(See Energy Level Diagrams for 6He)
Astrophysical questions: (1976VI1A).
Electromagnetic interactions: (1975VE01).
Pion capture and reactions (See reaction 10 in 6Li. See also reaction 5 here.): (1974CA24, 1974DE1C, 1974KO27, 1975BE1G, 1975MO22, 1976AL1F, 1976CA20, 1976TR1A, 1976TZ1A, 1977AR1C, 1977AU02, 1977BA1Q, 1977CO1E, 1977DO06, 1977RA1A, 1977SH1C).
The decay proceeds to the ground state of 6Li(Jπ = 1+) and is a super-allowed Gamow-Teller transition. The half-life is 808.1 ± 2.0 msec (1974WI14): see Table 6.2 (in PDF or PS) in (1966LA04) for a listing of earlier determinations. Using Qm and this τ1/2, log ft = 2.910 ± 0.002: see (1974WI14, 1978RO01).
The internal bremsstrahlung spectrum has been measured by (1965BI09). The electron-neutrino correlation results are in good agreement with pure axial vector interaction. An upper limit to the possible admixture of tensor interaction is 0.4% (1963JO15). See also (1974AJ01, 1978CA1H) and (1974KU06, 1974VE02, 1975BE42, 1975CA35, 1975DO05, 1975KR14, 1976BE1E, 1976KU07, 1976WA02, 1977AZ02, 1977KL09, 1977SA1G, 1977TE1B, 1977WA1F, 1978PA02; theor.).
The cross section for neutron production (reactions (a) and (b)) rises monotonically from 40 keV to 2.2 MeV: see (1974AJ01). The zero-energy cross section factor S0 ≈ 300 keV · b (1964PA1A). At Et = 1.90 MeV, the total cross section for production of α-particles is 106 ± 5 mb (1958JA06). The cross section for reaction (b) increases monotonically for Et = 34 to 160 keV (1977SE11). For a review of ann determinations see (1972KU08). See also (1974LA02). (1977BA23) find that the cross section for the reaction 3H(t, d)4He + e- + ν-bar is ≤ 0.3 nb/sr at Et = 1.4 MeV: see also reaction 1 in 6Be. For the earlier work on these reactions see (1974AJ01). See also (1975KU1C), (1975FO19; astrophysical questions) and (1974DE18, 1974NE1B; theor.).
The excitation of 6He*(0, 1.8) and possibly of (broad) states at Ex = 15.6 ± 0.5, 23.2 ± 0.7 and 29.7 ± 1.3 MeV is reported by (1973BA62) from Eγ measurements using a pair spectrometer. (4.4 ± 0.6)% of stopped pions were absorbed radiatively: the branching ratios of 6He*(0, 1.8) are (0.31 ± 0.04)% and (0.15 ± 0.03)% respectively (1973BA62). (1978RE05) find (0.34 ± 0.03)% and (0.11 ± 0.01)%, respectively.
Angular distributions have been measured at Ep = 4.71, 5.24 and 6.77 MeV (1977RO01: p0). At En = 14 MeV the p0 angular distribution is similar in shape to the angular distributions in 6Li(p, p')6Li*(3.56) and in 6Li(p, n)6Beg.s.. The ratios of the cross sections for these isobaric analog transitions are consistent with charge independence (1972ME05). For measurements at 800 MeV see (1977RI07). See also (1976KI1D, 1976MI1C), (1976SL2A) and 7Li.
The ground state angular distribution has been studied at Et = 17 MeV. The weighted average of the experimental ratio of the differential cross sections at twelve angles (θc.m. ≈ 28° → 72°) for population of 6Heg.s. to that for population of the analog state, 6Li*(3.56), reached in the (t, t') reaction [see 6Li] is 2.28 ± 0.16, rather than 2 as predicted by geometrical isospin considerations. Corrections based on a microscopic DWBA calculation of charge exchange and inelastic scattering do not appear to account for the difference (1976SH14). At Et = 22 MeV only 6He*(0, 1.80) are populated with Ex ≲ 8.5 MeV (1971ST05).
See (1970BA41; E(3He) = 53.2 MeV).
