(See Energy Level Diagrams for 12C)
Shell model: (1956KU1A, 1956PE1A, 1957KU58, 1960ME1C, 1960TA1C, 1960WE1C, 1961BA1E, 1961TR1B, 1963NA04, 1963VI1A, 1964AM1D, 1964CL1A, 1964GI1B, 1964GI1C, 1964NE1E, 1965BA2E, 1965CO25, 1965FA1C, 1965NE1C, 1966GI1A, 1966HA18, 1966VA1D, 1966YO1B, 1967CO32, 1967EV1C, 1967KU1N, 1968HI1H).
Collective model: (1959BA1F, 1959BR1E, 1961CL10, 1962CL13, 1962GO1R, 1962WA17, 1963GO1Q, 1964BR1H, 1964VO1B, 1965ST22, 1965UB1B, 1965UB1A, 1965VO1A, 1966BO1X, 1966DA1F, 1966DR1F, 1966KR02, 1967BA2N, 1967BO1G, 1967BO04, 1967BR1E, 1967KR1C, 1967LA09, 1967LA1G, 1967PA10, 1967RI1B, 1967RO1G, 1967SA1K, 1967SO1A, 1967SO07, 1967BA1K, 1967BA12, 1967BA2D, 1968MI1E, 1968SO1B).
Cluster model: (1956GL1B, 1956PE1A, 1959PI1B, 1960BI1E, 1960IN1B, 1960SH1A, 1966HE1C, 1962MA1H, 1963MA1D, 1963MA1E, 1964GR1M, 1964MA1G, 1965BA2D, 1965BE1H, 1965FA1C, 1965IN1A, 1965KU1E, 1965NE1B, 1965SH11, 1966DA1J, 1966DU1D, 1966HA1R, 1966HE1C, 1966KA1A, 1966BR1U, 1967NO1C, 1967TA1C, 1967UI01, 1968GO01).
Elastic scattering, studied for E(6Li) = 3.2 to 7.0 MeV follows the Mott formula at low energies, but deviates at energies ≳ 4.0 MeV. Angular distributions can be accounted for by the Blair cut-off model, but the parameters are not sharply defined (1966PI02).
For E(6Li) = 1.2 to 2.8 MeV, population ratios of 7Be*(0.43), 7Li*(0.48) and 10B*(0.72) remain approximately constant. Simple tunneling or compound nucleus models are not compatible with the data and a direct interaction through long-range tails is suggested (1962MC12). Absolute reaction cross sections at E(6Li) = 2.1 MeV are in reasonable agreement with estimates based on barrier penetration. A strong preference for α-emission suggests that the favored mechanism involves interacting clusters (1963HU02). A conspicuous feature of the α-spectrum involves a transition through highly excited states of 8Be, possibly those at Ex = 22.2 and 22.9 MeV with large 6Li + d parentage (1963KA20, 1964MA26). Angular distributions of α0 and α1 indicate stripping (1964MA26: 2.0 to 4.4 MeV). Noticeable fluctuations of protons angular distributions and of 0° α-yields in the range E(6Li) = 2.4 to 9.0 MeV (1966KI09), and 2.2 to 14.5 MeV (1967AL1F), indicate compound nucleus effects. See also (1962DE1F, 1963BA1Q, 1963CO35, 1963LE19, 1964CA1G, 1964GA1E, 1965NO1A), 8Be and 10B in (1966LA04), and 11B and 11C here.
The yield of capture γ-rays to the ground and 4.4 MeV states (reaction (a)) has been measured for E(3He) = 2 to 4.5 MeV. The cross section increases monotonically; at 4.5 MeV, σ(γ0) is ≈ 4 μb. A strong 17.6 MeV γ-ray is ascribed to reaction (f) (1964BL12). Evidence for decays via higher states of 12C is not conclusive (1963BL05, 1964BL12).
Excitation functions for neutrons (reaction (b)) have been determined by (1963DU12: 1.2 to 2.7 MeV; n0, n1, n2, n3, n4+5; θ = 0° and 81.5°), (1965DI06: 1.3 to 4.9 MeV; n0, n1; θ = 0°, 90° and 160°), (1965TO06: 3.5 to 5.8 MeV; n0, n1, n2+3, n4+5, n6, n7, n8; θ = 5° and 90°) and (1968OK1D: 3.5 to 9.9 MeV; n0, n1). See also (1959MA1D, 1962SE1A, 1966MA1R, 1967HA20). No sharp structure is observed but there is some suggestion from angular distribution data and excitation functions at forward angles for a broad structure (Γ ≈ 350 keV) at E(3He) ≈ 2 MeV: Ex = 27.8 MeV (1963DU12, 1965DI06). Comparison with (3He, p) shows reasonable similarity at low energies, but strong differences for E(3He) > 2.5 MeV (1965DI06, 1965TO06). For E(3He) = 3.5 to 5.8 MeV the reaction proceeds predominantly by direct interaction (1965TO06). The total cross section for 11C production shows a broad maximum, σ = 113 mb, at E(3He) = 4.3 MeV (1966HA21: 3.2 to 10 MeV). See also (1965BR42, 1967HA20). Angular distributions of the polarization of neutrons to 11C*(0, 2.0, 4.3 + 4.8) have been measured at nine 3He energies from 2.1 to 3.9 MeV by (1967TH1H). See also 11C.
Excitation functions and angular distributions for protons (reaction (c)) have been measured for E(3He) = 1.0 to 2 MeV (1967CO03: 90°; p2 to p9), 1.8 to 4.9 MeV (1959WO53: total and differential cross sections; p0 and p1), 3 to 5 MeV (1959WO53: p2 and p3) and 5.7 to 10.2 MeV (1960HI08: 10°, p0 to p9; 90°, p0, p1). From E(3He) = 5.7 to 10 MeV the majority of angular distributions are essentially independent of energy, showing pronounced forward peaking, consistent with a predominantly direct process. The excitation curves show only a slow and smooth increase (1960HI08). See also 11B.
Excitation functions for ground-state tritons (reaction (e)) for E(3He) = 2.4 to 4.1 MeV (θ = 20°, 40°, 55° and 90°) show a smooth rise with energy in the region explored (1960TA04). At θ = 20° the cross section then shows a broad maximum at E(3He) ≈ 4.5 MeV. Following this maximum, the cross section decreases to E(3He) ≈ 7.5 MeV and then rises slowly to 9 MeV. Angular distributions of ground-state tritons have been measured at 1 MeV intervals between E(3He) = 5.0 and 9.0 MeV (1967EA01). Near E(3He) = 10 MeV, excitation functions for tritons and deuterons show no detailed structure; angular distributions show characteristic direct interaction features. DWBA fits for tritons are less satisfactory than those for deuterons (1967CR04). See also (1960HI08).
The elastic scattering excitation function (reaction (g)) has been measured at 45° for E(3He) = 4.0 to 9.0 MeV: it decreases monotonically over this energy region (1967EA01). For reaction (f), see (1964BL12) and 8Be in (1966LA04).
Neutron groups corresponding to 12C levels at 0, 4.4, 7.7, 9.6, (10.1) and (10.8) MeV are reported: see (1960AJ04, 1962NI02). Observation of the γ-decay of the 15.1 MeV level is reported by (1954RA35, 1957WA04, 1957WA1F). Angular distributions have been studied by (1959SM98: 1.9 to 2.7 MeV; n0, n1), (1960RE02: 2 to 5.6 MeV; n0, n1, n2), (1960GA14: 3.35 and 5.10 MeV; n1), (1963ME11: 5.3 MeV; n1), (1960AJ04: 5.6 and 5.8 MeV; n0, n1, n2), (1961GA03: 5.5, 5.8 and 6.0 MeV; n0, n1, n2), (1967VE1D: 6 to 10 MeV; n0, n1, n2), (1962KJ02, 1962KJ04, 1962NI02: 9.8 to 14.2 MeV; n0, n1, n2, n3+4), (1962DE1G, 1963DE27, 1965DE1F: 12.9 to 23 MeV; n0, n1) and (1963KO03: 17.5 to 22.1 MeV; n0, n1, n3).
Doppler shift measurements on the transition 12C*(4.4 → g.s.) yield a mean life τm = 50 ± 6 fsec (1961DE38); 57+23-17 fsec, Γγ = (11.5+5-3.2) meV (1966WA10); ≤ 48 ± 10 fsec (1967CA02): see Table 12.8 (in PDF or PS). The internal pair conversion coefficient indicates an E2 transition (1954MI68): the pair angular correlation permits M1 or E2 and favors the latter (1954HA07, 1956GO1K, 1956GO73, 1958AR1B). Angular distributions of n1 and n1-γ correlations strongly indicate a direct interaction mechanism even at Eα = 3.3 and 5.5 MeV (1960GA14, 1962KJ01). Gamma ray angular distributions have been studied by (1955TA28, 1959SM98, 1963SE04). See also 13C.
