
^{6}Li (1959AJ76)(See the Energy Level Diagram for ^{6}Li) 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).
The relative intensities (43 ± 2, 6 ± 2, 51 ± 2) of reactions (a), (b) and (c), do not vary for E(^{3}He) = 225 to 600 keV. The deuterons are isotropic (c.m.) at E_{t} = 360 keV. The total cross section, reported for E(^{3}He) = 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).
See (1958CH35).
An upper limit for capture radiation at E_{d} = 1.055 MeV (2.18MeV state) is 0.1 mb (1954SI07). A search for resonant capture radiation at E_{d} = 3.1 MeV (^{6}Li* = 3.56) yields Γ_{dα} < 0.2 eV. It is concluded that the intensity of parity nonconserving parts of the wave functions, F^{2} ≲ 10^{7} (1958WI15).
Differential cross sections have been measured for E_{d} = 0.88 to 3.51 MeV (1949BL66), E_{d} = 1.0 to 1.2 MeV (1953LA28), E_{d} = 0.28 to 4.62 MeV (1955GA26), E_{d} = 6.5 MeV (1947GU1A), E_{d} = 7.94 MeV (1951BU1C), E_{d} = 10.3 MeV (1951AL26), E_{d} = 13.7 and 19.0 MeV (1954FR22): see (1957JA37). At E_{d} = 1.07 MeV, a strong resonance is observed (see Table 6.3 (in PDF or PS)) attributed to dwave formation of a J = 3^{+} state. The best fit is obtained with the pwave phase shift set equal to zero. If an interaction radius of R = 3.5 × 10^{13} cm is used, the swave phase shift requires a contribution from the ground state, whose width is thus determined to be θ^{2} = 0.51; for a radius R = 5.0 × 10^{13} cm, hardsphere 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 dwaves. Again the pwave phase shift differs from the hard sphere value, suggesting an odd parity state at higher energy. There is no indication of the 3.56MeV ^{6}Li level, consistent with its assumed J = 0^{+}, T = 1 character. The three states found here are presumed to be the components of a ^{3}D term; their spacings indicate a coupling rather close to the L  S limit (1955GA74: see (1953IN1A)).
Groundstate protons from reaction (a) and groundstate neutrons from (b) have been studied by (1957WA01: E_{d} = 14.8 MeV) and by (1956BO1F, 1956BO43: E_{d} = 13.5 MeV), respectively; see ^{5}He, ^{5}Li. 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 lowenergy neutrons have been measured in the range E_{d} = 3 to 6 MeV by (1955HE90). It is argued that reaction (c) probably involves production of the ^{1}S 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).
See ^{6}He.
The cross section is 0.3 ± 0.2 mb at E_{γ} = 6.2 MeV (1956ED15) and 0.5 ± 0.2 mb for E_{γ} = 14.8 to 17.6 MeV (1951TI06). See also (1957BA1H, 1957FO1A, 1958BE1C, 1958PR66, 1958RY77).
This reaction has been observed with photoplates for E_{γ}(max) = 30 to 80 MeV. The angular distribution is of the form a + bsin^{2}θ(1 + cosθ)^{2} (1956KL19). For 17.5MeV bremsstrahlung, no n  p correlation is observed; this result argues against a (^{4}He + d) model for ^{6}Li (1958PR66). See also (1955TI1A, 1958RY77).
The cross section is ≲ 5 μb in the range E_{γ} = 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 (^{4}He + d) model for ^{6}Li (1953JE1A: see also (1954VA1A, 1955DA1A, 1955DA1C, 1958PR66)).
The cross section is < 10 μb at E_{γ} = 17.6 MeV (1954TI25).
Elastic scattering of 187MeV electrons yields an r.m.s. radius of 2.78 × 10^{13} cm ± 2%: for the uniform model, R_{0} = 1.98 × 10^{13} cm (1955ST85, 1956HO93). At E_{e} = 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 2.18MeV state and to higher states (1957HO1D, 1957HO1E). See also (1957EH1A, 1957ME1B). Reactions (b) and (c) have been observed and angular distributions measured at E_{e} = 500 MeV (1957KE1A).
At E_{p} = 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 (Γ_{c.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 E_{x} = 5 MeV: the intensity limit of groups corresponding to such states is 3% of the intensity of the group corresponding to the 2.18MeV state. A group of ≈ 1 MeV breadth would have escaped detection unless its intensity were ≈ 3 times that of the 2.18MeV state group (1957BR12). At E_{p} = 14.8 and 19.0 MeV, the angular distributions of the protons to the 2.18MeV state peak at ≈ 40°. The protons from the 3.56MeV state appear weakly, if at all, on the side of the broad group from the 4.5MeV level (Γ ≈ 1.8 MeV) (1956SH1B). For a theoretical discussion of the data see (1957LE1E). See also (1952AJ38, 1952FR1B, 1958CH26).
