(See Energy Level Diagrams for 8He)
GENERAL: References to articles on general properties of 8He published since the previous review (1988AJ01) are grouped into categories and listed, along with brief descriptions of each item, in the General Tables for 8He located on our website at (nucldata.tunl.duke.edu/nucldata/General_Tables/8he.shtml).
The half-life of 8He is 119.0 ± 1.5 msec. The decay takes place (84 ± 1)% to 8Li*(0.98) [log ft = 4.20] and (16 ± 1)% via the neutron unstable states 8Li*(3.21, 5.4). A small decay branch (≈ 0.9%) populates 8Li*(9.67). (32 ± 3)% of the emitted neutrons then populate 7Li*(0.48). The decay to 8Li*(3.21, 5.4) suggests π = + for 8Li*(3.21) and 0+ or 1+ for 8Li*(5.4) (1981BJ03). Branching ratios for intermediate states are given in (1988BA67): see also reaction 11 in 8Li and Fig. 2. For discussion of 8He β-decay (1988BA67, 1991BO31, 1993BO24, 1996BA66, 1996GR16, 1997SH19). See also (1990ZH01, 1993CH06, 1994HA39).
Invariant mass spectroscopy was used to determine the 8He excitation spectra in a complete kinematics measurement of the 1H(8He, 8He + p) reaction at 72 MeV/A (1993KO34, 1995KO27). The ground state and an excited state at 3.55 ± 0.15 MeV were observed. The 3.55 MeV state has Jπ = 2+, Γ = 0.50 ± 0.35 MeV and Γ(α + 4n)/Γ(6He + 2n) ≤ 5% (1995KO27); possible evidence for a resonance at 5 - 6 MeV is seen.
The 1H(8He, 8He + p) scattering distribution at E(8He) = 674 MeV/A was analyzed using a Glauber scattering model and yields an 8He matter radius Rr.m.s. = 2.45 ± 0.07 fm (1997AL09). Elastic and inelastic scattering distributions from 1H(8He, 8He + p) at 72 MeV/A were evaluated in an eikonal approximation and indicate a matter radius Rr.m.s. = 2.52 fm and a deformation parameter β2 = 0.3 for the first 2+ excited state (1995CH19). A folding model analysis of the 8He first excited Jπ = 2+ state, using Ex = 3.57 MeV, indicates L = 2 and a deformation parameter β = 0.28 (2002GU02).
Evaluation of the four-momentum transfer distribution yields Rrms = 2.45 ± 0.07 fm at E(8He) = 800 MeVA (2002EG02) and Rrms = 2.53 ± 0.08 fm at E(8He) ≈ 700 MeVA (2002AL26). See also (2003LA22; E(8He) = 15.6 MeV/A), (2002WO08; E(8He) = 26 MeV/A),(1995KO10; E(8He) = 33 MeV/A), (1997KO06; E(8He) = 66 MeV/A), (1997KO12; E(8He) = 73.5 MeV/A), (1995NE04; E(8He) = 674 MeV/A), (2002EG02; E(8He) ≈ 700 MeV/A), and (1995BE26, 1995CR03, 1995GO32, 1998AN25, 2000GU19, 2000KA04, 2000WE03, 2001AV02, 2001SA79, 2003BA65; theor.).
The Generator-Coordinate Method was used to calculate 8He(α, α) scattering in an investigation of excited states in 12Be (2000BB06). A search for 4-neutron cluster contributions to the reaction was performed at E(8He) = 26 MeV/A, no evidence was observed (2003WO13).
The 2-neutron transfer reaction 1H(8He, t) was measured at E(8He) = 61.3 MeV/A. The results indicate a significant contribution of 6He*(1.8) in the 8He ground state (2003KO11); spectroscopic factors yield S(6Heg.s.)/S(6He*(1.8)) = 1.
Using Eπ- = 125 MeV, the 8He ground state was observed in the 9Be(π-, p) missing mass spectra; the measured 6He + 2n phase space appears to favor a di-neutron final state (1991SE06). The ground state and the 4.4 MeV state were observed in (1998GO30) following the capture of stopped π--mesons in 9Be(π-, p), Ex = 4.4 ± 0.2 MeV, Γ = 1.8 ± 0.2 MeV and in 11B(π-, p + d) Ex = 4.4 ± 0.4 MeV, Γ = 1.2 ± 0.2 MeV.
At E(7Li) = 83 MeV, θ = 10°, the population of 8Heg.s., an excited state at 2.8 ± 0.4 MeV (presumably Jπ = 2+) and a structure near Ex ≈ 7 MeV are reported by (1985AL29).