The 7.65 MeV state decays predominantly into 8Be + α (see reactions 15 and 28). The 7.7 MeV nuclear pairs have been observed: Γπ/Γ = (6.6 ± 2.2) × 10-6 (1959AL97, 1960AJ04, 1960AL04, 1961GA03). See also (1959AJ76) for a survey of the earlier work.
Not observed: see (1964CA05).
At Ed = 0.95 MeV, the upper limit to the capture cross section is 0.1 μb (1955SA1B).
The thin-target excitation function in the forward direction in the range Ed = 0.3 to 4.6 MeV shows some indication of a broad resonance near Ed = 0.9 MeV. Above Ed = 2.4 MeV, the cross section increases rapidly to 210 mb/sr at 3.8 MeV, and then remains constant to 4.6 MeV (1954BU06, 1955MA76). The 0° excitation function for ground state neutrons shows no structure for Ed = 3.2 to 9.0 MeV. The angular distributions all show a sharp peak around 20° and a smaller contribution in the background direction. DWBA produces a satisfactory fit to these distributions, but the parameters vary with energy (1967DI01). Cross-section ratios have been obtained at Ed = 1 to 5 MeV for the neutrons and protons to the second, third, fourth, and fifth excited states of the 11B and 11C mirror nuclei (1967SC1K). Polarization measurements have been carried out for Ed = 2.5 to 4.0 MeV (1967ME1N). See also (1955SA1B) and 11C.
Absolute yields and angular distributions are reported for various proton groups by (1952EN19, 1954BU06, 1954PA28, 1956MA69, 1956VA17, 1959CR1A, 1960CR1A, 1960HA08, 1964BR1A, 1965LE1B, 1967PO01) for Ed = 0.14 to 12 MeV. Although the excitation functions show several broad peaks, no clear resonances can be identified, and it is assumed that many overlapping resonances are involved (1956MA69) except possibly at Ed = 2.3 MeV (Ex = 27.1 MeV) where the effect of a broad resonance influences the cross section of the p1 and p3 groups (to 11B*(2.14, 5.03)) (1964BR1A). Studies of plane wave, distorted wave and Coulomb wave Born approximation angular distributions are reported by (1967PO01: see also (1966MO1H, 1967MO1N). There are no significant fluctuations in the yield of protons for Ed = 5 to 12 MeV (1965LE1B: θ = 50° and 150°). See also (1965BA31). Yields of gamma rays have been measured by (1955SA1B). Cross section ratios for the (d, n) and (d, p) reactions to mirror states have been measured by (1967SC1K) [see reaction 9].
Polarization measurements have been made by (1959HI1E: 6.9 MeV; p0), (1964BE08: 10 MeV; p0, p1, p2 + p3), (1962TA13, 1964PA1E: 11 to 13.8 MeV; p0), (1960TA27: 11.4 MeV; p0), (1963BO1J: 21 MeV; p0). The circular polarization of 2.12 and 4.44 MeV γ-rays has been investigated at Ed = 0.45 MeV (1960ER1A, 1961ZI02, 1962ZI01, 1963ER1A). See also 11B.
Excitation curves and angular distributions are reported for α0 and α1 groups (to 8Be*(0, 2.9)) by (1956MA69, 1960BE15, 1961LE10, 1963PU02, 1964AL1Q, 1964BR1A, 1966LO18, 1967LO1J) for Ed = 0.4 to 3.3 MeV. Broad maxima are observed in both excitation curves above Ed = 1 MeV. Preliminary data at Ed = 0.98 MeV indicate the formation of a level in 12C at 26.00 ± 0.01 MeV which decays via 8Be*(2.9) and 8Beg.s. (1967PE1B).
This reaction has been studied for Ed = 8 to 13.5 MeV. A DWBA calculation with L = 2 α-transfer gives a qualitative account of the angular distribution (1964GE10).
Proton groups observed by (1958MO99, 1959AL96, 1962BR10, 1967CO1F) are displayed in Table 12.9 (in PDF or PS). Angular distributions of many of these groups have been measured for E(3He) = 2.0 to 3.0 MeV (1965BH1A), 3.7 MeV (1962BR10), 10.1 MeV (1962AL01) and 14 MeV (1967CO1F). The following comments on individual levels derive largely from the 10B(3He, p)4He4He4He studies of (1964ET02, 1965AL1B, 1966WA16): see Table 12.9 (in PDF or PS).
12C*(7.65, 9.6) are observed to decay to 8Beg.s., confirming natural parity π = (-1)J for both states (1966WA16). Pair emission from 12C*(7.66) has been measured: Γπ/Γ = (6.6 ± 2.2) × 10-6 [see Table 12.8 (in PDF or PS)] as has the cascade through 12C*(4.4): Γγ/Γ = (3.3 ± 0.9) × 10-4 (1961AL23). By observation of 12C recoils, a value Γrad/Γ = (3.5 ± 1.2) × 10-4 is found by (1964HA23): compare to 14N(d, α)12C.
12C*(11.83, 12.71, 13.35) decay to 8Be*(2.9) but not to the g.s., indicating unnatural parity: this result is inconsistant with the assignment (1-) to 12C*(11.83): see (1964BR25). For 12C*(12.71) Γγ/Γα = (3 ± 1)% (1958MO99), (2 ± 1)% (1959AL96): the cascade through 12C*(4.4) is (20 ± 7)% relative to the ground-state transition (1960AL14): see Table 12.9 (in PDF or PS). The alpha breakup of 12C*(12.71) shows a triple-peaked α-particle spectrum, characteristic of the breakup of a Jπ = 1+ state (1967BH1B).
12C*(14.08) decays both to 8Be*(0, 2.9); the branching ratio Γ(α0)/Γ is 0.1 - 0.4. Proton α-correlations require J ≥ 2 (1966WA16).
12C*(15.11: Jπ = 1+; T = 1) decays by γ-emission to 12Cg.s. 97% and to 12C*(4.4) (3.1 ± 0.6)% (1959AL96), (4 ± 1)% (1960AL14); Γα/Γ < 0.2 (1960MI1E), < 0.05 (1965AL1B), < 0.10 (1966WA16), respectively. The strong inhibition of the transition to 8Be*(2.9) is cited as evidence for a high isospin purity (1965AL1B): see, however, (1958KA31).
12C*(16.11, 16.57) show decay to both 8Be*(0, 2.9). The consequent assignment of natural parity is consistent with Jπ = 2+ for the former but not with the accepted Jπ = 2- for the latter [see reaction 19]. For 12C*(16.11) observed values of Γα0/Γ are 0.05 - 0.12; the decay to 3α occurs rarely, if at all (1966WA16: see, however, 11B(p, α)8Be: (1965DE1R)).
The giant resonance excitation region has been searched for levels with a resolution of ≈ 20 keV at E(3He) = 14 MeV. Two peaks have been observed corresponding to 12C* ≈ 20.6 and 24.5 MeV, with Γ ≈ 200 and 50 keV, respectively. Angular distributions show forward maxima. peaks which could be associated with structure seen in photodisintigration experiments are not observed (1967CO1F). See also (1959JO1E, 1961AL10, 1961WO08, 1962KU02, 1965SI05, 1966BA01) and 13N.
At Eα = 21.2, 23.0 and 25.0 MeV angular distributions of the deuterons corresponding to 12C*(0, 4.4) have been measured (1967AL16).
At E(6Li)c.m. = 3.05 MeV, angular distributions have been obtained for α0, α1, α2 and α3. The α-particles to 12C*(11.83) and (12.71) have also been observed (1966MC05). At E(6Li) = 3.8 MeV, the population of the T = 15.11 MeV state is about 3 ± 2% of the T = 0 state at 12.71 MeV. It is pointed out that in this reaction the distance of closest approach is 15 fm and the dominant potential would be the Coulomb potential. It is therefore not neccessary to invoke a large T = 0 mixing in the 15.11 MeV state to explain the observation of the 15 MeV γ rays (1964CA18).
In the range Ep = 0 to 25 MeV, twenty-three resonances are reported. Their characteristics are displayed in Table 12.10 (in PDF or PS). Cross sections of astrophysical interest are discussed by (1967FO1B).
The Ep = 0.16 MeV resonance (12C* = 16.11 MeV) is well established as the Jπ = 2+; T = 1 analogue of the first excited states of 12B and 12N: see (1959AJ76). The resonant energy is 163.1 ± 0.2 keV; Γlab = 6.5 ± 0.6 keV [see (1955AJ61)]. The gamma decay of the 16.11 MeV state takes place to 12C*(0, 4.4, 9.6): see (1959AJ76, 1961CA13, 1961SE10). The decay to 12C*(9.6) is via a 6.45 ± 0.05 MeV γ-ray whose angular distribution, together with the known α-decay properties of 12C*(9.6), leads to Jπ = 3-. The intensity of the transition is 1% of the γ1 decay (1961CA13). See also (1961GR07, 1965CV1A, 1966SO1B).
