(See Energy Level Diagrams for 8Li)
GENERAL: References to articles on general properties of 8Li published since the previous review (1988AJ01) are grouped into categories and listed, along with brief descriptions of each item, in the General Tables for 8Li located on our website at (www.tunl.duke.edu/nucldata/General_Tables/8li.shtml).
Ground State Properties:
8Li atomic transitions: Atomic excitations in the lithium isotopes were analyzed in (2000YA05) where a theoretical framework was developed that correlates the atomic decay energies in neutral Li ions with the nuclear sizes.
The β- decay is mainly to the broad 2+ first-excited state of 8Be, which then breaks up into 2α [see reaction 24 in 8Be]. The weighted average of the 8Li half-life is 839.9 ± 0.9 ms based on measured values of 838 ± 6 ms (1971WI05), 836 ± 3 ms (1979MI1E) and 840.3 ± 0.9 ms (1990SA16). The log ft ≥ 5.6, using τ1/2 = 839.9 ms, Q = 16.0052 MeV and branching ratio ≤ 100%; other values in the literature that account for the decay to the broad Γ ≈ 1.5 MeV 8Be*(3.0) state are log ft = 5.37 (1986WA01) and log ft = 5.72 (1989BA31).
The quadrupole moment of 8Li was deduced by measuring the asymmetry in β-NMR spectra. We adopt Q(8Li) = +32.7 ± 0.6 mb, which results from a new method, modified β-NMR (NNQR), that is 100 times more sensitive than previous methods (1993MI34). This value is larger than 28.7 ± 0.7 mb (1988AR17) and the previous adopted value 24 ± 2 mb (1988AJ01). The sign of the 8Li quadrupole moment was measured and is positive (1994JA05).
The tilted foil technique was used to polarize atomic 8Li, and the hyperfine interaction led to a nuclear polarization of 1.2 ± 0.3% which was deduced from the measured β-decay asymmetry (1987NO04). The polarization quantum beat in the hyperfine interaction was measured by varying the foil separation distances (1993MO33, 1996NO11). See also (1987AR22) for discussion of hyperfine structure splitting in lithium isotopes.
The pure Gamow-Teller (ΔT = 1) β-decay of 8Li to the 8Be*(3.0) level has been measured in a search for time-reversal violation (1990SR03, 1992AL01, 1996SR02, 2003HU06); the present constraint for the time violating parameter is R = (0.9 ± 2.2) × 10-3. See also (1992DE07, 1995YI01, 1998KA51). Searches for second-class currents in 8Li β-decay have yielded negative results: see (1988HA21, 1989TE04, 2003SM02). For an analysis of the anti-neutrino energy distribution shape in 8Li β-decay, see (1987LY05, 2002BH03). For a comment on the usefulness of β-decay asymmetries to reveal information on spin dynamics in nuclear reactions involving polarized projectiles see (2001DZ02). A suggestion to use 8Li β-decay for calibration of the SNO detector is described in (1998JO09, 2002TA22). β-NMR is used to measure the 8Li quadrupole-coupling constants in Mg and Zn (1993OH11). For condensed matter applications of 8Li β-decay see (1993BU29, 1993NO08, 1994HO23, 1996EB01).See also (1993CH06, 1993MO28, 2003SU04).
Angular distributions have been obtained at Et = 23 MeV for the proton groups to 8Li*(0, 0.98, 2.26, 6.54 ± 0.03); Γcm for 8Li*(2.26, 6.54) are 35 ± 10 and 35 ± 15 keV, respectively. J for the latter is ≥ 4: see (1979AJ01). A multi-cluster model is used to calculate excitation function and γ-ray flux from 6Li(t, p1)8Li*(0.981), which is proposed as a diagnostic tool for fusion reactions (2000VO22, 2001VO02).
At En = 1.5 - 1340 eV agreement was found with the expected 1/v (velocity) energy dependence, and a thermal cross section of 40 ± 2(stat.) ± 4(syst.) mb was measured (1996BL10). (1998HE35) measured σave. = 101.9 mb for an energy bin for En = 1.7 - 20 meV, and σave. = 36.6 μb for En = 5 - 150 keV. A reanalysis of the ion chamber efficiencies used by (1989WI16) led to a revised cross section σ(En = 25 keV) = 57 ± 9 μb and Γγ = 0.18 eV (1998HE35). Measurements by (1991LY01), who analyzed σ(E) from Ethermal to 3.0 MeV, determined σthermal = 45.4 ± 3.0 mb and the γ-ray branching ratios at En = thermal (see 8.4 (in PDF or PS)). At En = 30 keV, (1991NA16, 1991NA19) measured σγ0 = 35.4 ± 6.0 μb and σγ1 < 9.1 μb. The excitation function shows the resonance corresponding to 8Li*(2.26): Eres = 254 ± 3 keV, Γn = 31 ± 7 keV, Γγ = 0.07 ± 0.03 eV: see 8.5 (in PDF or PS) and (1974AJ01). Theoretical models are discussed in (1988DE38, 1993KR18, 1994DE03, 1996SH02, 1997BA04, 1999BE25, 2000BE21, 2000CS01, 2001KO54). The decay of 8Li*(2.26) → 7Lig.s. + n in the interaction of 35 MeV/A 14N ions on Ag is reported by (1987BL13).