Angular distributions have been studied at E(6Li) = 32 and 36 MeV for the reactions involving the ground states of 6He and 6Be and for the inelastic scattering of 6Li to 6Li*(3.56) [see reaction 18 in 6Li]: these three states in 6He, 6Li and 6Be are analog states. At 32 MeV the ratios of the differential cross sections for populating 6Li*(3.56) to those for populating 6Heg.s. varies with angle from 0.17 to more than 1.65. These variations indicate that charge dependent effects are important and that additional analysis would require a better knowledge of the wave functions of these isospin multiplet states than is currently available (1974WH01, 1974WH02, 1974WH07, 1975WH01).
At Eγ = 60 MeV, the proton spectrum shows two prominant peaks attributed to 6He*(0 + 1.8, 18 ± 3) (1973GA16, 1976MA34). (1974DE52) using bremsstrahlung radiation (Ebs ≤ 28 MeV) suggest states at Ex = 2.5 and 8.5 MeV in 6He, in addition to 6He*(0, 1.8). See also (1975MA1E).
At Eγ = 60 MeV, the proton spectrum shows two prominant peaks attributed to centered at Ex = 13.6, 15.4 and 17.7 MeV (± 0.5 MeV) and a possible state or states (populated with an l-transfer ≥ 2) at Ex = 23.7 MeV. DWBA analyses of the d0 and d1 groups lead to S(1p3/2) = 0.62 for 6Heg.s. and to S(1p3/2) = 0.37 and S(1p1/2) = 0.32 for 6He*(1.8) (1977BR17). See (1974AJ01) for earlier measurements.
The summed proton spectrum at Ep = 100 MeV shows 6He*(0, 1.8) and a broad group centered at Ex ≈ 14 MeV: the angular correlation for 6Heg.s. is in quite good agreement with DWIA (1976BH02). In a lower resolution experiment (1967RO06) suggest states at Ex = (13.4 ± 0.5) MeV [γ = 1.2 MeV] and (15.3 ± 0.3) MeV [partially resolved]. See also (1966LA04).
On the basis of a very accurate measurement of the Q-value of this reaction (1978RO01) find the atomic mass excess of 6He = 17593.7 ± 1.1 keV. The weighted average of this value and of the previous determinations leads to 17594.0 ± 1.1 keV (1978RO01) which we adopt and use in calculating Qm.
Angular distributions of the 3He ions to 6He*(0, 1.80) have been measured at Ed = 14.4 and 22 MeV: they are consistent with l = 1 and therefore with Jπ = 0+, 2+ for these two states: see (1974AJ01). There is no evidence for any other states of 6He with Ex < 10.7 MeV (1971ST05). See also (1974DI1A, 1975DI1B) and reaction 28 in 6Li.
The energy of the first excited state is 1.797 ± 0.025 MeV, γ = 113 ± 20 keV (1965AJ01). 6He*(1.80) decays into 4He + 2n. The branching ratio γγ/γn is < 4 × 10-4 (1964HU1A), γγ/γα ≤ 2 × 10-6 (1966LI1A). Angular distributions of the α0 and α1 groups have been measured at Et = 13 and 22 MeV: they are consistent with l = 1 pickup and therefore with Jπ = 0+, 2+ for 6He*(0, 1.80): see (1974AJ01). No other α-groups are reported corresponding to 6He states with Ex < 12 MeV (1965AJ01), Ex < 24 MeV (1971ST05: Et = 16 → 22 MeV; region between Ex ≈ 13 and 16 MeV was obscured by presence of breakup α-particles). See also (1975KO1B).
Angular distributions have been measured at En = 12.2, 14.1 and 18.0 MeV (1976SM02: α0, α1), 13.99 MeV (1974PE06: α0, α1) and at 14.4 MeV (1967PA03: α0). No other states are observed below Ex ≈ 7 MeV (1974PE06). See also 10Be.
At Et = 21.5 and 23.5 MeV, angular distributions have been studied for the transitions to 6He*(0, 1.80) and 6Li*(0, 2.19, 3.56) (1973VO08, 1975VO08): see discussion in reaction 32 in 6Li. See also (1974CA04, 1975BR1E; theor.).
See 18F in (1978AJ03).