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6Li (1959AJ76)

(See the Energy Level Diagram for 6Li)

GENERAL: See also Table 6.2 [Table of Energy Levels] (in PDF or PS).

Theory: See (1954MO1C, 1955AD1A, 1955AU1A, 1955BA1J, 1955IR1A, 1955LA1C, 1955OT1A, 1956FE1A, 1956ME1A, 1956NE1B, 1957FR1B, 1957LE1E, 1957LY1A, 1957SO1A, 1957TA1A, 1958PI1A, 1958SK1A).

1. (a) 3H(3He, d)4He Qm = 14.319 Eb = 15.790
(b) 3H(3He, p)5He Qm = 11.136
(c) 3H(3He, p)4He + n Qm = 12.093

The relative intensities (43 ± 2, 6 ± 2, 51 ± 2) of reactions (a), (b) and (c), do not vary for E(3He) = 225 to 600 keV. The deuterons are isotropic (c.m.) at Et = 360 keV. The total cross section, reported for E(3He) = 100 to 800 keV, varies from 0.5 mb to 0.18 b, without showing resonance behaviour, the main variation being accounted for by the Coulomb barrier effect (1953MO61). See also (1953AL1A) and (1957JA37).

2. 3He(α, p)6Li Qm = -4.004

See (1958CH35).

3. 4He(d, γ)6Li Qm = 1.471

An upper limit for capture radiation at Ed = 1.055 MeV (2.18-MeV state) is 0.1 mb (1954SI07). A search for resonant capture radiation at Ed = 3.1 MeV (6Li* = 3.56) yields Γ < 0.2 eV. It is concluded that the intensity of parity non-conserving parts of the wave functions, F2 ≲ 10-7 (1958WI15).

4. 4He(d, d)4He Eb = 1.471

Differential cross sections have been measured for Ed = 0.88 to 3.51 MeV (1949BL66), Ed = 1.0 to 1.2 MeV (1953LA28), Ed = 0.28 to 4.62 MeV (1955GA26), Ed = 6.5 MeV (1947GU1A), Ed = 7.94 MeV (1951BU1C), Ed = 10.3 MeV (1951AL26), Ed = 13.7 and 19.0 MeV (1954FR22): see (1957JA37).

At Ed = 1.07 MeV, a strong resonance is observed (see Table 6.3 (in PDF or PS)) attributed to d-wave formation of a J = 3+ state. The best fit is obtained with the p-wave phase shift set equal to zero. If an interaction radius of R = 3.5 × 10-13 cm is used, the s-wave phase shift requires a contribution from the ground state, whose width is thus determined to be θ2 = 0.51; for a radius R = 5.0 × 10-13 cm, hard-sphere scattering alone suffices (1955GA74).

The anomaly in the region 3.5 to 4.5 MeV requires two levels, J = 1+ and 2+ (see Table 6.3 (in PDF or PS)) formed by d-waves. Again the p-wave phase shift differs from the hard sphere value, suggesting an odd parity state at higher energy. There is no indication of the 3.56-MeV 6Li level, consistent with its assumed J = 0+, T = 1 character. The three states found here are presumed to be the components of a 3D term; their spacings indicate a coupling rather close to the L - S limit (1955GA74: see (1953IN1A)).

5. (a) 4He(d, p)5He Qm = -3.184 Eb = 1.471
(b) 4He(d, n)5Li Qm = -4.023
(c) 4He(d, pn)4He Qm = -2.226

Ground-state protons from reaction (a) and ground-state neutrons from (b) have been studied by (1957WA01: Ed = 14.8 MeV) and by (1956BO1F, 1956BO43: Ed = 13.5 MeV), respectively; see 5He, 5Li. The cross section for (a) is 25 ± 5 mb/sr at θ = 18° (c.m.) and 15 ± 5 mb/sr at 24° (c.m.) (1957WA01).

Cross sections for production of low-energy neutrons have been measured in the range Ed = 3 to 6 MeV by (1955HE90). It is argued that reaction (c) probably involves production of the 1S state of the deuteron and hence is isobaric spin forbidden in the present case: the low energy neutrons are therefore attributed to reaction (a) (1955HE90). See also (1951AL26, 1951BU1C, 1957JA37).

6. 6He(β-)6Li Qm = 3.536

See 6He.

7. 6Li(γ, n)5Li Qm = -5.494

The cross section is 0.3 ± 0.2 mb at Eγ = 6.2 MeV (1956ED15) and 0.5 ± 0.2 mb for Eγ = 14.8 to 17.6 MeV (1951TI06). See also (1957BA1H, 1957FO1A, 1958BE1C, 1958PR66, 1958RY77).

8. 6Li(γ, p)5He Qm = -4.655

This reaction has been observed with photoplates for Eγ(max) = 30 to 80 MeV. The angular distribution is of the form a + bsin2θ(1 + cosθ)2 (1956KL19). For 17.5-MeV bremsstrahlung, no n - p correlation is observed; this result argues against a (4He + d) model for 6Li (1958PR66). See also (1955TI1A, 1958RY77).