The character of the α-decay at the Ep = 0.16 MeV resonance has been studied with coincidence techniques by (1961DE31, 1962DE1H, 1964DE1J, 1965DE1R). Immediately below and above the resonance, a Dalitz plot indicates sequential decay via 8Be*(0, 2.9): in this region, the behavior is ascribed to tails of the Ep = 0.675 and 1.4 MeV resonances. At Ep = 0.163 MeV, the Dalitz plot shows a striking increase in population near the center, indicative of direct 3α breakup. Some part of the reaction also goes through 8Beg.s., and there is evidence for a final-state two-body interaction with l = 2 (1964DE1J): see, however, (1965MA1X, 1965PH1A, 1966CH1L, 1966WA16). See also (1959KA12, 1959KA13, 1960BO26, 1962BE21, 1964DE1H, 1964LO05, 1964LO1E, 1965KR1A, 1965KR1B, 1967KA09, 1967LO1H). Similar studies at Ep = 0.675, 1.388 (1965KA1G), 2.0, 2.65, 3.25, 3.73, 4.00, 5.08, and 5.64 MeV (1964PH1A, 1965BR18, 1965PH1A) indicate that sequential decays through 8Be*(0, 2.9) dominate; direct 3α decay is < 5%. The latter work shows evidence for "order-of-emission" interference in the shapes of α-groups: see also (1965DU1C, 1965SW1B). See also (1958FO1D, 1966AD1E, 1966SO1B, 1967LO1H, 1968CH1L).
The Ep = 0.67 MeV state (12C* = 16.58 MeV) has a proton width Γp ≈ 150 keV. Such a width indicates s-wave protons and therefore Jπ = 1- or 2-. These assignments are supported by the near isotropy of the two resonant exit channels, α1 and γ1. The size of the α1 cross section indicates 2J + 1 ≥ 5; therefore Jπ = 2-. The reduced width θ2(α1) ≈ 0.05 and the γ1 E1 transition has |M|2 ≈ 0.01 Weisskopf units, suggesting T = 1 (1957DE11, 1965SE06). See also (1959AJ76). (1962BL10) report a γ-branch to the 12.71 MeV state, ≈ 6% of the intensity of the 4.4 MeV transition. Such a branch may also be present at 12C*(17.23).
For the Ep = 1.4 MeV state (12C* = 17.23 MeV), (2J + 1)Γγ0 ≥ 115 eV. This indicates Jπ = 1-, with T = 1 most probable (1965SE06). Jπ = 1- is also required to account for the interference at lower energies in α0 and γ0: see (1957DE11); see also (1959AJ76). Two solutions for Γp are possible; the larger (chosen for Table 12.10 (in PDF or PS)) is favored by elastic scattering data (1965SE06). (1963SY01) find no evidence for resonance in α0 or α1 at this energy.
Jπ = 0+; T = 1 is consistent with all data for the Ep = 2.0 MeV resonance (12C* = 17.77 MeV) which decays via the α0 and α1 channels (1965SE06). The resonance in the yield of α0 requires natural parity, and the small α-widths suggest T = 1. For Jπ = 1- or 3- the small γ-widths would be surprising; Jπ = 2+ would lead to a larger elastic anomaly than is observed (1965SE06: see also (1963SY01)).
At Ep = 2.62 MeV (Ex = 18.36 MeV), the resonance for α0 again demands natural parity; the presence of a large P4 term in the angular distribution requires J ≥ 2 and lp ≥ 2. The assignment Jπ = 3-; T = 0 is consistent with the resonance data and with the angular distribution of α0 at the Ep = 1.98 MeV resonance (1965SE06: see also (1955GO10, 1963SY01, 1967FL1F, 1968CH1L)).
The Ep = 2.66 MeV resonance, distinguished from that at Ep = 2.62 MeV by its width, is not seen here: see 11B(p, p).
The Ep = 3.01 MeV resonance appears only in the angular distributions for α0: the small α-widths suggest T = 1 (1965SE06).
At Ep = 3.12 MeV (Ex = 18.82 MeV), the angular distribution of γ0 indicate E2 radiation, Jπ = 2+. This assignment is supported by the angular correlation in the cascade γ1 and by the behavior of σ( α0); T = 1 is suggested by the small Γα (1965SE06).
The structure near Ep = 3.5 - 3.7 MeV (Ex = 19.2 and 19.4 MeV) seems to require at least two levels. The large Γγ0 requires that one be Jπ = 1-; T = 1 and interference terms in σ(α0) require the other to have even spin and even parity: Jπ = 2+; T = 0 is favored (1963SY01, 1965SE06).
Levels at Ep = 4.93 and 5.11 MeV, seen in σ(γ1) (1955BA22) also appear in σ(α1), but not in σ(α0). Angular distributions suggest Jπ = 2+ or 3- for the latter (Ex = 20.64 MeV); the strength of γ1 and absence of γ0 favors Jπ = 3-; T = 1 (1963SY01).
In the range 4 < Ep < 14.5 MeV, σ(γ0) is dominated by the giant dipole resonance at Ep = 7.2 MeV (Ex = 22.6 MeV), Γcm = 3.2 MeV: ∫ σdE = 630 eV · b. σ(γ1) likewise shows a giant resonance centered at about 10.3 MeV (Ex = 25.4 MeV), Γcm ≈ 6.5 MeV, ∫ σdE = 850 eV · b. Both excitation functions show significant fine structure (Table 12.10 (in PDF or PS)): see (1964AL20, 1967FE04).
From Ep = 4 to 14 MeV the angular distributions of γ0 are given by W(θ) = 1 + a1P1(cosθ) + a2P2(cosθ) with the coefficient a2 almost constant at -0.6, in approximate agreement with the expectation from particle-hole calculations of Jπ = 1-; T = 1 states by (1962VI01). The a1 term exhibits fluctuations for Ep = 4 to 5.5 MeV in the region of narrow resonances; from 5.5 to 14 MeV it rises smoothly from +0.03 to +0.3. A sharp resonance in σ(γ1) at Ep = 10.15 MeV, ∫ σdE = 4 eV · b, may have T = 2 (1964AL20). See also (1959GE33, 1959GO89, 1961GO13, 1963BE18, 1964AL1J, 1964BL1D, 1964TA05, 1965TA1E, 1966HA1M, 1966ME1H, 1966UB01, 1967KA05, 1967KE1K, 1967MA11).
For 15 < Ep < 25 MeV, a resonance is found at Ex = 34.4 ± 0.5 MeV, Γ ≈ 4 MeV, ∫ σ(γ0)dE = 38 eV · b (1963RE09: see, however, (1968BR1M)). The resonance is ascribed to a component of the E1 giant resonance, but the cross section is an order of magnitude less than predicted (1963RE09). (1968BR1M) report no pronounced structure for Ep = 13 to 22 MeV.
Excitation functions are reported by (1955BA22: Ep = 2 to 5 MeV, long counter), (1959GI47: Ep = 2.6 to 5.5 MeV, 4π graphite sphere), (1964BA16: Ep = 4 to 14 MeV, 4π graphite sphere), (1961LE11: Ep = 4.9 to 11.4 MeV, 11C activity), (1965SE06: Ep = 3 to 4 MeV, 11C activity), and (1965OV01: Ep = 4 to 11.5 MeV: time-of-flight resolved groups). The excitation functions are characterized by numerous peaks (see Table 12.11 (in PDF or PS)) whose positions appear to correspond with 11B(p, γ)12C and with some of the (γ, n) and (γ, p) structure, suggesting that resonances, and not fluctuations, are involved. Angular distributions do not change as rapidly as might be expected from the pronounced structure in the excitation function (1965OV01). The strength of the pronounced peak at Ep = 6.03 MeV (Ex = 21.49 MeV) appears to demand J ≥ 4 (1961LE11). See also (1959GO89, 1960FU1A, 1961GO13).
A pronounced anomaly in the elastic scattering is observed near Ep = 0.67 MeV at all angles; the level is therefore formed by s-waves. The 0.3 to 1.0 MeV results are well accounted for by two resonances: Ep = 0.67 MeV, s-wave, Jπ = 2-, Γ = 0.33 MeV. Γp/Γ = 0.5, and Ep = 1.4 MeV, Jπ = 1- (1957DE11). Higher energy structure in the yields of reactions (a) and (b) are displayed in Table 12.11 (in PDF or PS) (1955BA22, 1963AN12, 1965SE06). See also (1960SA28) and Table 12.10 (in PDF or PS).
Reported neutron groups are listed in Table 12.12 (in PDF or PS). Angular distributions of the neutrons to many of the 12C states up to Ex = 17.23 MeV have been reported for energies in the range Ed = 0.5 to 10 MeV: see (1957AM48, 1958AM13: 0.5 to 1.15 MeV), (1955WA30: 0.6 MeV), (1955IH1B: 0.69 MeV), (1954GR53: 0.85 MeV), (1957BI78: 0.92 MeV), (1962SA09: 1.0 to 2.0 MeV), (1963KI02: 1.6 to 2.7 MeV), (1961ZD02: 1.1 to 2.8 MeV), (1965SI12: 1.1 to 3.2 MeV), (1965CL02: 1.5 to 4.7 MeV), (1961GA04: 2.7 and 5.4 MeV), (1966HU1H, 1967WI1J: 3.0 to 5.5 MeV), (1965AL17: 2.4 to 9.7 MeV), (1964RO1F, 1966RO1X, 1966WE1B: 6.3 MeV), (1953GI05: 8.1 MeV), (1956MA83: 9 MeV), and (1958ZE01: 10 MeV). See also (1959AJ76) and (1960WA1G, 1961HO1D, 1962LE1A, 1963LA1E, 1964NA02, 1965SI13, 1967DI01).
