(See the Energy Level Diagram for 8Be)
GENERAL: See also Table 8.3 [Table of Energy Levels] (in PDF or PS).
Theory: See (1955HE1E, 1956KU1A, 1956PE1A, 1957BI1C, 1957FR1B, 1958WI1E).
Recent Q-values are 93.7 ± 0.9 keV (1957CO59: 9Be(p, d)8Be), 90 ± 5 keV (1955TR03: 11B(p, α)8Be): the weighted mean of all measurements is 94.1 ± 0.7 keV (1957VA11). The width of the ground state is 4.5 ± 3 eV (1956RU41: 15% of Wigner limit), ≤ 3.5 eV (1956HE57). The second value leads to τm ≥ 2 × 10-16 sec. (Combination of these values places the mean life in the range τm = 2 to 4.5 × 10-16 sec.) An upper limit to the mean life is 6 × 10-15 sec (1955TR03). See also (1955AJ61).
Absolute differential cross sections are reported for Eα = 0.15 to 3.0 MeV (1956HE57), Eα = 3.0 to 5.9 MeV (1956RU41), Eα = 12.9 to 21.6 MeV (1953ST52), Eα = 12.3 to 22.9 MeV (1956NI20), Eα = 20 and 20.4 MeV (1951BR92, 1951MA1B), Eα = 30 MeV (1951GR45, 1952GR1A), Eα = 38.5 MeV (1957BU13), and Eα = 44.7 MeV (1957CO63). See also (1958CH35).
Phase shifts summarizing the work of (1956HE57), (1956RU41) and (1956NI20) are presented in (1956RU41) and (1958NI05). These three sets of data appear to join smoothly, but do not appear to fit well with the data of (1953ST52). For Eα < 3 MeV, only the s-wave phase shift is important. A careful survey in the range 146 - 202 keV reveals no effect of the ground state and places an upper limit of Γ ≤ 3.5 eV on this state (1956HE57): see Table 8.5 (in PDF or PS). Analysis of the 0 to 6 MeV data by effective range theory leads to a value Γ = 4.5 ± 3 eV for the ground-state width; θ2 ≈ 0.15 of the Wigner limit (with R = 5.7 × 10-13 cm). Some evidence of shape-dependence is found in this analysis (1956RU41). According to (1958NI05) a good account of the s-wave phase shift below 6 MeV is given by hard-sphere scattering plus resonance scattering from the ground state with a width θ2 = 0.75 (R = 4.44 × 10-13 cm). There is no indication of other S-states below Ex = 11.5 MeV (1958NI05).
The d-wave phase shift first becomes appreciable near Eα = 2.5 MeV (1956HE57) and appears to pass through resonance at 6.0 MeV (1956RU41). The g-wave shift rises continuously from Eα = 11 to 23 MeV; a broad 8Be level is indicated at Ex = 11.7 MeV (1956NI20, 1958NI05). The course of the phase shifts appears to be consistent with a simple two-body interaction with an attractive potential near R ≈ 5 × 10-13 cm and a repulsive core at a smaller radius; some dependence of the well shape on l is required (1951HA1B, 1956HE1B, 1956RU41, 1958NI05, 1958VA1B). A detailed comparison with the model of (1951HA1B) is made by (1958NI05). At 38.5 MeV, the s and d phase shifts are large, while the δ4, δ6 and δ8 phase shifts are small. These results are consistent with 0+ and 2+ states near 19 MeV (see 7Li(p, α)4He) and are not inconsistent with a 4+ state at ≈ 11 MeV (1957BU13). See also (1955AJ61), (1956HA1C, 1958MC1C; theor.) and (1954SN1A).
Not observed: see (1953SA1A, 1954SI07).
The excitation 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). At this energy the neutron yield to the 0.43-MeV state of 7Be is isotropic, while at Ed = 600 keV and above, the angular distributions indicate a strong admixture of stripping process (1956NE13). A sharp resonance at Ed = 2.12 MeV is reported by (1952BA64). However, (1957SL01) find that 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. This is confirmed by (1954BU1B) who reports no appreciable change in slope at Ed ≈ 1.8 MeV and suggests that the increase in neutron yield observed by (1952BA64) might have been due to oxygen contamination.