9. 6Li(γ, d)4He Qm = -1.471

The cross section is ≲ 5 μb in the range Eγ = 2.6 to 17 MeV (1952GL1A, 1952TI1A, 1953JE1A, 1954TI25, 1955TI1A). Electric dipole absorption is forbidden by isobaric spin selection rules (1953GE1B); the relative weakness of both electric and magnetic interaction may be made plausible on a simple (4He + d) model for 6Li (1953JE1A: see also (1954VA1A, 1955DA1A, 1955DA1C, 1958PR66)).

10. 6Li(γ, t)3He Qm = -15.790

The cross section is < 10 μb at Eγ = 17.6 MeV (1954TI25).

11. (a) 6Li(e, e')6Li
(b) 6Li(e, p)5He Qm = -4.655
(c) 6Li(e, d)4He Qm = -1.471

Elastic scattering of 187-MeV electrons yields an r.m.s. radius of 2.78 × 10-13 cm ± 2%: for the uniform model, R0 = 1.98 × 10-13 cm (1955ST85, 1956HO93). At Ee = 426 MeV, the data are fitted by an r.m.s. radius of (2.2 ± 0.2) × 10-13 cm. Inelastic scattering has been observed to the 2.18-MeV state and to higher states (1957HO1D, 1957HO1E). See also (1957EH1A, 1957ME1B).

Reactions (b) and (c) have been observed and angular distributions measured at Ee = 500 MeV (1957KE1A).

12. 6Li(p, p')6Li*

At Ep = 7.0 to 7.5 MeV, and three angles, proton groups are observed corresponding to the ground state, to a state at 2.188 MeV (Γc.m. = 25 keV) and to a state at 3.559 MeV (Γ < 5 keV): (see Table 6.4 (in PDF or PS)). No other sharp levels (Γ < 100 keV) are seen below Ex = 5 MeV: the intensity limit of groups corresponding to such states is 3% of the intensity of the group corresponding to the 2.18-MeV state. A group of ≈ 1 MeV breadth would have escaped detection unless its intensity were ≈ 3 times that of the 2.18-MeV state group (1957BR12). At Ep = 14.8 and 19.0 MeV, the angular distributions of the protons to the 2.18-MeV state peak at ≈ 40°. The protons from the 3.56-MeV state appear weakly, if at all, on the side of the broad group from the 4.5-MeV level (Γ ≈ 1.8 MeV) (1956SH1B). For a theoretical discussion of the data see (1957LE1E). See also (1952AJ38, 1952FR1B, 1958CH26).

13. 6Li(d, d')6Li*

At Ed = 7.0 and 7.5 MeV (θ = 60° and 90°), deuteron groups are observed corresponding to the ground state and to a level at 2.186 MeV (Γc.m. = 24 keV: see Table 6.4 (in PDF or PS)). At Ed = 7.5 MeV, θ = 60°, an upper limit of 0.9% of the intensity of the 2.18-MeV group is given for a group corresponding to the 3.56-MeV state. The fact that the 2.18-MeV state was observed in this experiment but that the 3.56-MeV state was not is consistent with the assignments T = 0 and T = 1, respectively, for these states. No other sharp groups with Ex < 5 MeV were observed: their intensity limits are 1% of the intensity of the 2.18-MeV state. A 1-MeV broad group would have escaped detection unless its intensity were > 2 times that of the 2.18-MeV state group (1957BR12). At Ed = 15 MeV, the angular distribution of the deuterons to the 2.18-MeV state has been measured by (1956HA90). Comparison is made with predictions based on nuclear and electric interactions (1956HA90).

Excitation of 6Li states at 2.2, ≈ 4.5, and 76. ± 0.3 MeV is observed in deuteron bombardment, Ed(max) = 13.8 MeV, of Li-loaded emulsions. Observed stars are reported to correspond to d + α decay of the first two states and decay into α + p + n of the last (1956SO21). At Ed = 17.5 ± 0.25 MeV, these three states and two additional states at 5.9 and 8.3 MeV are reported (1956SO33).

14. 6Li(α, α')6Li

Angular distributions of elastic and inelastic scattering have been studied at Eα = 31.5 MeV by (1956WA29). Inelastic groups corresponding to the 2.18 and 4.52-MeV levels are observed; the isobaric spin-forbidden group corresponding to the 3.56-MeV level is < 4% as intense. The angular distributions of the inelastic groups are well described by the direct interaction theory of (1953AU1A) with R = 6.6 and 5.8 × 10-13 cm, respectively (1956WA29).

15. 6Li(p, 3He)4He Qm = 4.022

Angular distributions observed at Ep = 15 and 18.5 MeV indicate a deuteron pickup process. An analysis based on Born approximation theory leads to deuteron reduced widths for the ground state of 6Li of θ2d = 0.30 and 0.45 (1956LI37). See also 7Be.

