(See Energy Level Diagrams for 8Be)
GENERAL: See (1956KU1A, 1957FR1B, 1958WI1E, 1959BA1F, 1959BA1D, 1959BR1E, 1959WI1B, 1960BI1E, 1960KU05, 1960PE11, 1960PH1A, 1960PH1C, 1960TA1C, 1961BA1E, 1961CL10, 1961VA17, 1962IN02, 1962IN1A, 1962IW1A, 1963BR1N, 1963BU1C, 1963DA1C, 1963FR1G, 1963KU03, 1963MA1E, 1963MO1H, 1963NA1E, 1963SH1G, 1964AM1D, 1964BA1Y, 1964BE1N, 1964BE1M, 1964BR1H, 1964DA1G, 1964DU1D, 1964GR1J, 1964MA1G, 1964VO1B, 1965BA2H, 1965BE1H, 1965MA2B, 1965MA1G, 1965NE1C, 1965TR1B, 1965YU1D).
The weighted mean of direct Q determinations to 1957 is Q = 94.1 ± 0.7 keV (1957VA11). Reported widths are 4.5 ± 3 eV (1956RU41), 6.8 ± 0.6 eV (1962BA1C), ≤ 3.5 eV (1956HE57), > 0.1 eV (1955TR03): see Table 8.8 (in PDF or PS). (1966BE05) find Q = 92.12 ± 0.05 keV, Γ = 6.8 ± 1.7 eV.
At Eα = 39 MeV, σ < 0.7 mb (1952WA31).
Reported differential cross section measurements are cited in Table 8.7 (in PDF or PS). The course of the derived phase shifts with increasing energy from Eα = 0.15 to 120 MeV is exhibited in (1956RU41, 1958NI05, 1960JO03, 1963TO02, 1965DA1A).
The s-wave phase shift, δ0, decreases smoothly from 180° at 0.3 MeV. The absence of measurable effects in the range Eα = 0.15 to 0.2 MeV yields an upper limit of 3.5 eV on the width of the ground state (1956HE57). Analysis of the 0 - 6 MeV data leads to Γ(g.s.) = 4.5 ± 3 eV (1956RU41), 6.8 ± 0.6 eV (1962BA1C): see Table 8.8 (in PDF or PS).
The d-wave phase shift becomes appreciable for Eα > 2.5 MeV and passes through resonance at Eα = 6 MeV (J = 2+, Ex = 3.18 MeV, Γ = 1.5 MeV). The g-wave shift rises from Eα ≈ 11 MeV and indicates a broad Jπ = 4+ level at Ex = 11.4 MeV. The l = 6 and l = 8 phase shifts become active above Eα ≈ 30 and 50 MeV respectively. Dispersion formula fits are somewhat unsatisfactory, but indicate Jπ = 6+ and 8+ levels at Ex ≈ 28 and ≈ 57 MeV respectively. Both the excitation energies and the reduced widths for all five levels are approximately proportional to J(J + 1) (1965DA1A).
Sharp oscillations in the excitation functions at 15° and 45° are observed corresponding to 8Be*(16.6) and (16.9). Since no oscillations were observed at 27.8°, these states are assigned Jπ = 2+ (1964SH19).
Optical model analysis of α - α scattering is discussed by (1960IG02, 1965DA1A); analysis in terms of two-nucleon forces is given by (1959VA1F, 1961SC1B). See also (1959BU07, 1959WI1F, 1960BI1E, 1961GO1T, 1961SH1F, 1962IG1B, 1962SH1F, 1963WI1H, 1964EN1C, 1965OK1B).
The yield curve has been measured for Ed = 0.06 to 5.5 MeV (1952BA64, 1954HI34, 1956NE13, 1957SL01). A broad s-wave resonance is indicated at Ed = 0.41 MeV, Γ = 0.45 MeV (1952BA64, 1956NE13). The forward cross section rises from ≈ 22 mb/sr at Ed = 1.1 MeV to ≈ 57 mb/sr at 5.5 MeV without sharp resonances (1957SL01). Above Ed = 0.6 MeV, angular distributions indicate a strong admixture of stripping process (1956NE13).
