(See Energy Level Diagrams for 7Be)
GENERAL: See (1957FR1B, 1960PH1A, 1960SH1A, 1960TA1C, 1961KU1C, 1961TA05, 1961TO04, 1962GL1A, 1962IN02, 1964AR22, 1964BA2A, 1964GR1J, 1964HO31, 1964LI1F, 1964MO1K, 1964NE1H, 1964PA1K, 1964PH1A, 1964RA1B, 1964SA1F, 1964ST1B).
The decay is complex: see 7Li.
In the range Eα = 0.42 to 5.80 MeV the cross section rises from 0.02 to 4 μb. The branching ratio γ0 (to g.s.)/γ1 (to 0.4 MeV state) remains at 73/27 for Eα = 1.1 to 3.2 MeV (1963PA12) .The zero-energy intercept of the cross section factor S = 0.47 ± 0.05 keV · b (1963PA12). See also (1958BA59, 1959HO03, 1961FA02, 1964PA1A). A direct capture calculation, assuming an α + 3He model with phase shifts obtained from 4He(3He, 3He), gives good agreement with the excitation function of (1963PA12) with R = 2.8 fm, θ2α = 1.25 and 1.05 for 7Be(0) and 7Be*(0.43), respectively. The capture proceeds mainly by E1, with both s- and d-waves contributing above Eα = 1 MeV (1963TO06). See also (1961CH1C, 1961TO04, 1962HE1C). The bearing of this reaction on 4He production in stars is discussed by (1958FO1A, 1964PA1A).
Elastic scattering studies have been reported for E(3He) = 3 to 5.5 MeV (1958MI92), 2.5 to 5.7 MeV (1964BA09), 4 to 12 MeV (1963TO04), 8 to 18 MeV (1964SP04), 29 to 30 MeV (1960BR19, 1960MC1E), Eα = 11 to 28 MeV (1960BR1J), 28 to 41 MeV (1961CH09). Two resonances are reported in the f-wave phase shifts: at Ex = 4.55 MeV (2F7/2) and Ex = 6.51 MeV (2F5/2); see Table 7.10 (in PDF or PS). In the range E(3He) = 2.5 to 12 MeV, the s-wave and d-wave phase shifts reflect hard-sphere scattering with R = 2.8 fm, although some systematic deviation seems to appear in the s-waves for E(3He) > 5 MeV (1963TO04, 1964BA09). The p3/2 phase shift exhibits a contribution from the ground state, with θ2α(g.s.) = 0.15, but the p1/2 phase shift cannot be adequately accounted for by 7Be*(0.43) (1961TO04, 1964BA09). The 4P5/2 level (Ex = 7.18 MeV) seems to have no influence on the scattering: θ2p < 0.02 (1963TO04). See also (1964PH1A). A broad resonance has been observed at Ex = 9.2 ± 0.5 MeV. It is not clear whether it can be identified with the (3/2-) state at 9.9 MeV (1964SP04).
At the higher energies, distinct minima are observed in the angular distribution at θ = 45°, 100° and 140°. No evidence of sharp resonances is observed for Ex > 13 MeV (1960BR19, 1960BR1J, 1961CH09). Optical model calculations are reported by (1963SQ1A), resonating group calculations by (1963TA10). See also (1963SC1M).
Polarizations have been calculated from the observed phase shifts by (1959PH37, 1963TO04, 1964BA09). Measurement of scattering of α-particles from a polarized 3He target at Eα = 6.53 and 7.33 MeV confirms the expected reversal of polarization between these two energies (1962PH1B).
Reaction (b) has been studied for E(3He) = 8 to 12 MeV by (1963TO04) and at 29 MeV by (1960BR19, 1960MC1E). A peak appears in the excitation function at E(3He) = 9.8 MeV, corresponding to the 4P5/2 level at Ex = 7.18 MeV (1963TO04). The resonance corresponding to 7Be*(9.2), observed in the elastic scattering, is strongly present in the yield of 6Li*(2.18) protons but not in the yield of ground state protons (1964SP04). See also (1962TE1D).
Gamma transitions are observed to the ground and 0.43 MeV states. 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)% to the ground state, 38% to the 0.43 MeV state, < 4% to the 4.6 MeV state (1955BA59, 1956WA03). The 90° differential cross section at 750 keV is 0.02 μb/sr (1956WA03).
The angular distributions of γ0 and γ1 are the same at Ep = 0.75 MeV, W(θ) = 1 + (1.05 ± 0.15)cos2 θ. Neither s- nor d-wave capture yields such a distribution, and p-wave, J = 3/2-, is indicated (see, however, 4He(3He, 3He)4He); a direct non-resonant capture process seems also possible (1956WA03). For Ep = 1 to 2 MeV, large cos2 θ terms appear: the yield shows no resonance behavior in this range (1963MC09). See also (1959GR1A, 1961TO04, 1965LA03).
The yield of neutrons increases approximately monotonically from threshold to Ep = 14.3 MeV (1964BA16). The excitation function for slow neutrons near threshold indicates that both s- and p-wave neutrons contribute significantly (1964HOZZ). The cross section for production of ground state neutrons is 5 ± 1 mb at Ep = 9 MeV (1957BO1F), 0.5 mb/sr at Ep = 10.5 MeV, θc.m. = 104° (1959AJ81).
