(See Energy Level Diagrams for 12B)
Astrophysical questions: (1970BA1M).
Q = (+0.030 ± 0.008) b (1970WI17);
Q = (0.0151 ± 0.0014) b (1970SU04);
Q = (0.0171 ± 0.0016) b (1971MI06).
Effective μ in various hosts have been measured by (1974MC12): μ ranges from 1.00282 (2) nm in Si, 1.00291 (2) nm in Ge and 1.00287 (2) in SiC.
The half-life of 12B is 20.41 ± 0.06 msec: see Table 12.2 (in PDF or PS) here and Tables 12.1 (in PDF or PS) and 12.2 (in PDF or PS) in (1968AJ02). The decay is complex; 12B decays to 12C*(0, 4.4, 7.7, 10.3): see Table 12.16 (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 MeV (1967GA06: p0 → p3) and E(7Li) = 3.78 to 5.95 MeV (1967KI03: p0, p1, p2, p3+4, p5). Except for p2 the distributions are nearly isotropic. See also (1968AJ02).
Angular distributions have been measured at E(7Li) = 2.10 to 5.75 MeV (1969CA1A: d0, d1, d2, d3+4). The gamma decay of the first four excited states has been studied by (1963CA09): 12B*(0.95) decays of course to the ground state. So, primarily, do 12B*(1.67) [> 98%] and 12B*(2.72) [> 80%], while 12B*(2.62) decays [> 90%] via 12B*(0.95, 1.67). [See also Table 12.7 (in PDF or PS).] The mean lifetimes of 12B*(0.95, 2.62) are 295 ± 37 fsec and < 48 fsec, respectively (1969TH01). See also (1967CA1D, 1974CE06).
Yields of elastically scattered tritons have been measured for Et = 0.60 to 1.70 MeV (1969NA04) and 1.0 to 2.1 MeV (1970CO04). The yields of α-particles [α0, α1] have been obtained for Et = 0.52 to 1.70 MeV by (1969NA04). See also (1970CO04). There is no evidence of the resonance structure observed in reaction 4. See also 8Li and 9Be in (1974AJ01). For reaction (a) see (1969NA04) and 10Be in (1974AJ01).
Observed α-particle groups are displayed in Table 12.4 (in PDF or PS) (1961HO19, 1969GL07, 1971AJ1B). Angular distributions have been measured at E(7Li) = 3.3, 3.5, 3.75 MeV (1961HO19: α0 → α4), 5.6 to 6.2 MeV (1969SN02: α0 → α4) and 30.3 MeV (1969GL07: to most states listed in Table 12.3 (in PDF or PS)). See also (1970OG1A).
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).
Excited states of 12B observed by (1960JA17) at Et = 5.5 MeV and by (1964MI04) at Et = 10 MeV are displayed in Table 12.5 (in PDF or PS). See also (1968CO1H), (1967OG1A) and (1969SO02, 1970BA58, 1970MA04, 1970MA38, 1970NE1F, 1971KA1E, 1974GU1D; theor.).
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). For astrophysical implications see (1968FO1A).
The thermal (bound) scattering cross section is 3.9 ± 0.2 b. The scattering amplitude (bound) is a = 6.1 ± 0.1 fm (1973MU14). See also (1969BA1P). Parameters of observed resonances in σtot are shown in Table 12.6 (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*(3.388) has Jπ = 3-. The neutron width [3.1 ± 0.6 eV] is about the Wigner limit (1969MO10). The 0.43 MeV resonance [12B*(3.76)] is formed by l = 1; γ2S=1 = γ2S=2; Jπ = 2+ (1955WI25, 1962EL01, 1970LA21).