Comparison of the yields n0/n1 (7Be(0) and 7Be*(0.43)) and p0/p1 (7Li(0) and 7Li*(0.48)) over the energy range Ed = 0.4 to 3.2 MeV shows that the angular distributions are closely similar for n and p. The yield ratios are also closely equal over this range, consistent with the assumption of charge symmetry. The ratio n0/n1 increases rapidly as Ed falls below 0.8 MeV, suggesting a change in the reaction mechanism there (1957WI24, 1963BI27, 1963CR08).
Cross sections and angular distributions have been measured for Ed = 30 keV to 5.4 MeV ( 1959AJ76, 1963BI27, 1963ME09, 1964PA06). A broad maximum near Ed = 1.0 MeV is interpreted as indicating a level at Ed = 0.4 MeV (1950WH02). In the range Ed = 1 to 5 MeV there is evidence for both direct interaction and compound nucleus formation (1963BI27, 1963ME09, 1964PA06): at back angles the (d, p1) data show evidence of the Ed = 3.7 MeV resonance (see 6Li(d, α)4He). See also 6Li(d, n)7Be and (1964FE01).
Cross sections and angular distributions have been measured for Ed = 0.03 to 12 MeV (1948HE01, 1950WH02, 1962HA15, 1962JE02, 1963AN10, 1963ME09, 1964MA2D, 1964PA06, 1965MA13). Maxima are observed at Ed = 0.8 MeV, Γlab ≈ 0.8 MeV, and Ed = 3.75 MeV, Γlab ≈ 1.4 MeV. Analysis of the angular distributions in terms of two-level interference favors Jπ = 2+ for the upper level, and Jπ = 0+ or 2+ for the lower (1963ME09, 1964PA06). The upper level appears to have a large deuteron width, θ2d ≈ 0.2, and small θ2α ≈ 0.025 and θ2p < 0.005 (1964PA06). A more elaborate analysis appears to require that both levels, Ex = 22.54 and 25.23 MeV, have Jπ = 2+ and that a third broad Jπ = 0+ level exist at Ex = 24.02 MeV (1965FR02): see Table 8.10 (in PDF or PS). The same parameters give a good account of the data up to Ed = 9.5 MeV. There is no evidence of further resonances (1965MA13). See also (1959HA29, 1960HA14, 1963EL1D, 1963FR1G, 1964FE01).
At E(3He) = 1.2 to 2 MeV, proton groups are reported corresponding to 8Be(0), 8Be*(2.9) and possibly 8Be*(12.3) (1956MO19, 1956SC01). At E(3He) = 3.5 to 4.2 MeV, groups with Q = 0.163, -0.143 and -0.854 MeV (± 10 keV) are observed, corresponding to the 16.6, 16.9 and 17.6 MeV states (1961ER01): see Table 8.11 (in PDF or PS).
The differential cross sections for ground-state protons show backward peaking at E(3He) = 5, 6 and 7 MeV, forward peaking at E(3He) = 13 and 17 MeV, and both forwards and backward peaks in the intermediate region (1963MA02). Analysis of the angular distribution of protons leading to the Ex = 16.6 MeV level indicates stripping, with L = 0 and a large deuteron width (1963MO1K, 1964MO1L). See also (1963WE1B, 1965KA1F, 1965RO1R, 1965YO1D).
At Eα = 43 MeV, deuteron groups corresponding to 8Be(0) and 8Be*(2.9) are reported (1959ZE1A); at Eα = 48 MeV, an additional group ascribed to 8Be*(11.3 ± 0.4) is observed (1962CE01). Angular distributions of d0 and d1 show forward peaking, with that of d0 exhibiting pronounced oscillatory characteristics. Attempts to fit the observations with direct interaction theory are only moderately successful (1959ZE1A, 1962CE01). See also (1959ST30, 1962KO13, 1963BL20, 1963DE1G, 1963DE29, 1963WE1B, 1964DE1K).
