
^{6}He (1984AJ01)(See Energy Level Diagrams for ^{6}He) GENERAL: See also (1979AJ01) and Table 6.1 [Table of Energy Levels] (in PDF or PS). Model Calculations: (1979SHZV, 1980FI1D, 1981KU13, 1982FI13, 1982KR1B, 1982LE11, 1982VO01). Special states: (1982FI13, 1983DE16, 1983KR05, 1983LE01). Electromagnetic transitions: (1982AW02). Complex reactions involving ^{6}He: (1978DU1B, 1978VO1A, 1979BO22, 1979VI05, 1980BO31, 1980WI1L, 1981BO1X, 1981CU05, 1981VO10, 1982BO1Q, 1982BO35, 1982BO40, 1982GU1H, 1982HE1D, 1983JA1C). Muon and neutrino capture and reactions: (1978SE1B, 1979DE1D, 1979WA1D, 1980BR1A, 1980MU1B). Reactions involving pion and other mesons: (1978FU09, 1979BA16, 1979BA1M, 1979DO1C, 1979PE1C, 1979SH1D, 1979WI1A, 1980BE20, 1981BA1M, 1981DU1H, 1981NI03, 1981SH06, 1982BO11). Hypernuclei: (1978PO1A, 1978SO1A, 1980VE2B, 1981BA2P, 1981WA1J, 1982BA1Y, 1982BA2N, 1982BA2R, 1982KO1L). Other topics: (1978FI12, 1979SHZV, 1979VA1A, 1981PAZZ, 1982AW02, 1982MO1Q, 1982NG01, 1982ZO1B, 1983BA2F, 1983BA3A, 1983DE16, 1983NA03). Ground state of ^{6}He: (1978SM02, 1981AV02, 1981PAZZ, 1982FI13, 1982KR1B, 1982LE11, 1982NG01, 1982VO01, 1983LE01).
The decay proceeds to the ground state of ^{6}Li [J^{π} = 1^{+}] and is a superallowed GamowTeller transition. Recent halflife measurements are 808.1 ± 2.0 msec (1974WI14), 798.1 ± 1.0 msec (1981BA58), 805.4 ± 2.0 msec (1982AL17): see Table 6.2 (in PDF or PS) in (1966LA04) for a listing of earlier determinations. We adopt the mean of the (1974WI14, 1982AL17) values, τ_{1/2} = 806.7 ± 1.5 msec; log ft = 2.910 ± 0.002. See also (1979AJ01, 1979DO1A) and (1977SA1A, 1978SE1B, 1979DE15, 1981PAZZ; theor.).
The cross section for neutron production (reactions (a) and (b)) rises monotonically from 40 keV to 2.2 MeV. The zeroenergy cross section factor S_{0} ≈ 300 keV · b. The cross section for reaction (b) increases monotonically for E_{t} = 34 to 160 keV. (1981JA1F) suggests that the (t, 2n) cross sections below E_{t} = 300 keV are poorly known and may be subject to large systematic errors. For the earlier work on these reactions, and for references, see (1974AJ01, 1979AJ01). See also (1979OH1B, 1981HA1P), (1982SL1A) and (1979HA1C, 1980HA1Y, 1981GE1C; theor.).
Angular distributions of the protons to ^{6}He*(0, 1.80) have been measured at E_{t} = 22 and 23 MeV. [No Lvalues were assigned.] No other states are observed with E_{x} ≲ 4.2 MeV: see (1979AJ01).
See (1978GL03, 1982GL01: E_{α} = 61.5 → 158.2 MeV). See also ^{8}Be.
From measurements at E_{e} = 170, 180 and 195 MeV, angular distributions of (γ, π^{+}) to ^{6}He*(0, 1.80) are derived by (1979SH1D, 1981SH06).
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. See also (1979AJ01). The chargeexchange process with stopped pions (reaction (b)) has been studied by (1981BA16).