The ratio of 430-keV γ-radiation from this reaction and 477-keV γ-radiation from the mirror reaction, 6Li(d, p)7Li, has been measured for Ed = 0.2 to 1.8 MeV. This ratio, which measures Γn/Γp, rises from 1.1 to about 1.13 at Ed = 0.45 MeV, falling to 0.98 at Ed = 1.8 MeV. The theoretical ratio, assuming charge symmetry, rises from 0.96 at low energy to 0.98 at Ed = 1.8 MeV. It is concluded that the predictions of charge independence are borne out within 15%, and that the slight deviation observed may be connected with the resonance near Ed = 0.45 MeV (1957WI24). See also (1954HI34).
Cross sections and angular distributions have been measured for Ed = 30 keV to 3 MeV by (1950KR1A, 1953SA1A: see (1950WH02, 1954NI10, 1957JA37)). A broad maximum near Ed = 1.0 MeV is interpreted by (1950WH02) as indicating a level at Ed = 0.4 MeV, Γ ≈ 0.5 MeV. The angular distributions at Ed > 1 MeV indicate stripping effects, with ln = 1 (1954NI10). See also the discussion of the work of (1957WI24) in the preceding section. See also (1955AJ61).
The cross section for tritium production rises rapidly to 190 mb at 1 MeV, then more slowly to 290 mb near 4 MeV. There is evidence of deviation from isotropy near 0.4 MeV (1955MA20). See also 5Li.
Cross sections have been measured for Ed = 30 keV to 1.6 MeV (1953SA1A: see (1950WH02, 1954HI34, 1957JA37)). A broad maximum is observed at Ed = 0.6 MeV which is interpreted in terms of a resonance at Ed = 0.35 MeV, Γ ≈ 0.5 MeV (1950WH02: see, however, (1954HI34)). See also (1956PO1A), (1956SA1B; theor.) and (1952AJ38).
At E(3He) = 1.25 MeV, proton groups are observed to the ground state, the 2.9-MeV state and possibly to a state at ≈ 12.3 MeV (Γ ≈ 2 MeV, intensity ≈ 6% of 2.9-MeV transition). It is suggested that the 12.3-MeV state may be that observed in 4He(α, α)4He. No other states are observed with Ex ≲ 14 MeV. The upper limits on the intensities of groups leading to such states are 1% (of 2.9-MeV transition) for sharp states and 3% for levels 1 MeV wide (1956MO19). These results are confirmed by (1956SC01) at E(3He) = 1.5 and 2 MeV: no group of width ≲ 1 MeV appears for Ex < 14 MeV with an intensity as much as 2% of the 8Be*(2.9) group. See also (1953KU24, 1955AL57).
This reaction has been observed at Eα = 31.5 MeV (1956WA29).
The cross section has been studied from Ep = 30 keV (1957JA37) to 7.7 MeV. Resonances are observed at Ep = 0.44, 1.03 and 2.1 MeV: see Table 8.6 (in PDF or PS). There is no further structure up to 5 MeV (1952BA1B). The radiation comprises two components, one from the ground-state transition, with Eγ = 17.2 + 7/8 Ep and the other from the transition to the 2.9-MeV broad excited state, with Eγ ≈ 14.3 + 7/8 Ep. Both are resonant at the 0.44-MeV resonance, but only the lower energy transition shows the 2.1-MeV resonance (1957NE22). The intensity ratio of the higher to the lower-energy radiation increases from 0.5 at Ep = 0.2 MeV to 1.7 at Ep = 0.44 MeV and falls to 1.0 at Ep = 0.6 MeV (1956CA1A: θ = 90°). Between Ep = 1.0 and 3.5 MeV the ratio is constant within 30% at 2/3 (1955WI1D, 1957NE22). Evidence for a component at ≈ 12 MeV is discussed by (1953TI1C: see also (1955CA19)). A broad resonance appears near Ep = 5.8 MeV probably corresponding to the "giant" (γ, p) resonance, Ex = 22.3 MeV (1959GE33).