In addition to α-groups to the ground and 2.9 MeV states, there is evidence for a cluster reaction mechanism in which 6Li combines with a deuteron cluster to produce a state of 8Be at 20.95 ± 0.3 MeV, Γ = 3.4 MeV. At E(6Li) = 1.9 MeV, the cross section is 1.3 mb, or 23 times that for ground-state alpha particles (1963KA20). On the other hand (1964QU01) find that the α-particle spectrum is consistent with the involvement of a 8Be level near 22 MeV, with Γ ≈ 0.4 MeV. See also (1962CO05). The 8Be state decays by proton and α emission: Γp/Γα = 1.64 (1962GA21). At E(6Li) = 3.2 MeV, alpha groups corresponding to 8Be* = 16.6, 16.9, (17.6), 18.15 MeV are reported (1964CA1G). See also (1960SH01, 1962CO21, 1963LE19, 1963TA1B, 1964GA1E, 1964SH05, 1965MA2A, 1965NO1A).
Cross sections and angular distributions have been reported from Ep = 30 keV to 14.5 MeV. Two γ-rays are observed, γ0 to the ground state and γ1 to the broad, 2+, excited state at 2.9 MeV: Eγ = (17.2, 14.3) + 7/8 Ep. Resonances for both γ0 and γ1 occur at Ep = 0.44 and 1.03 MeV, and for γ1 alone at 2 MeV (see, however, (1963PE15)): see Table 8.12 (in PDF or PS). In the range Ep = 2.5 to 9 MeV, broad resonances are reported at Ep ≈ 5 MeV (γ0), Γ ≈ 5 MeV (1959GE33, 1963MI08, 1963PE15 (Γ = 5.5 MeV)), at Ep ≈ 7.3 MeV (γ1), Γ ≈ 8 MeV, and possibly at Ep ≈ 6 MeV (γ1). The Ep ≈ 5 MeV resonance (Ex ≈ 21.6 MeV) is presumed to represent the giant dipole resonance based on 8Be(0), while the γ1 resonance, 2.0 ± 0.5 (1963MI08), 3 ± 0.5 MeV (1963RE09) higher is similarly related to 8Be*(2.9). Angular distributions from Ep = 2.0 to 7.5 MeV are approximately isotropic (except at 2.59 MeV), suggesting l = 0 capture (1963MI08, 1963PE15). The integrated (γ, p) cross section, determined by reciprocity is 22 ± 4 MeV · mb (1963RE09), 20 MeV · mb (1964TA05). See also (1965TA1E).
Angular distributions near the Ep = 0.44 MeV resonance show strong cos θ interference terms which vanish at resonance. At resonance the radiation is nearly isotropic, consistent with p-wave formation, Jπ = 1+, with channel spin ratio σ(Jc = 2)/σ(Jc = 1) = 5. A detailed study of the angular distributions of γ0 and γ1 yields a small cos2θ term for γ0: A2 = 0.067 ± 0.025 (1961ME10), A2 = 0.028 ± 0.003 (1958NE17, 1961BR02), while γ1 is isotropic: A2 = -0.004 ± 0.008 (1961ME10); see, however, (1958NE17, 1961BR02). Analysis of these data together with γ-α correlations leads to Γ(E2)/Γ(M1) for γ1 = 0.018 ± 0.009 (1961ME10), 0.044 ± 0.014 (1960GR21). With Γγ = 25 eV, one obtains Γ(γ0) = 16.7 eV, Γ(γ1, M1) = 8.15 ± 0.07 eV, Γ(γ1, E2) = 0.15 ± 0.07 eV. The observed E2 strength is about 15 times smaller than predicted by intermediate coupling shell model (1961ME10). See (1957KU58). The channel spin mixture σ(2)/σ(1) is 3.2 ± 0.5 (1961ME10), 4.1 ± 0.1 (1958NE17).
The relative intensity of γ0/γ1 at resonance is 2.0 (1961ME10), 2.3 ± 0.04 (1960MA23: θ = 90°), 1.8 ± 0.2 (1963BA58); the ratio falls rapidly above resonance and is approximately constant at 0.54 ± 0.08 for Ep = 0.8 to 1.0 MeV (1960MA23: see also (1961BR02)). There is evidence for an increase in the ratio at Ep = 1.03 MeV (1964SC19). At Ep = 5.4, 6.5, 7.5 and 8.6 MeV, the ratios are 0.5 ± 0.07, 0.3 ± 0.04, 0.2 ± 0.04 and 0.2 ± 0.04, respectively (1964TA05). Angular distributions in the range Ep = 0.2 to 1.1 MeV indicate interference with s-wave and d-wave non-resonant radiation (1960MA33: see also (1961BR02)); at Ep = 1.03 MeV the resonant radiation is isotropic, suggesting p-wave, J = 1+, σ(2)/σ(1) = 5 (1960MA33).