The thermal cross section is 0.97 ± 0.04 b [see (1981MUZQ)], σfree = 1.07 ± 0.03 b (1983KO17). The real coherent scattering length is -2.22 ± 0.01 fm. The complex scattering lengths are b+ = -4.15 ± 0.06 fm and b- = 1.00 ± 0.08 fm (1983KO17); see also (1979GL12). See (1984AJ01) for earlier references.
Total and elastic cross sections have been reported for En = 5 eV to 49.6 MeV: see (1979AJ01, 1984AJ01, 1988AJ01). Cross sections have also been reported for n0, n0+1 and n2 at En = 6.82, 8.90 and 9.80 MeV. (1987SC08; n2 at the two higher energies).
A pronounced resonance is observed at En = 254 keV with Jπ = 3+, formed by p-waves: see 8.5 (in PDF or PS). A good account of the polarization is given by the assumption of levels at En = 0.25 and 3.4 MeV, with Jπ = 3+ and 2-, together with a broad Jπ = 3- level at higher energy. Broad peaks are reported at En = 4.6 and 5.8 MeV (± 0.1 MeV) [8Li*(6.1, 7.1)] with Γ ≈ 1.0 and 0.4 MeV, respectively, and there is indication of a narrow peak at En = 5.1 MeV [8Li*(6.5)] with Γ ≪ 80 keV and of a weak, broad peak at En = 3.7 MeV: see (1974AJ01, 1984AJ01, 1988AJ01). A multi-level, multi-channel R-matrix calculation is reported by (1987KN04). This analysis leads to predictions for the cross section for elastic scattering, for (n, n') to 7Li*(0.48, 4.68, 6.68) and for triton production. A number of additional (broad) states of 8Li, unobserved directly in this and in other reactions, derive from this analysis (1987KN04). See (1989FU03) for a resonating group study of 8Li*(6.53) [Jπ = 4+; T = 1]; see also (2002GR25). See also references cited in (1988AJ01).
The excitation function for 0.48 MeV γ-rays shows an abrupt rise from threshold (indicating s-wave formation and emission) and a broad maximum (Γ ≈ 1 MeV) at En = 1.35 MeV. A good fit is obtained with either Jπ = 1- or 1+ (2+ not excluded), Γlab = 1.14 MeV. A prominent peak is observed at En = 3.8 MeV (Γlab = 0.75 MeV) and there is some indication of a broad resonance (Γlab = 1.30 MeV) at En = 5.0 MeV. At higher energies there is evidence for structure at En = 6.8 and 8 MeV followed by a decrease in the cross section to 20 MeV: see (1979AJ01, 1984AJ01). The total cross section for (n0 + n1) and n2 have been reported at En = 8.9 MeV (1984FE1A). For R-matrix analyses see (1987KN04) in reaction 5 and (1984AJ01).
The cross section for reaction (b) rises from threshold to ≈ 360 mb at En ≈ 6 MeV and then decreases slowly to ≈ 250 mb at En ≈ 16 MeV: see (1985SW01, 1987QA01). Cross sections for tritium production have been reported from threshold to En = 16 MeV (1983LI1C), 4.57 to 14.1 MeV (1985SW01), 7.9 to 10.5 MeV (1987QA01), 14.74 MeV (1984SMZX) and at 14.94 MeV (1985GO18: 302 ± 18 mb). At En = 14.95 MeV the total α production cross section [which includes the (n, 2n d) process] is 336 ± 16 mb (1986KN06). Spectra at 14.6 MeV may indicate the involvement of states of 4H (1986MI11). See also references cited in (1988AJ01).
Angular distributions and analyzing powers for the transitions to 8Li*(0, 0.98, 2.26) have been studied at Ep = 200.4 MeV. [The (p, π-) reaction to the analog states in 8B is discussed: see reaction 4 in 8B.] The (p, π+) cross sections are an order of magnitude greater than the (p, π-) cross sections and show a much stronger angular dependence (1987CA06). Angular distributions of cross section and Ay have also been measured at Ep = 250, 354 and 489 MeV to the first three states of 8Li. Those to 8Li*(0, 2.26) have differential cross sections which exhibit a maximum near the invariant mass of the Δ(1232) and Ay which are similar to each other and to those of the p-bar p → dπ+ reaction. 8Li*(6.53) is populated (1987HU12, 1988HU11).
