(See Energy Level Diagrams for 6He)
Ground state: J = 0 (1958CO68).
The decay proceeds to the ground state of 6Li(1+) and is a super-allowed Gamow-Teller transition. Recently reported end points are: Eβ = 3.50 ± 0.05 (1952WU22), 3.50 ± 0.02 (1956SC40), 3.508 ± 0.015 (1963VI06), 3.508 ± 0.004 MeV (1963JO04). The weighted mean decay energy, including recoil energy of 1.4 keV, leads to 6He - 6Li = 3509.8 ± 3.8 keV (1963JO04). Half lives are listed in Table 6.2 (in PDF or PS). With a half life of 802 msec and Eβ-(max) = 3508.4 keV, ft = 802 (1965BA2C).
The electron-neutrino correlation has the form W(θ) = 1 + α(v/c)cosθ with α = -0.39 ± 0.05 (1958HE46, 1959AL10), α = -0.353+0.033-0.053 (1961RI03), α = -0.319 ± 0.028 (1963VI06), α = -0.3343 ± 0.0030 (1963JO15), in good agreement with the expected value α = -1/3 for pure axial vector interaction. An upper limit to the possible contribution of tensor interaction is 0.4% (1963JO15). See also (1958CS88, 1959AL1E, 1959UB1A, 1961KO1H, 1961YU01, 1963CA1H, 1963DA04, 1963KI1D, 1964DE1G, 1965LO1H).
At Et = 1.9 MeV, the α-spectrum observed at 30° extends from 1 to 7 MeV, with peaks at Eα = 2 and 5 MeV. The same general shape is observed at other angles and for Et = 0.95 to 2.1 MeV. These peaks are attributed to a two-stage process involving formation and breakup of 5He in the P3/2 and P1/2 states and are superposed on the three-body spectrum, reaction (b). Structure observed near the end point may indicate a correlation between the two neutrons (1958JA06). Alpha spectra observed at θ = 90°, Et = 0.2 to 1.0 MeV, likewise exhibit a structure indicating appreciable contribution of 5Heg.s.; a peak ascribed to α + n2 is reported (1961GO18).
At Et = 1.48 MeV, the neutron spectrum shows a continuum from 0 to 12 MeV with a broad peak at 11.3 MeV, corresponding to formation of 5He in the ground state (1957BA10).
The cross section for neutron production rises monotonically from 0.1 to 2.2 MeV (1951AG30, 1957JA37, 1958JA06). The total cross section rises monotonically from 10 mb at Et = 60 keV to 82 mb at 1.14 MeV (1962GO1Q). At Et = 1.90 MeV, the total cross section for production of α-particles is 106 ± 5 mb (1958JA06). The zero-energy cross section factor S0 ≈ 300 keV · b (1964PA1A). See also (1964BA2B).
Differential scattering cross sections have been measured at Et = 1.58 to 2.01 MeV by (1956HO12). At Et = 1.90 MeV, θ = 30°, σ(θ) = 286 mb/sr (± 5%) (1958AL05). A phase shift analysis shows only 1S hard-sphere scattering, with R = 2.35 fm (1955FR1C).
The summed proton spectrum at Ep = 155 to 450 MeV shows two peaks with Q ≈ -11.6 and -25.4 MeV corresponding to 6He*(0, 1.80), and an excited state with J = 1- or 2- at Ex ≈ 15 MeV: see Table 6.3 (in PDF or PS). Angular distributions indicate that the higher energy peak corresponds to ejection of a p-proton while the lower results from removal of an s-proton: see (1957TY35, 1958MA1B, 1958TY49, 1961GA09, 1961PU1A, 1962GA09, 1962GA23, 1962GO1P, 1962TI01, 1964TI02, 1965RI1A, 1965TY1A). See also 7Li.
At Ed = 14.4 MeV, the ground state and the 1.80 MeV level are observed. The angular distributions analyzed by pick-up theory indicate even parity for both states (1955LE24); θ2 = 0.025 and 0.008 (1960MA32). See also (1960HA14).
The energy of the first excited state is 1.71 ± 0.01 MeV, Γ ≲ 100 keV (1954AL35, 1960AL10); 1.797 ± 0.025 MeV, Γ = 113 ± 20 keV (1965AJ01). Evidence gas also been reported for states at 3.4 ± 0.2 MeV (1960AL10), (6.0 ± 0.9) and for one or more states at 9.3 ± 0.7 MeV (1956MA1R, 1956MA50): see, however (1965AJ01). The width of the proton group corresponding to the 1.8 MeV state and the absence of 6He* recoils implies that this state decays predominantly into 4He + 2n (1954AL35, 1963MA1P, 1965AJ01). The branching ratio Γγ/Γn is < 4 × 10-4 (1964HU1A). Angular distributions at Et = 0.24 keV are consistent with J = 0 and 2 for the ground state and 1.8 MeV level, respectively (1954AL38). At Et = 13 MeV, the two angular distributions are strongly peaked forward (1965AJ01). See also (1956MA09, 1961HO01, 1961HO23) and 10Be.