Angular distributions from Ep = 0.8 to 1.7 MeV appear to require interference of the Ep = 0.44 and 1.03 (1+) MeV levels with at least two odd-parity levels, possibly those at Ep = 2.1 and 5.8 MeV (1964SC19).
A pair-spectrometer measurement yields Eγ = 17.647 ± 0.015 MeV for γ0 at the Ep = 440 keV resonance: from the spectrum of γ1 an excitation energy of Ex = 2.6 ± 0.15, Γ = 0.7 ± 0.1 MeV is obtained for the first excited state (1963BA58). See Table 8.9 (in PDF or PS).
At Ep = 440 keV, α-particles from 8Be*(16.63) are observed, indicating the existence of the γ-transition 8Be*(17.64 → 16.63). The total resonant cross section is 6.45 ± 0.7 μb (1965WIZZ). The coincidence spectrum at resonance shows the 1.02 MeV γ-ray (17.64 → 16.63), and a weak branch (5.9 ± 2)% (1965WI07), (7.5 ± 2)% (1965KO05), to the 16.92 MeV state. There is no indication of resonance in the yield of α-particles from 8Be*(16.6) at Ep = 1.03 MeV (1965WI07). Evidence from this and other reactions involving 8Be*(16.6) and (16.9) indicates that neither has a pure isospin character (1965KO05, 1965MA1G). See also (1964PA13).
A careful study of γ - α coincidence spectra at Ep = 441 keV reveals only the 2.9 MeV level in the range Ex = 2 to 7 MeV; the α-spectra can be matched with a single-level dispersion formula with Eλ = 5.95 MeV, γ2α = 11.9 MeV · fm, θ2 = (1.7), R = 4.48 fm (1960GE07). A report of other γ - α groups by (1962CA13) is ascribed to contamination (1964MA25, 1964MI10, 1964PR04, 1964WE1C).
This reaction is widely used as a source of monochromatic neutrons: see summary by (1960GI1A). The threshold is Ep = 1880.36 ± 0.22 keV (1963MA1R: see 7Be); a second threshold, for neutrons leading to 7Be*(0.43), occurs at Ep = 2376 ± 2 keV (1960MA1G), and above Ep = 7 MeV, neutrons corresponding to 7Be*(4.55) may be produced (1963BO06).
Reaction cross sections and angular distributions have been reported by (1948TA16: to 2.55 MeV), (1964BU08: 2.4 to 3.0 MeV; n1), (1957NE22, 1958MA07: to 5.5 MeV), (1959GA08: to 3.25 MeV), (1961BE05: 2.6 to 4.2 MeV; n0 and n1 groups separately), (1961NI04: 2.7 to 3.5 MeV), (1963BO06: 3 to 13 MeV; n0, n1, n2), (1964BA16: 4 to 14 MeV) and by (1960HI04: 8 to 14 MeV; n0, n1, n2). See also (1959AJ81, 1960GI1A, 1962AU01, 1963PA1K). The yield of ground state neutrons (n0) rises steeply from threshold and shows pronounced resonances at Ep = 2.25 (σ ≈ 0.57 b) and 4.9 MeV (σ ≈ 0.36 b) (1963BO06). The yield of n1 also rises steeply from threshold (1964BU08) and exhibits a broad maximum near Ep = 3.5 MeV (1961BE05).