Angular distributions of the p_{0} group are reported at E_{n} = 4.7 to 6.8 MeV and at 14 MeV [see (1979AJ01)] and at E_{n} = 59.6 MeV (1982BR04). There is no evidence for the excitation of ^{6}He*(1.8) (1982BR04). See also (1980MI02) and ^{7}Li.
At E_{d} = 55 MeV the population of ^{6}He*(0, 1.8) [the latter weak] is observed: no other states are observed with E_{x} ≲ 25 MeV (1979ST15).
The groundstate angular distribution has been studied at E_{t} = 17 MeV. At E_{t} = 22 MeV only ^{6}He*(0, 1.8) are populated with E_{x} ≲ 8.5 MeV. See (1979AJ01) for additional discussion and for references. See also (1983AB1A).
Angular distributions have been studied at E(^{6}Li) = 32 amd 36 MeV for the transitions to ^{6}He_{g.s.}, ^{6}Be_{g.s.} and, in inelastic scattering of ^{6}Li [see ^{6}Li], to the analog state ^{6}Li*(3.56): for a discussion of these see the references quoted in (1979AJ01).
At E(^{7}Li) = 78 MeV, ^{6}He*(0, 1.8) are populated (1982AL1G).
At E_{γ} = 60 MeV, the proton spectrum shows two prominent peaks attributed to ^{6}He*(0 + 1.8, 18 ± 3): see (1979AJ01). For reaction (b) see (1980AS02: E_{e} = 108, 163 and 198 MeV). See also ^{7}Li.
At E_{n} = 60 MeV, the deuteron spectrum shows two prominent peaks attributed to states centered at E_{x} = 13.6, 15.4 and 17.7 MeV (± 0.5 MeV) and a possible state or states (populated with a l_{p} transfer ≥ 2) at E_{x} = 23.7 MeV. DWBA analyses of the d_{0} and d_{1} groups are consistent with l_{p} = 1 and S(1p_{3/2}) = 0.62 for ^{6}He_{g.s.} and to S(1p_{3/2}) = 0.37, 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: see (1979AJ01).
Angular distributions of the ^{3}He ions to ^{6}He*(0, 1.8) have been measured at E_{d} = 14.4 and 22 MeV: they have a l_{p} = 1 character and therefore these two states have J^{π} = (0  3)^{+}. There is no evidence for any other states of ^{6}He with E_{x} < 10.7 MeV: see (1979AJ01).
The energy of the first excited state is 1.797 ± 0.025 MeV, Γ = 113 ± 20 keV. ^{6}He*(1.80) decays into ^{4}He + 2n. The branching ratio Γ_{γ}/Γ_{α} ≤ 2 × 10^{6}: for Γ_{c.m.} = 113 ± 20 keV, Γ_{γ} ≤ 0.23 eV. Angular distributions of the α_{0} and α_{1} groups have been measured at E_{t} = 13 and 22 MeV. No other αgroups are reported corresponding to ^{6}He states with E_{x} < 24 MeV (region between E_{x} ≈ 13 and 16 MeV was obscured by the presence of breakup αparticles): see (1979AJ01). See also ^{10}Be.
At E(^{3}He) = 120 MeV a kinematically complete experiment has been carried out by (1980VO1B): in addition to ^{6}He_{g.s.} a structure corresponding to E_{x} = 17 MeV, Γ < 5 MeV is reported.
See (1981BA58).
Angular distributions have been reported for E_{n} = 12.2 to 18.0 MeV (α_{0}, α_{1}). No other states are observed with E_{x} ≲ 7 MeV: see (1979AJ01). See also ^{10}Be.
At E_{t} = 21.5 and 23.5 MeV, angular distributions have been studied for the transitions to ^{6}He*(0, 1.8) and ^{6}Li*(0, 2.19, 3.56): see (1979AJ01). See also reaction 38 in ^{6}Li and (1983WE02; theor.).
