(See the Energy Level Diagram for 11B)
Three resonances are reported below Eα = 2.5 MeV (1951BE13, 1954HE22): see Table 11.2 (in PDF or PS). Study of α-γ and γ-γ angular correlations, taken together with the relative γ-intensities, leads to the following assignments: 9.28 MeV level, J = 5/2+; 9.19 MeV, J = 5/2-; 8.93 MeV, J = 3/2 or 5/2; 6.81 MeV, J = 3/2-; 4.46 MeV, J = 5/2- ((1952JO1B) and D.H. Wilkinson, private communication). The strength of the transition (8.92 → g.s.) implies E1 radiation (1958BI31). Angular distributions of several gamma rays at each resonance are tabulated by (1957ME1D); no terms higher than cos2θ are indicated. The absence of the transition (9.28 → 2.14) speaks for J = 1/2- for the latter (1957WI26). Angular distributions of γ-rays from the 9.28 MeV level have been measured by (1958FE70). The results are in agreement with assignments J = 5/2+ and 5/2- for 11B*(9.28, 4.46), respectively. The angular distribution of the transition 9.28 → 6.76 strongly favors J = 7/2- for the latter. See also (1958ME77): 11B(γ, α)7Li, and 10B(d, p)11B.
For Eα < 5.8 MeV, two resonances are observed; at Eα = 4.7 MeV (broad), and 5.15 ± 0.08 MeV (Γ = 0.22 MeV) corresponding to 11B*(11.68 ± 0.10, 11.95 ± 0.08) (1957BI84). A further resonance at Eα = 7.15 MeV, 11B*(13.22), is reported by (1958MA1J, 1959GI47). Calculation of σ(10B(n, α)7Li) from the observed yield of 7Li(α, n)10B gives good agreement for En = 0 to 0.8 MeV. The 5.15 MeV resonance corresponds to that reported in σ(n, α) at 0.52 MeV: Ex = 11.95 MeV. It is concluded that the resonance is formed by s or p-wave neutrons, J = 5/2+ or 3/2-, Γn ≈ 20 keV and Γα ≈ 300 keV (1959GI47). See also (1950HO80).
Observed resonances in the yield of 0.48 MeV γ-radiation are exhibited in Table 11.3 (in PDF or PS) (1954HE22, 1954LI48, 1957BI84). Sum rule limits give J ≤ 5/2 for the 9.88 MeV level and J ≤ 7/2 for the 10.26 MeV level (1954LI48).
This reaction has not been observed: at Ed = 0.9 MeV, σ < 1.8 μb; at Ed = 1.5 MeV, σ < 20 μb (1955AL16).
The cross section follows the Gamow function for Ed = 70 to 110 keV (1955RA14). The fast neutron and γ-ray yield rise smoothly to Ed = 1.8 MeV except for a broad resonance at Ed = 1 MeV (1949EV1A, 1955BO1A, 1957SH65). This resonance is observed in the total neutron yield and in the yield of the fast neutrons to each of the first five states of 10B. Angular distributions change markedly through the resonance except for that corresponding to 10B*(3.58), which is dominated by stripping throughout (1957SH65). On the other hand, (1958NE38, 1959NE1A) have obtained integrated cross sections for three separate neutron groups from Ed = 0.5 to 2.0 MeV and find no evidence of a resonance near 1 MeV. See also 10B.
In the range Ed = 1.02 to 1.44 MeV, two resonance anomalies are reported by (1956JU17) at Ed = 1.162 and 1.348 MeV corresponding to 11B*(16.77, 16.93) (Γ ≈ 70 and 120 keV, respectively). See also 9Be.
Reaction (a) exhibits a simple Gamow dependence to 250 keV (1953SA1A). Angular distributions for Ed < 1.5 MeV are reported by (1952DE24, 1955JU10, 1955JU1B, 1957HY1A, 1957SM78, 1958JU38). The distributions of long-range protons for Ed = 0.1 to 0.2 MeV are analyzed in terms of two 11B states with J = 1/2- and 1/2+ or J = 3/2- and 1/2+ (1957SM78). (1952CA19) reports broad maxima in the 90° yield of ground-state protons at Ed ≈ 0.9, (1.3) and 2.1 MeV. (1957MC35) observe broad resonances at 1.3 and possibly at 1.8 MeV in the yield of 3.37 MeV γ-rays, in the range Ed = 1.0 to 5.6 MeV. No resonances are observed in the yield of 6 MeV γ-rays for Ed = 2.0 to 5.6 MeV. The cross section for production of 10Be (reaction (a)) rises to a peak value of ≈ 0.34 b at ≈ 4 MeV and then falls almost linearly to ≈ 0.08 b at ≈ 21.5 MeV (1955HE83). See also (1957CO54).