Measurements in the vicinity of the Ecm = 0.61 MeV 9Be*(17.3) resonance found σ[7Li(d, p)] = 143.6 ± 8.9 mb (1996ST18), σ[7Li(d, 8Li)p: 8Li β- / → 8Be → 2α] = 151 ± 20 mb (1996ST18), and σ[7Li(d, p)] = 155 ± 8 mb (1998WE05). An extensive review in (1998AD12) presented the results found in 8.6 (in PDF or PS). However, (1998WE05) suggest that systematic errors may persist in the (1998AD12) evaluation.
Angular distributions of the p0 and p1 groups [ln = 1] at Ed = 12 MeV have been analyzed using DWBA: Sexpt. = 0.87 and 0.48 respectively for 8Li*(0, 0.98). Angular distributions have also been measured at several energies in the range of Ed = 0.49 to 3.44 MeV (p0) and 0.95 to 2.94 MeV (p1). The lifetime of 8Li*(0.98), determined from 2H(7Li, p)8Li via the Doppler-shift attenuation method, is 10.1 ± 4.5 fsec: see (1979AJ01). See also references cited in (1988AJ01).
The 7Li(d, p)8Li β- / → 8Be → 2α reaction was studied in the range of 0.4 - 1.8 MeV to investigate a mechanism where the 8Li reaction products are backscattered out of the target which introduces up to a 20% systematic error in measurements of the reaction yield (1998ST20). They determined that 8Li reaction products are increasingly backscattered out of the target with: (i) increasing the Z of the backing material, (ii) decreasing the thickness of the deposited Li/Be target, and (iii) decreasing the incident projectile energy.
See reaction 1 in 8He.
The triton spectrum observed in 8He β-decay was analyzed in a single-level R-matrix model that indicated the triton emission branching ratio is (8.0 ± 0.5) × 10-3 (1991BO31, 1993BO24). The R-matrix fit indicates a level at 8Li*(9.3 ± 1.0 MeV, Jπ = 1+) with a reduced width γreduced = 0.978 ± 0.012 MeV1/2 that decays primarily by triton emission; this corresponds to B(GT) = 5.18 and log ft = 2.87 [B(GT) = 8.29, using the definition given in the introduction]. A subsequent analysis of the (1993BO24) data used a multi-level, multi-channel R-matrix model that included low-lying 1+ states in 8Li that participate in 8He β-decay (see 8.7 (in PDF or PS)) and suggests Ex = 9.67 MeV, B(GT) = 4.75 and log ft = 2.91 (1996BA66) [B(GT) = 7.56, using the definition given in the introduction]. Branching ratios for 8Li states are given in (1988BA67). See also Fig. 2.
The 9Be(γ, p0) reaction was measured in the range from Eγ = 22 - 25.5 MeV and was evaluated in a simple cluster model (1999SH05). The analysis indicated that mainly E1 and E2 multipolarities contribute to the breakup cross section. The photodisintegration of 9Be was measured at Eγ = 180 - 240 MeV, and the (γ + nucleon) reaction dynamics were studied by measuring 9Be(γ, p) at Eγ = 187 - 427 MeV in the Δ(1232) resonance region (1988TE04).
The total cross section for 9Be(γ, pπ0) was measured with bremsstrahlung γ-rays in the range of Eγ = 200 - 850 MeV (1987AN14).
For reaction (a) see (1984AJ01) and (1985KI1A). The summed proton spectrum (reaction (b)) at Ep = 156 MeV shows peaks corresponding to 8Lig.s. and 8Li*(0.98 + 2.26) [unresolved]. In addition, s-states [Jπ = 1-, 2-] are suggested at Ex = 9 and 16 MeV, with Γcm ≈ 6 and 8 MeV; the latter may actually be due to continuum protons: see (1974AJ01). At Ep = 1 GeV the separation energy between 5 and 8 MeV broad 1p3/2 and 1s1/2 groups is reported to be 10.7 ± 0.5 MeV (1985BE30, 1985DO16). See also (1987GAZM).
For reaction (b) angular distributions were measured at 70 MeV. The data were evaluated using the distorted wave T-matrix approximation (DWTA) where it was determined that the 1s and 1p shells dominate in the nucleus-nucleon single-particle-knockout reaction mechanism (2000SH01).
Angular distributions have been reported for the 3He ions to 8Li*(0, 0.98, 2.26, 6.53) at Ed = 28 MeV [C2S (abs.) = 1.63, 0.61, 0.48, 0.092] and 52 MeV. The distributions to 8Li*(6.53)[Γ < 100 keV] are featureless: see (1979AJ01).
