
^{12}B (1990AJ01)(See Energy Level Diagrams for ^{12}B) GENERAL: See also (1985AJ01), 2 [Electromagnetic Transitions in A = 1112] (in PDF or PS), 12.2 [Table of Energy Levels] (in PDF or PS). Model calculations: (1988WO04). Special states: (1984KA1H, 1986XU02). Astrophysical questions: (1988AP1A). Applications: (1988NO1C). Complex reactions involving ^{12}B: (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). Polarization of ^{12}B: (1984KO1A, 1984KU11, 1984TA1M, 1985TA09, 1986AU1E, 1986KU05, 1986NO1C, 1986TA13, 1987AU1D, 1987CO1L, 1987FU13, 1988NO1C). Muon and neutrino capture and reactions (See also reaction 19.): (1984KU11, 1985GR1A, 1986KU05, 1987BR1L, 1987FU13, 1987MI17, 1988BR24, 1988FU08, 1988MI11, 1988MI20, 1989GM1A). Pion and kaon capture and reactions (See also reactions 18, 20, and 21.): (1983AS01, 1984AS05, 1984ER06, 1984KA31, 1984MA1U, 1985DO1G, 1985SA06, 1986DI07, 1986SU18, 1988HA12, 1989MR01). 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). Other topics: (1984KA1H, 1985AN28, 1986YA1F). Groundstate properties of ^{12}B: (1985AN28, 1986GL1A, 1987VA26, 1988VA03, 1988WO04).
Q = 1.34 ± 0.14 fm^{2} [(1978MI19); see also (1978LEZA, 1985NA1A).] Interaction cross sections at 790 MeV/A by ^{12}B ions with Be, C, and Al are reported by (1988TA10). The interaction radius and the r.m.s. radius for the nucleon distribution in ^{12}B have also been derived (1988TA10). See also (1989SA10). ^{12}B*(3.39) has been produced in the interaction of a 490 MeV ^{14}N beam with Ag: its 19 keV neutron decay to ^{11}B_{g.s.} has been observed (1989DE05).
The halflife of ^{12}B is 20.20 ± 0.02 ms (1978AL01). See also (1988SA04). The decay is complex. ^{12}B decays to ^{12}C*(0, 4.4, 7.7, 10.3): see 12.14 (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^{+}. The βmomentum spectrum has recently been measured by (1987NA08). For measurements of the angular distribution functions from oriented ^{12}B 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 ^{12}B. Angular distributions have been measured at E(^{6}Li) = 3.5 to 5.95 MeV. The distributions are generally featureless. Angular distributions for d_{0}, d_{1}, d_{2}, d_{3+4} have been measured at E(^{7}Li) = 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 < E_{x} < 14.7 MeV: see 12.3 (in PDF or PS) in (1975AJ02). The yield of 2.12 MeV γrays has been measured for E_{t} = 1.5 to 3.3 MeV and E(^{9}Be) = 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 γrays (from the (t, αn) reaction). Elastically scattered tritons have been studied for E_{t} = 0.60 to 2.1 MeV and E_{pol. t} = 15 and 17 MeV (also A_{y}). The yields of α_{0} and α_{1} have also been reported for E_{t} = 0.52 to 1.70 MeV: see (1975AJ02). The analyzing powers of the reactions leading to ^{6}He_{g.s.} and ^{6}Li*(0, 3.56) have been measured at E_{ pol. t} = 17 MeV. For references see (1985AJ01). See also (1988AJ01).
See unpublished work in (1985AJ01). See also (1986AU1E).
At E(^{6}Li) = 32 MeV ^{12}B*(0, 0.95, 1.67, 3.38, 3.76) and some unresolved states are populated (1986AS02).
Observed αparticle groups are displayed in 12.3 (in PDF or PS). Angular distributions have been measured at E(^{7}Li) = 3.3 to 6.2 MeV, at 20 MeV and at 30.3 MeV: see (1975AJ02, 1980AJ01). At E(^{7}Li) = 20 MeV angular distributions to the first seven states are rather featureless and have approximate symmetry about 90°. The integrated cross sections go as 2J_{f+1} consistent with a compound nucleus mechanism for the transitions populating the lowlying states of ^{12}B. It is suggested that the sharp states of ^{12}B at high excitation energies correspond to states of high angular momenta with cluster configurations.