The cross section for reaction (b) shows a simple Gamow rise to Ed = 250 keV (1953SA1A). Angular distributions have been measured for Ed = 0.3 to 0.7 MeV by (1951RE01): see also 7Li. The cross section for reaction (c) has been measured for Ed = 0.15 to 0.62 MeV by (1952DE24), for Ed = 0.6 to 1.5 MeV by (1955JU10, 1955JU1B), and at several energies in the range Ed = 3 to 19 MeV by (1955HE83). The forward yield of tritons shows a peak at Ed = 1.38 MeV (1955JU10, 1955JU1B). There seems also the possibility of a resonance at Ed = 0.87 MeV (1958JU38). Triton angular distributions for Ed = 0.1 to 0.2 MeV are analyzed in terms of two 11B states with J = 5/2- and 3/2+ (1957SM78). See also (1957HY1A) and 8Be.
Angular distributions of the protons to the ground state and several excited states of 11B have been determined at various bombarding energies from 2 to 6 MeV: see 12C. See also (1956HL01, 1956WO1A, 1957JO1B).
Gamma rays from the first nine excited states have been observed at E(3He) = 2.1 MeV in coincidence with proton groups. The direct ground state gamma-transition is observed for all the levels. In addition the cascade through the 2.14 MeV state is observed from the 5.03, 6.81, 7.99 and 8.57 MeV states and, possibly, from the 6.76 and 8.92 MeV states, generally with an intensity comparable with that of the corresponding ground state transition. The gamma-ray width, Γγ, of the 8.92 MeV level is found to be comparable with its Γα. The decay scheme derived from this work is shown in Fig. 17 (1957FE1B, 1958FE70). The branching of the 5.03 and 7.30 MeV levels agrees with the shell-model assignments 3/2- and 5/2- (1957KU58, 1958FE70). At E(3He) = 5.7 MeV, proton groups are reported to levels at 2.126 ± 0.010, 4.459 ± 0.010, 5.037 ± 0.010, 6.756 ± 0.010, 6.807 ± 0.010, 7.296 ± 0.010, 7.987 ± 0.010, 8.569 ± 0.010, 8.927 ± 0.010, 9.191 ± 0.010, 9.278 ± 0.010 and 9.87 ± 0.02 MeV. The 9.87 MeV state is broad, Γ ≈ 150 keV (S. Hinds and R. Middleton, private communication). See also (1954MO1E, 1958BR1D, 1958SW63) and 7Li(α, γ)11B.
Deuteron groups have been observed at Eα = 21.6 MeV to the ground and 2.14 MeV states of 11B. A search in the region Ex = 0.4 to 1.0 MeV showed no deuteron groups with intensity greater than 0.1 of the intensity of the two observed groups (1955RA41).
The thermal neutron capture cross section is 0.5 ± 0.2 b. Observed capture γ-rays are listed in Table 11.4 (in PDF or PS). The relative weakness of the ground state transition suggests J = 7/2+ for the capturing level.
The epithermal scattering cross section (free) is 3.6 ± 0.2 b (1958HU18). Broad maxima appear in the total cross section at En = 1.9 and 2.8 MeV (1951BO45) and at 4 MeV (1957HU1D); additional peaks near 0.2 and 0.4 MeV may be indicated (1951BO45). Differential cross sections have been measured at En = 0.55, 1.00 and 1.50 MeV: derived phase shifts are δ0 = -53.5° at En = 0.55 MeV, δ0 = -60.7°, δ1 = -4.0° at En = 1.00 MeV, and δ0 = -66.9°, δ1 = -10.3°, δ2 = -2.9° at En = 1.50 MeV (1955WI25). The total cross section decreases from ≈ 1.9 b at 4.4 MeV to 1.6 b at 5.6 MeV and then remains approximately constant at ≈ 1.5 b from 6 to 9.7 MeV (1954NE1A, 1956BE98). At En = 14 MeV it is 1.47 ± 0.03 b (1952CO41) and it remains nearly constant to 18 MeV (1954CO16: σ = 1.45 ± 0.02 b). See (1957HU1D).
At En = 14 MeV, the integrated cross sections (0° to 90°, c.m.) for the transitions to the ground and the 2.4 MeV states of 9Be are 21 ± 3 mb and 16 ± 2 mb, respectively (1954RI15). See also (1956FR18).