For Ed less than 6 to 7 MeV, angular distributions of n0 and n1 are characterized by both forward and backward peaks, with broad minima near 90°. Detailed shapes are strongly dependent on bombarding energy. The general behavior in the lower-energy range is ascribed to heavy-particle stripping by (1957OW03, 1958AN32, 1961ZD02), but others (1961GA04, 1965AL17, 1966ST1L, 1967WI1J), find reasonable agreement with DWBA, possibly also with compound nucleus effects (1963KI02, 1965CL02). For Ed > 7 MeV, DWBA gives a quite satisfactory account of n0 and n1 distributions, except for a single example (n1 at 8.8 MeV: (1965AL17)). See also (1968YA1G).
Angular correlations of neutrons and 4.4 MeV γ-rays have been studied by (1963HU05: 0.7 and 1 MeV), (1963RI03, 1964RI1D: 1 and 1.2 MeV), (1961ZD02: 1.1 MeV), (1961GA04: 2.65 and 5.35 MeV), (1966RO1X, 1966WE1B: 6.3 MeV). Angular correlations have also been determined for neutrons and 15.1 MeV γ-rays: see (1963KI14: 2.3 and 2.6 MeV), (1960FE01, 1960FE13: 5 and 8 MeV), (1964RO1F: Ed = 6.3 MeV). The formation of the 15.11 MeV state involves lp = 1 with a 12 : 1 ratio between channel spins 2 and 1. The reduced proton width is approximately equal to the single-particle width (1960FE01, 1960FE13).
In the range Ed = 1.0 to 5.5 MeV, two slow neutron thresholds are observed at 1.627 ± 0.004 MeV (Ex = 15.109 ± 0.005 MeV) and near 4.1 MeV (broad; Ex = 17.2 MeV) (1955MA76). At the lower threshold, 15.1 MeV γ-rays are observed: Ed = 1.633 ± 0.003 MeV, width less than 2 keV (1958KA31) [Ex = 15.110 ± 0.003 MeV].
A study of the angular distributions and energy spectra of α-particles from the decay of 12C states shows that the 12.71 and 11.83 MeV states decay sequentially via 8Be; the former via 8Be*(2.9), the latter 90% via 8Be*(2.9) and 10% via 8Be(0). There is some evidence that the 10.84 MeV state decays primarily to 8Be(0). Jπ = 3- for the 9.64 MeV state is favored on the basis of the angular distribution of the α-particles to 8Be(0). There is no evidence for direct 3α decay of 12C levels in the range Ex = 9 to 13 MeV, nor does 12C*(10.3) appear to participate in this reaction (1965OL01).
Observed deuteron groups are displayed in Table 12.13 (in PDF or PS) (1960FO01, 1961HI08). Excitation functions near 10 MeV show no structure; angular distributions exhibit characteristic direct interaction features (1967CR04). See also (1959HO01, 1959WO1B, 1961HO1F) and (1967AD1F).
Differential cross sections have been obtained for the ground-state tritons and for the tritons to 12C*(4.4), in the forward direction at Eα = 43 MeV: single-proton transfer seems to be the dominant reaction mode (1967DE1K). The angular distributions appear to be affected by final state spin (1967SI1A).
The decay is mainly to the ground state of 12C; branching ratios to other states are listed in Table 12.14 (in PDF or PS). The half-life is 20.41 ± 0.06 msec (Table 12.2 (in PDF or PS)). Since transitions to 12C(0+) and (2+) are allowed, Jπ of 12B is 1+.
Since the decays of 12B and 12N are mirror transitions, comparison of the ft values has some interest. According to (1964KA08), ft(12B) = 11.800 ± 70 sec, ft(12N) = 13.060 ± 90 sec; the ratio 12N/12B is 1.11 ± 0.01: see also (1963FI05, 1963PE10, 1964FI02, 1964NA1C, 1964WU01, 1966BA1A). Possible explanations of the difference are discussed by (1963HU10, 1964EI1C, 1965BL1F, 1966MA2R, 1966OK1B).
Comparison of shape factors for the ground-state β-transitions for 12B - 12N provides a test of the conserved vector current theory. The correction to the simple Fermi shape should have the form 1 + AE for 12B and 1 - AE for 12N where A is determined mainly by the (experimental) M1 width of the 12C*(15.1) T = 1 level: A(theor.) = 0.52 ± 0.12% MeV-1 (1958GE1C, 1959GE1D, 1963KI1D, 1964WU01): see also (1959FL41, 1959MO27, 1960WE1C, 1961MO1C, 1967HU10). The experimental values of (1964WU01) are A = 0.55 ± 0.10 and 0.52 ± 0.06 for 12B and 12N, respectively: see also (1961MA16, 1962LI05, 1962MA22, 1962MC14, 1963GL04, 1963LE05, 1965WU1A).
The level at Ex = 7.6 MeV has special interest for helium burning processes in stars (1963SE17, 1963SE23, 1967FO1B). Observation of the α-spectrum yields Q(12C* - 8Be - 4He) = 278 ± 4 keV (1957CO59). With Q(8Be - 24He) = 92.12 ± 0.05 keV (1966BE05), Q(12C* - 34He) = 370 ± 4 keV. Using (1965MA54) masses for 8Be and 4He, 12C* = 7647.0 ± 4.1 keV. Although the level decays mainly by α-emission, both the gamma branch to 12C*(4.4) and pair emission to 12Cg.s. have been observed in various reactions. The relevant parameters are given in Table 12.8 (in PDF or PS) (see (1963AL15, 1963SE23)). The fact that the β-decay is allowed indicates Jπ = 0+, 1+ or 2+ for 12C*(7.6); observation of the α particles eliminates Jπ = 1+, and the large Γα requires Jπ = 0+ (1957CO59).
The 10.3 MeV level is observed in both 12B and 12N decays. The excitation energy given by (1966SC23) is 10.3 ± 0.3 MeV, Γ = 3.0 ± 0.7 MeV. The decay is to 8Beg.s.. With R = 5.2 fm, θ2α = 1.5 and 7.5 for Jπ = 0+ and 2+, respectively (1958CO66). It is suggested by (1966MO08) that this level is a highly deformed rotational state with Jπ = 2+, related to 12C*(7.66). Some fraction of the observed α-spectrum is presumed to result from a "ghost" of the lower level (1962BA1C, 1963WI05, 1964BL12).
In 12N, allowed transitions are observed to 12C*(12.7) and (15.1) (AL66J, 1966SC23: see also (1963GL04, 1963WI05)). The 12.7 MeV level decays primarily to 8Be*(2.9; Jπ = 2+); the absence of decay to 8Beg.s. is in agreement with the assignment Jπ = 1+ (1966SC23). The ft values for both β-transitions (see Table 12.25 (in PDF or PS)) agree well with shell model calculations of (1965CO25). In particular, the agreement strongly suggests a close relation between 12C*(12.71; Jπ = 1+; T = 0) and 12C*(15.11; Jπ = 1+; T = 1). A search for the transition from 12B to 12C*(12.71) was unsuccessful (1967AL03). See also (1959JA1B, 1960FA02, 1964SH1J, 1966DU1E).
Resonance scattering and absorption by the 15.1 MeV level have been studied by (1957HA13, 1959GA09, 1960BU14, 1960HA1H, 1961BU1E, 1963SC21, 1967KU11): partial widths are displayed in Table 12.15 (in PDF or PS). The scattering angular distribution is W(θ) = 1 + cos2θ, indicating dipole radiation (1959GA09); the azimuthal distribution of scattered polarized radiation indicates M1 (1960JA01) and the large Γ(M1) indicates T = 1.
The ground-state γ-width of 12C*(16.11) is reported as Γγ = 7.5 ± 1.9 eV (1959KE19: see, however, (1961SE10)). For 12C*(17.22), the scattering cross section is 1.0 ± 1.0 μb, consistent with Γγ given by 11B(p, γ) (1963SC21). See also (1960WE1C).
At higher energies, elastic scattering studies show the giant resonance peak at ≈ 24 MeV (see 12C(γ, n)11C), dσ/dΩ(135°) = 4 μb/sr. A considerable tail is visible, extending to > 40 MeV (1959PE32). See also (1958WO53, 1961DE22, 1961WI1G, 1962SE02, 1967LO1B).
