(See Energy Level Diagrams for 6Be)
The yield of γ-rays to 6Be*(1.7) (reaction (a)) increases smoothly from 0.4 to 9.3 μb (assuming isotropy) for 0.86 < E(3He) < 11.8 MeV (90°). No transitions were observed to 6Be(0) [σ < 0.01 μb at E(3He) = 1.4 MeV]. This is understood in terms of a direct capture of 3He by 3He in the singlet spin state and with zero angular momentum: the 0+ → 0+ γ transition is forbidden. Reaction (a) is thus of negligible astrophysical importance compared to reaction (c) (1967HA24) [see below]. The capture cross section from E(3He) = 11 to 27 MeV continues to increase smoothly with energy at first and then shows a broad structure centered at E(3He) = 23 ± 1 MeV [Ex = 23.0 ± 0.5 MeV], Γc.m. ≈ 5 MeV (1972VE1C, 1973VE1B, 1973VE1E) this appears to be a 33F cluster resonance which decays by an E1 transition to 6Be*(1.7). The γ-ray angular distributions are consistent with Jπ = 3- (1973VE1B).
Measurements of the total cross section for reaction (c) have been carried out for E(3He) = 60 to 300 keV (1973DW1A) and 0.16 to 2.2 MeV (1969DW1A, 1971DW01). The measurements of (1973DW1A), down to Ec.m. = 30 keV, eliminate the possibility of a resonance [which might help explain the observed absence of solar neutrinos], unless it is extremely narrow (Γ ≲ 100 eV): θ2p ≈ 3 × 10-6 (1973DW1A). (1972BA10) has commented that such a high 3He + 3He cluster purity, a 6Be state at ≈ 11.5 MeV is not expected. The cross section factor S(Ec.m.) = [5.5 - 3.1 Ec.m. + 1.4 E2c.m.] MeV · b [error in S is ± 20% for Ec.m. > 40 keV] (1973DW1A). Cross sections are also reported for E(3He) = 0.5 to 1.7 MeV by (1966WA18). See (1966LA04) for a discussion of earlier work and (1966BA1N, 1967SH1F, 1967TO1B, 1968BA1W, 1968MA1U, 1969BA1M, 1970MA57, 1972BA2M, 1972BB10, 1972FE12, 1972FO1H, 1972KO1M, 1972TO1D, 1973BA2C, 1973FO1J, 1973PA1Q, 1973TR1E) for a discussion of the astrophysical implications of reaction (c). From a kinematically complete study of reaction (c) (1972KU08, 1972KU1N) find app = -7.6 ± 0.6 fm. (1970GR17) report app = -7.52 ± 0.22 fm. A study of the proton spectrum at E(3He) = 6.9, 7.9 and 9.1 MeV indicates that reaction (c) proceeds mainly via a direct mechanism (1972DE46). See also (1970BO1R, 1973RO2A) and 5Li.
The elastic scattering (reaction (e)) has been studied for E(3He) = 3 to 12 MeV (1963TO03), 9.1 and 11.1 MeV (1968IV01), 11.9 to 18.9 MeV (1968BA76), 17.9 to 32.0 MeV (1970JE02) and 18 to 80 MeV (1968BA1V). The excitation function shows a smooth monotonic behavior except for an anomaly at E(3He) = 25 MeV in the l = 3 partial wave corresponding to a broad state in 6Be at Ex ≈ 24 MeV (1968BA1V, 1970JE02). Polarization measurements have been reported at E(3He) = 4.33 to 9.83 MeV (1972BO42) and 9.3 to 17.5 MeV (1972HA64): the polarization is consistent with zero at θ ≈ 63°, consistent with a description of the scattering which leaves the P- and F-wave phase shifts unsplit (1972BO42) and 9.3 to 17.5 MeV (1972HA64).
Neutron groups have been observed to 6Be*(0, 1.7) at E(3He) = 19.4 MeV (1967AD05), 26 MeV (1966EC01) and 29.8 MeV (1966GU09): see Table 6.8 (in PDF or PS). There is no evidence for other states of 6Be with Ex ≲ 5 MeV (1966EC01). See also (1970BA1V).
Neutron groups have been observed to 6Be*(0, 1.7) as has the ground state threshold. The width of the ground state is 95 ± 28 keV (1967HO01). The parameters of 6Be*(1.7) are displayed in Table 6.8 (in PDF or PS) (1965BA39, 1966EC01, 1968BA41, 1968WA11, 1972AR22). Higher excited states have been reported by (1965BA39, 1968BA41, 1968WA11): there is very strong evidence that states analog to these do not exist in 6He. At Ep = 24.9 MeV no evidence is found for a 6Be state near Ex = 11.5 MeV (1973WO1D). See also reaction 4.
Triton groups have been observed to 6Be*(0, 1.7). The width of the ground state is 89 ± 6 keV (1966WH01). The parameters of the excited state are displayed in Table 6.8 (in PDF or PS) (1966MA36, 1966RO06). No other excited states have been seen with Ex < 13 MeV (1966MA36: E(3He) = 40 MeV), < 10 MeV (1966RO06: E(3He) = 31 MeV). An attempt has been made at E(3He) = 25.5 MeV (1973PA1C) and at 46.3 MeV (1973HA45) to observe a possible 6Be state at Ex = 11.5 MeV, of astrophysical interest: dσ/dΩ ≲ 0.19 μb/sr at θc.m. = 45° at the lower energy; dσ/dΩ ≤ 1.6 μb/sr at 8.4° and at 46.3 MeV. Upper limits for the spectroscopic factor are S ≤ 0.006 (1973PA1C) and ≤ 0.001 (1973HA45). The ground-state angular distribution shows a pronounced oscillatory character, consistent with l = 0; that for the 1.7 MeV state is relatively structureless (1966RO06). Angular distributions are also reported by (1969NU1A; n0: 14, 18 MeV) and by (1971GI1E, 1972GI07; n0: 27.0 MeV). The α-decay spectra of 6Be*(0, 1.7) have been studied by (1973GE1H).