
^{12}B (1975AJ02)(See Energy Level Diagrams for ^{12}B) GENERAL: See also (1968AJ02) and Table 12.2 [Table of Energy Levels] (in PDF or PS). Model calculations: (1968FU1B, 1968GU11, 1969MO1F, 1969VA1C, 1970TA1J, 1973HA49, 1973SA30). Special levels: (1968CE1A, 1968GU11, 1970FR1C, 1973SA30). Electromagnetic transitions: (1969VA1C, 1973HA49, 1973SA30). Special reactions: (1969AR13, 1969GA18, 1971AR02, 1973KO1D, 1973WI15, 1974FO22). Muon capture (See also reaction 16.): (1969VA37, 1970BU1B, 1970HI09, 1971MO1Q, 1971MU20, 1972BA1X, 1972DE50, 1972MI15, 1973CH16, 1973MU04, 1973MU11, 1973MU1B, 1974DE1V, 1974IM1C, 1974PO05, 1975SU1G). Pion capture and pion reactions: (1970HI09, 1970HI10, 1970RA39, 1971GU13, 1971SK03, 1971SK02, 1973AN1J, 1973KA1D, 1973UB01, 1974DE1U, 1974SE02). Astrophysical questions: (1970BA1M). Other topics: (1971BA2Y, 1972AN05, 1972GA1L, 1972OC01, 1972PN1A, 1973JU2A, 1973WI15, 1974IR04, 1975KI1M, 1975ZI1D). Ground state: μ = +(1.00285 ± 0.00015) nm (1970WI17, 1970WI1L); 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. See also (1967SH14, 1968WE12, 1969FU11, 1969VA1C, 1969WE1G, 1971SH26, 1972VA36, 1972WI08, 1973HOYD, 1973KU1L, 1973MA1K, 1973MIYZ, 1973SA30, 1973ST1P, 1973SU1B, 1974CO1P, 1974MC07).
The halflife of ^{12}B 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; ^{12}B decays to ^{12}C*(0, 4.4, 7.7, 10.3): see Table 12.16 (in PDF or PS). The transitions to ^{12}C*(0, 4.4) are allowed: hence the J^{π} of ^{12}B_{g.s.} is 1^{+}.
At E(^{7}Li) = 2 MeV, eleven groups of protons are reported to known states of ^{12}B (1959MO12). Angular distributions have been measured at E(^{6}Li) = 3.5 MeV (1967GA06: p_{0} → p_{3}) and E(^{7}Li) = 3.78 to 5.95 MeV (1967KI03: p_{0}, p_{1}, p_{2}, p_{3+4}, p_{5}). Except for p_{2} the distributions are nearly isotropic. See also (1968AJ02).
Angular distributions have been measured at E(^{7}Li) = 2.10 to 5.75 MeV (1969CA1A: d_{0}, d_{1}, d_{2}, d_{3+4}). The gamma decay of the first four excited states has been studied by (1963CA09): ^{12}B*(0.95) decays of course to the ground state. So, primarily, do ^{12}B*(1.67) [> 98%] and ^{12}B*(2.72) [> 80%], while ^{12}B*(2.62) decays [> 90%] via ^{12}B*(0.95, 1.67). [See also Table 12.7 (in PDF or PS).] The mean lifetimes of ^{12}B*(0.95, 2.62) are 295 ± 37 fsec and < 48 fsec, respectively (1969TH01). See also (1967CA1D, 1974CE06).
Reported resonances in the yield of neutrons at θ = 0° are listed in Table 12.3 (in PDF or PS) (1961VA1C). See also ^{11}B.
Yields of elastically scattered tritons have been measured for E_{t} = 0.60 to 1.70 MeV (1969NA04) and 1.0 to 2.1 MeV (1970CO04). The yields of αparticles [α_{0}, α_{1}] have been obtained for E_{t} = 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 ^{8}Li and ^{9}Be in (1974AJ01). For reaction (a) see (1969NA04) and ^{10}Be in (1974AJ01).
See (1968AJ02).
