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6Be (2002TI10)

(See Energy Level Diagrams for 6Be)

GENERAL: References to articles on general properties of 6Be published since the previous review (1988AJ01) are grouped into categories and listed, along with brief descriptions of each item, in the General Tables for 6Be located on our website at (nucldata.tunl.duke.edu/NuclData/General_Tables/6be.shtml).

See also Table 6.14 preview 6.14 [Table of Energy Levels] (in PDF or PS).

1. (a) 3He(3He, γ)6Be Qm = 11.4884
(b) 3He(3He, p)5Li Qm = 11.17 Eb = 11.49
(c) 3He(3He, 2p)4He Qm = 12.8596
(d) 3He(3He, 3He)3He
(e) 3He(3He, pd)3He Qm = -5.4935

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 are observed to 6Beg.s. [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): see (1979AJ01). 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], Γcm ≈ 5 MeV. 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-: see (1979AJ01). See also (1989IS1B). Thermonuclear reaction rates for this reaction calculated from evaluated data are presented in the compilation (1999AN35).

Ay has been measured for E(3He) = 14 to 30 MeV [reaction (b)] by (1983KI10) using a polarized target. See also 5Li.

Measurements of the total cross section for reaction (c) have been carried out for E(3He) = 60 keV to 2.2 MeV [see (1979AJ01)] and for 36 to 685 keV (1987KR09). The measurements are consistent with a non-resonant reaction mechanism, at least down to Ecm = 24.5 keV. Upper limits for ωγ for a resonance below that energy (and with ER (cm) as low as 16.2 keV) [which might help explain the low observed flux of solar neutrinos], are given in (1987KR09). [It should be noted that a corresponding mirror state in 6He has not been observed.] The best fit to the data is given by S(0) = 5.57 ± 0.31 MeV · b (1987KR09). See (1979AJ01) for the earlier work. See also (1966LA04, 1974AJ01). For work on astrophysical considerations see references cited in (1988AJ01), and see also the following work: thermonuclear reaction rates calculated from evaluated data (1988CA26, 1999AN35); dynamic screening (1988CA1J); neutrino astrophysics (1989BA2P); reaction rates (1989SC25); plasma fusion (1988PO1J); S factors, RGM (1989VA20); cross sections, extended elastic model (1990SC15); cross sections, microscopic study (1991TY01); phase shifts, generator coordinate method (1990KR12); astrophysical S-factor, potential model (1992WI09); cross sections, microscopic analysis (1994DE27); S factor, electron screening effects (1989BE08); and nucleosynthesis around black holes (1989JI1A). (1985SI12) report α-d correlation measurements at E(3He) = 13.6 MeV, which suggest the breakup of the diproton (2He) into 2H + e+ + ν.

The elastic scattering (reaction (d)) has been studied for E(3He) = 3 to 32 MeV and at 120 MeV. 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. Polarization measurements have been carried out at E(pol. 3He) = 17.9 to 32.9 MeV. A two level R-matrix analysis of the phase shifts (L ≤ 5) suggests three broad F-wave states at Ex ≈ 23.4, 26.2 and 26.7 MeV [Jπ = 4-, 2- and 3-, respectively], in disagreement with the capture γ-ray results described above: see (1979AJ01). Calculations using the generator coordinate method have been reported for phase shifts (E(3He) < 5 MeV) (1990KR12), and for differential cross sections and astrophysical S factors (E(3He) = 2 - 6 MeV) (1994DE27). See also (1984AJ01) and (1986FO04).

A kinematically complete experiment (reaction (e)) has been performed at E(3He) = 120 MeV: large peaks were observed which appear to correspond to 3He-d quasi-free scattering followed by p-d FSI: see (1984AJ01).

The total reaction cross sections σR = 156.7 ± 3.8, 250 ± 14 and 296 ± 12 mb at E(3He) = 17.9, 21.7 and 24.0 MeV (1987BR02) [see also for partial cross sections for the breakup reactions and for unpublished results for σR for E(3He) = 3.0 to 17.9 MeV]. See also (1984AJ01) and references cited in (1988AJ01).

2. 4He(3He, n)6Be Qm = -9.0892

Neutron groups to 6Be*(0, 1.7) have been observed at E(3He) = 19.4 to 38.61 MeV: see Table 6.8 preview 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, nor for a state near the 3He threshold at 11.5 MeV: see (1979AJ01).

3. (a) 6Li(p, n)6Be Qm = -5.070
(b) 6Li(p, pn)5Li Qm = -5.39

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. The parameters of 6Be*(1.7) are displayed in Table 6.8 preview 6.8 (in PDF or PS) of (1974AJ01). Angular distributions have been reported at Ep = 8.3 to 144 MeV [see (1979AJ01, 1984AJ01)] and at 800 MeV (1986KI12). The transverse spin transfer coefficient, DNN(0°), at Ep = 160 MeV for the ground-state transition is -0.37 ± 0.04 in agreement with results in other light nuclei (1984TA07). See also 7Be and references cited in (1988AJ01).

In more recent work, evidence for a proportionality between σpn(0°) and Gamow-Teller transition strengths were examined (1987TA13). See also (1989RA1G). Measurements are reported at: Ep = 60 - 200 MeV [DNN(0°) (1990RA08)]; Ep = 256, 800 MeV [double differential cross sections (1993ST06)]; Ep = 186 MeV [polarization observables (1993WAZX, 1993YAZZ, 1994RA23), quasi-free excitations (1994WA22, 1999WAZV), dipole excitations (1995YA12)]; Ep = 392 MeV [σ(θ), Ay(θ) (1994TO08)]; Ep = 300, 400 MeV [quasi-free excitations, DNN(0°) (1994SA43)]; Ep = 295 MeV [spin-flip strength, DNN(0°) (1995WA16)]; Ep = 200 MeV [Ay(θ) (1995WAZW)]; Ep = 35 MeV [σ(θ) (1996ORZZ, 1998OR1B)]; and Ep = 280 MeV [σ(θ), isospin-symmetry test (1990MI10)]. For recent applications see (1998HA24, 1998WA12). Calculations with a dynamical multicluster model are discussed in (1991DA08, 1993SH1G). See also the review of two-particle neutron halo nuclei in (1996DA31).

In reaction (b) some evidence has been reported at Ep = 47 MeV for sequential decay via 6Be*(15.5 ± 2, 24 ± 2): see (1979AJ01). See also (1988MIZX).

4. 6Li(3He, t)6Be Qm = -4.3063

Triton groups have been observed to 6Be*(0, 1.7). The width of the ground state is 89 ± 6 keV. The parameters of the excited state are displayed in Table 6.8 preview 6.8 (in PDF or PS) of (1974AJ01). No other excited states have been seen with Ex < 13 MeV. There is no evidence for a state near 11.5 MeV: see (1979AJ01). (1987BO39) have studied the decay of 6Be*(1.7) at E(3He) = 38.7 MeV: they report that the branching ratio for decay via the emission of 2He [T = 1, S = 0] is 0.60 ± 0.15: see also reactions 21 in 6He and 38 in 6Li and (1984BO49, 1985BO56, 1988BO1J). See also (1984AJ01), (1987DA31; theor.) and 9B in (1988AJ01).

In more recent work, kinematically complete experiments for 6Li(3He, t)6Be*(0, 1.7) → α + p + p were reported in (1988BO38, 1989BO1N, 1989BO25, 1989BO42) and in (1992BO25, 1993BO38 [studied decay mechanism]). Measurements of differential cross sections at E(3He) = 93 MeV are described in (1994DOZW).