search in the range En = 2.0 to 2.65 MeV (Ex = 9 to 9.55 MeV) revealed no evidence for a level reported in 6Li(γ, n) (1959PU75). See also (1964GI1F).
Proton spectra and angular distributions have been studied at Ed = 1.5 MeV (1964RI1E), 1.6 MeV (1960AN02), 3.4 to 5.2 MeV (1963ME09), 7 to 7.5 MeV (1957BR97), 8.0 MeV (1953HO48), 14 to 15 MeV (1955LE24, 1959HA29, 1960HA14). Groups corresponding to the ground state and to the states at 0.48, 4.6 and 7.5 MeV have been identified: see Table 7.3 (in PDF or PS). The first two and the last show stripping patterns with ln = 1, while the angular distribution of p2 (Ex = 4.6 MeV) is isotropic. It is noted that stripping to this level is forbidden if it has the character 22F. Ratio of observed θ2n are consistent with assignments 22P to the g.s. and 0.48 and 24P to the 7.5 MeV state (1960HA14, 1960MA32).
The angular correlation between the protons and the 0.48 MeV γ-rays is isotropic (see (1955AJ61)) indicating J = 1/2 for 7Li*. The mean lifetime is reported to be (7.7 ± 0.8) × 10-2 psec (1956BU83: see also 9Be(d, α)7Li) see Table 7.4 (in PDF or PS). See also (1959BO1C, 1959HE1C, 1960NE1C, 1961HA1F, 1963BI27). The polarization of p0 and p1 protons has been observed at Ed = 1.6 MeV by (1961VA03). The circular polarization of the 0.48 MeV γ-rays has been studied by (1964SC1J). See also (1964PA16, 1965HE1B).
Reports on the structure of the (γ, n) cross section differ widely. According to (1954GO1A, 1958RY77: Ebrems. = 24 MeV), a broad maximum appears at Eγ = 16.8 MeV, with σmax = 2.3 mb and a width Γ = 9.3 MeV. Fine structure corresponding to levels at Ex = 9.66 ± 0.04, 10.8, 12.4, 14.0 and 17.5 MeV is also reported (1954GO39, 1958RY77). Up to Ebrems. = 19 MeV, the work of (1964AL08) confirms the gross structure of (1958RY77): additional levels at Ex = 13.6, 15.3 and 16.5 MeV are found: see Table 7.5 (in PDF or PS). (1955HE51) and (1959RO62) find, on the other hand, a weak rise at 8 MeV, a pronounced narrow peak at 14 MeV, followed by a deep minimum at 17 MeV and a sharp rise thereafter. It is suggested by (1964AL08) that the discrepancies are to be traced to different spectral sensitivities in the neutron detectors. Evidence is found for enhanced emission of slow neutrons from a level near Ex = 17.25 MeV (1964AL08). With Ebrems. = 57 MeV, (1960FA06) find a broad maximum near 19 MeV, σ = 3.2 ± 0.8 mb, and a slow tailing off to 57 MeV: see also (1963CO1D). Reported integral cross sections, ∫ σdE, are 0 - 24 MeV: 33 MeV · mb (1954GO1A), 18 MeV · mb (1958RY77); 0 - 25 MeV: 39 MeV · mb (1960FA06); 0 - 50 MeV: 93 MeV · mb (1960FA06). (1964GR40) report cross sections for Eγ = 7.4 to 10.8 MeV: a value Γγ = 0.9 ± 0.4 eV is obtained for 7Li*(7.48). See also (1960KU1C).
The (γ, p) cross section determined by 6He production shows a splitting of the giant resonance into 2 components, at 15.5 and 19.6 MeV: smaller resonances are reported at 12.7, 25.4, 32.0 and 38.0 MeV (1963CL03: see also (1954RU27)). Peaks at 12.5, 13.5 and 14.3 MeV are reported by (1962SH24), while (1963KU25) report 6 peaks in the range Ex = 11 to 24.5 MeV. According to (1954TI16, 1962GR08) only a single maximum occurs, at ≈ 15.6 MeV, with a width of ≈ 4 MeV. Polarization of photoprotons at Eγ = 335 MeV is small and consistent with zero (1962LI13). See also (1963FU1D, 1963KI1C).
Elastic scattering has been studied at Ee = 41.5 MeV (1963GO04: θ = 180°) and 187 MeV (1955ST85). At the lower energy the magnetic scattering is consistent with that expected from a point dipole (1963GO04). The higher energy results yield an r.m.s. charge radius of about 2.1 fm (1957HO1E).