At E_{d} = 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 (Γ_{c.m.} = 24 keV: see Table 6.4 (in PDF or PS)). At E_{d} = 7.5 MeV, θ = 60°, an upper limit of 0.9% of the intensity of the 2.18MeV group is given for a group corresponding to the 3.56MeV state. The fact that the 2.18MeV state was observed in this experiment but that the 3.56MeV state was not is consistent with the assignments T = 0 and T = 1, respectively, for these states. No other sharp groups with E_{x} < 5 MeV were observed: their intensity limits are 1% of the intensity of the 2.18MeV state. A 1MeV broad group would have escaped detection unless its intensity were > 2 times that of the 2.18MeV state group (1957BR12). At E_{d} = 15 MeV, the angular distribution of the deuterons to the 2.18MeV state has been measured by (1956HA90). Comparison is made with predictions based on nuclear and electric interactions (1956HA90). Excitation of ^{6}Li states at 2.2, ≈ 4.5, and 76. ± 0.3 MeV is observed in deuteron bombardment, E_{d}(max) = 13.8 MeV, of Liloaded 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 E_{d} = 17.5 ± 0.25 MeV, these three states and two additional states at 5.9 and 8.3 MeV are reported (1956SO33).
Angular distributions of elastic and inelastic scattering have been studied at E_{α} = 31.5 MeV by (1956WA29). Inelastic groups corresponding to the 2.18 and 4.52MeV levels are observed; the isobaric spinforbidden group corresponding to the 3.56MeV 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).
Angular distributions observed at E_{p} = 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 ^{6}Li of θ^{2}_{d} = 0.30 and 0.45 (1956LI37). See also ^{7}Be.
Angular distributions at E_{n} = 1.5 and 2.0 MeV reported by (1954WE11) and E_{n} = 14 MeV by (1954FR03) are analyzed by (1955DA1A, 1955DA1B, 1955DA1C, 1955SA1C) in terms of pickup theory in Born approximation, using a (^{4}He + d), twobody model for ^{6}Li. (1956LI37) calculate θ^{2} = 0.5 from the data of (1954FR03) on reaction (a). See also ^{7}Li.
See ^{7}Li and (1955TI1A, 1958TI1A).
At E_{p} = 17.5 MeV, angular distributions of the deuterons to the ground state and the 2.18MeV level, analyzed by pickup theory, indicate l_{n} = 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 (^{7}Li) are θ^{2}_{n} = 0.05 and θ^{2}_{n} = 0.035 for ^{6}Li(0) and ^{6}Li*(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 E_{p} = 18 and 31.8 MeV, a deuteron group has also been observed leading to the 3.56MeV state (1952FR1B, 1957SI36). See also (1955HI1A, 1957MA04, 1958EL1A).
At E_{d} = 14.5 MeV, the angular distributions of the tritons analyzed by pickup theory, indicate l_{n} = 1, and hence even parity, for the ground state and the 2.18MeV state. Peak cross sections are 32.4 and 16.0 mb/sr at θ_{c.m.} = 11° and 16°, respectively (1955LE24). The corresponding reduced widths are θ^{2} = 0.11 and 0.061 (1957FR1B: compare ^{7}Li(p, d)^{6}Li). See also (1953HO48, 1956HA90, 1957BR12, 1958EL1A). At E_{d}(max) = 13.8 MeV, stars are observed in Liloaded emulsions corresponding to excitation of ^{6}Li states at 2.2, ≈ 4.5 and 7.5 MeV with subsequent disintegrations into (α + d) (1956SO21). At E_{d} = 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 E_{x} = 4.4 to 8.5 MeV has revealed only the 5.4MeV level; no other level appears with Γ < 80 keV (1958HA10, 1958HA1G).
Alphaparticle groups observed at E(^{3}He) = 700 to 900 keV are listed in Table 6.5 (in PDF or PS). ^{6}Lirecoils corresponding to the 3.56MeV state are observed, indicating that the state decays by γemission. The states at 2.2, 4.3 and (5.6) MeV are presumably those observed in ^{4}He(d, d)^{4}He. The small width of the 5.35 and (8.37)MeV states suggests that they have T = 1 (1955AL1C).
See (1955AJ61).
At E_{p} = 7.2 to 7.5 MeV (θ = 30° and 60°), alphaparticle groups are observed to the ground state and to levels at 2.188 (Γ_{c.m.} = 24 keV) and 3.560MeV (see Table 6.4 (in PDF or PS)). No other sharp levels with E_{x} < 5 MeV are seen with an intensity greater than 5% of the intensity of the groups to the 2.18 and 3.56MeV states (1957BR12). The 3.56MeV state is observed to decay by γradiation: E_{γ} = 3.572 ± 0.012 MeV. The internal pair spectrum is consistent with an M1 transition (1954MA26). Both the γray angular distribution and the ( α  γ) correlation are isotropic at E_{p} = 2.56 MeV, consistent with J = 0 for ^{6}Li*(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 groundstate and 2.18MeV state αparticles do not show resonance at E_{p} = 2.56 MeV is consistent with the assumption that the first two levels have T = 0: see ^{10}B (1954MA1C). See also (1956RA32).
See ^{10}B.
At E_{n} = 12.2 to 19.5 MeV, this reaction proceeds partly through the 2.18MeV state of ^{6}Li (1956FR18).