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, respectively: the yields for both reactions vary smoothly with energy. No resonances are observed: see (1975AJ02).
Observed excited states are displayed in 12.3 (in PDF or PS). Angular distributions have been studied at E_{t} = 10 and 23 MeV: see (1980AJ01). See also (1982CI1B; theor.).
The thermalneutron capture cross section is 5.5 ± 3.3 mb [see (1981MUZQ)]. The capture cross section shows resonances at E_{n} = 20.8 ± 0.5 keV and at 0.43, 1.03, 1.28 and 1.78 MeV, with Γ_{γ} = 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 ^{11}B. Total crosssection measurements have been reported for E_{n} = 0.3 to 18.0 MeV: see (1968AJ02, 1980AJ01, 1985AJ01). Polarization measurements have been studied at E_{n} = 75 keV to 2.2 MeV [see (1980AJ01)] and at E_{pol. n} = 10 to 17 MeV (1985WA1P, 1986MU08; n_{0}; prelim.). Results from the most recent Rmatrix 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 E_{n} = 14.7 to 16.9 MeV and that for reaction (b) has been investigated for E_{n} = 12.6 to 20.0 MeV and at 25 and 38 MeV: see (1975AJ02). A study of reaction (d) is reported at E_{n} = 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 E_{p} = 200 MeV ^{12}B*(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 γrays are displayed in 12.5 (in PDF or PS). The J^{π} assignments for ^{12}B*(0.95, 1.67) are derived as follows [see (1968AJ02) for a 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. τ_{m} is too short for pure E2 and hence J ≠ 3, which is confirmed by studies of the polarization of γ_{1}. The results are consistent with J^{π} = 1^{+} or 2^{+}. The latter is fixed by γγcorrelations in the cascade 1.67 > 0.95 > g.s. The mixing ratio δ = 0.08 ± 0.06. 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^{}. An assignment of 1^{} to ^{12}B*(2.62) is made in a similar manner. See 12.12 (in PDF or PS) in (1980AJ01) for a comparison of the properties of the first seven T = 1 states in ^{12}B and in ^{12}C.
See (1984KO25).
At E(^{7}Li) = 34 MeV angular distributions have been studied to ^{12}B*(0, 0.95, 1.67, 2.62 + 2.72, 3.39, 4.52, 5.61 + 5.73) (1987CO16) [see for spectroscopic factors].
See ^{12}Be.
Using monoenergetic photons with E_{γ} = 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 quasifree pion production and FSI processes. At E_{e} = 195 to 205 MeV the π^{+} energy distributions show contributions from ^{12}B*(0, 0.95, ≈ 4.5, 7.0): see (1986SH14, 1988SH36) and (1985AJ01). The 2^{} and 4^{} states at E_{x} ≈ 4.5 MeV have been compared with their isobaric analogs in ^{12}C at E_{x} ≈ 19.5 MeV. At E_{e} = 400 MeV π^{+} with E_{π} = 32 MeV have been studied: double differential cross sections are obtained for the transitions to ^{12}B*(0, 0.95, 1.67), and single differential cross sections to ^{12}B*(0, 0.95). The cross section (at θ = 54°) is the same whether virtual or real photons are used in producing the pions. At E_{γ} = 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 γtransitions have led to the determination of the capture rates to ^{12}B*(0, 0.95, 1.67, 2.62) [J^{π} = 1^{+}, 2^{+}, 2^{}, 1^{}]: see (1985AJ01). The ratio of the polarization of ^{12}B_{g.s.}, P_{av}, and of the longitudinal polarization leads to a neutrino helicity h_{ν} = 1.08 ± 0.11, in agreement with the partial conservation of axialvector current (PCAC) hypothesis: see (1985AJ01). The polarization of ^{12}B has also been studied recently by (1984KU20): P_{av} 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, g_{p}/g_{a} = 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 ^{12}B*(0, 4.4, 7.9), and branching ratios have been obtained for these and other transitions. That to ^{12}B_{g.s.} is (6.22 ± 0.35)% (absolute branching ratio per stopped pion) (1986PE05). For the earlier work see (1980AJ01).