In inelastic scattering studies at Ee = 41.5 MeV, θ = 180°, weak broad peaks, ascribed to M1 transfer, are reported at Q = -6.9, -10.5, -14.0 MeV (1963BA19: see Table 7.6 (in PDF or PS)). At Ee = 102 to 177 MeV, excitation of states at 0.48, 4.63 ± 0.05, (5.7 ± 0.1), 6.8 ± 0.1 and 7.5 ± 0.08 MeV is reported. Transitions to the Ex = 0.48 MeV state show both longitudinal and transverse E2 contributions (see (1963WI1B)), while the others are mainly longitudinal E2: reported B(E2) are listed in Table 7.6 (in PDF or PS) (1963BE26, 1963BE53, 1964BI03). See also (1959ME1D, 1962BA1D, 1964GU1B, 1964MA2C, 1965CH11).
At En = 14 MeV, evidence is reported for states at Ex = 4.6 ± 0.25, ≈ 6.5, 7.5 ± 0.25, and (9.25) MeV (1954AL24). For En = 1.5 to 7.5 MeV, the excitation of the 4.6 MeV state shows no evidence of direct interaction (1963BA50). A DWBA analysis of the distribution of this group at En = 14 MeV has been carried out by (1960PE1A). See also (1960HE1F, 1962WO07).
Elastic scattering and polarization has been studied at Ep = 40 MeV (1959CH1B, 1960CH1B), 150MeV (1962NE12), 155MeV (1964TA02), 156MeV (1964JA03) and 160 to 180MeV (1959JO43, 1960JO14, 1961JO18, 1962RO1F). Analysis in terms of optical parameters is reported by (1961JO18, 1964SA1L).
Inelastic proton groups have been observed corresponding to the excited states of 7Li at 0.48, 4.63, 6.56 and 7.48 MeV: see (1952AJ38). At Ep = 185 MeV, proton groups are observed to these states, as well as to states at 5.5 ± 0.3 MeV (Γ ≈ 0.4 MeV) and 9.6 ± 0.2 MeV. The width of the 6.5 MeV state is reported to be ≈ 1 MeV. Angular distributions have been measured for the 4.6 and 6.5 MeV states (1965HA17). A check of the isotropy of the 477 keV radiation (J = 1/2 → 3/2) yields an upper limit F2 < 10-4 for the intensity of a parity non-conserving part of the wave function (1958WI38). Relative intensities of the Q = -4.6, -6.6 and -7.5 MeV groups, observed at Ep = 17.5 MeV, are consistent with the assignments 22F7/2, 22F5/2, 24P5/2, respectively (1957LE1E, 1957MA04). At Ep = 150 MeV, the Ex = 4.6 MeV state is strongly excited compared to the Ex = 0.48 and 6.5 MeV states, consistent with the assumed rotational character of these levels (1962NE12, 1964JA03). See also (1960HA21, 1960HE1F, 1962RU04, 1963CH08, 1963RI1B, 1964ST15, 1965WE1E).
The summed proton energy spectrum, observed at Ep = 155 to 450 MeV, shows two peaks, with Q ≈ -11.6 and Q ≈ -25.4 MeV, corresponding to formation of 6He(g.s. + 1.80) and an excited state near 15 MeV (see 6He). Angular distributions indicate that the higher energy peak corresponds to the removal of a p-proton while the lower results from removal of an s-proton (1958MA1B, 1959MA1F, 1960HI10, 1961GA09, 1962BE1J, 1962BE1K, 1962DI1A, 1962GA09, 1962GO1P, 1962IN02, 1962IN1A, 1962ST1E, 1962ST1F, 1962TI01, 1963BE1A, 1963BE42, 1963EL1C, 1963JO07, 1963RI1B, 1963TA1D, 1964BA1C, 1964LI1D, 1964TI02, 1965RI1A, 1966TY01).
Reduced widths derived from PWBA analysis of pickup reactions (a) and (b) are listed in Table 7.7 (in PDF or PS). For reaction (c), θ2 = 0.025 and 0.008 for 6He(0) and 6He*(1.80) (1957FR1B, 1960MA32).