Observed αparticle groups are displayed in Table 12.4 (in PDF or PS) (1961HO19, 1969GL07, 1971AJ1B). Angular distributions have been measured at E(^{7}Li) = 3.3, 3.5, 3.75 MeV (1961HO19: α_{0} → α_{4}), 5.6 to 6.2 MeV (1969SN02: α_{0} → α_{4}) 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 ^{8}Li (reaction (b)) and of ^{11}Be (reaction (a)) have been measured for E_{d} = 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 ^{12}B observed by (1960JA17) at E_{t} = 5.5 MeV and by (1964MI04) at E_{t} = 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 E_{n} = 20.8 ± 0.5 keV (see also reaction 11) with Γ_{γ} = 0.025 ± 0.008 eV (1969MO10). In the range 140 to 2325 keV, resonances are observed at E_{n} = 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 σ_{tot} are shown in Table 12.6 (in PDF or PS). Comparison of ^{11}B + n and of data from ^{11}B(d, p) shows that the E_{n} = 20.8 keV resonance is dwave and that ^{12}B*(3.388) has J^{π} = 3^{}. The neutron width [3.1 ± 0.6 eV] is about the Wigner limit (1969MO10). The 0.43 MeV resonance [^{12}B*(3.76)] is formed by l = 1; γ^{2}_{S=1} = γ^{2}_{S=2}; J^{π} = 2^{+} (1955WI25, 1962EL01, 1970LA21). The polarization and differential cross sections have been measured for 0.075 ≤ E_{n} ≤ 2.2 MeV by (1970LA21) as has σ_{t} for 0.3 ≤ E_{n} ≤ 2.05 MeV. A twochannel Rmatrix analysis fits both σ(θ) and P(θ) assuming broad 2^{} (l = 0) and 4^{} (l = 2) states at E_{x} = 4.37 and 4.54 MeV [E_{n} = 1.09 and 1.28 MeV] in addition to the sharp state ^{12}B*(4.30) fitted with J^{π} = 1^{}. The analysis also confirms J^{π} = 1^{+} for ^{12}B*(4.99) (1970LA21). Differential cross sections have also been measured for 2.2 < E_{n} < 4.5 MeV by (1973NE1F, 1973NE19) and analyzed by a threelevel Rmatrix calculation. The fit is quite good if assignments 1^{} and (1^{+}) are made for broad states of ^{12}B at E_{x} = 5.8 and 6.8 MeV, respectively [E_{n} = 2.7 and 3.8 MeV]. ^{12}B*(5.61, 5.73, 6.6) are assigned J^{π} = 3^{+}, 3^{} and (1^{+}), respectively (1973NE19). See also (1974BI07; theor.). For a study of the excitation function of ^{11}B(n, n'γ_{1}) see (1961LI04). Total cross sections from E_{n} = 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 < E_{x} < 17.3 MeV. Limitations of statistical accuracy exclude observation of J = 0 levels above E_{n} = 4 MeV, and of J = 1 levels above E_{n} = 12 MeV in this work (1961FO07). The σ_{t} for natural boron has been measured for E_{n} = 2.5 to 15 MeV (1971FO1A). Cross section measurements are also reported at E_{n} = 7.55 MeV (1969HO1G: elastic, and inelastic to ^{11}B*(2.12, 4.45)), 9.72 MeV (1970CO12: elastic, and inelastic to ^{11}B states below 6.8 MeV), 14.1 MeV (1970AL08: elastic, and inelastic to ^{11}B states below 8.6 MeV) and 14.5 MeV (1970AN1F: ^{nat}B). See also (1972LA1F), (1969AN1E, 1969MA39, 1969RO1F, 1970PO1E) and (1967BE1F; theor.).
The cross section for reaction (a) has been measured for E_{n} = 14.7 to 16.9 MeV (1962KA1A, 1966ST17, 1967FL16, 1967NA1C). See also (1971CU1B, 1971PR09, 1972ED01). For reaction (b) see ^{10}Be in (1974AJ01). For reaction (c) see (1958WY67, 1972BI1E). See also ^{9}Be in (1974AJ01). The cross section for reaction (d) has been measured by (1956AR21, 1959SA04, 1970SC29, 1973BO26) in the range E_{n} = 12.6 to 20.0 MeV, and at E_{n} = 25 and 38 MeV (1972KI19): no resonances are observed. See also ^{8}Li in (1974AJ01) and (1968AJ02).