The angular distributions of both γ-rays show small deviations from isotropy at the Ep = 0.44 MeV resonance and exhibit strong interference effects nearby. The observed distribution of the 17.6-MeV radiation at resonance is consistent with p-wave formation if the channel spin ratio χ ≡ σ(1)/σ(2) = 1/4: this ratio implies an intermediate coupling with a/K = 2 to 3 (1958NE17: see also (1950DE1A, 1957FR1B)). Angular distributions in the range Ep = 0.9 to 1.2 MeV are reported by (1954KR06) an 0°/90° cross sections for Ep = 1.5 to 3.5 MeV by (1957NE22). The latter observations indicate that some process other than direct s-wave capture is responsible for the background between resonances (1954WI1A, 1957NE22).
A study of (γ - α) coincidences at Ep = 0.45 MeV yields an angular correlation which rules out the assignment J = 0+ to the 2.9-MeV 8Be level and indicates that the 14.7-MeV γ-radiation contains a mixture of E2 and M1 radiation (1956BO1H: see also (1954DE1D)). The alpha spectra, taken singly (1956LA1A) and in coincidence with γ-rays (1958ME78) show no evidence of weakly excited levels reported by (1954IN1A: see also (1955TI1B)). In the work of (1958ME78), the 2.9-MeV level appears to have a width of 1.9 MeV; compare (1950BU1B). There is some evidence for a broad level near Ex = 10 MeV (1956LA1A).
See also (1955RI1A, 1956PO1A) and (1957FR1B, 1957KU58; theor.).
The cross section has been studied from the threshold at Ep = 1.8811 MeV (see 7Be) to 10 MeV (1957BO1F, 1957JA37, 1957KA1C: see also (1958TA03)). Resonances are indicated at Ep = 1.93, 2.25, (3.0) and 5.0 MeV (see Table 8.7 (in PDF or PS)). Absolute cross sections are given by (1948TA16) and (1958MA07): see also (1959MA20: footnote 14). Angular distributions in the range Ep = 2.0 to 2.5 MeV show strong (cos θ) terms, suggesting interference of states of opposite parity (1948BR1A, 1948TA16). (1954AD1A) has shown that the behavior in this region can be accounted for by ascribing the 2.25-MeV resonance to p-wave formation, with J = 3+ and γ2p = γ2n = 0.8 × 10-13 MeV-cm (presumably the T = 1 analogue of 8Li*(2.28)), and assuming a background due to s-wave, J = 1- and 2-, levels of undetermined location. According to (1957NE22), a better account of the cross section below 2.4 MeV is obtained with the J = 3+ level assumed to have γ2n/γ2p = 5.5 and a single s-wave J = 2- level at Ep = 1.9 MeV, with γ2n/γ2p = 5.5. With this large ratio the low energy cross section can be accounted for in detail and can be made to agree with that derived from the inverse reaction 7Be(n, p)7Li (1957NE22, 1958MA07: see also (1955HA34)). [It is of interest to note that a similar apparent deviation from charge independence occurs in 10B(α, n)13N and 10B(α, p)13C (see 14N). On the other hand, see 6Li(d, p)7Li and 6Li(d, n)7Be.] Using the stacked-foil method, (1957KA1C) report structure in the excitation function corresponding to 8Be levels at 21.5, 22.5, 23.85, (24.9) and (25.6) MeV. At Ep = 10 MeV, the cross section for production of 7Be is 120 ± 20 mb (1957KA1C), 100 ± 20 mb (1957BO1F).
The relative intensity of the low-energy neutrons (to 7Be*(0.43)) to the high-energy (ground state) neutrons varies with energy: see Table 8.8 (in PDF or PS). In the range Ep = 2.5 to 2.9 MeV, the low-energy neutrons are practically isotropic (c.m. system). From the shape of the excitation function, (1955BA1L) conclude that the reaction to 7Be* proceeds by s-wave protons in and s-wave neutrons out.
It is pointed out by (1954AD1A) that the existence of the J = 3+ level, apparently well separated from the other components J = 1+ and 2+ which can be formed with channel spin 2, indicates a strong spin-orbit interaction, which should lead to polarization of the neutrons and scattered protons. Polarization measurements are reported by (1954AD1A, 1954WI42, 1955OK01, 1956WI1E, 1958CL98, 1958CR85, 1958ST28). See also (1957RO1C, 1958GI15).
