(See Energy Level Diagrams for 12B)
Complex reactions involving 12B:(1979AL22, 1979BO22, 1980MA24, 1980NA1C, 1980TR1F, 1980WI1L, 1981GR08, 1981ME13, 1981MO20, 1982GO1E, 1982LY1A, 1983EN04, 1983MA06, 1983OL1A, 1983SA06, 1983WI1A, 1984GR08, 1984HI1A).
Muon and neutrino capture and reactions (See also reaction 17.):(1979HW02, 1979KI1G, 1979PA19, 1979PR1C, 1979WU10, 1980MU1B, 1981AM05, 1981CI05, 1981EI1A, 1981GI08, 1981PA1G, 1981RO05, 1981RO15, 1982BO1W, 1982MI05, 1982RO1F, 1982RO13, 1983FU17, 1983GM01, 1983KU1L, 1983NO12, 1984BO1M).
Pion and kaon capture and reactions (See also reactions 13 and 16.):(1978GO18, 1979AL21, 1980AR01, 1980DU20, 1980HA18, 1980OH07, 1980RA05, 1980SO05, 1980TR1A, 1981DE01, 1981DE1U, 1981DO1F, 1981DU1H, 1981EI01, 1981RA16, 1982AR06, 1982BE1D, 1982BO11, 1982GO01, 1982GOZX, 1982RA28, 1983BE1A, 1983CO1L, 1983DE2G, 1983GI08, 1983HS01, 1983MO1J, 1983SH2C, 1983TR1J).
Ground-state properties of 12B, including its polarization:(1980FU1G, 1980TR1F, 1981AV02, 1981KO1D, 1981MI1C, 1981MI1D, 1981NE1B, 1981SE06, 1982BA37, 1982DE16, 1982NG01, 1982RE01, 1983ANZQ, 1983FU15, 1983KU1L, 1983MI28, 1983NO05, 1983TA1K, 1984KU07).
The half-life of 12B is 20.20 ± 0.02 msec (1978AL01). The decay is complex. 12B decays to 12C*(0, 4.4, 7.7, 10.3): see Table 12.14 (in PDF or PS). The transitions to 12C*(0, 4.4) are allowed: hence the Jπ of 12Bg.s. is 1+. See also (1983AD1B) and (1979HW03, 1979KI1G, 1979MA31, 1979NO01, 1979PR1C, 1979RH1A, 1980OK01, 1980SY02, 1981CH1B, 1981HO06, 1981KO27, 1982GU09, 1983MO1N; theor.).
Eleven groups of protons are reported to known states of 12B. Angular distributions have been measured at E(6Li) = 3.5 to 5.95 MeV. The distributions are generally featureless: see (1975AJ02).
Thirteen resonances have been reported corresponding to 13.6 < Ex < 14.7 MeV: see Table 12.3 (in PDF or PS) in (1975AJ02). The yield of 2.12 MeV γ-rays has been measured for Et = 1.5 to 3.3 MeV and E(9Be) = 10 to 16 MeV by (1983MI08): no resonances are observed.
Yields of elastically scattered tritons have been measured for Et = 0.60 to 2.1 MeV: see (1975AJ02). Differential cross sections and analyzing power for elastic tritons have been measured at Epol. t = 15 and 17 MeV (1978SC02). The yields of 0.32 MeV (reaction (a)), 0.98 MeV (reaction (d)) and 0.48 MeV γ-rays (from the (t, αn) reaction) have been studied for Et = 1.5 to 3.3 MeV and E(9Be) = 10 to 16 MeV: no resonances are observed (1983MI08). The yields of α0 and α1 have also been reported for Et = 0.52 to 1.70 MeV: see (1975AJ02). The analyzing powers of the reactions leading to 6Heg.s. and 6Li*(0, 3.56) have been measured at Epol. t = 17 MeV (1979FL03). See also (1983CE01), (1981ME1D, 1983WE02; theor.), 11Be, and 6He, 6Li, 8Li and 9Be in (1984AJ01).
Observed α-particle groups are displayed in Table 12.3 (in PDF or PS). Angular distributions have been measured at E(7Li) = 3.3 to 6.2 MeV, at 20 MeV and at 30.3 MeV: see (1975AJ02, 1980AJ01). At E(7Li) = 20 MeV angular distributions to the first seven states are rather featureless and have approximate symmetry about 90°. The integrated cross sections go as 2Jf + 1 consistent with a compound nucleus mechanism for the transitions populating the low-lying states of 12B. It is suggested that the sharp states of 12B at high excitation energies correspond to states of high angular momenta with cluster configurations.
The cross sections for production of 8Li (reaction (b)) and of 11Be (reaction (a)) have been measured for Ed = 0.67 to 3.0 MeV and 2.3 to 12 MeV, respectively: the yields for both reactions vary smoothly with energy. No resonances are observed: see (1975AJ02).
The thermal-neutron capture cross section is 5.5 ± 3.3 mb [see (1981MUZQ)]. The capture cross section shows resonances at En = 20.8 ± 0.5 keV and at 0.43, 1.03, 1.28 and 1.78 MeV, with Γγ = 25 ± 8 meV and 0.3, 0.3, 0.2 and 0.9 eV (± 50%): see Table 12.4 (in PDF or PS) and (1968AJ02). See also (1980DO1C; astrophys.).
