(See Energy Level Diagrams for 12B)
Electromagnetic transitions: (1974HA1C).
Special reactions: (1975HU14, 1976AB04, 1976BE1K, 1976BU16, 1976LE1F, 1976OS04, 1977AR06, 1977SU02, 1977TA1L, 1977UD1A, 1978AB08, 1978GE1C, 1978HE1C, 1978IS01, 1978IS1C, 1978KO01, 1978TA20, 1978UD01, 1979IS03).
Muon capture (See also reaction 17.): (1975KI2A, 1976HO1G, 1977BA1P, 1977DE1U, 1977LE1K, 1977PO1B, 1977PR1B, 1978DE15, 1978HW1A, 1978KO31, 1978MU04, 1978PA1F, 1978SE1B, 1978WU01, 1979OH1A, 1979PA1J, 1979PE1C, 1979PR1D, 1979RO03, 1979RO1H, 1979SH1N, 1979TR05, 1979WI1A).
Pion capture and pion reactions (See also reaction 16.): (1974CA1K, 1976AL1J, 1976TR1A, 1976TZ1A, 1977BA1Q, 1977BA2G, 1977DO06, 1977FU11, 1977RA1A, 1977SH14, 1978ER1C, 1978FU04, 1978FU09, 1978KO34, 1978SI1E, 1979BA1M, 1979NA1Q, 1979SO1C).
Applied topics: (1978MC1H).
Q = 1.34 ± 0.14 fm2 (1978MI19).
The half-life of 12B is 20.20 ± 0.02 msec (1978AL01): see Table 12.2 (in PDF or PS) of (1968AJ02) for a summary of earlier values. The decay is complex. 12B decays to 12C*(0, 4.4, 7.7, 10.3): see Table 12.15 (in PDF or PS). The transitions to 12C*(0, 4.4) are allowed: hence the Jπ of 12Bg.s. is 1+.
At E(7Li) = 2 MeV eleven groups of protons are reported to known states of 12B (1959MO12). Angular distributions have been measured at E(6Li) = 3.5 to 5.95 MeV. The distributions are generally featureless: see (1975AJ02).
Angular distributions have been measured at E(7Li) = 2.10 to 5.75 MeV (1969CA1A: d0, d1, d2, d3+4). The mean lifetimes of 12B*(0.95, 2.62) are 295 ± 37 fsec and < 48 fsec, respectively (1969TH01). For γ-decay results see (1975AJ02).
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 Et-bar = 15 and 17 MeV (1978SC02). The yield of α0 and α1 (reaction (d)) have been obtained for Et = 0.52 to 1.70 MeV (1969NA04). There is no evidence of the resonance structure reported in reaction 4. The analyzing powers of the reactions leading to 6Heg.s. and 6Li*(0, 3.56) have been measured at Et-bar = 17 MeV (1979FL03). See also 6He, 6Li, 8Li and 9Be in (1979AJ01) and 11Be here.
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 30.3 MeV [see (1975AJ02)] and at 20 MeV (1975AJ03). 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 (1975AJ03).
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: the yields for both reactions vary smoothly with energy. No resonances are observed (1970GO11, 1973GO09).
The thermal neutron capture section is 5 ± 3 mb (1962IM01) [(1973MU14) adopt 5.5 ± 3.3 mb]. The capture cross section shows a resonance at En = 20.8 ± 0.5 keV (see also reaction 11) with Γγ = 0.025 ± 0.008 eV (1969MO10). 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). See also (1976GAYV).
The thermal (bound) scattering cross section is 3.9 ± 0.2 b. The scattering amplitude (bound) is a = 6.1 ± 0.1 fm (1973MU14). Parameters of observed resonances in σtot are shown in Table 12.5 (in PDF or PS).
Comparison of 11B + n and of data from 11B(d, p) shows that the En = 20.8 keV resonance is d-wave and that 12B*(3.389) has Jπ = 3-. The neutron width [3.1 ± 0.6 eV] is about the Wigner limit. The 0.43 MeV resonance [12B*(3.76)] is formed by l = 1; γ2S=1 = γ2S=2; Jπ = 2+.
The polarization and differential cross sections have been measured for 0.075 ≤ En ≤ 2.2 MeV by (1970LA21) as has σt for 0.3 ≤ En ≤ 2.05 MeV. A two-channel R-matrix analysis fits both σ(θ) and P(θ) assuming broad 2- (l = 0) and 4- (l = 2) states at Ex = 4.37 and 4.54 MeV [En = 1.09 and 1.28 MeV] in addition to the sharp state 12B*(4.31) fitted with Jπ = 1-. The analysis also confirms Jπ = 1+ for 12B*(5.00) (1970LA21). Differential cross sections have also been measured for 2.2 < En < 4.5 MeV by (1973NE19) and for 4.0 to 8.0 MeV by (1977WH1B, 1978WH1B). See also (1976LA1C, 1977WH1A).
Recent additional measurements (1978WH1B) and R-matrix analysis for 2.6 < En < 4.0 MeV confirm Jπ = 1- (ln = 0, 2) for the 12B* state at Ex = 5.8 - 6.0 MeV, and give 1- (ln = 0, 2) for 12B*(6.9) instead of 1+ (1973NE19). In the region 7.5 < Ex < 8.5 MeV 1- and 2- states of ln = 0, 2 are present (1977WH1B, 1978WH1B) and (R. Lane, private communication). Assignments of states at higher energies are displayed in Table 12.5 (in PDF or PS) (1977WH1B, 1978WH1B). See also (1975AJ02).