At E(9Be) ≈ 11 MeV/A, the ground state and three excited states are populated at Ex = 1.3 ± 0.3 MeV, Ex = 2.7 ± 0.3 MeV, Γ = 0.5 ± 0.3 MeV and Ex = 4.0 ± 0.3 MeV, Γ = 0.5 ± 0.3 MeV (1988BE34).
At E(12C) = 357 MeV, population of the ground state and 3.6 MeV state are reported. Excited states are also observed at Ex = 4.54 ± 0.15 MeV [Γ = 0.70 ± 0.25 MeV], 6.03 ± 0.10 MeV [Γ = 0.15 ± 0.15 MeV] and 7.16 ± 0.04 MeV [Γ = 0.10 ± 0.10 MeV] (1995ST29, 1999BO26). The narrow width of the 7.16 MeV state leads to a preliminary Jπ = (3-) assignment (1999BO26).
At E(11B) = 87 MeV the ground state of 8He is populated and excited states are reported at Ex = 1.3, 2.6 and 4.0 MeV ( ± 0.3 MeV). The width of the latter is 0.5 ± 0.3 MeV (1987BE2B). In (1988BE34) the ground state and a state at 2.7 ± 0.3 MeV with Γ = 1.0 ± 0.5 MeV are reported. See also (1988BEYJ).
A measurement to determine muon induced background rates in large-volume scintillation solar neutrino detectors found σ = 2.12 ± 1.46 μb for natC(μ, 8He or 9Li) at Eμ = 100 GeV (2000HA33).
At E(8He) = 227 MeV/A structures are seen in reaction (a) corresponding to sequential decay through the Jπ = 3/2- 7Heg.s. (Eres = 0.44 MeV, Γ = 0.16 MeV), and a suggested Jπ = 1/2- resonance at Eres = 1.2 ± 0.2 MeV with Γ = 1.0 ± 0.2 MeV (2001MA05). A reconstruction of the 6He + 2n reaction kinematics indicated that 8He*(2.9 ± 0.2 MeV, Γ = 0.3 ± 0.3 MeV (2+) and 4.15 ± 0.20 MeV, Γ = 1.6 ± 0.2 MeV (1-)) participate in the breakup. Cross sections for the one- and two-neutron knockout reactions (i.e., where one or none of the removed neutrons is observed) were determined as σ1n = 129 ± 15 mb and σ2n = 29 ± 23 mb. Contributions for various cluster configurations in 8He were estimated to be 45% 6He* + 2n (p3/2, p1/2), 33% 6He + 2n (p3/2) and 22% 6He + 2n (p1/2). See (1996NI02) for earlier work at E(8He) = 240 MeV by this group, where Ex = 3.72 ± 0.24 MeV and Γ = 0.53 ± 0.43 MeV, were reported for the first excited state, and where the total 2-neutron removal cross section was determined as σ2n = 0.27 ± 0.03 b.
Complete reaction kinematics were measured for reactions (b, c, d) in (8He, 6He + 2n) on Al, Sn and Pb targets at E(8He) 24 MeV/A (2000IW05). Observation of a peak in the 6He + n relative energy spectra indicates a substantial participation (40 - 60%) of sequential decay via 7He + n. A peak in the missing mass spectra corresponds to the first excited state of 8He, which is assumed to dominate in nuclear breakup since it cannot be excited by E1 Coulomb processes. By integrating the remaining excitation strength up to 3 MeV (assumed to be E1 Coulomb) B(E1) = 0.091 ± 0.026 e2 · fm2 was determined.
Measurements of 8He breakup on C and Pb are presented in (2002ME09); the results indicate that the 8He Coulomb dissociation cross section is 3 times smaller than the Coulomb dissociation cross section for 6He. The measurements of (2002ME09) also support Jπ = 1- for 8He*(4.15). The two-neutron- and four-neutron-removal cross sections were measured for reaction (e) at 800 MeV/A (1992TA18), and for reaction (f) at E(8He) = 20 - 60 MeV/A (1996WA27). The large neutron removal cross sections indicate a 8He matter radius of 2.49 ± 0.04 fm (1992TA18). Analysis indicates that 8He is well represented as four neutrons that are bound to a 4He core. See also (1994ZH14, 1995SU13, 2001CA50; theor.), and a review of nuclear radii deduced from interaction cross sections in (2001OZ04).
A double folding model was used to predict the influence of the 8He neutron skin on 14C(8He, 8He) elastic-scattering angular-dependent cross sections at 20, 30, 40, and 60 MeV (1988KN02).