For En = 5.20 MeV, production of tritons appears to be mainly via 10B(n, α)7Li*(4.7) and direct three-body breakup (1956FR18). Cross sections at En = 4, 5.6, 9.6 and 14.1 MeV are 95 ± 10, 230 ± 25, 125 ± 15 and 85 ± 6 mb, respectively (1958WY67).
Recent values for the thermal neutron absorption cross section in natural boron are 749 ± 4 b (1953CA45), 755 ± 3 b (1953HA1C), 744 ± 20 b (1954SC87), 764 ± 3 b (1954VO1A) and 760 ± 3 b (1956CO1E). (1958HU18) give 755 ± 2 b for the thermal absorption cross section in "U. S. standard" boron and 3813 b for the thermal isotropic cross section; see also (1957HU1D). The cross section follows the 1/v law from 7 × 10-4 eV to 100 keV (1955HU1B, 1957BI84). (1957BI1F) report σtotal = 642/√En + 2.45 b for En = 3 to 70 keV. The data from En = 0 to 250 keV can be satisfactorily interpreted in terms of the 11B levels at 11.46 and 11.68 MeV (En = 0 and 140 keV) observed in 7Li(α, α')7Li* (1957BI84). (1957BE71) find, on the other hand, that deviations from the 1/v law for En < 1 MeV indicate a single broad level at En ≈ 250 keV with J = 5/2+ or 7/2+, Γα ≈ 400 keV, Γn ≈ 200 keV. A pronounced resonance is observed at En = 1.86 MeV: Γc.m. = 0.45 MeV ((1957BI84) and (1951PE18); see also (1950ST1A) and (1952BI1A)). There are indications of less pronounced resonances at 0.53, 2.8 and 4.1 MeV (with Γc.m. = 0.10, 0.3 and 0.5 MeV) in the energy range to En = 4.8 MeV (1957BI84). The ratio of ground state to excited state transition varies strongly with energy: see (1952AJ38, 1954DE38, 1958BU02).
Cross sections for production of 2α + t either through the 4.6 MeV level of 7Li or via three-body breakup have been determined for En = 4 to 19.5 MeV. A strong maximum near En = 5.6 MeV may indicate a resonance near 11B*(16.6) (1956FR18, 1958WY66). See also (1955AJ61).
Proton groups reported by (1951VA1A) and (1953EL12) are listed in Table 11.5 (in PDF or PS). No other levels are observed below Ex = 11.46 MeV: the known levels observed in 7Li(α, α')7Li are presumably too wide to be seen here. See also (1954KH1A, 1955KH35).
The angular distribution of protons leading to the ground state (J = 3/2-) shows a well-developed ln = 1 stripping pattern at all energies ≳ 2.0 MeV (1954EV1A, 1958EV01: 7.7 MeV), (1953HO48: 8 MeV), (1956ZE1A: 10 MeV), (1957LE1F: 15.1 MeV), (1956MA69: 1.0 to 3.0 MeV); see, however, (1957CO54). Even below Ed = 1 MeV, stripping appears to play an important role in formation of this level (1954BU06, 1954PA28, 1957HA1H). The relatively large neutron capture probability suggests a single-particle character (1953HO48, 1954EV1A). See also (1957CO54, 1957HA1H). The polarization has been studied by (1958HE47, 1958HI74).
For the 2.1 MeV state, the evidence is not so clear: the pattern though generally of an ln = 1 shape (see, however, (1954PA28, 1957LE1F)) shows strong variations with energy (1954EV1A, 1956MA69, 1956ZE1A, 1957CO54, 1957LE1F, 1958EV01). An ln = 1 formation implies 3/2- ≤ J ≤ 9/2-, in contradiction to the shell-model expectation that the state should have J = 1/2-; however to form such a state requires ln = 3, involving rearrangement of several nucleons and hence a low probability (1956MA69, 1958EV01). The more probable mode of formation would appear to involve a nucleon exchange (1958EV01) or a spin reversal of the outgoing proton (1957WI26). The observation that the polarization of protons for this state is opposite to those corresponding to the ground state is consistent with either point of view, and permits 1/2- ≤ J ≤ 11/2- (1958HE47). Support for the assignment J = 1/2 is found in the observed isotropy of the p-γ correlation (see below): see also 11B(p, p')11B* and (1957WI26, 1958BO1C).
