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6He (1959AJ76)
(See the Energy Level Diagram for 6He)
GENERAL: See also Table 6.1 [Table of Energy Levels] (in PDF or PS).
Spin of 6He: In a Stern-Gerlach experiment, (1958CO68) find μ(6He) < 0.16 nuclear magnetons if J is taken as 1; it is concluded that J(6He) = 0.
Theory: See (1955BA1J, 1958SK1A).
| 1. 6He(β-)6Li | Qm = 3.536 |
The β-spectrum is simple, with an end point Eβ(max) = 3.50 ± 0.05 (1952WU22), 3.50 ± 0.02 MeV (1956SC40). Recently reported half-lives are 0.852 ± 0.016 sec (1956VE10), 0.83 ± 0.02 sec (1958HE46), 0.85 ± 0.03 sec (1955RU06). The weighted mean of half-lives cited here and in (1950HO80, 1955AJ61) is 0.813 ± 0.007 sec. Using Qm, log ft is 2.92.
The electron-neutrino correlation is found to be W(θ) = 1 + α(p/W)cosθ, with α = -(0.39 ± 0.02), in good agreement with the value α = -1/3 for pure axial vector interaction (1958HE46, 1959HE1E, 1959PL52). An earlier report by (1955RU06) appears to have been in error: see (1958AL1G, 1958WU60). See also (1955BA1J, 1955LA1D, 1957LE1E; theor.).
| 2. (a) 3H(t, n)5He | Qm = 10.371 | Eb = 12.272 |
| (b) 3H(t, α)2n | Qm = 11.328 | |
| (c) 3H(t, α)2n |
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). 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). At Et = 1.90 MeV, the total cross section for production of α-particles is 106 ± 5 mb (1958JA06).
| 3. 3H(t, t)3H | Eb = 12.272 |
Differential scattering cross sections have been measured at Et = 1.58 to 2.01 MeV by (1956HO12). At Et = 1.90 MeV, θ(lab) = 30°, σ(θ) = 286 mb/sr (± 5%) (1958AL05). A phase-shift analysis shows that the distributions at Et = 1.80 and 2.01 MeV are adequately accounted for by a 1S phase shift corresponding to a hard sphere of radius 2.35 × 10-13 cm. There is no evidence of p-waves or of resonance in this region (1955FR1C).
| 4. 3H(α, p)6He | Qm = -7.540 |
Not reported.
| 5. 6Li(n, p)6He | Qm = -2.753 |
See 7Li.
| 6. 6Li(t, 3He)6He | Qm = -3.518 |
Not reported.
| 7. 7Li(γ, p)6He | Qm = -10.006 |
A γ-emitting level at Ex = 1.6 ± 0.2 MeV is reported by (1954TI16). This evidence, based on the colinearity of p and 6He tracks in photoplates, appears to conflict with other indications that this state decays predominantly to 4He + 2n (1954AL35: see, however, (1956MA1R, 1956MA50)). See 7Li(t, α)6He.
| 8. 7Li(n, d)6He | Qm = -7.779 |
See 8Li.
| 9. 7Li(p, 2p)6He | Qm = -10.006 |
The summed proton spectrum at Ep = 185 MeV shows two peaks, attributed to formation of 6Heg.s. by removal of a p-proton from 7Li and 6He* ≈ 15 MeV, formed by removal of an s-proton (1957TY35, 1958MA1B, 1958TY49).
| 10. 7Li(d, 3He)6He | Qm = -4.512 |
At Ed = 14.5 MeV, the ground state and the 1.71-MeV level are observed. The angular distributions analyzed by pick-up theory indicate even parity for both states. Peak differential cross sections are 8.0 mb/sr at θc.m. = 17° and 2.0 mb/sr at θc.m. = 16.5° for the ground and 1.7-MeV states, respectively (1955LE24); θ2 = 0.055 and 0.017 (1957FR1B).
| 11. 7Li(t, α)6He | Qm = 9.807 |
| Q0 = 9.79 ± 0.03 (1954AL35); | |
| Q0 = 9.79 ± 0.14 (1956MA09). |
The energy of the first excited state is 1.71 ± 0.01 MeV, Γ ≲ 100 keV (1954AL35, 1955AL1C). Preliminary results may indicate a state at 3.4 MeV, Γ < 0.3 MeV (1954AL35, 1955AL1C). (1956MA1R, 1956MA50) report evidence for a state at (6.0 ± 0.9 MeV) and for one or more states at 9.3 ± 0.7 MeV (3.8-MeV tritons from 9Be(d, t)8Be were used). Angular distributions at Et = 240 keV are consistent with J = 0 and 2 for the ground state and the 1.7-MeV level, respectively (1954AL38: see 10Be). At Et = 0.84 MeV, θ = 90°, the cross sections for formation of 6He(0) and 6He*(1.7) are 16 and 26 mb/sr, respectively (1956MA09).
The absence of 6He recoils corresponding to the 1.7-MeV state implies that this state decays predominantly into 4He + 2n (1954AL35). See also (1955CU17, 1956BA1E) and (1952AJ38).
| 12. 9Be(n, α)6He | Qm = -0.628 |
See 10Be.