
^{6}He (1979AJ01)(See Energy Level Diagrams for ^{6}He) 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 ^{6}Li. 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 ^{6}He: The mass excess of ^{6}He is 17593.7 ± 1.1 keV [see ^{7}Li(d, ^{3}He)^{6}He] (1978RO01). Including previous values of the mass excess, the adopted mass excess of ^{6}He is 17594.0 ± 1.1 keV.
The decay proceeds to the ground state of ^{6}Li(J^{π} = 1^{+}) and is a superallowed GamowTeller transition. The halflife 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 electronneutrino 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 zeroenergy cross section factor S_{0} ≈ 300 keV · b (1964PA1A). At E_{t} = 1.90 MeV, the total cross section for production of αparticles 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_{n}n determinations see (1972KU08). See also (1974LA02). (1977BA23) find that the cross section for the reaction ^{3}H(t, d)^{4}He + e^{} + νbar is ≤ 0.3 nb/sr at E_{t} = 1.4 MeV: see also reaction 1 in ^{6}Be. For the earlier work on these reactions see (1974AJ01). See also (1975KU1C), (1975FO19; astrophysical questions) and (1974DE18, 1974NE1B; theor.).
Differential cross sections have been measured for E_{t} = 1.58 to 2.01 MeV: see (1974AJ01). See also (1975AB1C; theor.).
At E_{t} = 22 (1971ST05) and 23 MeV (1978AJ02), angular distributions of the protons to ^{6}He*(0, 1.80) have been measured. No other states are observed with E_{x} ≲ 4.2 MeV (1971ST05). See also (1974LA1A).
The excitation of ^{6}He*(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 ^{6}He*(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 E_{p} = 4.71, 5.24 and 6.77 MeV (1977RO01: p_{0}). At E_{n} = 14 MeV the p_{0} angular distribution is similar in shape to the angular distributions in ^{6}Li(p, p')^{6}Li*(3.56) and in ^{6}Li(p, n)^{6}Be_{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 ^{7}Li.
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 ^{6}He_{g.s.} to that for population of the analog state, ^{6}Li*(3.56), reached in the (t, t') reaction [see ^{6}Li] 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 ^{6}He*(0, 1.80) are populated with E_{x} ≲ 8.5 MeV (1971ST05).
See (1970BA41; E(^{3}He) = 53.2 MeV).
Angular distributions have been studied at E(^{6}Li) = 32 and 36 MeV for the reactions involving the ground states of ^{6}He and ^{6}Be and for the inelastic scattering of ^{6}Li to ^{6}Li*(3.56) [see reaction 18 in ^{6}Li]: these three states in ^{6}He, ^{6}Li and ^{6}Be are analog states. At 32 MeV the ratios of the differential cross sections for populating ^{6}Li*(3.56) to those for populating ^{6}He_{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).
At E_{γ} = 60 MeV, the proton spectrum shows two prominant peaks attributed to ^{6}He*(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 ^{6}He, in addition to ^{6}He*(0, 1.8). See also (1975MA1E).
See ^{7}Li.
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 ltransfer ≥ 2) at E_{x} = 23.7 MeV. DWBA analyses of the d_{0} and d_{1} groups lead to S(1p_{3/2}) = 0.62 for ^{6}He_{g.s.} and to S(1p_{3/2}) = 0.37 and S(1p_{1/2}) = 0.32 for ^{6}He*(1.8) (1977BR17). See (1974AJ01) for earlier measurements.
The summed proton spectrum at E_{p} = 100 MeV shows ^{6}He*(0, 1.8) and a broad group centered at E_{x} ≈ 14 MeV: the angular correlation for ^{6}He_{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).
On the basis of a very accurate measurement of the Qvalue of this reaction (1978RO01) find the atomic mass excess of ^{6}He = 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 ^{3}He ions to ^{6}He*(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 ^{6}He with E_{x} < 10.7 MeV (1971ST05). See also (1974DI1A, 1975DI1B) and reaction 28 in ^{6}Li.
The energy of the first excited state is 1.797 ± 0.025 MeV, γ = 113 ± 20 keV (1965AJ01). ^{6}He*(1.80) decays into ^{4}He + 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 α_{0} and α_{1} 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 ^{6}He*(0, 1.80): see (1974AJ01). No other αgroups are reported corresponding to ^{6}He 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 αparticles). See also (1975KO1B).
See (1970BA41: E(^{3}He) = 53.2 MeV). See also (1976WA12).
See (1968ST12).
Angular distributions have been measured at E_{n} = 12.2, 14.1 and 18.0 MeV (1976SM02: α_{0}, α_{1}), 13.99 MeV (1974PE06: α_{0}, α_{1}) and at 14.4 MeV (1967PA03: α_{0}). No other states are observed below E_{x} ≈ 7 MeV (1974PE06). See also ^{10}Be.
See (1970DE17).
At E_{t} = 21.5 and 23.5 MeV, angular distributions have been studied for the transitions to ^{6}He*(0, 1.80) and ^{6}Li*(0, 2.19, 3.56) (1973VO08, 1975VO08): see discussion in reaction 32 in ^{6}Li. See also (1974CA04, 1975BR1E; theor.).
See (1968ST12).
See ^{18}F in (1978AJ03).
