A=6He (1979AJ01)

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⁶He (1979AJ01)

(See Energy Level Diagrams for ⁶He)

GENERAL: See also (1974AJ01) and Table 6.1 [Table of Energy Levels] (in PDF or PS) here.

Model calculations: (1974GH01, 1974IR04, 1975FI1C, 1975FI1D, 1975VE01, 1976CE03, 1976IR1B).

Astrophysical questions: (1976VI1A).

Electromagnetic interactions: (1975VE01).

Special reactions: (1974BO08, 1975FE1A, 1975ZE01, 1976BO08, 1976VA29, 1977FE1B, 1977YA1A).

Muon and neutrino capture and reactions: (1973MU1B, 1974CA04, 1975DO1F, 1976WA02, 1977PR1B, 1978DE15, 1978HW01).

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).

Other topics: (1973DZ1A, 1974GH01, 1974IR04, 1974KA1E, 1974MC04, 1975KU08, 1976IR1B, 1976RO1D, 1978PA02).

Ground state properties: (1974GH01, 1975BE31, 1976BE1G, 1976HA1E).

Mass of ⁶He: The mass excess of ⁶He is 17593.7 ± 1.1 keV [see ⁷Li(d, ³He)⁶He] (1978RO01). Including previous values of the mass excess, the adopted mass excess of ⁶He is 17594.0 ± 1.1 keV.

1. ⁶He(β^-)⁶Li Q_m = 3.507

The decay proceeds to the ground state of ⁶Li(J^π = 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 Q_m and this τ_1/2, 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.).

2. (a) ³H(t, n)⁵He Q_m = 10.44 E_b = 12.305
(b) ³H(t, 2n)⁴He Q_m = 11.3321
(c) ³H(t, d)⁴H Q_m = -9.2
(d) ³H(t, 3n)³He Q_m = -9.2457

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 E_t = 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 E_t = 34 to 160 keV (1977SE11). For a review of a_nn 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 E_t = 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.).

3. ³H(t, t)³H E_b = 12.305

Differential cross sections have been measured for E_t = 1.58 to 2.01 MeV: see (1974AJ01). See also (1975AB1C; theor.).

4. ⁴He(t, p)⁶He Q_m = -7.509

At E_t = 22 (1971ST05) and 23 MeV (1978AJ02), angular distributions of the protons to ⁶He*(0, 1.80) have been measured. No other states are observed with E_x ≲ 4.2 MeV (1971ST05). See also (1974LA1A).

5. ⁶Li(π^-, γ)⁶He Q_m = 13.060

The excitation of ⁶He*(0, 1.8) and possibly of (broad) states at E_x = 15.6 ± 0.5, 23.2 ± 0.7 and 29.7 ± 1.3 MeV is reported by (1973BA62) from E_γ 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.

6. ⁶Li(n, p)⁶He Q_m = -2.725

Angular distributions have been measured at E_p = 4.71, 5.24 and 6.77 MeV (1977RO01: p₀). At E_n = 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)⁶Be_g.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.

7. ⁶Li(t, ³He)⁶He Q_m = -3.489

The ground state angular distribution has been studied at E_t = 17 MeV. The weighted average of the experimental ratio of the differential cross sections at twelve angles (θ_c.m. ≈ 28° → 72°) for population of ⁶He_g.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 E_t = 22 MeV only ⁶He*(0, 1.80) are populated with E_x ≲ 8.5 MeV (1971ST05).

8. ⁶Li(³He, 3p)⁶He Q_m = -10.442

See (1970BA41; E(³He) = 53.2 MeV).

9. ⁶Li(⁶Li, ⁶Be)⁶He Q_m = -7.795

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 ⁶He_g.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).

10. ⁷Li(γ, p)⁶He Q_m = -9.976

At E_γ = 60 MeV, the proton spectrum shows two prominant peaks attributed to ⁶He*(0 + 1.8, 18 ± 3) (1973GA16, 1976MA34). (1974DE52) using bremsstrahlung radiation (E_bs ≤ 28 MeV) suggest states at E_x = 2.5 and 8.5 MeV in ⁶He, in addition to ⁶He*(0, 1.8). See also (1975MA1E).