Absolute differential scattering cross sections are reported for Ep = 0.4 to 1.4 MeV (1953WA27), Ep = 1.4 to 3.0 MeV (1956MA12), and Ep = 14.5, 20.0 and 31.5 MeV (1956KI54). Anomalies appear at Ep = 0.44, 1.03, 1.88, 2.1 and 2.5 MeV (see Table 8.7 (in PDF or PS)). Both the 0.44- and the 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, 1953LI1A, 1955LI1B: compare 7Li(p, γ)8Be).
The anomaly at Ep = 1.88 MeV coincides with the 7Li(p, n)7Be threshold and is ascribed to the abrupt change in total width of a broad (2- ?) resonance when neutron emission becomes possible (1956MA12, 1957NE22). The observed structure at 2.0 - 2.25 MeV may reflect interference of the p-wave 2.25-MeV (J = 3+) resonance with one at Ep = 2.1 MeV, also formed by p-waves (1956MA12). Preliminary results of a phase shift analysis suggest, on the other hand, interference between a J = 3- level at Ep = 2.1 MeV with a 2- level at Ep = 1.9 MeV, and interference of the 3+ level at Ep = 2.25 MeV with a broad 1+ level near 3 MeV (J. Olness quoted in (1957NE22)).
A pronounced resonance appears in the yield of inelastically scattered protons (1951BR10, 1954MO04) and 0.48-MeV γ-rays (1954KR06) at Ep = 1.030 ± 0.005 MeV, Γ = 168 keV. The angular distribution of the protons is approximately isotropic at resonance, σ = 42 mb, and asymmetric above it, consistent with an s- or p-wave resonance interfering with a non-resonant wave of opposite parity (1954MO04: see also (1955LI1B)).
The yield of 480-keV radiation rises smoothly from Ep = 1.5 to 3.0 MeV except for a pronounced cusp at 1.881 MeV (1955HA34, 1957NE22). Analysis of the excitation function suggests that the inelastic process is enhanced by the J = 2- level at 1930 keV and that the cusp results from the sudden increase in the total width when neutron emission becomes possible (1957NE22). See also (1951BA79).
The cross section, which has been measured to 3.8 MeV, exhibits a broad maximum at Ep = 3 MeV which is interpreted in terms of a level ≈ 1 MeV wide, with J = 2+, at Ep ≈ 3 MeV, and a several-MeV broad level of J = 0+, underlying the region: see (1948HE01, 1948HE1B, 1948IN1A, 1953SA1A, 1957JA37). Absolute differential cross sections are reported by (1958FR03) for Ep = 1.0 to 1.5 MeV: at 1.01 MeV, dσ/dω (lab, 90°) = 0.67 mb/sr, see also (1956MA12). Differential cross sections have also been measured at Ep = 15.0 and 18.5 MeV; there are indications of a triton pickup process at these energies (1957MA1F). See also 4He(α, p)7Li, (1955AJ61, 1955RI1A, 1956BA1E, 1956CR47, 1958BU38).
A careful study of the neutron spectrum at Ed = 2 MeV at several angles reveals only two distinct groups, corresponding to 8Be(0) and 8Be*(2.9). No other levels below Ex = 10 MeV appear in this work: upper limits for groups leading to levels near Ex = 4 to 5 MeV and Ex = 7.5 MeV are 10% and 20%, respectively, of the ground state group. Angular distributions of the ground state and 3-MeV state neutrons exhibit lp = 1 stripping patterns at forward angles (1954TR1A, 1955TR1B). Other workers have reported neutron groups corresponding to levels at 2.2, 2.9, 4.1, 5.1 and 7.6 MeV: see for instance, (1953TR1B, 1954RE1A, 1955BE1D, 1955GI1B, 1955IH1A, 1955IH1B), as well as states at 10 MeV (1941RI1A), 11.1 and 14.7 MeV (1950WH1B).