At Et = 12.98 MeV, angular distributions of the α-particles to 8Li*(0, 0.98, 2.26, 6.53 ± 0.02 [Γcm < 40 keV]) have been measured: see (1974AJ01). Angular dependent differential cross sections for 9Be(t, α) at Et = 15 MeV were compared with DWBA and coupled-channel Born approximation calculations to extract the relative and absolute C2S factors for 8Li + p: see 8.8 (in PDF or PS) (1988LI27). At Et = 17 MeV, σ(θ) and Ay measurements, analyzed by CCBA, lead to Jπ = 4+ for 8Li*(6.53): see (1984AJ01). For 8Li*(0.98), τm = 14 ± 5 fsec, Ex = 980.80 ± 0.10 keV: see (1974AJ01).
At Ep = 45 MeV, 3He ions are observed to a state at Ex = 10.8222 ± 0.0055 MeV (Γcm < 12 keV): the angular distributions for the transition to this state, and to its analog (8Be*(27.49)), measured in the analog reaction [10Be(p, t)8Be] are very similar. They are both consistent with L = 0 using a DWBA (LZR) analysis: see (1979AJ01).
The 11B(π+, 3p) reaction was studied at 50, 100, 140 and 180 MeV using a large solid angle detector to measure the missing energy spectra (1992RA11).
The excitation function for 11B(n, α)8Li was measured at En = 7.6 - 12.6 MeV to determine, via detailed balance, the astrophysical rate for the 8Li(α, n) reaction in the vicinity of the 12B*(10.58) level (1990PA22).
Angular distributions of the α0 and α1 groups have been measured at En = 14.1 and 14.4 MeV: see (1974AJ01, 1984AJ01, 1988AJ01). Energy dependent 8Li(α, α) elastic scattering phase shifts, which are important for calculating the 11B(n, α)8Li reaction rate, were calculated in the range of Ecm < 4 MeV (1996DE02).
At E(7Li) = 34 MeV angular distributions have been studied involving 8Li*(0, 0.98) and 10Bg.s. (1987CO16).
The π+ absorption reaction mechanism was studied by measuring protons produced in 12C + π+ reactions at 30 - 135 MeV (2000GI07).
Nuclear effects in the spallation reaction mechanism (i.e., even-odd and odd-odd nucleon pairing) were studied via 12,13C(p, 6,7,8,9Li) reactions at 1 GeV (1992BE65).
Angular distributions were measured at E(7Li) = 9 MeV/A, and a DWBA analysis was used to determine the ratio of p1/2/p3/2 contributions, and the Asymptotic Normalization Constant (ANC) for 7Li + n → 8Li (2003TR04). Then, using charge symmetry, the 7Be + p → 8B ANC was deduced, which corresponds to S17(0) = 17.6 ± 1.7 eV · b.
Elastic and inelastic scattering of 8Li on natC were measured at E(8Li) = 13.8 - 14 MeV (1991SM02). Optical model parameters were deduced for the 2+ ground state and the first 1+ excited state at ≈ 1 MeV and B(E2)(↑) = 30 ± 15 e2 · fm4 was deduced. In addition natAu(8Li, 8Li) was measured for comparison with Rutherford scattering.
The 8Li first 1+ excited state at 1.0 ± 0.1 MeV was observed in Coulomb excitation on natNi at E(8Li) = 14.6 MeV (1991BR14) and B(E2)(↑) = 55 ± 15 e2 · fm4 was determined for this excitation. See (2003BE38) for elastic and inelastic scattering on Pb at E(8Li) = 20 - 36 MeV.
A measurement to determine muon induced background rates in large-volume scintillation solar neutrino detectors found σ = 2.93 ± 0.80 μb and 4.02 ± 1.46 μb for natC(μ, 8Li) at Eμ = 100 and 190 GeV, respectively (2000HA33).
Total cross sections and charge-changing cross sections for the lithium isotopes on C and Pb were measured at 80 MeV/A (1992BL10); it was deduced that post-abrasion evaporation plays a minor role in these reactions. For reaction (b) the energy-dependent total reaction cross sections at 20 - 60 MeV/A were measured (1996WA27) and compared with microscopic and shell model predictions. A review of nuclear radii deduced from interaction cross sections is given in (2001OZ04).
Population of the 8Li ground state and 2.255 MeV neutron unbound state was reported in reactions (a) and (b) at 35 MeV/A. The reaction nuclear temperature was estimated (1987BL13). In a similar study of 35 MeV/A 14N on 165Ho, (1987KI05) deduced that the 8Li*(2.255) state has Γ = 33 keV from the 7Li + n relative energy spectrum.