11. ⁷Li(e, ep)⁶He Q_m = -9.976

See ⁷Li.

12. ⁷Li(n, d)⁶He Q_m = -7.751

At E_γ = 60 MeV, the proton spectrum shows two prominant peaks attributed to centered at E_x = 13.6, 15.4 and 17.7 MeV (± 0.5 MeV) and a possible state or states (populated with an l-transfer ≥ 2) at E_x = 23.7 MeV. DWBA analyses of the d₀ and d₁ groups lead to S(1p_3/2) = 0.62 for ⁶He_g.s. and to S(1p_3/2) = 0.37 and S(1p_1/2) = 0.32 for ⁶He*(1.8) (1977BR17). See (1974AJ01) for earlier measurements.

13. ⁷Li(p, 2p)⁶He Q_m = -9.976

The summed proton spectrum at E_p = 100 MeV shows ⁶He*(0, 1.8) and a broad group centered at E_x ≈ 14 MeV: the angular correlation for ⁶He_g.s. is in quite good agreement with DWIA (1976BH02). In a lower resolution experiment (1967RO06) suggest states at E_x = (13.4 ± 0.5) MeV [γ = 1.2 MeV] and (15.3 ± 0.3) MeV [partially resolved]. See also (1966LA04).

14. ⁷Li(d, ³He)⁶He Q_m = -4.482

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 Q_m.

Angular distributions of the ³He ions to ⁶He*(0, 1.80) have been measured at E_d = 14.4 and 22 MeV: they are consistent with l = 1 and therefore with J^π = 0⁺, 2⁺ for these two states: see (1974AJ01). There is no evidence for any other states of ⁶He with E_x < 10.7 MeV (1971ST05). See also (1974DI1A, 1975DI1B) and reaction 28 in ⁶Li.

15. ⁷Li(t, α)⁶He Q_m = 9.838

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 γ_γ/γ_n is < 4 × 10^-4 (1964HU1A), γ_γ/γ_α ≤ 2 × 10^-6 (1966LI1A). Angular distributions of the α₀ and α₁ groups have been measured at E_t = 13 and 22 MeV: they are consistent with l = 1 pickup and therefore with J^π = 0⁺, 2⁺ for ⁶He*(0, 1.80): see (1974AJ01). No other α-groups are reported corresponding to ⁶He states with E_x < 12 MeV (1965AJ01), E_x < 24 MeV (1971ST05: E_t = 16 → 22 MeV; region between E_x ≈ 13 and 16 MeV was obscured by presence of breakup α-particles). See also (1975KO1B).

16. ⁷Li(³He, ⁴Li)⁶He Q_m = -12.9

See (1970BA41: E(³He) = 53.2 MeV). See also (1976WA12).

17. ⁷Li(⁷Li, ⁸Be)⁶He Q_m = 7.281

See (1968ST12).

18. ⁹Be(n, α)⁶He Q_m = -0.600

Angular distributions have been measured at E_n = 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 E_x ≈ 7 MeV (1974PE06). See also ¹⁰Be.

19. ⁹Be(p, ⁴Li)⁶He Q_m = -24.1

See (1970DE17).

20. ⁹Be(t, ⁶Li)⁶He Q_m = -5.383

At E_t = 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.).

21. ⁹Be(⁷Li, ¹⁰B)⁶He Q_m = -3.390

See (1977KE09) and ¹⁰B.

22. (a) ¹⁰B(⁷Li, ⁶He)¹¹C Q_m = -1.284
(b) ¹¹B(⁷Li, ⁶He)¹²C Q_m = 5.982

See (1968ST12).

23. ¹⁸O(⁶Li, ¹⁸F)⁶He Q_m = -5.161

See ¹⁸F in (1978AJ03).