Thresholds for slow neutron production indicate 8Be levels at 16.08, 16.67, 17.61 and 18.20 MeV (1954BO79, 1957SL01) (see Table 8.9 (in PDF or PS)). It is suggested that the 16.67-MeV level is the lowest T = 1 level of 8Be, and that the levels at 17.61 and 18.20 MeV correspond to those seen in (7Li + p) at Ep = 0.44 and 1.03 MeV (1954BO79). A search for nuclear pairs from possible pair-emitting states of 8Be yielded an upper limit of 2 × 10-5 mb at Ed = 0.33 MeV for excitation of such states in the range Ex = 5.0 to 8.5 MeV (1955BE62). See also (1955CA1A, 1955CA1C, 1955PE1C, 1956BO1F, 1956BO43, 1956CA1B, 1956RI37, 1957CA14).
See (1954MO92) and (1955AL57).
At thermal energies, the (n, p) cross section is (5.1 ± 0.6) × 104 b (1955HA34) while the (n, α) cross section is < 25 mb (1958SE08). These observations are consistent with the odd parity of 7Be. Less than 10% of transitions involve 7Li*(0.48). Comparison of the (n, p) cross section with the cross section for 7Li(p, n)7Be gives evidence for an l = 0 level in 8Be within 20 keV below the neutron threshold, with Γ < 30 keV (1955HA34). See, however, (1957NE22, 1958MA07), and see also (1954AD1A). Comparison of the thermal cross section with the (p, n) cross section observed in the inverse reaction supports the assignment J = 3/2 for 7Beg.s. (1957NE22).
At Ed = 0.85 MeV, θ = 30°, 90° and 270°, proton groups are observed corresponding to 8Beg.s. and the broad level, Ex = 2.95 MeV, Γ = 1.6 ± 0.4 MeV. No other prominent groups appear for Ex < 5.8 MeV (1958SP1A).
The observed β-spectrum closely matches the mean of reported α-spectra (1955FR29) for Eβ > 3 MeV and is consistent with 89% branching via 8Be*(2.9), with log ft = 5.67 and 11% to higher states, possibly 8Be*(11.7), log ft = 4.6. Less than 1% of transitions involve 8Beg.s.: log ft > 8 (1958VE20: see also (1955AJ61)). Upper limits to transitions to sharp states in 8Be with Ex = 2, 4 and 6 MeV are, respectively, 2.5, 1 and 0.5% (1955FR29: see also (1956AR21)).
The α - β angular correlation is isotropic within a few per cent for all β-energies: see (1955AJ61). It is pointed out by (1955MO1A) that a small, ≈ 0.5%, anisotropy may be expected at high β-energies because of the increased importance of l = 1 and 2 emission, even in an allowed transition. Anisotropy in the β-decay from partially oriented 8Li nuclei is reported by (1957BU44: see also (1958SH1A)). The distribution of recoil momenta and the neutrino-recoil correlation establish that the decay is at least 90% Gamow-Teller and that the Gamow-Teller portion is at least 90% axial vector in character. The observations also require J = 2+ for the 8Li ground state (1958BA1E, 1958LA07, 1958LA08: see also (1958MO1D)). An upper limit of 0.2 ± 0.1 % is reported by (1956TA07) on the number of disintegrations leading to 4.9-MeV γ-radiation: see also (1953BU35). See also (1955GI1A) and (1955JA1C, 1955LA1D; theor.).
The observed positron spectrum matches the 8Li - 8B α-spectra for Eβ+ > 6 MeV. About 80% of transitions involve 8Be*(2.9), log ft = 5.72, with ≈ 19% to higher states, possibly 8Be*(11.7), log ft = 4.6. Less than 5% go to 8Beg.s., log ft > 7.3 (1958VE20). See (1950AL57, 1952KI1A, 1954GI1A). See also (1955GI1A).
At Eγ = 6 MeV, most of the transitions are to the 2.9-MeV state (1954CA1A). See also 9Be and (1956CO56). For reaction (b), see 9Be and 10Be.
Angular distributions of ground-state deuterons are remarkably similar for Ep = 5, 10, 16.5 and 22 MeV and show strong contributions from the pickup process (1951HA1A, 1953CO1C, 1955SU1A, 1956RA32, 1956RE04, 1956SU1A, 1958SU14). The significance of this result, which is not consistent with the simple Butler theory, is discussed by (1955DA1D, 1956GL25: see also (1955DA1E, 1955SA1D, 1956DA1D, 1956KO1B, 1957GR1C)). At Ep = 16.5 MeV, the distribution is consistent with ln = 1, R0 = 3.0 × 10-13 cm, and θ2 = 0.024 for 8Be(0) + n (1956RE04). At higher energies, the distributions appear to be affected by pickup from within the nuclear volume (1956BE14, 1956SE1A). For Ep = 31 and 95 MeV, unresolved 8Be states near 17 MeV appear, possibly representing pickup of a 1s neutron in 9Be (1956BE14, 1956SE1A: see 9Be).