The total absorption, mainly (γ, p) + (γ, n), in the range Eγ = 13 to 30 MeV is dominated by the giant resonance peak at 23.2 MeV, Γ = 3.2 MeV [Ex = 22.6 MeV from 11B(p, γ)12C]. This single peak accounts for 64% of the total strength below 27 MeV. The total integrated cross section to 35 MeV is 144 MeV · mb, about 80% of the classical sum rule, 60 NZ/A MeV · mb: see Table 12.16 (in PDF or PS). Other resonant structure is reported at Eγ = (16.5), 17.5, (19.1), 26 and 29 MeV: see Table 12.17 (in PDF or PS) (1960ZI01, 1963BU1G, 1965WY02). See also (1958MO1F, 1958WO53, 1958ZI1B, 1959BU1H, 1959KO55, 1959KU84, 1959PE21, 1960CA09, 1960TA15, 1960WY1A, 1962DE03, 1962MI07, 1963BU18, 1963CO1D, 1965MA1N, 1967BR1P, 1967DR1E).
Shell-model calculations on the structure of the giant resonance ascribe the main effect to four Jπ = 1-; T = 1 levels formed from the particle-hole configurations p-13/2s1/2, p-13/2d5/2, p-13/2d3/2 and s-11/2p1/2, of which the second contributes most of the E1 strength. The computed energies are 19.6, 23.3, 25.0 and 35.8 MeV [the most probable identifications are Ex = 19.2, 22.6, 25.4 and 34.4 MeV (Table 12.7 (in PDF or PS))] ((1964LE1D), and references therein). Calculations with a deformed potential suggest a somewhat more complicated structure (1962NI1D). See also (1961BA1D, 1962VI01, 1963BO21, 1963MI1B, 1964GI1C, 1964MI1E, 1965DA1D, 1966FU02, 1966LO04, 1966UB01, 1967LE1H, 1967MA1P).
Observations of σ(γ, n) show a giant resonance centered at about 22.5 MeV, σ(peak) ≈ 8 mb, Γ ≈ 3 MeV, with a long tail slopping off to about 40 MeV. The integrated cross section to 40 MeV is about 60 MeV · mb; integrated to 250 MeV, it is about 80 MeV · mb (Table 12.16 (in PDF or PS)). In the giant resonance region, the angular distribution of ground-state neutrons is W(θ) = 1 + 1.5sin2θ, consistent with ln = 2 ejection (1960EM02, 1964AL33, 1965VE03: see, however, (1967HA1P)). For Eγ = 28 to 145 MeV, comparison with (e, e'n) indicates mainly E1 processes, with ≲ 8% E2 (1958BA60: see also (1961RO1H)).
The giant resonance appears to have a fine structure (Table 12.18 (in PDF or PS)): at least two major components are identified at Ex = 22.0 and 23.5 MeV (1966FU02, 1966LO04: see also (1966CO09, 1967FI1E)). A satellite at Ex = 25.5 MeV is also well established: a large fraction of the decay of this level (6 MeV · mb) involves excited states of 11B and 11C (1966MA1T: see also (1966CO09, 1966FU02)). Up to 28 MeV, about 83% of the neutrons leave 11C in the ground state (1966FU02). Several broad levels are indicated in the range Ex = 27 to 35 MeV (1966CO09, 1966FI1D, 1966FU02, 1967FI1E). A possible level near 52 MeV is reported by (1966FO06). See also (1962BO1H, 1966BI1B, 1966CO09). Considerable fine structure in the range Ex = 19 to 23 MeV is reported by (1959SA06, 1960GE06, 1961TH03, 1966CO09): see also (1959CO62, 1959VA1D, 1960EM02, 1962FU11, 1965MI03).
The photoproton cross section exhibits a single broad giant resonance peak centering at Eγ = 22.5 MeV, (Table 12.19 (in PDF or PS)). There appears to be a significant difference from σ( γ, n) both in shape and peak cross section (1963HA1E, 1966FU1C, 1966ME1H): such a difference could result from a comparatively small isospin mixing (1957BA1K). Some fine structure is suggested by the work of (1956CO59, 1962DO1A, 1963MU08, 1963WA18, 1964SH24, 1964TA1G): compare 11B(p, γ)12C. The angular distributions near the giant resonance are consistent with W(θ) = 1 + 1.5sin2θ expected on the IPM (1962DO1A). For higher energies, an appreciable cosθ term makes its appearance (1959PE22, 1961VA10, 1963WA18). See also (1965DA1D, 1967FR1E). The azimuthal distribution observed with polarized γ-rays at Eγ = 21.3 - 21.6 MeV is not inconsistent with E1 (1966KE06). Evidence for involvement of excited states of 11B for Eγ > 30 MeV is reported by (1959PE22). See also (1960BA1P, 1963BU1G, 1963FI1B, 1966MA1T, 1966RA1E, 1967LE1H).
At high energies, Eγ ≳ 150 MeV, the cross section for photoproton production remains relatively large (≈ 100 μb) and the angular distributions show forward peaking. The high internal momenta thus implies may be understood on a quasi-deuteron picture, in which the interaction involves neutron-proton pairs in close proximity within the target nucleus (1951LE1B). This picture is strongly supported by the observation that most, if not all, high energy photoprotons are accompanied by neutrons with the proper kinematical relation (1954MY1A, 1958BA1C, 1958WH35); see, however, (1965RE1B).
Experimentally derived momentum distributions are reported by (1961CE1B, 1963KI03, 1963KI1C). See also (1958MA1A, 1959CH25, 1961MA36, 1961SH1C, 1962CH26, 1962PA08, 1964JI1A). Polarization of the photoprotons has been studied by (1962LI13).
For Ebrems = 90 MeV, the ratio of yields of deuterons to protons is ≈ 2%, for particle energies 15 to 30 MeV. For higher particle energies, the ratio decreases. Angular distributions are similar for (γ, d) and (γ, p), with strong forward peaking. These observations are consistent either with a quasi-deuteron mechanism or with a two-stage pickup process (1962CH26: see (1962MA1F, 1964SH1B)). According to (1964SH1B) the high apparent threshold for ( γ, d) reflects the presence of continuum states of 10B with a high parentage in 12C. See also (1959CH25, 1960SH1D, 1963BA1K, 1964KI1D, 1965KI04, 1967SM1A).
The cross section exhibits broad peaks at about 18 MeV and ≈ 29 MeV; a pronounced minimum occurs at 20.5 MeV: to what extent the peaks have fine structure is not clear (1953GO13, 1955CA19, 1955GO59, 1955JO1C, 1964TO1A).
For Eγ < 22 MeV, transitions are mainly to 8Beg.s. and 8Be*(2.9) with the g.s. transition dominating for Eγ ≲ 14 MeV. For Eγ > 26.4 MeV, 8Be (T = 1) levels near 17 MeV are strongly excited (1955GO59). Integrated cross sections are 0.82 ± 0.03 MeV · mb (1964TO1A: to 20.5 MeV), 1.21 ± 0.16 MeV · mb (1953GO13: to 20.5 MeV), 2.8 ± 0.4 MeV · mb (1953GO13: 20.5 to 42 MeV) and < 0.2 MeV · mb (1953GO13: 42 to 60 MeV). Alpha energy distributions show surprisingly strong E1 contributions below Eγ ≈ 17 MeV (1955GO59, 1964TO1A). See also (1958MA1A, 1960GA16, 1961SE13, 1964WA1J, 1965RO1J), and (1959YO1B, 1964LE1C, 1965DZ1A, 1965DZ1B).
Reported integrated cross sections for reaction (a) are for 25 to 40 MeV: 1.61 MeV · mb (1962MO16), 1.35 MeV · mb (1956LI05), 3.85 MeV · mb (1958MA1A); for 25 to 120 MeV: 1.9 ± 0.4 MeV · mb (1962MO16). For production of 7Be (reaction (b)), the integrated cross section to 57 MeV is 6.0 ± 0.4 MeV · mb (1966AR01).
Elastic scattering has been studied up to 800 MeV: momentum transfers q2 ≤ 11.5 fm-2 (1959ME24, 1964CR11, 1966CR07). The form factor is well accounted for by a harmonic-well model with R(r.m.s.) = 2.40 ± 0.02 fm. Only one diffraction minimum is observed (1966CR07). R = 2.42 ± 0.04 fm (1967EN1C), R = 2.58 fm (1967EL1B), R = 2.35 ± 0.1 fm (1966AF1A: see also (1967AF02, 1967AF04, 1968AF1A)). See also (1960IN1A, 1963GO04, 1965MU1B, 1965RA1F, 1967BE1P, 1967BO1X, 1967EL1B, 1967FR1D, 1968BR1N).
