(See the Energy Level Diagram for 7Be)
The decay is complex; see 7Li.
In the range E(3He) = 0.48 to 1.32 MeV, the capture cross section increases from 0.04 to 1.2 μb. At E(3He) = 1.32 MeV, about 50% of the transitions involve the 0.43-MeV state (1959HO03). See also (1958BA59, 1958HE1F). The significance of this reaction for energy generation in stars is discussed by (1958CA1C, 1958FO1A).
Elastic scattering studies for E(3He) = 3 to 5.5 MeV (θ = 54° to 126°) indicate a prominent resonance at E(3He) = 5.17 MeV, Γ = 0.180 MeV (7Be* = 4.53 MeV, Γc.m. = 0.102 MeV). Phase-shift analysis indicates that the state is F7/2, with γ2α = 3.0 × 10-13 MeV-cm, θ2α = 0.36, R = 4.4 × 10-13 cm. The behavior of the p-wave phase shifts suggests that the ground state, and possibly 7Li*(0.48), have widths which approach θ2α = 1. The s-wave phase shift is consistent with hard-sphere scattering, with R = 2.8 × 10-13 cm (1958MI92). See also (1958CH35).
A search for this reaction at Eα = 39 MeV, 0.5 MeV (c.m.) above threshold, led to a negative result; the cross section is < 0.7 mb. This result is consistent with the assumption that 7Be has negative parity (1952WA31).
Gamma transitions to the ground state and to the 0.43-MeV state are observed. The yield shows no evidence of resonance for Ep = 0.4 to 1.0 MeV, and the branching ratio remains approximately constant at 62 ± 5%, (1955BA59: 65%) to the ground state, 38% to the 0.43-MeV state, (1955BA59: < 4% to the 4.6-MeV state). The 90°-differential cross section at 750 keV is 0.02 μb/sr (1956WA03). (1955BA59) estimate 0.2 μb/sr from observation of (430 ± 20)-keV radiation. (Because of the extremely low cross section of the present reaction, small amounts of contaminants may have a strong influence; in particular, earlier reports of a resonance at Ep ≈ 1.6 - 1.8 MeV may be due to 10B(p, α)7Be: see (1956WA03).)
The angular distribution of both γ-rays (to the ground state and the 0.43-MeV level) is the same at Ep = 750 keV, W(θ) = 1 + (1.05 ± 0.15)cos2θ. Neither s- or d-wave capture yields such a distribution, and p-wave, J = 3/2-, is indicated; a direct non-resonant capture process seems also possible (1956WA03).
At Ep = 9 MeV, the cross section for production of ground state neutrons is dσ/dΩ = 0.19 + 0.23cosθ + 0.70cos2θ mb/sr; σ = 5 ± 1 mb (1957BO1F).
The excitation function follows the Gamow function from Ep = 19 to 250 keV (1953SA1A, 1957JA37). The total cross section exhibits two resonances, one at ≈ 1 MeV (broad) and one at 1.83 MeV (1951BA79, 1956MA91). (There is some discrepancy in the reported absolute cross sections.) In the range Ep = 1.3 to 3.1 MeV, the elastically scattered protons, θ = 164°, exhibit a single maximum at Ep = 1.75 MeV, with Γ = 0.5 MeV (1951BA79): see Table 7.2 (in PDF or PS). Preliminary results of a study at θ = 156° for Ep = 0.45 to 1.8 MeV clearly show the 1.75-MeV anomaly, but no marked effect near 1 MeV (1958MC65). Analysis of α-particle angular distributions in the range 0.6 ≤ Ep ≤ 2.5 MeV is consistent with J = 3/2+, 5/2- for the two states at 6.35 and 7.18 MeV. A large, non-resonant background may indicate other states or some direct interaction (1956MA91): see also 6Li(p, γ)7Be. At Ep = 15 and 18 MeV, the angular distributions of 3He-particles are sharply peaked forward (1956LI37: see 6Li). See also (1955SA1C; theor.).
Two neutron groups are reported, corresponding to the ground- and 0.43-MeV states. The γ-ray energy is given as 428.9 ± 2 keV (corrected for Doppler shift): the 7Li* - 7Be* difference is 48.5 ± 1.0 keV (1952TH24). At Ed = 3.5 MeV, the angular distributions of the neutron groups, analyzed by stripping theory indicate lp = 1, J ≤ 5/2- for both states. Reaction (b) is also observed (1952AJ1B). Pronounced stripping patterns for neutrons leading to the 0.43-MeV state, observed at Ed = 0.6 to 1.5 MeV, also indicate lp = 1. Neutron-gamma correlations, with the neutron counter at a fixed angle, are isotropic; taken together with the stripping results, this observation indicates J = 1/2- for the excited state (1956NE13, 1958NE38, 1958SA17); see also (1951TH1A, 1953TH1B). A search for neutron groups leading to levels of 7Be in the range 0.3 to 2.0 MeV revealed no others with intensities >10% of the main group (1956NE13).
Broad maxima observed in the ratio of low energy to high-energy neutrons at Ed = 4.2 and 5.1 MeV are attributed to 7Be state at 6.5 MeV (Γc.m. = 1.2 MeV) and 7.2 MeV (Γc.m. = 0.5 MeV). The 4.6-MeV state produces no such effect, presumably because of the high angular momentum of the neutrons (1957SL01): see 4He(3He, 3He)4He. At Ed = 1.8 MeV, a search for 4.2-MeV γ-rays (from the 4.58 → 0.43 transition) was unsuccessful: I(4.2-MeV γ) < 0.1 I(0.4-MeV γ) (1957WI24). See also (1955CA1A).
A weighted mean of measurements of the ground state threshold gives 1.8811 ± 0.0005 MeV (1954JO10). Using a two-meter absolute electrostatic analyzer, (1958BO76) obtain Ethresh. = 1.8812 ± 0.0009 keV.
The weighted mean value of five independent observations for the energy of the excited state is 431 ± 5 keV; see Table II (7) (in PDF or PS) in (1952AJ38). A neutron threshold determination gives 434 ± 4 keV (1955MA84). At Ep = 9 MeV, neutrons corresponding to 7Be*(4.6) are reported by (1957BO1F). At Ep = 18.3 MeV, 7Be levels are observed at 4.6 ± 0.2 and 7.1 ± 0.2 MeV (1952TH1C).
The weighted mean of five determinations of the energy of the first excited state gives 430.3 ± 2 keV: see Table III (7) (in PDF or PS) in (1952AJ38), and (1952CR30); Eγ = 432 ± 3 keV (1954DA20). The mean lifetime of this state is (2.7 ± 1.0) × 10-13 sec (1956BU83: Doppler shift). This value agrees with a shell model calculation by (1955LA1D).
At Ep = 18 MeV, α-groups are reported corresponding to 7Be* = 0, 0.49 ± 0.10, 4.72 ± 0.08, 6.27 ± 0.10, 7.21 ± 0.10 and 14.6 ± 0.3 MeV. The last group is ten times as intense as any of the others. It is not completely excluded that it may be due to 10B(p, 3He)8Be (1955RE16).