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*(4.30) fitted with Jπ = 1-. The analysis also confirms Jπ = 1+ for 12B*(4.99) (1970LA21). Differential cross sections have also been measured for 2.2 < En < 4.5 MeV by (1973NE1F, 1973NE19) and analyzed by a three-level R-matrix calculation. The fit is quite good if assignments 1- and (1+) are made for broad states of 12B at Ex = 5.8 and 6.8 MeV, respectively [En = 2.7 and 3.8 MeV]. 12B*(5.61, 5.73, 6.6) are assigned Jπ = 3+, 3- and (1+), respectively (1973NE19). See also (1974BI07; theor.). For a study of the excitation function of 11B(n, n'γ1) see (1961LI04).
Total cross sections from En = 3.4 to 15.5 MeV have been studied by (1961FO07): see Table 12.6 (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). Cross section measurements are also reported at En = 7.55 MeV (1969HO1G: elastic, and inelastic to 11B*(2.12, 4.45)), 9.72 MeV (1970CO12: elastic, and inelastic to 11B states below 6.8 MeV), 14.1 MeV (1970AL08: elastic, and inelastic to 11B states below 8.6 MeV) and 14.5 MeV (1970AN1F: natB). See also (1972LA1F), (1969AN1E, 1969MA39, 1969RO1F, 1970PO1E) and (1967BE1F; theor.).
The cross section for reaction (a) has been measured for En = 14.7 to 16.9 MeV (1962KA1A, 1966ST17, 1967FL16, 1967NA1C). See also (1971CU1B, 1971PR09, 1972ED01). For reaction (b) see 10Be in (1974AJ01). For reaction (c) see (1958WY67, 1972BI1E). See also 9Be in (1974AJ01). The cross section for reaction (d) has been measured by (1956AR21, 1959SA04, 1970SC29, 1973BO26) in the range En = 12.6 to 20.0 MeV, and at En = 25 and 38 MeV (1972KI19): no resonances are observed. See also 8Li in (1974AJ01) and (1968AJ02).
Angular distributions have been studied at many energies: see (1968AJ02) for earlier references and Table 12.7 (in PDF or PS). Recent interest has centered on the first unbound state in 12B, at Ex = 3.39 MeV: a DWBA analysis, when combined with information from 11B + n (see reactions 10 and 11), finds ln = 2 and is consistent with Jπ = 3- for 12B*(3.39) (1969FO10). See also (1971BU02; theor.). Assuming ln = 2 to this state (1972DZ06, 1973DO02) analyzed the data in terms of a peripheral stripping model and calculated Γn = (1.9+0.8-0.6) eV for 12B*(3.39) [see also Table 12.6 (in PDF or PS)]. The probability p3/2 for transfer of a neutron with angular momentum 3/2 has been determined for the p0 and p1 groups using vector polarized deuterons with Ed = 10 and 12 MeV (1970FI07).
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+, 2+ 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 12.13 (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).
Observation of γ-transitions has led to the determination of the capture rates to 12B*(0, 0.95, 1.67, 2.62) (1970BU1B, 1972MI15); those to 12B*(0.95, 1.67) are consistent with zero (1972MI15). Using polarized muons, (1974PO05) have determined the average polarization of 12Bg.s. to be 0.43 ± 0.10. See also (1970HI09).
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).
At Ed = 28 MeV angular distributions have been measured for the 3He groups to 12B*(0, 0.95) (1972BR27). [See reaction 66 in 12C for a discussion of the population of the analog states in 12C in the mirror reaction 13C(d, t)12C.]
At Ep = 63.4 MeV, the excitation of the lowest T = 2 state in 12B is stated to be at Ex = 12.710 ± 0.020 MeV (1971NE1B, 1971NE1E: unpublished). This state has also been observed at Ep = 54 MeV as has another at Ex = 14.7 ± 0.1 MeV. The angular distribution of the lower state is consistent with L = 0; that for the higher state is rather featureless (D. Ashery, private communication) [see also reaction 70 in 12C]. See also (1970OL1B).
At E(14N) = 118 MeV the excitation of 12B*(0, 0.95) is reported by (1974AN36).