(See Energy Level Diagrams for 12B)
Model calculations: (1988WO04).
Astrophysical questions: (1988AP1A).
Complex reactions involving 12B: (1984HO23, 1984TA1K, 1985JA18, 1985NO1E, 1985PO11, 1985TA09, 1986AV1B, 1986BI1A, 1986CS1A, 1986GA1P, 1986ME06, 1986PO06, 1986UT01, 1986WE1C, 1986XU02, 1987AN1A, 1987BA38, 1987BA1T, 1987EL14, 1987GE1B, 1987JA06, 1987LY04, 1987PE1B, 1987PO23, 1987SA25, 1987VI02, 1987YA16, 1988AU1A, 1988CA06, 1988FO03, 1988KI05, 1988KI06, 1988RU01, 1988SA19, 1988TA1N, 1989DE05, 1989SA10, 1989SE03, 1989ST1G, 1989YO02).
Hypernuclei: (1983SH1E, 1984CH1G, 1984CO1L, 1984MA1U, 1984SH1J, 1984ZH1B, 1985AH1A, 1985CO1H, 1985CO14, 1985DO1G, 1985YA1F, 1986AN1R, 1986BE1P, 1986CO1U, 1986DA1B, 1986FE1A, 1986HA39, 1986KO1A, 1986MA1C, 1986RO25, 1987BE1S, 1987MA08, 1987MA2A, 1987MI38, 1987PO1H, 1988MO1L, 1988RO11, 1988TA29, 1989MR01).
Antinucleon interactions: (1986AU1D).
Q = 1.34 ± 0.14 fm2 [(1978MI19); see also (1978LEZA, 1985NA1A).]
Interaction cross sections at 790 MeV/A by 12B ions with Be, C, and Al are reported by (1988TA10). The interaction radius and the r.m.s. radius for the nucleon distribution in 12B have also been derived (1988TA10). See also (1989SA10).
12B*(3.39) has been produced in the interaction of a 490 MeV 14N beam with Ag: its 19 keV neutron decay to 11Bg.s. has been observed (1989DE05).
The half-life of 12B is 20.20 ± 0.02 ms (1978AL01). See also (1988SA04). The decay is complex. 12B decays to 12C*(0, 4.4, 7.7, 10.3): see 12.14 (in PDF or PS). The transitions to 12C*(0, 4.4) are allowed; hence the Jπ of 12B(g.s.) is 1+. The β-momentum spectrum has recently been measured by (1987NA08). For measurements of the angular distribution functions from oriented 12B nuclei see, for instance (1986MI1P, 1986MI1T, 1987MI1P). See also the "Polarization" section in (1985AJ01), (1986CU01, 1986RO27, 1987MI20), (1985GR1A, 1985MI1A), (1985BA2L, 1987RO03; applied) and (1984DU1E, 1984TA1M, 1985MO13, 1986KE1A, 1986KO2C, 1986TO1A, 1987DR1A, 1987KE1B, 1987SA1N, 1988AL1E, 1988SA12, 1989SA1H; theor.).
Eleven groups of protons (reaction (a)) 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. Angular distributions for d0, d1, d2, d3+4 have been measured at E(7Li) = 2.10 to 5.75 MeV. See (1975AJ02) for references. See also (1984KO25).
Thirteen resonances have been reported in reaction (a) corresponding to 13.6 < Ex < 14.7 MeV: see 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: no resonances are observed. This is also the case for the yields of 0.32 MeV (reaction (b)), 0.98 MeV (reaction (e)) and 0.48 MeV γ-rays (from the (t, αn) reaction). Elastically scattered tritons have been studied for Et = 0.60 to 2.1 MeV and Epol. t = 15 and 17 MeV (also Ay). 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 E pol. t = 17 MeV. For references see (1985AJ01). See also (1988AJ01).
At E(6Li) = 32 MeV 12B*(0, 0.95, 1.67, 3.38, 3.76) and some unresolved states are populated (1986AS02).
Observed α-particle groups are displayed in 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 12.4 (in PDF or PS) and (1968AJ02). For a summary and the ENDF projections see (1988MCZT). See also (1985BA2L; applied) and (1988MA1U; astrophysics).
The thermal (bound) scattering cross section is 3.9 ± 0.2 b. The scatttering amplitude (bound) is a= 6.65 ± 0.04 fm, σ(free) = 4.84 ± 0.04 b (1983KO17). Parameters of observed resonances are shown in 12.4 (in PDF or PS). See also (1981MUZQ). For a summary and the ENDF projections see (1988MCZT). See also (1982GL02). For differential cross sections see 11B. Total cross-section measurements have been reported for En = 0.3 to 18.0 MeV: see (1968AJ02, 1980AJ01, 1985AJ01). Polarization measurements have been studied at En = 75 keV to 2.2 MeV [see (1980AJ01)] and at Epol. n = 10 to 17 MeV (1985WA1P, 1986MU08; n0; prelim.).
Results from the most recent R-matrix analysis are displayed in 12.4 (in PDF or PS) (1983KO03). For a discussion of the earlier work see (1980AJ01). See also (1986BAYL, 1986DR10), (1985BO1C) and (1988HAZT; 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 study of reaction (d) is reported at En = 14.4 MeV: see (1985AJ01). For a summary and the ENDF projections see (1988MCZT). See also (1985FO1D), (1986WI1B; applied) and (1985BO1D).
The cross section for π+ production near threshold has been measured. At Ep = 200 MeV 12B*(0, 0.95, 1.67, 2.62, 3.39, 3.76, 4.30 + 4.52, 5.00, 5.61) are reported: see (1985AJ01). See also the "General" section here.