See (1987CL02) for a study of delta production at E_{π} = 475 MeV. See also (1989BRZX).
At E_{n} = 59.6 MeV the angular distribution of the p_{0} group is reported. Broad (unresolved) structures are also observed with E_{x} < 10 MeV: see (1985AJ01). The 0° differential cross section to ^{12}B_{g.s.} at E_{n} = 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 ^{12}B*(4.52) [u?] is reported at E_{n} = 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 E_{d} = 55 and 99.2 MeV to ^{12}B*(0, 4.5(u)) [and, at the lower energy to ^{12}B*(0.95)] [see (1985AJ01)] and at E_{d} = 70 MeV (1986MO27, 1988MO11; to ^{12}B*(0, 4.5(u)); also VAP, TAP) [^{12}B*(0.95, 8.2) are also populated]. See also (1986EL1C, 1987EL14).
At E(^{7}Li) = 150 MeV, giant resonances at E_{x} = 21.8 ± 0.4 (Γ = 1.3 ± 0.4) and 23.9 ± 1.0 (Γ = 6.0 ± 1.0) MeV, with J^{π} = 3^{} and 1^{}, respectively, have been reported by (1987NA16). For the earlier work see (1985AJ01). See also (1983PU01), (1987COZZ) and (1984BA53; theor.).
At E(^{12}C) = 35 MeV/A forwardangle differential cross sections have been measured for the groups to ^{12}B*(0, 0.95, 1.67, 4.5[u]). A broad peak E_{x} = 7.8 MeV is also populated (1986WI05). The groups to ^{12}B*(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 Δresonance 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(^{13}C) = 30 MeV/A for the groups to ^{12}B*(0, 0.95, 4.5[u]) and to structures at E_{x} ≈ 5.5, 7.8, 10.1 and 18.2 MeV (1987AD07). See also (1988VO08). ^{12}B*(1.67, 2.62, 3.38) are also populated (1988VO06; 0°; 29.2 MeV/A). At E(^{13}C) = 50 MeV/A the GDR peak is located at 7.7 ± 0.1 MeV (Γ_{c.m.} = 1.9 ± 0.1 MeV). Forwardangle differential cross sections have been measured also for ^{12}B*(0, 0.95) (1989BE50) [and M. Buenerd, private communication]. See also (1986VO02, 1989VO1D).
Angular distributions have been measured for the transitions to ^{12}B*(0, 0.95) at E_{d} = 24.1 to 29 and at 52 MeV. Distributions to ^{12}B*(2.72, 3.76, 5.00) have been reported at E_{d} = 52 MeV: see (1980AJ01). The spectroscopic factors for ^{12}B*(0, 0.95) are S = 1.1 ± 0.2 and 2.0 ± 0.5. C^{2}S = 1.09 [assuming 1p_{3/2}], 2.17, 0.14, 0.07, 0.22 [assuming 1p_{1/2}] for ^{12}B*(0, 0.95, 2.72, 3.76, 5.00) [from measurements at E_{d} = 52 MeV]. See (1980AJ01) for references. For a summary of information on analog states of ^{12}B and ^{12}C see 12.12 (in PDF or PS) in (1980AJ01).
At E_{p} = 54 MeV, in addition to transitions to ^{12}B*(0, 0.95, 5.61), the population of T = 2 states at E_{x} = 12.72 ± 0.07 and 14.82 ± 0.10 MeV is reported. The angular distribution of ^{3}He ions to ^{12}B*(12.75) is fittted by L = 0; that to ^{12}B*(14.82) is rather featureless [its T = 2 character is assigned from the energies of the analog states]: both states have Γ_{c.m.} ≲ 200 keV (1976AS01). See also reaction 70 in ^{12}C.