Observed proton groups and gamma rays are displayed in Table 12.7 (in PDF or PS). 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 ^{12}B, at E_{x} = 3.39 MeV: a DWBA analysis, when combined with information from ^{11}B + n (see reactions 10 and 11), finds l_{n} = 2 and is consistent with J^{π} = 3^{} for ^{12}B*(3.39) (1969FO10). See also (1971BU02; theor.). Assuming l_{n} = 2 to this state (1972DZ06, 1973DO02) analyzed the data in terms of a peripheral stripping model and calculated Γ_{n} = (1.9^{+0.8}_{0.6}) eV for ^{12}B*(3.39) [see also Table 12.6 (in PDF or PS)]. The probability p_{3/2} for transfer of a neutron with angular momentum 3/2 has been determined for the p_{0} and p_{1} groups using vector polarized deuterons with E_{d} = 10 and 12 MeV (1970FI07). The J^{π} assignments for ^{12}B*(0.95, 1.67) are derived as follows [see (1968AJ02) for detailed listing of earlier references]: 0.95 MeV: l_{n} = 1 leads to J^{π} = 0^{+}, 1^{+}, 2^{+} or 3^{+}. The γradiation is anisotropic and therefore J ≠ 0 (1963WA20, 1968CO14). τ_{m} is too short for pure E2 and hence J ≠ 3, which is confirmed by studies of the polarization of γ_{1}, most recently by (1968GO17: E_{d} = 0.5 to 5.5 MeV). The results are consistent with J^{π} = 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). Γ_{γ} = 2.2 ± 0.25 meV (1968OL01). See also (1968GO18, 1974KA29). 1.67 MeV: l_{n} = 0 and therefore J^{π} = 1^{} or 2^{}. The state decays primarily to ^{12}B_{g.s.}. Gammagamma correlations lead to J^{π} = 2^{} (1968CH05). An assignment of 1^{} to ^{12}B*(2.62) is made in a similar manner. (1971MO14) have analyzed existing ^{11}B(d, p)^{12}B and ^{11}B + p reactions and have listed the properties of the first seven T = 1 states in ^{12}B and ^{12}C: see Table 12.13 (in PDF or PS). Neutron reduced widths γ^{2}_{λn} for the first six excited states in ^{12}B were calculated from spectroscopic factors and compared with 2γ^{2}_{λp} for the corresponding ^{12}C states. The agreement was quite good once new values for the partial widths of ^{12}C*(16.11) [see (1974AN19)] became available (1971MO14). See also (1967SP09, 1967TI1A, 1970WI17, 1973HOYD), (1973FI1C) and (1970VO09; theor.).
Not reported.
Not reported.
Observation of γtransitions has led to the determination of the capture rates to ^{12}B*(0, 0.95, 1.67, 2.62) (1970BU1B, 1972MI15); those to ^{12}B*(0.95, 1.67) are consistent with zero (1972MI15). Using polarized muons, (1974PO05) have determined the average polarization of ^{12}B_{g.s.} to be 0.43 ± 0.10. See also (1970HI09).
At E_{n} = 54 MeV strong excitation of ^{12}B*(0, 4.4, 7.8) is reported (1973KI1K). See also (1968AJ02) and ^{13}C in (1976AJ04).
Not reported.
At E(^{7}Li) = 52 MeV the population of ^{12}B*(0, 0.95, 1.71, 2.70, 2.87) and ^{7}Be(0) [and in the case of ^{12}B(0), of ^{7}Be*(0.43)] is reported by (1973BA34).
At E_{bs} = 15 to 40 MeV, ^{12}B*(0.95) is weakly excited (1971WI1N). See also (1973KI1J; theor.) and ^{13}C in (1976AJ04).
At E_{d} = 28 MeV angular distributions have been measured for the ^{3}He groups to ^{12}B*(0, 0.95) (1972BR27). [See reaction 66 in ^{12}C for a discussion of the population of the analog states in ^{12}C in the mirror reaction ^{13}C(d, t)^{12}C.]
Not reported.
Not reported.
At E_{p} = 63.4 MeV, the excitation of the lowest T = 2 state in ^{12}B is stated to be at E_{x} = 12.710 ± 0.020 MeV (1971NE1B, 1971NE1E: unpublished). This state has also been observed at E_{p} = 54 MeV as has another at E_{x} = 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 ^{12}C]. See also (1970OL1B).
See (1968AJ02).
At E(^{14}N) = 118 MeV the excitation of ^{12}B*(0, 0.95) is reported by (1974AN36).
See (1948JE03).
See (1974SW04).