Sharp inelastic peaks are reported corresponding to 12C*(4.4, 7.7, 9.6, 15.1 and 16.1 MeV) (1956FR27, 1956HE83, 1959BA36, 1959EH1A, 1960BA47, 1961BO32, 1961DU09, 1962ED02, 1963BO36, 1963GO1P, 1964BR1N, 1964CR11, 1964GO14, 1965GU04, 1966CR07, 1967AR1A, 1967CR01, 1967PE07). Observed widths are reported in Table 12.20 (in PDF or PS). Additional structure in the range Ex = 16.6 to 25.5 MeV is reported by (1963BO36, 1966PR1C). The variation of the form factor F(q2) with momentum transfer yields unambiguous assignments of Jπ = 2+, 0+ and 3- for the first three levels (1960BA38, 1964CR11, 1967HA1U). There is some evidence of a diffraction structure in F(12C*(4,4)) at high momentum transfer (1966CR07). Transverse and longitudinal scattering form factors from 12C*(4,4) have been measured by (1967BE43).
The 15.1 MeV transition is a strong M1: Jπ = 1+; T = 1: the indicated Γγ is somewhat lower than the mean from 12C(γ, γ) (1964GO15, 1964GU05, 1965FO1F). The theoretical value is sensitive to the spin-orbit coupling parameter, but a pure p43/2 ground state seems to be excluded (1964BI1H, 1964KU1G). The 16.1 MeV level corresponds to that observed in 11B(p, γ)12C. Γγ0 = 0.184 ± 0.045 eV is obtained from the electron data, and T = 1 is confirmed (1961DU09, 1964BI1H, 1965BI1B, 1966PR1C). See also (1960IN1A, 1962KU1C, 1963BO36, 1964GI1A, 1965IN1A, 1965SE1D, 1967SH1L).
Inelastic excitation of the giant resonance has been studied by (1959BA36, 1960BA47, 1961BO32, 1963BO36, 1963GO1P, 1963LE1H, 1964GO14, 1966PR1C). There appears to be evidence for structure at 18.1 ± 0.05, 19.5 ± 0.05 (Γ = 0.5 ± 0.1), ≈ 24 and ≈ 34 MeV (1964GO14: see also (1967CR02)). The variation of F(q2) with q2 in the range 0 - 0.6 fm -2 shows good agreement with the calculations of (1964LE1D) which assumes four 1- particle-hole states at Ex = 19.6, 23.3, 25.0 and 35.8 MeV (see also (1967CR02)). See also (1960FA1E, 1965LE1D, 1966FR1H, 1966UB01, 1967BI1K). Reported integrated cross sections are 75 MeV · mb (1960BA47), 50 MeV · mb (1963GO1P). See also 12C(γ, n)11C. The behavior of the 19.2 MeV level suggests ascription to the expected giant magnetic quadrupole state Jπ = 2-; this state is not likely to have been seen in 11B(p, γ)12C (1965DE1C, 1965DE1K, 1966BE1Q, 1967BI1K, 1967CR02, 1967DR1C). A positive parity state with a large longitudinal matrix element may also be present (1967BI1K).
Electron spectra in the region of large energy loss show a broad peak which is ascribed to quasi-elastic processes involving ejection of single nucleons from bound shells: see (1961BO32, 1963CZ1A, 1966DE1K). A study of e' - p coincidences for incident energies around 550 - 600 MeV reveals peaks corresponding to ejection of 1p and 1s protons: the results are consistent with observations in (p, 2p) (1961JA1L, 1964AM1C, 1967AM1E). Angular distributions for the two groups are analyzed by (1967AM03) in terms of proton momenta.
Elastic and inelastic scattering have been studied at many energies up to 350 MeV. The data are summarized by (1963GO1M, 1964ST25); some later references are listed in Table 12.21 (in PDF or PS). At En = 14 MeV, the elastic angular distribution shows a forward-peaked diffraction structure indicating direct interaction. Optical model fits in the forward hemisphere are satisfactory but fail at back angles (1963LU10, 1964CL05). A strong-coupling calculation has been made by (1965BL09). polarization studies at En = 24 MeV are in fair agreement with optical model predictions and yield a sign for the spin-orbit coupling in agreement with shell model predictions (1962WO08: see also (1960SA1E, 1965BR1E)). See also (1959GL57, 1959KE1A, 1959WI1C, 1960HO14, 1960RO24, 1964CR1B).
At 14 MeV the cross sections for n0, n1 (4.4) and n2 (9.6) are about 800, 220 and 100 mb, respectively: see (1963BO15, 1964ST25, 1966GR1L, 1967GR1P). Angular distributions at En = 14 MeV of neutrons corresponding to 12C*(4.4) show direct interaction, but DWBA optical model fits are unsatisfactory (1964CL05: see also (1959GL57, 1960PE02, 1960RO24)). (1967GR1P) find that coupled channel calculations, with 12C*(0, 4.44, 9.64, 10.84) taken as collective states yield fair agreement with the experimental angular distributions at En = 14 MeV. See also (1962BA15, 1962BA25, 1962CO1E, 1963HO08, 1964JO1E). The angular correlation (n', γ4.4) has forward neutron angles the distributions are similar to (p, p'γ) while a strong difference is apparent at θn = 135° (1963BE31, 1966BE1R). See also (1961AS1B, 1963MO04, 1964MO1D, 1965LA1D). See also (1958AN32, 1958NA09, 1959GA1D, 1959HA06, 1959SI79, 1960DE10, 1960HE10, 1960PE1A, 1961BR08, 1961JA19, 1961ST22, BL62C, 1962BO06, 1962BR11, 1962ST18, 1962TE05, 1963ED1B, 1963GA1G, 1963KO1C, 1963MC1B, 1963OP1A, 1963SE1P, 1964AD1B, 1964EN1B, 1964OL1B, 1964PE20, 1966BR1D, 1966CI1A, 1966FE1C, 1966JO1B, 1966KA1E, 1966KO1D, 1966LI1F, 1966MO1C, 1967BR23, 1967CH1R, 1967LE1G) and 13C.
At En = 14.2 MeV, reaction (c) proceeds about 50% through 12C states which then decay via 8Beg.s. and 8Be*(2.9). Most of the remainder of the observed events take place through 12C(n, α)9Be* → n + 8Be* and 12C(n, α)9Be* (α) 5He* (n) α. The reaction 12C(n, 5He*)8Be* cannot be excluded. Four-body break-up is not necessary to explain the results. The α-decay of the 10.8 and 11.8 MeV states, together with stripping results, suggest Jπ = 1- and 2- for these states (1964BR25: see also (1966MO05)). See also (1959TS1A, 1960VA10, 1962BA15, 1962BA25, 1964SA1E, 1965GR1V, 1965GR1W, 1966CI1A, 1966FE1C, 1966GR1M).
Up to Ep ≈ 12 MeV, the elastic scattering exhibits resonance structure: see 13N. References to elastic and inelastic studies at higher energies are listed in Table 12.22 (in PDF or PS). Excitation functions for both (p, p) and (p, p1(4.43)) exhibit pronounced resonance-like structure for Ep = 14 to 20 MeV (1957PE14, 1961NA02, 1964DA03). An optical model analysis gives a good account of the data, but the parameters vary strongly with energy (1962NO03: see also (1962RO14)). Between 18 and 30 MeV the elastic scattering and polarization angular distributions show strong energy dependence, while those for inelastic groups are much smoother (1963DI04, 1963DI16, 1966CR04). An optical model analysis with resonances is discussed by (1964TA1E); see also (1965BA1M). At 40 MeV an 11-parameter optical model fit is satisfactory for elastic differential cross sections, but unsatisfactory for polarization data (1966BL19, 1967FR20: see also (1965BA54, 1965FR17)). The same problem arises with the data at 46 MeV (1967SA13) and 49 MeV (1966CR14). For analysis of the 140 MeV small angle results, see (1966JA08). See also (1958BE1B, 1959WI1C, 1962RO1F, 1964CR1B, 1967BE1Q, 1967TA1B). At 1 MeV, the differential elastic scattering cross section exhibits diffraction-like structure associated with the multiple scattering of protons by nucleons inside the nucleus (1967PA25).
At Ep = 40 MeV, angular distributions of inelastic protons corresponding to 12C*(4.4, 7.7, 9.6) confirm the assigned parities even, even, odd, respectively. Comparison of deformation parameters for 12C*(4.4) determined by (p, p'), (α, α') and (e, e') show considerable differences (1964ST15): see, however, (1966BA2K, 1967SA13). Angular distribution measurements at Ep = 46 MeV (1967PE05) have been analyzed by (1967SA13): a large quadrupole deformation was found (β2 ≈ 0.6); the inelastic scattering agrees best with deformation of both the real and imaginary parts of the optical potential; the angular distribution for 12C*(7.6) is best described by double quadrupole excitation via 12C*(4.4). 12C*(14) is interpreted as the 4+ rotational state. The similarities of distributions corresponding to 12C*(10.8, 11.8) suggest that they have the same spin (1967SA13). Asymmetries observed with 40 MeV polarized protons on 12C*(4.4) disagree with DWBA predictions (1966FR1G: see also (1967SA13)). See also (1960BA38, 1963DI16, 1964DA03, 1965FR17, 1966BL19, 1967FA06, 1967LE13, 1967LE1G, 1967PA1L, 1967SA1L, 1968GA1H, 1968TA1P).
