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 WWW TUNL    ## 12B (1968AJ02)

(See Energy Level Diagrams for 12B)

GENERAL: See Table 12.1 [Table of Energy Levels] (in PDF or PS).

μ = +1.003 ± 0.001 nm (1967SU03).

 1. 12B(β-)12C Qm = 13.370

Measured values of the half-life are displayed in Table 12.2 (in PDF or PS). The decay is complex; 12B decays to the ground state of 12C and to several excited states: see 12C. The transition to 12Cg.s. and (4.4) are allowed: hence J(12B) = 1+.

 2. 6Li(7Li, p)12B Qm = 8.337

At E(7Li) = 2 MeV, eleven groups of protons are reported to known states of 12B (1959MO12). At E(7Li) = 2.6 MeV, a 1.67 MeV γ-ray is reported by (1962BE24). Angular distributions of protons have been determined by (1967GA06: E(6Li) = 3.5 MeV; p0, p1, p2, p3) and by (1967KI03: E(7Li) = 3.78 to 5.95 MeV; p0, p1, p2, p3+4, p5). Except for p2, the distributions are nearly isotropic. See also (1957NO14, 1962BE16).

 3. 7Li(7Li, d)12B Qm = 3.309

The gamma decay of the first four excited states has been studied by (1963CA09): besides the 0.95 MeV state, the states at 1.67 and 2.72 MeV decay primarily to the ground state ( > 98% and > 80%, respectively), while the 2.62 MeV state decays primarily via cascades through the 0.95 or 1.67 MeV states ( > 80%). τm(12B*(0.95)) = 295 ± 40 fsec; τm(12B*(1.67)) < 50 fsec (1967THZX). See also 11B(d, p)12B. See also (1962BE24, 1967WY1B) and 14C in (1970AJ04).

 4. 9Be(t, n)11B Qm = 9.561 Eb = 12.930

Reported resonances in the yield of neutrons at θ = 0° are listed in Table 12.3 (in PDF or PS) (1961VA1C). See also (1962SE1A).

 5. 9Be(α, p)12B Qm = -6.884

See (1951MC57, 1955RA41, 1962WE1C).

 6. 9Be(7Li, α)12B Qm = 10.463

At E(7Li) = 3.5 MeV, α-groups are seen to the ground state of 12B and to levels at 0.90, 1.61, 2.58, 3.27, 3.60, 4.22, 4.39, 4.90, 5.60, 5.80, 6.61, 7.00, 7.42, 7.77, 8.05, 8.34 and 9.06 MeV. Angular distributions have been obtained for the first five states for E(7Li) = 3.3, 3.5 and 3.75 MeV. The total cross sections range from 1.1 to 2.3 mb. All angular distributions are characterized by backward maxima (1961HO19). See also (1966RO1E).

 7. 9Be(11B, 2α)12B Qm = 1.799

See (1963HO1E).

 8. 10B(t, p)12B Qm = 6.343

Eleven excited states of 12B have been observed by (1960JA17) at Et = 5.5 MeV and by (1964MI04) at Et = 10 MeV. Widths and Jπ values derived from angular distribution analyses (1964MI04) are displayed in Table 12.4 (in PDF or PS). See also (1963HO19) and (1965SH1E, 1966SH1F, 1967BA1E; theor.).

 9. 11B(n, γ)12B Qm = 3.369

The thermal neutron capture cross section is 5 ± 3 mb (1962IM01). The excitation function of β- particles (from 12B decay) shows a resonance at En = 20 keV, with ωΓγ ≈ 0.4 eV, Γ < 10 keV (1964MO07). In the range 140 to 2325 keV, resonances are observed at En = 0.43, 1.03, 1.28 and 1.78 MeV, with radiation widths of 0.3, 0.3, 0.2 and 0.9 eV, respectively (± 50%) (1962IM01).

 10. 11B(n, n)11B Eb = 3.369

The thermal (bound) scattering cross section for B is 4.4 ± 0.2 b (1958HU18, 1961WI1A). The scattering amplitude (bound) is a = 6.6 ± 0.3 fm (1965DO14), 6.53 ± 0.35 fm (1966DO11). Parameters of observed resonances in σtot are listed in Table 12.5 (in PDF or PS). The resonance at En = 20 keV (1966MO09) is also observed in 11B(n, γ)12B (1964MO07). Angular distributions near the En = 0.43 MeV resonance (J = 2) show cosθ terms, indicating interference with s-wave potential scattering. The best fit to the data is obtained with l = 1 formation of the resonance, either all channel spin 1 or all channel spin 2 (1955WI25). Polarization results can be explained by interference of s- and p-wave neutrons at this resonance (1962EL01). The En = 1.28 MeV resonance can be fitted with J = 3, l = 2 with the partial width in channel spin 2 equal to 10 times that in channel spin 1. Potential scattering at En = 1.5 MeV is nearly all s-wave, δ0 = -90° (1955WI25). A resonance not seen in earlier work is reported by (1962IM01) at En = 1.027 MeV. The observed cross section is consistent with s-wave formation, Jπ = (1)-. Polarization and differential cross section measurements have been carried out at 70 neutron energies from 0.075 to 2.25 MeV. Large polarizations are observed from the resonance at En = 1.28 MeV up to 2.25 MeV. Resonance behavior of the interference terms indicates that the parities of 12B*(4.54) and (5.00) must be opposite (1967LA1N).

Total cross sections from En = 3.4 to 15.5 MeV have been studied by (1961FO07): see Table 12.5 (in PDF or PS). There is no evidence of sharp structure in the range 9.7 < Ex < 17.3 MeV. Limitations of statistical accuracy exclude observation of J = 0 levels above En = 4 MeV, and of J = 1 levels above En = 12 MeV in this work. Total cross sections from En = 14.1 to 18.0 MeV are reported by (1954CO16).