The behavior of the cross section near the n0 threshold indicates a broad resonance with Jπ = 2-, T = (0), (l = 0) at Ep = 1.9 MeV with γ2n/γ2p ≈ 5.0 (Table 8.13 (in PDF or PS)). The structure at Ep = 2.25 MeV is ascribed to a 3+, T = (1), l = 1 resonance with Γn ≈ Γp and γ2n/γ2p = 3 to 10 (1957NE22, 1958MA07, 1959WE1A, 1960GI1A, 1961BE05). The broad peak at 4.9 MeV can be fitted by Jπ = 3(+) with Γ = 1.1 MeV, γ2n ≈ γ2p (1963BO06). An additional resonance at Ep = 3.0 MeV may be indicated (1957NE22, 1961BE05). The behavior of the n1 cross section can be fitted in terms of a single level with Jπ = 1- at Ep = 3.55 MeV (1964BU08).
Polarization of the neutrons has been measured by (1961DA04, 1962EL01: 2.0 to 2.4 MeV), (1959CR84: 3.5 to 5 MeV), (1961AU02: 2 to 3 MeV), (1965MO1L: 3.1 to 3.3 MeV), (1965AN09: 3.0 to 4.0 MeV), (1959BA34, 1960BA27: 3 to 6 MeV), (1963MI01, 1963MI06, 1963MI20, 1964MI14: 4 to 5 MeV), (1962BE11: 4 to 10 MeV) and others (1959AJ76, 1963HA1G). Dominant effects are ascribed to the narrow 3+ level at Ep = 2.25 MeV and a broad 2- background level. At least one level is needed near threshold, and at the higher energies (Ep > 2.6 MeV) two additional levels are indicated (1961AU02).
Absolute differential cross sections are reported for Ep = 0.4 to 12 MeV (1953WA27, 1956MA12, 1965GL03) and for Ep = 14.5, 20.0 and 31.5 MeV by (1956KI54). Anomalies appear at Ep = 0.44, 1.03, 1.88, 2.1, 2.5, 4.2 and 6.0 MeV (see Table 8.14 (in PDF or PS)). Both the 0.44 and 1.03 MeV resonances are ascribed to p-waves, J = 1+, with channel spins 1 and 2 in a ratio of 1 to 5 (1953CH1A, 1955LI1B: compare 7Li(p, γ)8Be). The structure at Ep = 2.1 MeV may indicate a state with Jπ ≤ 3+ (1956MA12), 3- (1957NE22). The Ep = 4.2 MeV resonance is ascribed to an almost pure single-particle level in 8Be at 20.9 MeV (1965GL03).
A pronounced resonance appears in the yield of inelastically scattered protons (1951BR10, 1954MO04) and 0.48 MeV γ-rays (1954KR06) at Ep = 1.03 MeV (see Table 8.14 (in PDF or PS)); it is an s- or p-wave resonance interfering with a non-resonant wave of opposite parity (1954MO04: see also (1955LI1B)). Excitation functions for the proton group to the 0.48 MeV state have been measured for Ep = 2.3 to 12 MeV: a peak is observed at 5.6 MeV (1965GL03). See also (1964FA08). The yield of 0.48 MeV γ-rays rises smoothly from Ep = 1.5 to 3.0 MeV except for a pronounced cusp at 1.881 MeV (1955HA34, 1957NE22). See also (1962CA13).
Excitation functions and angular distributions have been reported by (1962LU01: 0.4 to 3.3 MeV), (1948HE01: 0.5 to 3.8 MeV), (1958CO62: 0.1 to 0.6 MeV), (1963SA10: 3.0 to 5.5 MeV), (1962CA12: 1.5 to 4.8 MeV), (1960AL18: 12 MeV), (1961HA27: 2.8 to 12 MeV), (1962TE04, 1962TE07: 3.3 to 6.6 MeV), (1964MA51: 4 to 12 MeV), (1962MA40: 15, 18.6 MeV) and (1962CA13, 1963CA1K: 0.5 to 2.5 MeV; see, however, (1964MA25, 1964MI10)). Polarization effects have been studied by (1964AN08: 0.5 to 2.0 MeV), (1962BE14: 1 to 3.2 MeV), (1964AS04: 2 to 3.5 MeV), (1965BO07: 3.2 to 5.3 MeV).
A broad resonance occurs at Ep = 3.0 MeV, Γ ≈ 1 MeV, σmax ≈ 90 mb (1948HE01): see Table 8.15 (in PDF or PS). A second prominent peak appears at Ep = 5.6 MeV, Γ ≈ 1 MeV (1961HA27, 1962TE04, 1964MA51). Some structure is reported near Ep = 6.0 to 6.5 MeV, and a further peak occurs at Ep = 9.0 MeV (1964MA51).