Inelastic deuteron groups are observed corresponding to the ground and 0.48, 4.6 and 7.5 MeV states. At Ed = 14.8 MeV, the Q = -7.46 MeV group is only weakly excited (1960HA14). At Ed = 28 MeV, the Q = -4.6 MeV group is strong: the angular distribution is strongly forward. A fit with PWBA requires l = 0 + 2 (1962SL02). See also (1952AJ38, 1959AJ76).
Inelastic alpha groups are observed corresponding to the 0.48 and 4.6 MeV states: see (1955AJ61). At Eα = 13.2, 31.8 and 48 MeV, the angular distributions of the Q = -4.6 MeV group show a prominent peak in the forward hemisphere (1956CO61, 1957SI36, 1960MA15). Analysis by PWBA yields R = 5.6 fm, l = 2 (1960MA15). See also (1962MA59). For reaction (b), see (1960BL1B).
Coulomb excitation of 7Li*(0.48) in reaction (c) has been studied at E(20Ne) = 9 to 11 (1960ST17), 15.4 (1962RI09) and 16 MeV (1961AN07). The observed intensity leads to a value of the reduced transition matrix element B(E2) = 7.6 e2fm4 (1962RI09), 7.3 e2fm4 (1960ST17) corresponding to a partial half life τ = 1.5 nsec. Comparison with the quoted τ1/2 of the state, 0.08 psec, yields Γ(E2)/Γ(M1) = 5 × 10-5 (1960ST17): see Tables 7.4 (in PDF or PS) and 7.6 (in PDF or PS). See also (1963BE1R).
The decay proceeds to the ground and 0.48 MeV states. Reported branching ratios are listed in Table 7.8 (in PDF or PS). The γ-ray energy is 477.8 ± 0.3 keV (see (1955AJ61, 1957DU37)). The weighted mean value of the half life is 53.37 ± 0.11 days (1949SE20, 1953KR16, 1956BO36, 1957WR37); ft = 2.00 × 103 for the ground state transition and 3.45 × 103 for the excited state (1965BA2C). Both transitions are super-allowed (1954MA1D, 1956CH1B). The internal conversion coefficient of the 0.48 MeV γ is 5.8 - 16.6 × 10-7 indicating an E2/M1 ratio between 0 and 0.8 (1959LE30). Calculations of electron capture and nuclear matrix elements support the conclusion that 7Li is close to LS coupling (1962BA1X). See also (1959BL1C, 1963CH08, 1963KI1D).
At Ep = 43.7 MeV, 3He groups are observed corresponding to the 7Li levels at 0, 0.48, 4.6 and 7.5 MeV and to a new state at 11.13 ± 0.05 MeV, with Γ = 268 ± 30 keV. From the similarity of the angular distribution and cross section to that in the (p, t) mirror reaction, it is concluded that the level has Jπ = 3/2-; T = 3/2 (1965DE08).
A number of α-groups have been observed with deuteron energies up to 27.5 MeV. These correspond to levels at 480 ± 2 keV (1948BU31, 1953CO02, 1961JA23), 4.62 ± 0.02 MeV (1953GE01: see also (1964MA04)), and 7.5 MeV (1951GO47, 1964MA04). Angular distributions have been studied for the ground and Ex = 0.48 MeV states at Ed = 0.4 to 2.4 MeV (1962BI11), 10 MeV (1962WE04) and 13.6 MeV (1962IV1A); those to the 4.6 and 7.5 MeV states at Ed = 27.5 MeV (1964MA04). The widths of the 4.6 and 7.5 MeV states are, respectively, 93 ± 25 and 80 ± 20 keV (1966HA09). The upper limit to the intensity of an α-particle group to a state at Ex ≈ 5.5 MeV is 5% of the intensity of the group to 7Li*(7.48) (1966HA09: Ed = 11.1 MeV). See also (1964YA1A).
The (α, γ) angular correlation has been observed for Ed = 0.40 and 0.84 MeV, (1953UE01, 1954CO17). There is no significant departure from isotropy, in agreement with J = 1/2 for the 0.48 MeV level. The mean life of this state is reported to be (7.7 ± 0.8) × 10-2 psec (1956BU83): see Table 7.4 (in PDF or PS).
With thermal neutrons, two groups of α-particles are observed, corresponding to 7Li*(0, 0.48); the fraction of transitions leading to the ground state is about 6%: see 11B. The γ-ray energy is 478.5 ± 1.5 keV (1948EL1A), 478 ± 4 keV (1956DA23); the mean life is (7.5 ± 2.5) × 10-2 psec (1949EL07): see Table 7.4 (in PDF or PS).