
^{6}He (1966LA04)(See Energy Level Diagrams for ^{6}He) GENERAL: See (1960PH1A, 1960TA1C, 1961AH1A, 1961BA1E, 1962IN02, 1962IN1A, 1963BO1K, 1963MO1F, 1963VL1A, 1964GR1J, 1964WA1E, 1965BO1C, 1965LO1G). See also Table 6.1 [Table of Energy Levels] (in PDF or PS). Ground state: J = 0 (1958CO68).
The decay proceeds to the ground state of ^{6}Li(1^{+}) and is a superallowed GamowTeller transition. Recently reported end points are: E_{β} = 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 ^{6}He  ^{6}Li = 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 E_{β}(max) = 3508.4 keV, ft = 802 (1965BA2C). The electronneutrino 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 E_{t} = 1.9 MeV, the αspectrum observed at 30° extends from 1 to 7 MeV, with peaks at E_{α} = 2 and 5 MeV. The same general shape is observed at other angles and for E_{t} = 0.95 to 2.1 MeV. These peaks are attributed to a twostage process involving formation and breakup of ^{5}He in the P_{3/2} and P_{1/2} states and are superposed on the threebody spectrum, reaction (b). Structure observed near the end point may indicate a correlation between the two neutrons (1958JA06). Alpha spectra observed at θ = 90°, E_{t} = 0.2 to 1.0 MeV, likewise exhibit a structure indicating appreciable contribution of ^{5}He_{g.s.}; a peak ascribed to α + n^{2} is reported (1961GO18). At E_{t} = 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 ^{5}He 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 E_{t} = 60 keV to 82 mb at 1.14 MeV (1962GO1Q). At E_{t} = 1.90 MeV, the total cross section for production of αparticles is 106 ± 5 mb (1958JA06). The zeroenergy cross section factor S_{0} ≈ 300 keV · b (1964PA1A). See also (1964BA2B).
Differential scattering cross sections have been measured at E_{t} = 1.58 to 2.01 MeV by (1956HO12). At E_{t} = 1.90 MeV, θ = 30°, σ(θ) = 286 mb/sr (± 5%) (1958AL05). A phase shift analysis shows only ^{1}S hardsphere scattering, with R = 2.35 fm (1955FR1C).
Not reported.
Not reported.
See ^{8}Li.
The summed proton spectrum at E_{p} = 155 to 450 MeV shows two peaks with Q ≈ 11.6 and 25.4 MeV corresponding to ^{6}He*(0, 1.80), and an excited state with J = 1^{} or 2^{} at E_{x} ≈ 15 MeV: see Table 6.3 (in PDF or PS). Angular distributions indicate that the higher energy peak corresponds to ejection of a pproton while the lower results from removal of an sproton: see (1957TY35, 1958MA1B, 1958TY49, 1961GA09, 1961PU1A, 1962GA09, 1962GA23, 1962GO1P, 1962TI01, 1964TI02, 1965RI1A, 1965TY1A). See also ^{7}Li.
At E_{d} = 14.4 MeV, the ground state and the 1.80 MeV level are observed. The angular distributions analyzed by pickup theory indicate even parity for both states (1955LE24); θ^{2} = 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 ^{6}He* recoils implies that this state decays predominantly into ^{4}He + 2n (1954AL35, 1963MA1P, 1965AJ01). The branching ratio Γ_{γ}/Γ_{n} is < 4 × 10^{4} (1964HU1A). Angular distributions at E_{t} = 0.24 keV are consistent with J = 0 and 2 for the ground state and 1.8 MeV level, respectively (1954AL38). At E_{t} = 13 MeV, the two angular distributions are strongly peaked forward (1965AJ01). See also (1956MA09, 1961HO01, 1961HO23) and ^{10}Be.
(1964GA11) report αgroups to excited states at E_{x} = 1.66 ± 0.26, 3.29 ± 0.38 and 6.05 ± 0.26 MeV. See also (1963AL18) and ^{10}Be.