An earlier suggestion that a J = 0 resonance near Ep ≈ 0 is involved appears to be contradicted by polarization data (1962BE14). The states at Ex = 19.9 MeV and 22.1 MeV (Ep = 3.0 and 5.5 MeV resonances) appear to have J = 2+ (1959AJ76, 1961HA27, 1962TE04, 1962TE07, 1965FR02) and to have 6Li + d parentage (1962TE01, 1965FR02). Levels at Ex = 23 MeV (Ep = 6.5 MeV) and 25.2 MeV (Ep = 9.0 MeV) are assigned Jπ = 4+ and 2+, respectively (G. Temmer, private communication).
For Ed = 3.6 to 7.3 MeV, neutron time-of-flight spectra indicate states in 8Be at 0, 2.9, 16.64 ± 0.015, 16.9 ± 0.05, 17.64 and 18.15 MeV. Angular distribution of neutrons corresponding to the 16.6, 17.6 and 18.15 MeV states are strongly forward; ln = 1 for the 16.6 MeV state, consistent with J = 2+ (1960DI02). At Ed = 2 MeV, recoil proton spectra show only the ground state and Ex = 2.9 MeV groups; in the range Ex < 9 MeV, no others appear with intensity > 10% of the ground-state group. The spectrum yields Ex = 3.1 ± 0.1, Γ = 1.75 ± 0.1 MeV; comparison with the shape calculated from known α - α phase shifts suggest a considerable contribution of three-body processes (1964JO04). Reported slow neutron thresholds are listed in Table 8.16 (in PDF or PS). See also (1964JO1D, 1964JO1F, 1965DI1F).
Alpha-particle spectra indicate groups corresponding to the break up of 8Be*(16.6) and (16.9) (1963BI22, 1963PA04, 1964GL03). Alpha particles attributed to 8Be*(11.4) are also reported (1963PA04). In the range Ex = 15.0 to 18.5 MeV, no α-emitting states other than the 16.6 and 16.9 MeV levels are observed (1964GL03). See also (1965BI1F, 1965JO19, 1965MA1G, 1965MA1K).
A large number of additional states with Ex < 16.1 MeV has been suggested in addition to 8Be*(0, 2.9, 11.4): for a listing of early references, see (1959AJ76, 1964JO04). See also (1958CA1E, 1960AG03, 1960JU04, 1963MI1M, 1964MI1J).
At E(3He) = 24.3 MeV, angular distributions of deuterons corresponding to the ground and first excited states of 8Be have been measured (1963WE1B). At E(3He) = 9 to 10 MeV, excitation of 8Be*(17.6) is observed (1965GR08). See also (1964DU1E).
The angular distributions of the tritons to the ground state of 8Be have been determined at a number of energies up to 48 MeV; the distributions indicate strong contribution of direct interaction (1959ST1B, 1960GO04, 1960MA15, 1960VL03, 1962MA59, 1963WE1B). At Eα = 40 MeV, higher states are also observed (1960VL03).
At thermal energies, the (n, p) cross section is (5.1 ± 0.6) × 104 b (1955HA34), the (n, α) cross section is ≤ 0.1 mb (1962BA1B, 1963BA34) and the (n, γα) cross section is 155 mb (1963BA34). These observations are consistent with odd parity of 7Be; the small value of σ(n, α) leads to an estimated F2 < 4 × 10-10 for the intensity of a positive parity admixture (1963BA34). Comparison of the thermal cross section for reaction (a) with the (p, n) cross section observed in the inverse reaction supports the assignment J = 3/2 for 7Be(0) (1957NE22). See also (1959AJ76).
For Ed = 0.8 to 1.7 MeV, proton groups are observed corresponding to the ground state and the 2.9 MeV level: Γ(2.9) = 1.6 ± 0.4 (1959SP1A), 1.53 ± 0.04 MeV (1960KA17). A dispersion formula fit yields parameters Ex = 2.90 ± 0.06 MeV, γ2 = 0.60 MeV, θ2 = 0.64, R = 5.75 fm (1960KA17): see Table 8.9 (in PDF or PS).