At Ep = 7.4 MeV, a search for 8Be levels revealed only the ground-state and the 2.9-MeV state in the range Ex = 0 to 6.5 MeV (1956CA1C). See also (1954FI35, 1955GI1A, 1956ST30, 1957BE49) and (1955LA1C; theor.).
At Ed ≈ 1.2 and 3.5 MeV, the ground and first excited states are observed: see (1952CU1A, 1953CU1B, 1953GE01, 1956GE1A, 1956JU1D). For the first excited state, (1955CU16) finds Ex = 2.8 ± 0.1 MeV, Γ = 0.8 MeV. At Ed = 0.5 MeV (θ = 60° and 90°), there is no evidence for excited states of 8Be with Ex = 3.4 to 4.8 MeV: the upper limit to the intensity of the corresponding groups is 2% of the 2.9-MeV group (1956GE1A). At Ed = 14.8 MeV (θ = 15°), there is no evidence for states with Ex = 7.1 to 15.4 MeV (1956CA1C).
Recent studies of the angular distribution of ground-state tritons for Ed = 0.1 to 15 MeV have been reported by (1955JU10, 1955JU1B, 1956HA90, 1957SM78: see also (1955AJ61)). Below Ed ≈ 0.5 MeV, the tritons show strong backward peaking, suggestive of interference of compound nucleus states of opposite parity (1957SM78: see 11B). At high energies, the reaction proceeds mainly by pickup, with ln = 1 (1956HA90). See also (1955DA1E; theor.), (1957HA1F) and 9Be.
At E(3He) = 0.90 MeV, the ground (weak) and 2.9-MeV (strong) states are observed: see (1955AJ61).
8Be states up to Ex = 10 MeV are reported to be involved in this reaction: see (1954TI1C, 1955AJ61, 1955TI1A).
See (1951PE1B, 1954RI15, 1955JA18, 1956FR18, 1957TI1A) and 11B.
All observers agree that transitions occur to the ground state and a state at Ex ≈ 2.87 ± 0.06 MeV, Γ = 0.93 ± 0.15 MeV, (weighted mean of (1951WH1A, 1953CU1C, 1953TR04)). However, there is conflicting evidence on whether other states with Ex < 15 MeV are involved in this reaction. (1954CU1A) report additional states at (4), 5.1 and 7.5 MeV (see also (1955AJ61)). However, (1953TR04: Ed = 0.6 to 1.07 MeV) report no other low lying states; (1956BO1J: Ed = 5 MeV, θ = 50° and 90°) have not observed any other states below Ex = 9 MeV; (1955HO48: Ed = 1.4 to 3.2 MeV, several angles) find no evidence for excited states other than the 2.9-MeV level below Ex ≈ 10 MeV; and (1956KA1A, 1958KA31: Ed = 1.7 MeV) find no groups corresponding to 8Be states with Ex = 9.8 to 14.8 MeV above the continuum attributed to 10B(d, α)4He4He.
The observed α-spectrum corresponding to the 2.9-MeV level may be reasonably well accounted for by the Breit-Wigner formula with lα = 2, Eλ = 5.29 MeV, γ2α = 13.4 × 10-13 MeV-cm, R = 4.48 × 10-13 cm [θ2 ≈ 2] (1953TR04).
These reactions have been observed in boron-loaded photoplates. Six states of 8Be below Ex = 5 MeV are reported to be involved in reaction (a) (1953ER1A). See also (1955TI1A).
Alpha-particle groups corresponding to the ground state and to the 2.9-MeV state are reported; Ex = 2.94 ± 0.06 MeV, Γ = 0.84 MeV (1951LI1B, 1953BE61: see (1955AJ61)). Excitation of several additional levels is reported by (1953GL1A); however, a careful search by (1955HO48) reveals no evidence for any levels with Ex < 7 MeV except the ground state and that at 2.9 MeV.