See also (1963BA1F, 1963KU1F, 1963NE1H, 1965MO1J, 1967MA1K) and (1963LU10, 1966AG1A; theor.) and (1959AJ76).

 11. (a) 11B(n, d)10Be Qm = -9.004 Eb = 3.369 (b) 11B(n, t)9Be Qm = -9.561 (c) 11B(n, α)8Li Qm = -6.632 (d) 11B(n, p)11Be Qm = -10.731

Reaction (a) has not been reported. At En = 14.1 MeV, the cross section for reaction (b) is 15 ± 5 mb (1958WY67). The cross section for reaction (c) decreases from 27 mb at En = 12.6 MeV to 16 mb at En = 20.0 MeV (1956AR21). At En = 14.1 MeV, the cross section is 35 ± 7 mb (1959SA04; see also (1962KA37, 1963CH20, 1966MO09)). The cross section for reaction (d) has been measured for En = 14.7 to 16.9 MeV (1962KA37, 1966ST17). See also (1959AL83, 1964ST25).

 12. 11B(d, p)12B Qm = 1.144

Observed proton groups and gamma rays are displayed in Table 12.6 (in PDF or PS). Angular distributions of protons leading to the ground and 0.95 MeV states are of special interest because of the low Q-value: see studies by (1953HO48, 1961GO27, 1961GO28, 1961PU1B, 1962GO24, 1963RO22, 1963SE1F, 1963SE1G, 1964FI1D, 1964TI03, 1965BE1V, 1965FI05, 1965GA02, 1965LO02, 1965SA15, 1966GA09, 1966HA10, 1967BO17, 1967MO1Q, 1967SC29). Spectroscopic factors are listed by (1965GA02, 1966GA09, 1967MO1Q, 1967SC29). Polarization of the recoil 12B is reported by (1959CH1D, 1967SU03).

The 0.95 MeV level is formed by p-wave capture, Jπ = 0+, 1+, 2+ or 3+. The observed anisotropy of the γ-radiation (1960KO03, 1963WA20, 1965BE1V) excludes J = 0. The mean life of the state is 0.30 ± 0.033 psec (1968OL01). So short of a life excludes pure E2 radiation and hence J ≠ 3, and limits δ2 ≡ E2/M1 intensity ratio to < 0.02 for J = 1 or 2 (1963WA20). The observed polarization of the γ-ray also excludes J = 3 (1965BE1V). Gamma-γ correlations in the cascade 1.67 → 0.95 → g.s. fix J = 2; the amplitude mixing ratio δ ≡ E2/M1 for γ (0.95) is in the range -0.124 → 0.00 (1968CH05). The energy of the level is 953.14 ± 0.60 keV (1966WI01). The observed Γγ = 2.2 ± 0.25 meV and the mixing rates E2/M1 are in satisfactory agreement with IPM predictions (1968OL01). See also (1961GO27, 1961GO28, 1966BE31, 1967BO17).

The 1.67 MeV state is formed by s-wave capture, Jπ = 1- or 2-. The gamma decay is mainly direct, with a (3.2 ± 0.5)% (1968OL01), 3.0 ± 0.6% (1968CH05) branch via 12B*(0.95): τm < 50 fsec (1968OL01). See also (1964CH04, 1963WA20, 1965GA02). Gamma-γ correlations lead to an assignment Jπ = 2- (1964CH04, 1965GA02, 1968CH05). The energy of the level is 1673.65 ± 0.60 keV (1966WI01).

The 2.62 MeV level is formed by s-wave capture, Jπ = 1- or 2- (1962GO24, 1965GA02). The level decays mainly via 12B*(0.95) (80 ± 3)%. The ground-state branch is (6 ± 1)% (1968OL01): τm < 70 fsec (1968OL01). See also (1963WA20, 1965GA02). Gamma-γ correlations lead to Jπ = 1-. Ex = 2620.8 ± 1.2 keV (1968OL01), 2618.5 ± 3.5 keV (1964CH04, 1966WI01). See also (1965GA02).

The 2.72 MeV level is only weakly excited; most probably ln = 1, Jπ < / = 3+, although ln = 0, 2 are not excluded (1965GA02). The decay is mainly, if not entirely, ( > 85%) to the g.s. (1968OL01): see also 7Li(7Li, d)12B.

The 3.39 MeV state is formed by ln = 1 (1953HO48); Γγ/Γ < 0.1 (1965GA02). See also (1963BR16, 1966BE1E, 1966GO1L, 1966SC23, 1967CH19).

 13. 11B(t, d)12B Qm = -2.888

Not reported.

 14. 11B(α, 3He)12B Qm = -17.209

Not reported.

 15. 12C(n, p)12B Qm = -12.588

See (1948JE03, 1959AL83, 1967ME11) and 13C in (1970AJ04).

 16. 12C(t, 3He)12B Qm = -13.352

Not reported.

 17. 13C(γ, p)12B Qm = -17.535

See (1963NE02) and 13C in (1970AJ04).

 18. 13C(d, 3He)12B Qm = -12.041

Not reported.

 19. 13C(t, α)12B Qm = 2.280

Not reported.

 20. 14C(n, t)12B Qm = -17.229

Not reported.

 21. 14C(p, 3He)12B Qm = -17.993

Not reported.

 22. 14C(d, α)12B Qm = 0.361

See (1950HU72, 1956DO41).

 23. 14N(n, 3He)12B Qm = -17.366

Not reported.

 24. 15N(n, α)12B Qm = -7.623

See (1948JE03).  