Observed proton groups and γ-rays are displayed in 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 confirmed by studies of the polarization of γ1. The results are consistent with Jπ = 1+ or 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 12B(g.s.). Gamma-gamma correlations lead to Jπ = 2-. An assignment of 1- to 12B*(2.62) is made in a similar manner.
At E(7Li) = 34 MeV angular distributions have been studied to 12B*(0, 0.95, 1.67, 2.62 + 2.72, 3.39, 4.52, 5.61 + 5.73) (1987CO16) [see for spectroscopic factors].
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 to 205 MeV the π+ energy distributions show contributions from 12B*(0, 0.95, ≈ 4.5, 7.0): see (1986SH14, 1988SH36) and (1985AJ01). The 2- and 4- states at Ex ≈ 4.5 MeV have been compared with their isobaric analogs in 12C at Ex ≈ 19.5 MeV. At Ee = 400 MeV π+ with Eπ = 32 MeV have been studied: 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). The cross section (at θ = 54°) is the same whether virtual or real photons are used in producing the pions. At Eγ = 176 - 187 MeV the giant resonance region as well as some lower groups have been studied by (1987MIZZ, 1987MI1O; prelim.). See also (1988ADZN).
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-]: see (1985AJ01). The ratio of the polarization of 12Bg.s., Pav, and of the longitudinal polarization leads to a neutrino helicity hν = -1.08 ± 0.11, in agreement with the partial conservation of axial-vector current (PCAC) hypothesis: see (1985AJ01). The polarization of 12B has also been studied recently by (1984KU20): Pav is deduced to be 0.462 ± 0.053 which yields a ratio of the induced pseudoscalar to the axial vector coupling constant in the hadronic weak current, gp/ga = 10.1+2.4-2.6 which is consistent with the prediction of ≈ 7 from PCAC (1984KU20). See also the "General" section here and (1988DO05).
The photon spectrum from stopped pions is dominated by peaks corresponding to 12B*(0, 4.4, 7.9), and branching ratios have been obtained for these and other transitions. That to 12Bg.s. is (6.22 ± 0.35)% (absolute branching ratio per stopped pion) (1986PE05). For the earlier work see (1980AJ01).
At En = 59.6 MeV the angular distribution of the p0 group is reported. Broad (unresolved) structures are also observed with Ex < 10 MeV: see (1985AJ01). The 0° differential cross section to 12Bg.s. at En = 198 MeV has been measured. Its ratio to that for the H(n, p) reaction, R, is 0.180 ± 0.006 (1988JA01). The angular distribution to 12B*(4.52) [u?] is reported at En = 300 MeV (1987WAZY, 1989POZY; prelim.). See also (1985FO1C, 1985FR07, 1985SOZX, 1986DO12, 1986SOZY, 1987FR16, 1987HE24, 1987VE08, 1988RAZX, 1989SOZY) and (1985BR1G, 1986ALZJ, 1986FO1E, 1986VO1G, 1987BR32, 1987HE22, 1988HA1K).
Angular distributions are reported at Ed = 55 and 99.2 MeV to 12B*(0, 4.5(u)) [and, at the lower energy to 12B*(0.95)] [see (1985AJ01)] and at Ed = 70 MeV (1986MO27, 1988MO11; to 12B*(0, 4.5(u)); also VAP, TAP) [12B*(0.95, 8.2) are also populated]. See also (1986EL1C, 1987EL14).
At E(7Li) = 150 MeV, giant resonances at Ex = 21.8 ± 0.4 (Γ = 1.3 ± 0.4) and 23.9 ± 1.0 (Γ = 6.0 ± 1.0) MeV, with Jπ = 3- and 1-, respectively, have been reported by (1987NA16). For the earlier work see (1985AJ01). See also (1983PU01), (1987COZZ) and (1984BA53; theor.).
At E(12C) = 35 MeV/A forward-angle differential cross sections have been measured for the groups to 12B*(0, 0.95, 1.67, 4.5[u]). A broad peak Ex = 7.8 MeV is also populated (1986WI05). The groups to 12B*(4.5)[u] dominate the spectra at this energy (1986WI05) and at 30 MeV/A (1986VO1J). For preliminary work at 70 MeV/A see (1988BO1R, 1989ANZZ). For a recent review of the excitation of the Δ-resonance see (1988RO1H). See also (1986BA16, 1988ANZY), (1986RO1Q, 1987AU04, 1987EL14, 1988AN06, 1988RO17, 1988VO06) and (1986BA1D, 1986DE34, 1986DU1N, 1986LE1N, 1988LE12, 1989LE03; theor.).
Differential cross sections at θ = 1.8° are reported at E(13C) = 30 MeV/A for the groups to 12B*(0, 0.95, 4.5[u]) and to structures at Ex ≈ 5.5, 7.8, 10.1 and 18.2 MeV (1987AD07). See also (1988VO08). 12B*(1.67, 2.62, 3.38) are also populated (1988VO06; 0°; 29.2 MeV/A). At E(13C) = 50 MeV/A the GDR peak is located at 7.7 ± 0.1 MeV (Γc.m. = 1.9 ± 0.1 MeV). Forward-angle differential cross sections have been measured also for 12B*(0, 0.95) (1989BE50) [and M. Buenerd, private communication]. See also (1986VO02, 1989VO1D).
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 summary of information on analog states of 12B and 12C see 12.12 (in PDF or PS) in (1980AJ01).
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 fittted 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 reaction 70 in 12C.