The alpha particles leading to the ground state are strongly anisotropic at the Ep = 163-keV resonance (12C* = 16.11, J = 2+; T = 1); it is thus unlikely that J = 2 (1952TH1B). The directional correlation of successively emitted α-particles at Ep = 163 keV indicates isotropic breakup of 8Be(0) and hence J = 0, with J = 2 excluded. From the angle between the α-particles resulting from the breakup, Q = 90 ± 5 keV is obtained; the half-life is < 4 × 10-15 sec (1955TR03). The angular correlation of alpha particles leading to the 2.9-MeV state with those resulting from the subsequent breakup is consistent with J = 2+ for the 2.9-MeV state (1955GE1A). Certain peculiarities in the relative yields of ground state and 2.9-MeV excited state α-particles suggest that the latter level may have a significant T = 1 admixture: see (1953BE61, 1955HO48).
Nuclear pairs have been reported with E(π) = 7 MeV (1951PH1B); see, however, (1955BE62). See also (1955TI1B).
For Eγ < 40 MeV, the reaction involves mainly states of 8Be at 0, 2.9, (4.1) (16.5 ± 0.2; Γ < 0.4), 16.8 ± 0.2 (Γ < 0.3), 17.6 ± 0.2 (Γ < 0.3) MeV, with indications of further states near 6, 10 and 15 MeV. There is no evidence for three-body reactions in this work (1955GO59). Evidence for levels at 3.2, 4.0, (7.5) and 9.0 MeV is reported by (1955GL1A: see also (1954TI1C)). The ground state decay energy is given as 87 ± 8 keV; for the first excited state, Ex = 3.06 MeV, Γ = 0.9 MeV (1955GO59). The excitation function is characterized by a number of resonances, suggesting that the process takes place via definite energy levels of 12C (see 12C); the principal types of levels involved being J = 1-; T = 1, (E1 absorption) and J = 2+; T = 0, 1 (E2) (1955GO59: see also (1953GE1B, 1955TI1A, 1957MU1C)). For Eγ < 25 MeV, the reaction proceeds mainly to 8Be(0) and 8Be*(2.9); angular correlations indicate J = 0 and J = 2, respectively for these states. Wide variations in the branching ratio with energy are attributed to differences in isobaric spin impurities, estimated as 0.05 × 10-3 for the ground state, and ≥ 10-3 for 8Be*(2.9).
For Eγ > 26 MeV the reaction changes radically, now involving the 17 to 18 MeV states of 8Be, with E1 absorption. The fact that these levels are so strongly excited in this manner suggests that they have T = 1. Angular distributions indicate J = 2 for 8Be*(16.8) and J = 2 (or possibly 0) for 8Be*(17.6). It is noted that the latter level cannot be identified with the well-known 17.63-MeV, J = 1+ level (see 7Li + p) (1955GO59: see, however, (1953WA27)). Excitation of proton-emitting levels near Ex = 18 and 22 MeV is reported by (1956LI05). See also (1953GU1A, 1955HA1D, 1955TI1A).
Reaction (a) has been studied for En = 12.3 to 20.1 MeV by (1955FR35) who find evidence for transitions through the ground state and the 2.9-MeV level. See also (1955AJ61) and 12C.
Reaction (b) at Ep = 29 MeV appears to proceed predominantly through the ground state and the 2.9-MeV level. It is not clear whether higher levels in 8Be are involved (1955NE18). See also (1955CU1C, 1956SA1C, 1956SA1D, 1957JA1B).
At Eγ ≈ 22 MeV, the reaction appears to proceed mainly via the 9.6 and 10.8-MeV states of 12C to the ground state of 8Be. For Eγ > 24 MeV, transitions through the 15(?) and 16-MeV T = 1 state(s) of 12C, to the 2.9-MeV state of 8Be appear to dominate: see (1955AJ61) and (1955HA1D, 1955TI1A, 1956DA1C).
At Ep = 29 MeV, more than half the transitions are through the ground state of 8Be; there is no evidence for participation of any excited states of 8Be (1955KO1A). See also (1957JA1B).