8Li decays mainly to the broad 2.9 MeV, 2+ level of 8Be, which decays into two α-particles. The β-spectrum, which extends to ≳ 14 MeV, has been studied by (1950HO01), (1960FA02) and others, while the α-spectrum, extending to ≈ 10 MeV, has been reported by (1938RU01, 1948BO20, 1955FR29, 1960BI10, 1960BI06, 1960FA04, 1961DE1E) and others: see (1955AJ61). The two spectra correspond well, except perhaps for low β-energies (1960GR10). For Eα ≲ 5 MeV, the spectrum is closely matched by a density-of-states derived from the experimental d-wave phase shift of α - α scattering; at higher energies, an increasing excess of α-particles appears which may reflect transition into the tail of the Jπ = 2+ level at Ex = 16.67 MeV (1960GR10, 1963AL19, 1963DA05). See also 8B(β+).
Studies of the distribution of recoil momenta and neutrino-recoil correlation indicate that the decay is at least 90% GT, and that the GT portion is at least 90% axial vector. The observations indicate J = 2+ for 8Li (1958BA1E, 1958LA08, 1959LA11, 1959LA05).
The angular correlations W(θβα) have been measured for the decays of 8Li and 8B as a test of the conserved vector current theory of β-decay. The observed correlation is of the form W(θ) = 1 + Acos θ + Bcos2θ; reported values of A and B are listed in Table 8.17 (in PDF or PS). With shell model calculations of the matrix element Γγ(M1) for 8Be*(T = 1) → 8Be*(2.9 MeV), the CVC theory predicts δ ≡ B(8Li) - B(8B) = (7 ± 2) × 10-3 Wβ (1960KU05, 1960WE1A); conversely, if CVC is assumed correct, the experiment yields Γ(M1) = 1.9 ± 0.6 eV (1962NO02). See also (1963IS04).
The decay proceeds mainly to 8Be*(2.9). The α and β spectra correspond closely to those of 8Li, when account is taken of the different Q-values (1960FA04). The ratio ft(8B)/ft(8Li) = 1.096 (1964TO1D). Detailed study of the high energy portion of the α-spectrum reveals a maximum near Eα = 8.3 MeV, corresponding to transitions to 8Be*(16.62). The observed shape is reproduced with a Breit-Wigner density-of-states function with parameters Ex = 16.67 MeV, Γ = 150 to 190 keV; parameters Ex = 16.62, Γ = 95 keV are less good, but acceptable. With the former set, log ft = 2.9 (using τ1/2 = 0.774 ± 0.005 sec); with the latter, log ft = 3.03. In either case, the low ft-value supports the identification T = 1, J = 2+ for 8Be*(16.63) (1964MA35). See, however, (1965MA1G).
For reaction (a), see 9Be. The (n, 2n) reaction appears to proceed largely via excited states of 9Be, with subsequent decay to 8Be, mainly 8Be*(2.9) (1959CH1E, 1961MY01, 1963JE05, 1964BO31): see 9Be and 10Be. For reaction (c), see (1962ST12) and 9Be. See also (1959WI41, 1960KO1G, 1961JE01, 1962CU05, 1965LO1K).
At Ep = 5.2 MeV, the α-spectrum shows a sharp ground-state group α0, and a broad group, α1, corresponding to 8Be*(2.9). A pronounced anomaly appears at Ex ≈ 0.75 MeV, on the side of the α1 group, which is apparently a "ghost" of α0. A quantitative account is given with a density-of-states function derived from α - α scattering phase shifts (1961BE1E: see also (1962BA1C, 1962HA23, 1963AL19)).
At Ep = 95 to 155 MeV, 8Be states at 0, 2.9, 11.4, 16.6, 18.8 and (23) MeV are excited (1956SE1A, 1963BA1R, 1963RA01). See also (1959AJ76, 1959FI1B, 1960PH1B, 1964SH07, 1964TO1D, 1964YA1A), 9Be and 10B.
