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6Be (1979AJ01)

(See Energy Level Diagrams for 6Be)

GENERAL: See also (1974AJ01) and Table 6.6 [Table of Energy Levels] (in PDF or PS) here.

Model calculations: (1974IR04, 1976CE1B, 1976HE07, 1976IR1B).

Other topics: (1973WE18, 1974DA1B, 1974MC04, 1975BE31, 1975FE01, 1977SI1D).

1. (a) 3He(3He, γ)6Be Qm = 11.488 Eb = 11.488
(b) 3He(3He, p)5Li Qm = 10.89
(c) 3He(3He, 2p)4He Qm = 12.8596
(d) 3He(3He, 3p)3H Qm = -6.9544
(e) 3He(3He, 3He)3He
(f) 3He(3He, d)4Li Qm = -8.4
(g) 3He(3He, 2p)2H2H Qm = -10.987

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 negligable astrophysical importance compared to reaction (c) (1976HA24) [see below]. The capture cross section from E(3He) = 12 MeV 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 (1973VE1B, 1974VE01). This appears to be a 33F cluster resonance which decays by an E1 transition to 6Be*(1.7). The γ-ray angular distributions are coinsistent with Jπ = 3- (1974VE01).

For reaction (b) see 5Li and (1976IR02; polarization measurements at E(3He) = 13.6 MeV).

Measurements of the total cross section section for reaction (c) have been carried out for E(3He) = 60 to 300 keV (1974DW01) and 0.16 to 2.2 MeV (1969DW1A, 1971DW01). The measurements of (1974DW01), 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 (1974DW01). (1972BB10) has commented that such a high 3He + 3He cluster for a 6Be state at ≈ 11.5 MeV is not expected. The cross section factor S(Ec.m.) = [5.2 - 2.8 Ec.m. + 1.4 E2c.m.] MeV · b [error in S is ± 20% for Ec.m. > 40 keV] (1974DW01). For the earlier work see (1966LA04, 1974AJ01). For polarization measurements (reaction (c)), see (1976SL1A). The cross section of reaction (c) has been compared with that for 3H(3He, α)d [see reaction 1 in 6Li] at the same c.m. energy (16 MeV). At E(3He) = 13 MeV a deviation from the ratio of 2 is observed, which is qualitatively accounted for by a distorted wave zero-range calculation (1974RO01).

The elastic scattering (reaction (e)) has been studied for E(3He) = 3 to 32 MeV [see (1974AJ01)], at 32 MeV (1974RO01) and at 120 MeV (1977TA1A; also inelastic processes). The optical model fit at 120 MeV appears to be poor (1977TA1A). 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 (1970JE02). Polarization measurements have been reported at E(3He) = 4.33 to 17.5 MeV: 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, 1972HA64). Polarization measurements have also been carried out at E(3He) = 8.9 to 34.8 MeV on a polarized 3He target (1974BA1G, 1975BI1C, 1976BA1H; abstracts) and at eight energies in the range E(3He-bar) = 17.91 to 32.9 MeV (1978VL01). A two level R-matrix analysis of the phase shifts (L ≤ 5) suggests three broad F-wave states at Ex ≈ 23.4 (4-), 26.2 (2-) and 26.7 MeV (3-) (1978VL01), in disagreement with the capture γ-ray results described above. Measurements of various of these reactions in order to obtain a total reaction cross section at E(3He) = 17.9 and 21.7 MeV have been carried out by (1975PO1B; abstract). For reaction (f) see also (1974AJ01). Reaction (g) has been studied at 50 and 78 MeV to look at the quasi-free scattering of the two deuterons. The cross sections are an order of magnitude smaller than those predicted by PWIA (1978AL21).

(1977DA11) have searched at E(3He) = 15 and 25 MeV for the 3He + 3He → d + α + e+ + ν reaction [which had been reported by (1975SL01, 1976PIZS, 1977SL1B) to occur with a cross section of 3.4 nb/MeV · sr at E(3He) = 13.6 MeV]. (1977DA11) find no evidence for this process with a cross section twenty times lower than reported by (1975SL01); thus the cross section for the p + p process at solar energies is not in error and cannot account for the solar neutrino puzzle.

At 120 MeV (1977FU1A) have studied 3He + 3He → p + d + 3He and have interpreted the results as possible evidence for a 3N resonance. See also (1971BA1A, 1974UL1B, 1975FE01, 1976BA1J, 1976MC04, 1976NE1A, 1976NO1C, 1978RO1D; astrophysical considerations), (1975TO1A, 1976MI1F, 1978SL1B), (1977MC1C; applied) and (1974DE18, 1976AS05, 1976HE07; theor.).

2. 4He(3He, n)6Be Qm = -9.090

Neutron groups to 6Be*(0, 1.7) have been observed at E(3He) = 19.4 to 38.61 MeV: see Table 6.8 (in PDF or PS) in (1974AJ01) for the parameters of the first excited state. There is no evidence for other states of 6Be with Ex ≲ 5 MeV (1966EC01), nor for a state near the 3He threshold at 11.5 MeV: for the latter the differential cross section is ≲ 7 μb/sr at θlab = 7.3° (1977MC10; E(3He) = 38.61 MeV), ≲ 7.5 μb/sr at θ = 0° (1975VI04; 36.2 MeV).

3. (a) 6Li(p, n)6Be Qm = -5.070
(b) 6Li(p, pn)5Li Qm = -5.66

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) of (1974AJ01). Angular distributions have been reported at Ep = 8.3 to 49.4 MeV [see (1974AJ01)] and at 14.9 and 17.8 MeV (1974AR05; n0). For a study at Ep = 800 MeV see (1977RI07). In reaction (b) (1977WA05) report, at Ep = 47 MeV, some evidence for sequential decay via 6Be*(15.5 ± 2, 24 ± 2). See also (1976SL2A) and (1975FE01; theor.).

4. 6Li(3He, t)6Be Qm = -4.306

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) of (1974AJ01). 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 the possible 6Be state at Ex = 11.5 MeV, of astrophysical interest: dσ/dΩ ≤ 0.19 μ/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). See also (1975CH1D). 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 (1972GI07; n0; 27.0 MeV). The α-spectrum following the α + p + p decay of 6Beg.s. has been measured by (1977GE02): the yield of low energy α-particles appears to be enhanced compared with calculations based on the available phase space. See also (1977HA19) and (1975FE01, 1978HA1H; theor.).

5. 6Li(6Li, 6He)6Be Qm = -7.795

See reactions 9 in 6He and 18 in 6Li (1974WH01, 1974WH02, 1974WH07, 1975WH01).

6. 9Be(3He, 6He)6Be Qm = -9.690

See (1974AJ01).