A number of inelastic groups have been studied at Ep = 155 to 185 MeV: see Table 12.23 (in PDF or PS). When treated in the impulse approximation, the cross sections and angular distributions are closely related to the electric transition moments. Comparison of (p, p') and (e, e') form factors yield the transition multipolarities indicated in Table 12.23 (in PDF or PS) (1964JA03: see also (1960NI02, 1961BR08, 1961PI04, 1962BR11, 1962RO1F, 1962SA1G, 1963HO1D, 1963NI02, 1964HA1L, 1965HA17, 1965HA28, 1966HA51, 1967HA1U, 1967JO1F). The broad levels reported in the range Ex = 20 to 24 MeV are associated with the giant E1 resonance (1961SA1E, 1962SA1G, 1963DE35, 1965HA17).
The energy of the first excited state is 4440.0 ± 0.5 keV, from the γ-ray. The character of the Doppler broadening indicates rather little spin-flip contribution to the inelastic scattering (1967KO14: Ep = 23 MeV). Proton γ-angular correlations provide a sensitive measure of the spin-flip: for Ep = 10 to 15 MeV a considerable contribution is observed (1964SC07, 1966SC1L: see also (1961AD04, 1961CL1D, 1961GI1C, 1961GO13, 1962NO04, 1964BA14, 1964RO23, 1967GI1D, 1967KO1N) and Table 12.21 (in PDF or PS)).
The following is a list of other recent theoretical and review papers dealing with this reaction: (1959GL57, 1959HO95, 1959MC63, 1959PU1A, 1960BE31, 1960LU1B, 1960MA43, 1960MI1C, 1960SA1C, 1960SA1E, 1960SA1G, 1961DO1C, 1961GI1D, 1961MC1C, 1961SA1B, 1962KA1E, 1962MA1P, 1962NI1C, 1963BU1E, 1963LO1A, 1963VI1A, 1964DA07, 1964HO1C, 1964SA1L, 1965CL1E, 1966BO1P, 1966GI1A, 1966LI1F, 1966SA1D, 1967CH1R, 1967VA1K) and (1959AJ76).
Reaction (b) is widely used to monitor high-energy proton beams: see (1963CU05, 1963CU1B). For studies of recoil spectra, see (1962SI09, 1965BE1U). Possible emission of singlet deuterons is discussed by (1966NO1A). See also (1967HO1M).
In the summed proton spectrum of reaction (c), peaks are observed corresponding to the ejection of p- and s-shell protons: see 11B. Absolute cross sections are reported by (1965GI1F). See also (1962AU1A, 1962ST1F, 1964LI1D, 1964LI1E, 1965MC1F, 1965RI1A, 1966BE1B, 1966KO1F, 1966JA1A, 1967CO1V, 1967EL1C, 1967GO01, 1967JA1C, 1967RI08, 1968JA1G, 1968YU1B). At Ep = 57 MeV the reaction involves an excited state at 20.3 MeV (1967EP1B, 1968RO1L).
Reaction (e) has been studied up to Ep = 660 MeV. Various states in 12C appear to be involved. At Ep = 57 MeV, the sequential process dominates; α-decay to 8Beg.s. is observed via 12C*(21.1, 22.2, 26); comparison with other reactions suggests (Jπ; T) = (1+, 3+; 0), (1-; 0 + 1); (π = natural; significant T = 0 component) (1967EP1B). See also (1966RO1D, 1968RO1L). See also (1959VA15, 1960VA10, 1961VA17, 1962VA1A, 1962ZH1B, 1963JA07, 1963VA04, 1963ZH1A, 1964BA29, 1964KE1F, 1964SY02, 1964YU1A, 1965IS05, 1965KU14, 1965SA1K, 1965YU1C, 1965ZH1A, 1966JA1B, 1966ZE1A, 1967GA01) and (1959AJ76).
The angular distribution of elastically scattered deuterons has been studied at a number of energies from Ed = 2.8 to 650 MeV: see (1954FR24, 1960BU25, 1960CA24, 1961IG02, 1961IS02, 1961LO01, 1962SL02, 1963BU24, 1963CA1E, 1963FR1F, 1963VA23, 1965DI1C, 1965DU01, 1966CO24, 1966DO1B, 1966DU08, 1966GA09, 1966GE03, 1966NG01, 1967AU05, 1967DU1E, 1967FI07, 1967NE09, 1967PL1B, 1967WA1M). Inelastic groups corresponding to levels at 4.4 and 9.6MeV are reported by (1961JA02, 1962SL02, 1966NG01: see also (1951KE02, 1954FR24, 1956GR37, 1956HA90)). The angular distributions of inelastically scattered deuterons to these two states and to 12C*(7.7) have been measured at Ed = 25.9 MeV (1963VA23). A systematic optical model analysis yields a set of smoothly varying parameters which give a good account of the elastic angular distributions from Ed = 3 to 34 MeV. For the inelastic scattering, Q = -4.4, deformation of the potential corresponding to β = 0.4 - 0.6 was required (1966SA1C). See also (1961CI08, GO61I, 1963CA1J, 1963SA1G, 1968HI1H), (1958MA52, 1960EL09, 1960FA05, 1960LU1B, 1961RO1G, 1962SA1J, 1963ST1A, 1963ZA1B, 1964HE1H, 1964HO1C, 1964RU1A, 1964SA1K, 1965CA1F, 1965TJ1A, 1966BA2W, 1966JA1J, 1966MA2Q, 1967RE1E, 1967RU1A; theor.) and (1959AJ76).
Differential cross sections measured for Ed = 14.7 MeV are analyzed by DWBA to yield Sα, a measure of the composition 8Be + α in the wave function of 12Cg.s.. Subject to an uncertain normalization, Sα is found to be close to unity (1964DA1B, 1966DA1C, 1966DE09).
Angular distributions of elastically scattered tritons have been determined at Et = 1.0 to 1.75 MeV (1962GU01: see also (1968HE1N)), at 6.4, 6.8 and 7.2 MeV (1964PU01) and at 12 MeV (1965GL04: optical model analysis; see also (1966GL1B)).
Angular distributions of elastically scattered 3He particles have been determined for E(3He) = 2 to 6 MeV (1966SC12), 5.5 MeV (1961PA04), 8.5 and 10 MeV (1966SC22), 12 MeV (1965YO1B), 12.0 to 18.6 MeV (1967FO1F), 20.1 MeV (1967MA1G: also 12C*(4.4)), 24.5, 25.3 and 26.8 MeV (1964SE05), 25.7 MeV (1966DA1H: also 12C*(4.4, 7.7)), 26.1 to 32.6 MeV (1963PA15: also 12C*(4.4)) and 29 - 30 MeV (1961AG1A, 1961CA18, 1962AG01, 1962CA29, 1962GA17, 1965BU1H, 1967BA2P, 1967BR1N). See also (1961FO02, 1962WE1C, 1967AR17, 1967AS1B) and (1961HO1J, 1964GO1J, 1965FR1E, 1967GR1N, 1967PA1U; theor.). For reaction (b) see (1965DO1H).
Angular distributions of elastic and inelastic α-particles and of 4.43 MeV γ-rays have been measured at a number of energies: see Table 12.24 (in PDF or PS). DWBA fits to differential cross sections observed at 40.5 MeV for Q = -4.4 yield B(E2)(↓) = 13 e2 · fm4; for Q = -9.6, B(E3)(↓) = 50 - 70 e2 · fm6 (1966HA19). Except for the broad 10.1 MeV state all known levels of 12C with Ex < 14 MeV have been observed by (1966HA19). Angular distributions of α-particles and 4.4 MeV γ-rays have been studied at Eα = 22.5 (1964EI01) and at 43 MeV (1959SH62, 1962MC11). The studies of (1962MC11, 1964EI01) yield information on the polarization of the 12C*(4.4) as produced in inelastic scattering. The derived polarizations show strong dependence on scattering angle and are not explained by simple PWBA or adiabatic reaction mechanisms: DWBA gives qualitative agreement. Similar studies involving 12C*(9.6) confirm that Jπ = 3- (1963LA07). See also (1959CA14, 1962BR14, 1964BU1F, 1964LA07, 1967VE1C) and (1959BL31, 1959GL57, 1960RO1E, 1961EI1A, 1962RE1C, 1963DA1B, 1963HO1J, 1964DA1D, 1964GR1L, 1965JA1D, 1966JO1A, 1967BO1W, 1967JA1G, 1967LA1K; theor.) and (1959AJ76). The 9.6 MeV state decays predominantly through 8Beg.s. (1966BO28).
The angular distribution of alphas corresponding to 12C*(12.7; Jπ = 1+) at Eα = 28.5 MeV is not accounted for in DWBA. A spin-orbit interaction leading to spin flip appears to be involved (1967NA06).
For reaction (b) see (1953LI28). Angular correlations in 12C(α, 2α) (reaction (c)) observed at Eα = 915 MeV give evidence for strong α-clustering in 12C (1961GO1T). See also (1961VA38, 1962BR14, 1962VA25, 1965YA02).