Angular distributions of ground-state tritons have been measured at a number of energies up to 20 MeV: see (1959AJ76) and (1959VL24, 1960NE09, 1960NE11, 1961RE03, 1962BI11, 1965JA07). The width of the 2.9 MeV state is 0.8 MeV (1955CU16), 1.35 ± 0.15 MeV (1959VL24).
At E(3He) = 3.0 and 4.0 MeV, angular distributions of the α-particles to the ground-state of 8Be and to the levels at 2.9, 16.6, 16.9 and 17.6 MeV have been measured. The excitation energy and width of the first excited state are found to be 2.90 ± 0.04 MeV and 1.35 ± 0.15 MeV (1963DO08): see Table 8.9 (in PDF or PS). The parameters of the higher states (from (1961ER01) and (1963DO08)) are given in Table 8.11 (in PDF or PS). The angular distributions are amenable to analysis by direct interaction except in the case of the 16.6 MeV state which appears to involve compound nucleus formation (1963DO08) (compare 7Li(d, n)8Be). For angular distribution data at lower energies, see (1955AJ61) and (1963WE08).
The α-particles corresponding to the 2.9 MeV excited state are superposed on a broad, intense continuum extending to Ex > 16 MeV. Some part of this spectrum may be ascribed to a broad level at Ex = 12.5 MeV, with Γ ≈ 5 MeV, but the main contribution appears to be due to a direct 3α breakup (1963DO11: see also (1963WE08)). (1964MO19) find, on the other hand, that the sequential decay, via 8Be*(2.9) and (16.93) dominates the reaction. See, however, (1965MO1M). Angular correlation studies confirm Jπ = 2+ for 8Be*(16.9) (1964MO19, 1965MO1N).
Alpha groups are reported corresponding to 8Be states at 0, 2.9 (1960BE15, 1960BI11, 1961CI02, 1961LE10, 1963PU02, 1964YA1A; and see (1959AJ76)), 16.6 and 16.8 MeV (1961ER01: see Table 8.11 (in PDF or PS)). A coincidence study of the α1 group yields the following parameters for the level shape: Eres = 3.15 MeV, Γ = 2.04 MeV, Eλ = 4.00 MeV, γ2 = 3.4 MeV · fm, θ2 = 0.60, R = 5.5 fm (1963PU02: E relative to E(2α)). (1953TR04) reports Eλ = 5.29 MeV (relative to 2α), γ2 = 13.4 MeV · fm, θ2 ≈ 2, R = 4.48 fm: see Table 8.9 (in PDF or PS).
Arguments are presented for the assignments J = 2+, T = 1 for Ex = 16.63 and Jπ = 0+ or 2+, T = 0 for Ex = 16.93 MeV. Comparison of yields with 6Li(3He, p)8Be* indicates gross violation of the isospin selection rule, possibly to be ascribed to intermixture in the compound nucleus stage (1961ER01). See, however, (1965MA1G). There is no evidence for an earlier reported level at Ex = 16.08 MeV (1961ER01). See also (1961TE02, 1962TE02, 1965LE09).
Alpha groups are reported corresponding to the ground and 2.9 MeV state: see (1959AJ76) and (1962AL20). At Ep = 3 to 5 MeV, the α-spectrum shows an anomaly at Ex ≈ 0.75 MeV, ascribed to successive two-body decays with a final density-of-states related to α - α scattering phase shifts (1961BE1E: see 9Be(p, d)8Be and (1963BR03, 1964BR05, 1964DE1H)). See also 12C and (1959KA12, 1959KA13, 1961KO08, 1962BE21, 1963PH1A, 1964BA1C, 1964YA1A, 1965DU1C, 1965LE09, 1965PH1A, 1965SW1B).
At E(3He) = 3.0 MeV, 8Be*(2.9) is produced with an intensity 20 times that of the ground state (1964YO06).
Reaction (c) at Ep = 15 to 29 MeV proceeds predominantly through the ground state and the 2.9 MeV level. It is not clear whether higher levels of 8Be are involved (1955NE18, 1960VA10, 1962VA1A, 1963VA04). See also (1963JA07, 1964BA1C, 1964KE1F, 1964SY02, 1965YU1C, 1965YU1D).
See also 12C.
This reaction has not been observed: see (1962LA15).