16. (a) 6Li(n, t)4He Qm = 4.787
(b) 6Li(n, d)5He Qm = -2.428

Angular distributions at En = 1.5 and 2.0 MeV reported by (1954WE11) and En = 14 MeV by (1954FR03) are analyzed by (1955DA1A, 1955DA1B, 1955DA1C, 1955SA1C) in terms of pickup theory in Born approximation, using a (4He + d), two-body model for 6Li. (1956LI37) calculate θ2 = 0.5 from the data of (1954FR03) on reaction (a). See also 7Li.

17. 7Li(γ, n)6Li Qm = -7.252

See 7Li and (1955TI1A, 1958TI1A).

18. 7Li(p, d)6Li Qm = -5.026

At Ep = 17.5 MeV, angular distributions of the deuterons to the ground state and the 2.18-MeV level, analyzed by pickup theory, indicate ln = 1 and hence even parity, J ≤ 3 for both states. The absolute differential cross section (at ≈ 20° (c.m.)) for the formation of the ground state is 17 ± 4 mb/sr, in good agreement with the value computed from the observed cross section for the inverse reaction. The cross section for the formation of the excited state is about half that for the ground state. The derived reduced widths (7Li) are θ2n = 0.05 and θ2n = 0.035 for 6Li(0) and 6Li*(2.2), respectively. The ratio is in good agreement with that calculated from shell theory in intermediate coupling with 1.4 ≤ a/K ≤ 2.1, near the L - S limit (1956RE04). See also (1957SI36). At Ep = 18 and 31.8 MeV, a deuteron group has also been observed leading to the 3.56-MeV state (1952FR1B, 1957SI36). See also (1955HI1A, 1957MA04, 1958EL1A).

19. 7Li(d, t)6Li Qm = -0.994

At Ed = 14.5 MeV, the angular distributions of the tritons analyzed by pickup theory, indicate ln = 1, and hence even parity, for the ground state and the 2.18-MeV state. Peak cross sections are 32.4 and 16.0 mb/sr at θc.m. = 11° and 16°, respectively (1955LE24). The corresponding reduced widths are θ2 = 0.11 and 0.061 (1957FR1B: compare 7Li(p, d)6Li). See also (1953HO48, 1956HA90, 1957BR12, 1958EL1A). At Ed(max) = 13.8 MeV, stars are observed in Li-loaded emulsions corresponding to excitation of 6Li states at 2.2, ≈ 4.5 and 7.5 MeV with subsequent disintegrations into (α + d) (1956SO21). At Ed = 17.5 MeV, the decay of additional states at 5.2, 5.9, 6.7, 8.3, 9.5 and 10.1 MeV is also reported (1956SO33). A search in the region Ex = 4.4 to 8.5 MeV has revealed only the 5.4-MeV level; no other level appears with Γ < 80 keV (1958HA10, 1958HA1G).

20. 7Li(3He, α)6Li Qm = 13.325

Alpha-particle groups observed at E(3He) = 700 to 900 keV are listed in Table 6.5 (in PDF or PS). 6Li-recoils corresponding to the 3.56-MeV state are observed, indicating that the state decays by γ-emission. The states at 2.2, 4.3 and (5.6) MeV are presumably those observed in 4He(d, d)4He. The small width of the 5.35 and (8.37)-MeV states suggests that they have T = 1 (1955AL1C).

21. 9Be(γ, t)6Li Qm = -17.687

See (1955AJ61).

22. 9Be(p, α)6Li Qm = 2.125

At Ep = 7.2 to 7.5 MeV (θ = 30° and 60°), alpha-particle groups are observed to the ground state and to levels at 2.188 (Γc.m. = 24 keV) and 3.560-MeV (see Table 6.4 (in PDF or PS)). No other sharp levels with Ex < 5 MeV are seen with an intensity greater than 5% of the intensity of the groups to the 2.18 and 3.56-MeV states (1957BR12). The 3.56-MeV state is observed to decay by γ-radiation: Eγ = 3.572 ± 0.012 MeV. The internal pair spectrum is consistent with an M1 transition (1954MA26). Both the γ-ray angular distribution and the ( α - γ) correlation are isotropic at Ep = 2.56 MeV, consistent with J = 0 for 6Li*(3.56) (1956ST93). Determination of the Doppler shift establishes that the lifetime is < 3 × 10-14 sec (1957RO1B), < 3.3 × 10-14 sec (1957LE1D: see (1958WA1C)), consistent with M1 radiation but not with E2. The fact that the ground-state and 2.18-MeV state α-particles do not show resonance at Ep = 2.56 MeV is consistent with the assumption that the first two levels have T = 0: see 10B (1954MA1C). See also (1956RA32).

23. 10B(γ, α)6Li Qm = -4.459

See 10B.

24. 10B(n, dn'2α) Qm = -5.930

At En = 12.2 to 19.5 MeV, this reaction proceeds partly through the 2.18-MeV state of 6Li (1956FR18).