At E(12C) = 126 MeV, strong inelastic peaks corresponding to 12C*(0, 4.4, 14.0 ± 0.5) have been observed. Groups with Q = -9 and -19 MeV are also seen. Their interpretation is less clear. Angular distributions suggest that the 14 MeV state has Jπ = 4+; the large intensity indicates close association with 12Cg.s. (1962WA24, 1963GA05, 1966BA2K); deformation parameters are listed by (1966BA2K); see also (1962GA02, 1962WI09). The elastic scattering has also been studied for Ec.m. = 3 to 13.4 MeV (1961BR15) and at Ec.m. = 56.7 MeV (1962SM02). See also (1965GR1F), (1959AL1H, 1962BE43, 1962SE1G, 1963WI1G) and (1962BU1B, 1963BA1Y).
Differential cross sections have been determined for the 12Cg.s. transition at E(14N) = 21.5 to 27.3 MeV (1960HA16) and at Ec.m.(14N) = 62.5 MeV (1962SM02); those to the 4.4 MeV state have been measured at E(14N) = 27.3 MeV (1961HA04). See also (1961KU1D, 1961NE04, 1962WI09) and (1963KU1L).
Differential cross sections have been determined for the 12Cg.s. transition at Ec.m.(16O) = 8 to 13.7 MeV (1963KU12) and at E(16O) = 35 (1967GO1A, 1968VO1D), 42 (1964NE01), 67 (1959MC1D) and 168 MeV (1964HI09): those to the 4.4 MeV state have been measured at E(16O) = 42 and 168 MeV (1964HI09, 1964NE01).
The elastic scattering angular distributions in both (a) and (b) have been measured at E = 35 MeV (1967GO1A).
The decay is mainly to the ground state via an allowed transition. Branching fractions to other states of 12C are listed in Table 12.25 (in PDF or PS). The half-life is 10.97 ± 0.04 msec; see Table 12.28 (in PDF or PS). Since transitions to 12Cg.s. and 12C*(4.4) are allowed, Jπ(12N) = 1+. See discussion of 12B(β-) and (1965WU1A). See also (1962PO02, 1966DU1E, 1967HU10).
Angular distributions of deuterons to 12C*(0, 4.4) have been measured by (1966GL01: 8 and 12 MeV) and by (1961BE12: 17 MeV). For the ground state, θ2 = 0.036 at Ep = 8 MeV and 0.058 at 12 MeV; for the 4.4 MeV state θ2 = 0.051 at 12 MeV (1966GL01: PWBA). See also (1960NE1C, 1964TE1G). At Ep = 54.9 MeV, strong deuteron groups are observed to 12C*(0, 4.4, 12.7, 15.1, 16.1) and partial angular distributions of these groups have been observed. Spectroscopic factors (from DWBA) indicate that the summed transition strengths to the four excited states are approximately equal to the P3/2 neutron pickup strength in 12C(p, d) (1968TA1V).
Angular distributions of tritons have been obtained by (1954HO48: 2.2 and 3.3 MeV; t0), (1966GL01: 8 and 12 MeV; t0 and t1), (1960MA10: 14.8 MeV; t0, t1, t2). The relative θ2 for the ground and first two excited states are 1 : 0.76 : 0.039 (1960MA10, 1966GL01: PWBA). Assuming θ2 = 0.031 for 12Cg.s., θ2 = 0.024 for 12C*(4.4) and 0.0012 for 12C*(7.7) (1960MA32). See also (1959KU1C, 1963OG1A, 1965DE26, 1967DE1J).
Angular distributions have been determined at many energies: see (1962CH02: 1.6 to 3.3 MeV; α0, α1, α2), (1968MI1H: 1.66 to 3.12 MeV; α0, α1, α2), (1957HO63: 2 MeV; α0, α1), (1960BA25: 1.8 MeV; α0), (1957HO62: 4.5 MeV; α0, α1), (1964DE1E, 1965DE26: 8.8, 9.4, 10.3 MeV; α0, α1; DWBA), (1966KE08: 12, 15, 18 MeV; α0, α1, α2, α3, α5, α6, α7, α8; DWBA), (1967BA2G: 40 - 45 MeV). (1966KE08) find l = 1 or 0 for all the groups except α3 (to 12C*(9.6)) for which l = 2. Angular correlations of α-particles and 4.4 MeV γ-rays have been studied at E(3He) = 4.5 MeV (1962HO13). See also (1968NO1E). The 15.1 MeV γ-ray has been observed: see (1957BR18, 1959AL96, 1959BR79). See also (1959OW1A, 1961HO1F, 1964EL1B, 1965NE1B, 1967BH1B).
Alpha groups have been observed corresponding to all 12C states up to 12C*(16.11), with the exception of 12C*(10.3) and 12C*(14.7) (see (1965BR08)). See Table 12.26 (in PDF or PS) for a listing of energy parameters measurements. Angular distributions have been obtained at many energies: see (1966EU01: 0.5 to 2.2 MeV; α0), (1961SJ1B: 0.6 to 0.8 MeV; α0), (1965WI11; 0.8 to 1.9 MeV; α0, α1), (1965ST02: 0.9 to 1.2 MeV; α0, α1), (1960KA1H: 1.4 to 2.9 MeV; α0), (1961IS03: 1.5 to 3 MeV; α0), (1965SC12: 4 MeV; α0, α1, α2, α3, α5, α6, α7; 7.2 MeV; α7, α8, α9, α11), (1965BR08: 5.9 and 7.2 MeV; α7, α11, α12), (1964CH1C: 4, 6, 8 and 10 MeV; α0, α1, α2), (1962WI05: 10 MeV; α0, α1), (1960HU10: 10.3 to 11.4 MeV: α0, α1), (1966DR04: 11.3 and 12.6 MeV; α0, α1), (1961YA08: 14.7 MeV; α0, α1), (1959BO40, 1959HE1C: 20.0 MeV; α0), (1959FI30: 20.9 MeV; α0, α1, α3), (1965PE17: 24 MeV; α0, α1, α2, α3), (1966VI1A: 28.5 MeV; α1). Integrated cross sections for eight alpha groups have been obtained at Ed = 11.8 MeV (1966JA05). At Ed = 24 MeV, α1 is strongly favored over α0, and α3 is favored over α4. For α0, α1 L = 2 is preferred (1965PE17). Angular distributions at Ed = 4 MeV are nearly symmetric about 90°, suggesting a compound nucleus process involving many overlapping levels (1965SC12).
In a test of isospin conservation, (1965BR08) find, at Ed = 7.2 MeV, the cross section for excitation of 12C*(15.11, Jπ = 1+; T = 1) = 31% of that for 12C*(12.71, Jπ = 1+; T = 0); σ(16.11)/σ(12.71) = 4%. Violation of this order can be understood as being due to coulomb mixing in the compound nucleus.
Comparison of angular distributions of 14N(d, α)12C at Ed = 20 MeV and 12C(α, d)14N at Ed = 41.7 MeV suggest an upper limit of 3% for the time reversal non-conserving fraction of the Hamiltonian (1959BO40, 1959HE1C, 1965PE17). See also (1962AL09, 1966ME1E).
At Ed = 1.8 MeV, the alpha particles to the 7.65 MeV state were observed in coincidence with recoiling 12Cg.s. nuclei. If Γrad ≡ (Γγ + Γπ), the ratio Γrad/Γ = (2.8 ± 0.3) × 10-4: see 12B(β-)12C (1963SE23). See also (1967UI01).
Angular distributions of α-particles leading to the ground and 4.4 MeV states have been determined for Ep up to 18.6 MeV: (see (1963MI1H, 1963RO01, 1964EC03, 1965WA1N, 1967NO02)). At the higher energies the ground-state alpha particles show marked backward peaking, in agreement with the inverse reaction 12C(α, p)15N (1964EC03). See also (1963NA1C, 1964HO1D, 1966EV1B).
Angular distributions of the 7Li ions to the ground and 4.4 MeV states have been determined at Eα = 42 MeV (1966MI1M).
For reaction (a), see (1963MO04, 1964MO1D). In reaction (b), 4.4 MeV γ-rays are observed at Ep = 146 (1962FO03) and 150 MeV (1962RO25). See also (1957CH1A, 1964BA1C, 1965ZH1A, 1967CH04, 1967FU1A). In reaction (c), the angular distributions of 8Be nuclei (identified through the α-decay) leading to the ground and 4.4 MeV states of 12C have been determined for Eα = 35.5 to 41.9 MeV. The angular distributions and integrated cross sections vary strongly with energy (1965BR13). See also (1962DO1B, 1962ZU01, 1965KU1B, 1965ZE1B, 1967PA1T, 1967TA1C, 1968YA1C).
At Ed = 14.6 MeV, the ground state angular distribution suggests that direct interaction predominates (1964DA1B). Qualitative agreement with DWBA is reported (1966DA1C). See also (1963DR1B, 1964BL1C, 1965SL1C).