The thermal (bound) scattering cross section is 3.9 ± 0.2 b. The scattering amplitude (bound) is a = 6.65 ± 0.04 fm, σ(free) = 4.84 ± 0.04 b (1983KO17). Parameters of observed resonances are shown in Table 12.4 (in PDF or PS). See also (1981MUZQ). Differential cross sections have been studied for En = 75 keV to 4.5 MeV [see (1980AJ01)] and for 2.6 to 8.0 MeV (1980WH01; n0) and 4.86 to 7.55 MeV (1983KO03; n0, n1 and, at higher energies, n2, n3). Total cross-section measurements have been reported for En = 0.3 to 18.0 MeV [see (1968AJ02, 1980AJ01)] and 4.86 to 7.55 MeV (1983KO03). The polarization has been measured at En = 75 keV to 2.2 MeV: see (1980AJ01). See also (1983GO1H).
Results from the most recent R-matrix analysis are displayed in Table 12.4 (in PDF or PS) (1983KO03). See also (1980WH01, 1983DA22). For a discussion of the earlier work see (1980AJ01). See also (1979GLZV, 1980HA35; theor.).
The cross section for reaction (a) has been measured for En = 14.7 to 16.9 MeV and that for reaction (b) has been investigated for En = 12.6 to 20.0 MeV and at 25 and 38 MeV: see (1975AJ02). A recent study of reaction (d) at En = 14.4 MeV is reported by (1979AN18). See also (1980AJ01).
(1979MA38) have measured the cross section for π+ production near threshold. At Ep = 200 MeV the population of 12B*(0, 0.95, 1.67, 2.62, 3.39, 3.76, 4.30 + 4.52, 5.00, 5.61) is reported by (1980SO05). See also the "GENERAL" section here.
Observed proton groups and γ-rays are displayed in Table 12.5 (in PDF or PS). The Jπ assignments for 12B*(0.95, 1.67) are derived as follows [see (1968AJ02) for a listing of earlier references]: 0.95 MeV: ln = 1 leads to Jπ = 0+, 1+, 2+ or 3+. The γ-radiation is anisotropic and therefore J ≠ 0. τm is too short for pure E2 and hence J ≠ 3, which is conformed by studies of the polarization of γ1. The results are consistent with Jπ = 1+ and 2+. The latter is fixed by γγ correlations in the cascade 1.67 → 0.95 → g.s. The mixing ratio δ = -0.08 ± 0.06. 1.67 MeV: ln = 0 and therefore Jπ = 1- or 2-. The state decays primarily to 12Bg.s.. Gamma-gamma correlations lead to Jπ = 2-. An assignment of 1- to 12B*(2.62) is made in a similar manner.
Using monoenergetic photons with Eγ = 210 to 381 MeV (1982AR06) have measured the total cross section for π+ emission and the spectra of the positive pions. The latter show the influence of quasi-free pion production and FSI processes. At Ee = 195 and 200 MeV the π+ energy distributions show contributions from 12B*(0, 0.95, ≈ 4.5, 7.0) (1980MI08, 1980SH22, 1983SH1W). The 2- and 4- states at Ex ≈ 4.5 MeV are compared with their isobaric analogs in 12C at Ex ≈ 19.5 MeV (1980MI08). At Ee = 400 MeV (1983SC03, 1983SC11) have studied π+ with Eπ = 32 MeV: double differential cross sections are obtained for the transitions to 12B*(0, 0.95, 1.67) and single differential cross sections to 12B*(0, 0.95). (1983SC03) found that the cross section (at θ = 54°) is the same whether virtual or real photons are used in producing the pions.
Observations of γ-transitions have led to the determination of the capture rates to 12B*(0, 0.95, 1.67, 2.62) [Jπ = 1+, 2+, 2-, 1-] (1981GI08, 1981RO15). The ratio of the polarization of 12Bg.s., Pav, and of the longitudinal polarization, PL, has been determined by (1981RO05, 1981RO15, 1982RO13): this ratio leads to a neutrino helicity, hν = -1.08 ± 0.11, in agreement with the partial conservation of axial-vector current (PCAC) hypothesis. See also the "GENERAL" section here. For the earlier work see (1980AJ01).
At En = 59.6 MeV (1982BR04) have determined the angular distribution of the p0 group. Broad (unresolved) structures are also observed with Ex < 10 MeV (1984BR03). The use of this reaction for the production of polarized 12B nuclei is discussed by (1982DE16). For the earlier work see (1980AJ01).
Angular distributions are reported at Ed = 55 MeV to 12B*(0, 0.95, 4.5). 12B*(1.67, 5.6 and 8.0) are also populated (1979ST15). At Ed = 99.2 MeV σ(θ) at forward angles have been measured for the transitions to 12B*(0, 4.5). The differential cross sections are nearly the same for the two groups (1982BE33).
Angular distributions have been measured for the transitions to 12B*(0, 0.95) at Ed = 24.1 to 29 and at 52 MeV. Distributions to 12B*(2.72, 3.76, 5.00) have been reported at Ed = 52 MeV: see (1980AJ01). The spectroscopic factors for 12B*(0, 0.95) are S = 1.1 ± 0.2 and 2.0 ± 0.5. C2S = 1.09 [assuming 1p3/2], 2.17, 0.14, 0.07, 0.22 [assuming 1p1/2] for 12B*(0, 0.95, 2.72, 3.76, 5.00) [from measurements at Ed = 52 MeV]. See (1980AJ01) for references. For a discussion of analog states of 12B and 12C see reaction 67 in 12C. See also 15N in (1981AJ01), (1979SHZH, 1983AD1B) and (1980LE14; theor.).
At Ep = 54 MeV, in addition to transitions to 12B*(0, 0.95, 5.61), the population of T = 2 states at Ex = 12.72 ± 0.07 and 14.82 ± 0.10 MeV is reported. The angular distribution of 3He ions to 12B*(12.75) is fitted by L = 0; that to 12B*(12.82) is rather featureless [its T = 2 characater is assigned from the energies of the analog states]: both states have Γc.m. ≲ 200 keV (1976AS01). See also reaction 72 in 12C and (1983AR1K).