Elastic scattering has been studied from Ep = 1 to 3 MeV by (1951BA79), from 0.5 to 2.9 MeV by (1963MC09), from 1.2 to 5.6 MeV by (1964FA1D), from 2.4 to 12 MeV by (1963HA53), and at 31 MeV by (1963DE01). Two resonances are reported at Ep = 1.84 and 5 MeV. In the range 0.5 to 2.9 MeV, the data are consistent with p-wave formation of a 5/2- or 3/2- state, with Γc.m. = 836 keV, θ2p = 0.28, θ2α = 0.012 (see Table 7.2 (in PDF or PS)); an s-wave background is evident, possibly reflecting a 1/2+ state at higher energies (7Be* > 8 MeV). No evidence is found for a previously reported 3/2+ state near Ep = 1 MeV (1963MC09). The 5 MeV resonance, corresponding to Ex = 9.9 MeV, has a width of ≈ 1.8 MeV and exhibits a behavior much like that of the lower resonance, suggesting that it too is formed by p-wave: on this assumption, the reduced width γ2p = 3 ± 2 MeV · fm. A weak rise near Ep = 8 to 9 MeV may indicate a further level, 7Be* ≈ 13 MeV (1963HA53). See also (1964LE1E).
Polarization of elastically scattered protons has been studied by (1962RO20: 15 MeV), (1963HW01: 39 MeV), while polarization of inelastically scattered protons (to 6Li*(3.56)) has been studied by (1964MA1Y: 150 MeV). See also (1964VE1A).
Inelastic protons (to 6Li*(2.18)) studied from Ep = 3.6 to 9.4 MeV show the resonance at Ep = 5.5 MeV, Angular distributions suggest p-wave formation with Jπ = 3/2- or 5/2- (1963HA49). The yield of 3.6 MeV γ-rays (from 6Li*(3.56)) shows a broad maximum at ≈ 6 MeV, probably associated with 7Be*(9.9): Jπ = 3/2- is suggested. At Ep ≈ 6.3 MeV there is an abrupt decrease in the inelastic cross section, which is either due to the onset of the (6Be + n) channel or to interference with an other broad state with Jπ = 3/2- (1964HA37).
The cross section exhibits a broad, low maximum near Ep = 1 MeV and a pronounced resonance at Ep = 1.85 MeV (1951BA79, 1956MA91: see (1963MC09)). No other structure is reported up to Ep = 5.6 MeV (1963JE03, 1964FA03). From Ep = 3 to 12 MeV, θ = 70°, the excitation function shows only a smooth decrease (1962HE03, 1963TE1B). In the range Ep = 0.5 to 2.5 MeV, a strong cos θ term is observed, indicating interference between the p-wave resonance and s-wave background (1956MA91, 1963JE03). At Ep > 8 MeV, the angular distributions are characterized by forward and backward peaks (1956LI37, 1960AL18, 1962HE03, 1963TE1B). See also (1957JA37, 1960BO13, 1960SA28, 1961KH01, 1963BE08, 1964BE37) and 6Li.
Two neutron groups are reported, corresponding to the ground and 0.43 MeV states. The γ-ray energy is 428.9 ± 2 keV (corrected for Doppler shift): the 7Li* - 7Be* difference is 48.5 ± 1.0 keV (1952TH24). Angular distributions of the n0 and n1 groups have been determined at Ed = 0.56 to 2.9 MeV (1963BI1B), Ed = 0.6 to 1.5 MeV (1956NE13), Ed = 1.8 to 3.1 MeV (1963CR08) and Ed = 3.5 MeV (1952AJ1B). The distributions indicate lp = 1, J ≤ 5/2- for both states. Since the n - γ correlations are isotropic (1956NE13) J = 1/2- for the 0.43 MeV excited state is indicated.
Broad maxima are observed in the ratio of low-energy to high-energy neutrons at Ed = 4.2 and 5.1 MeV (7Be* = 6.5 MeV and 7.2 MeV, Γc.m. = 1.2 and 0.5 MeV, respectively) (1957SL01). See also (1964PA16, 1965MA1K).
The threshold for this reaction is used as a secondary standard for energy calibrations: the value recommended by (1963MA1R) is 1880.36 ± 0.22 keV (see Table 7.11 (in PDF or PS)). See also (1960BR20). Studies of target and shape effects at threshold are reported by (1959WE1A, 1963PA11, 1964BO10).
A determination of Q from the cone angle of neutrons above threshold confirms that the observed threshold corresponds to the true onset of the reaction (1963YO04). A second threshold, corresponding to the first excited state of 7Be, yields Ex = 433 ± 2 keV (1960MA1G). Neutrons corresponding to 7Be*(4.55) are observed for Ep ≳ 7 MeV (1959AJ81, 1960HI04). At Ep = 10 MeV, groups n0, n1 and n2 account for nearly all the neutrons observed (1963BO06). See also (1952TH1C, 1957BO1F, 1961NI04, 1962AU01). See also (1959GA08, 1960RO21, 1961TO06, 1962BO33, 1963BO1N, 1964BA16, 1964OL1C) and 8Be.
At Ep = 44 MeV, triton groups are observed corresponding to the 7Be levels at 0, 0.43 and 4.55 MeV, and to a new level at 10.79 ± 0.04 MeV with Γ = 298 ± 25 keV. From the similarity of the angular distribution to that in the (p, 3He) reaction to 7Li*(11.13), it is concluded that the level has Jπ = 3/2-; T = 3/2 (1965DE08). See also (1954CO02, 1956BE14).
Alpha groups corresponding to 7Be(0) and 7Be*(0.43) have been studied by many observers: see (1952AJ38, 1959AJ76). Some reported values for the energy of the first excited state are: 434.4 ± 4 keV (1951BR10), 431 ± 5 keV (1950VA01), 429 ± 3 keV (1952CR30), 428.5 ± 1.8 keV (1952TH24). The mean lifetime of this state is 0.27 ± 0.10 psec (1956BU83: Doppler shift). This value agrees with a shell model calculation by (1955LA1D).
At Ep = 18 MeV, α-groups are reported correspondingly 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).