Total cross sections from En = 3.4 to 15.5 MeV (1961FO07) and from 1 to 14 MeV (1979AU07) have been studied: 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 (1961FO07). The σt for natural boron has been measured for En = 2.5 to 15 MeV (1971FO1A). Other cross section measurements have been reported at En = 7.55 to 14.5 MeV [see (1975AJ02)] and 14.1 MeV (1974HY01). See also 11B and (1976GAYV).
The cross section for reaction (a) has been measured for En = 14.7 to 16.9 MeV: see (1975AJ02). See also (1974BO1E). For reaction (b) see (1976SL2A). For reaction (c) see (1978QA01). See also (1973BI1B) and (1976SL2A). The cross section for reaction (d) has been measured in the range En = 12.6 to 20.0 MeV and at En = 25 and 38 MeV: see (1975AJ02). No resonances are observed. See also (1974BO1E, 1974CA1J, 1976SL2A) and (1976GAYV).
Observed proton groups and γ-rays are displayed in Table 12.6 (in PDF or PS). The Jπ assignments for 12B*(0.95, 1.67) are derived as follows [see (1968AJ02) for detailed listing of earlier references]: 0.95 MeV: ln = 1 leads to Jπ = 0+, 1+, or 3+. The γ-radiation is anisotropic and therefore J ≠ 0 (1963WA20, 1968CO14). τm is too short for pure E2 and hence J ≠ 3, which is confirmed by studies of the polarization of γ1, most recently by (1968GO17: Ed = 0.5 to 5.5 MeV). The results are consistent with Jπ = 1+ or 2+. The latter is fixed by γ - γ correlations in the cascade 1.67 → 0.95 → g.s. (1968CH05). The mixing ratio δ = -0.08 ± 0.06 (1968GO17). Γγ = 2.2 ± 0.25 meV (1968OL01). See also (1968GO18, 1974KA29); 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- (1968CH05). An assignment of 1- to 12B*(2.62) is made in a similar manner.
(1971MO14) have analyzed existing 11B(d, p)12B and 11B + p reactions and have listed the properties of the first seven T = 1 states in 12B and 12C: see Table 12.12 (in PDF or PS). Neutron reduced widths γ2λn for the first six excited states in 12B were calculated from spectroscopic factors and compared with 2γ2λp for the corresponding 12C states. The agreement was quite good once new values for the partial widths of 12C*(16.11) [see (1974AN19)] became available (1971MO14). For the polarization of 12Bg.s. see (1977PO1B) and reaction 15.
The j mixing Als3/2/Als1/2 = -0.11 ± 0.03 or -0.90 ± 0.06 for 12Bg.s. and +0.12 ± 0.02 for 12B*(0.95) (1976TA07). See also (1975HU1H, 1975MC1E, 1975MI1F, 1976MI1J, 1977KA1V), (1976HA1J), (1977SA25; theor.) and 13C in (1981AJ01).
π+ electroproduction has been studied at Eπ ≈ 17 and 29 MeV to 12B*(0.95, 1.7, 2.6, 4.5) (1979PA06). See also (1979MI06) and reaction 39 in 12C. At Ee = 195 MeV, angular distributions are reported for the π+ to 12Bg.s. and to possibly unresolved excited states (1977SH14), including 12B*(4.5) (1979SH1R). For reaction (c) see (1979AL1M). See also (1975HU1D, 1978FU04, 1978FU09; theor.).
Observations of γ-transitions have led to the determination of the capture rates to 12B*(0, 0.95, 1.67, 2.62); those to 12B*(0.95, 1.67) are consistent with zero (1972MI15). The polarization of 12Bg.s., Pav = 0.452 ± 0.042 (1977PO1B). The longitudinal polarization, PL = -0.83 ± 0.21 (1979TR05). These two values lead to a neutrino polarization hν = -0.89 ± 0.23 [left-handed] (1979TR05). See also (1976SU03, 1978KO31; theor.).
At E(7Li) = 52 MeV the population of 12B*(0, 0.95, 1.71, 2.70, 2.87) and 7Be(0) [and in the case of 12B(0), of 7Be*(0.43)] is reported by (1973BA34).
Angular distributions have been measured for the transitions to 12B*(0, 0.95) at Ed = 24.1, 26.2 and 27.5 MeV (1977LI02), 28 MeV (1972BR27) and 52 MeV (1975MA41; also to 12B*(2.72, 3.76, 5.00)) and at Ed = 29 MeV (1979CO08). (1977LI02) find S = 1.1 ± 0.2 and 2.0 ± 0.5 for the spectroscopic factors to 12B*(0, 0.95). 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) (1975MA41). See also (1979CO08). For a discussion of analog states of 12B and 12C see reaction 73 in 12C and (1977LI02, 1979CO08). See also (1977RI08) and (1979CO08) in 15N (1981AJ01).
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*(12.72) is fitted by L = 0; that to 12B*(14.82) is rather featureless [its T = 2 character is assigned from the energies of the analog states]: both states have Γc.m. ≲ 200 keV (1976AS01). See also (1975AJ02), and reaction 80 in 12C.