The 4.46, 5.03 and 6.76 MeV states are also formed by ln = 1. The large neutron capture probability for the 6.76 MeV state indicates that it has a single-particle character (1954EV1A, 1958BI31). See also (1957CO54, 1957SJ1C). The next four states appear only weakly and probably arise from configuration mixing (1958BI31).
For the 8.92 MeV state, ln = 2 with a small admixture of ln = 0 is indicated; J = 5/2+, 7/2+. The large γ-width suggests J = 5/2+. The 9.19 and 9.28 MeV state are formed with ln = 0, J = 5/2+ or 7/2+; the intensity ratio is consistent with J(9.19) = 7/2+, J(9.28) = 5/2+. Again, the high relative intensities suggest that these three are single-particle states, formed by direct capture into the 1d, 2s, shell (1958BI31); see also 7Li(α, γ)11B.
Gamma rays reported by (1955BE81) and (1955SA1B) are listed in Table 11.6 (in PDF or PS). The p-γ angular correlation through the 2.14 MeV state is isotropic to ≈ 4%, consistent with J = 1/2: see (1953TH1B, 1955GO1D, 1956GO1L, 1956GO1M, 1956GO39, 1957GA1B). The angular correlation through 4.46 MeV state is consistent with J = 3/2- or 5/2-; that through the 6.76 MeV state rules out J = 3/2 and suggest J = 9/2 is unlikely for that state (1957CO54). See also (1956VA17).
The decay properties of 11Be are exhibited in Table 11.7 (in PDF or PS). The transition energy to the ground state is Eβ(max) = 11.48 ± 0.15 MeV; τ1/2 = 13.57 ± 0.15 sec, log ft = 6.77 (1958AL96, 1959WI49), τ1/2 = 14.1 ± 0.3 sec (1958NU40). The transition probabilities to 11Bg.s., J = 3/2-, and 11B*(2.1), J = 1/2-, suggest J = 1/2- for 11Be, but it is not clear why the transition should be so much inhibited as compared with other allowed transitions (1959WI49).
The mean life of the 4.46 MeV level, determined by resonance absorption and scattering is τm = 1.17 ± 0.17 × 10-15 sec, assuming J = 5/2. On the same assumption, the intensity ratio of quadrupole to dipole transitions is ≤ 0.2 (1958RA14). A mean life of ≈ 1.5 × 10-15 sec is calculated by (1957KU58).
A similar experiment yields τm = (4.6 ± 0.6) × 10-15 sec for the 2.1 MeV state, assuming J = 1/2. The result does not distinguish J = 1/2- from J = 1/2+, but the shortness of the lifetime excludes J > 1/2 (1958ME79): see (1957WI26: 11B(p, p')11B*). A calculation in intermediate coupling, assuming an M1 transition yields τm = (2.5 to 5) × 10-15 sec (1957KU58). See also (1958MC1D).
Resonance absorption of 9.19 MeV radiation yields Γ < 100 eV, (2J + 1)Γγ ≈ 0.8 eV for 11B*(9.19) (1958ME77).
At Ep = 3.58 MeV, a 2.134 ± 0.005 MeV γ-ray is observed (1957MC35); see also (1953HU29). The mean lifetime of the 2.14 MeV state has been determined by a Doppler shift measurement to be < 4 × 10-14 sec (1957WI26): see 11B(γ, γ)11B. It is pointed out that the observed isotropy in 10B(d, pγ)11B requires a nearly pure E2 transition if J = 3/2- and the present lifetime value excludes E2. On the other hand, the lifetime is quite consistent with M1 (1957WI26). The 2.14 MeV γ-ray exhibits < 2.0 × 10-3 part of circular polarization; this observation places an upper limit of F2 ≲ 1 × 10-7 for the intensity of the parity non-conserving part of the wave function (1958WI41).
At Ep = 185 MeV, inelastic groups with Q = -4.7, -6.6 and -8.5 MeV are observed; the latter shows strong forward peaking and is attributed to a spin flip of a target nucleon, indicating J = 1/2-, 3/2- or 5/2- (1958TY46). See also (1955AJ61, 1956ST1D) and 12C.
Ground state deuterons have been observed at Ed = 4.2 MeV (1955KH35).
Alpha-particle groups have been observed corresponding to 11B*(2.107 ± 0.017) (1951LI29) and 11B*(4.45, 6.83) (1953SP1A). A 4.46 MeV γ-ray observed by (1955BE62) in (13C + d) is definitely assigned to the present reaction by (1958RA13). See also (1955AJ61).