
^{12}C (1959AJ76)(See the Energy Level Diagram for ^{12}C) GENERAL: See also Table 12.4 [Table of Energy Levels] (in PDF or PS). Theory: See (1955FE1A, 1955HE1E, 1956CA1F, 1956EL1C, 1956GL1B, 1956HA1C, 1956HA1D, 1956KU1A, 1956MO1D, 1956NA1A, 1956PE1A, 1956RE1C, 1956WI1F, 1957BA1H, 1957BI1C, 1957HE1B, 1957KU58, 1957PA1A, 1957RE1A, 1957SA1B, 1958CA1G, 1958FR1C).
See (1957NO17).
The yields and angular distributions of protons leading to the ground state and several excited states of ^{11}B have been investigated by (1956AL1E: E(^{3}He) up to 2.7 MeV), by (1955HO1D, 1956HL01: E(^{3}He) = 2.0 MeV), by ((1956WO1A, 1956WO1C, 1957JO1B) and E. Wolicki, private communication: 2 to 4.5 MeV) and by (D.R. Sweetman, private communication: 6.05 MeV). The yield rises rapidly to E(^{3}He) = 1.8 MeV and remains approximately constant to 4.5 MeV, with no indication of resonance. Angular distributions show fore and aft asymmetry and vary only slowly with energy. At E(^{3}He) = 2 MeV, it appears that both direct interaction and compound nucleus formation, involving interfering resonances with l ≤ 3, may be taking place. At higher energies the forward peaking suggestive of direct interaction becomes more obvious. See also ^{11}B. For reactions (a), (c) and (d), see ^{11}C, ^{8}Be and ^{10}B.
Neutron groups corresponding to states of ^{12}C*(0, 4.4, 7.6) have been observed with E_{α} up to 5.3 MeV. At E_{α} = 5.3 MeV the forward yield of the group leading to the 7.6 MeV state is about 1/8 of that leading to the 4.4 MeV state: see (1952GU1A, 1955ST1C, 1956ME1B, 1957RI38); see also (1957DI1B). The energy of the γray from the first excited state is 4.425 ± 0.020 (1954MI68), 4.48 ± 0.06 MeV (1955BE1G) (both values corrected for Doppler shift). The internal pair conversion, coefficient indicates an E2 transition (1954MI68); the angular correlation of pairs admits M1 or E2, favoring the latter (1954HA07, 1956GO1K, 1956GO73, 1958AR1B). The angular distribution of γrays observed at several bombarding energies is consistent with J = 2^{+} (1955TA28, 1956PR1A). The nγ correlation at E_{α} = 5.3 MeV (thick target) is isotropic within 6.5% (1956ST1E: see also (1958TA05)). The mean lifetime of the 4.4 MeV level is (2.6 ± 0.9) × 10^{14} sec, about oneeighth of the singleparticle value for an E2 transition (1956DE22). The 7.7 MeV state appears to decay predominantly into ^{8}Be + α (see ^{12}B(β^{})^{12}C and ^{12}C(α, α')^{12}C*). A gamma ray of energy 3.1 MeV has been reported by (1953BF01, 1954UE01, 1956ST1E, 1956ST1F, 1957ST1E), but (1955BE1G, 1957GO89, 1957KR1A) find no evidence of the 7.7 MeV nuclear pairs which should accompany the decay to the ground state. (1955BE1G) estimate that at least 96% of the decays proceed to ^{8}Be + α; (1957KR1A, 1958KR70) find < 1.6 × 10^{5} 7MeV pairs per 4.4 MeV γray; assuming a population ratio of 1 : 8, this result yields Γ_{π}/Γ < 1.3 × 10^{4}. An upper limit of 1/600 for the ratio of 7.6/4.4 MeV pairs is reported by (1957GO89). (1954DI1A) find no nγ coincidences other than those associated with the 4.4 MeV level. See also (1957RO1E). At E_{α} = 21.7 and 175 MeV, γradiation from the 15 MeV, J = 1^{+}; T = 1 state (see ^{12}C(p, p')^{12}C*) is reported (1954RA35, 1957WA04). At the higher energy, the ratio of 15 MeV to 4.4 MeV radiation is 1.2 × 10^{2} (1957WA1F). See also (1954EL1B, 1955BR1A, 1955HA1E, 1956GO1N, 1957BR1J) and (1955MA1J; theor.).
At E_{d} = 0.95 MeV, the upper limit to the capture cross section is 0.1 μb (1955SA1B).
The thintarget excitation function in the forward direction in the range E_{d} = 0.3 to 4.6 MeV shows some indication of a broad resonance near E_{d} = 0.9 MeV. Above E_{d} = 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). Angular distributions seem to be dominated by the stripping process: see ^{11}C. The yield of 6.5 MeV γrays has been measured at four bombarding energies between 0.8 and 2.2 MeV (1955SA1B). See also ^{11}C.
Absolute yields and angular distributions are reported for various proton groups by (1952EN19, 1954BU06, 1954PA28, 1956MA69, 1956VA17) for E_{d} = 0.18 to 3.1 MeV. Although the excitation functions show several broad peaks, no clear resonances can be identified, and it must be assumed that many overlapping resonances are involved (1956MA69). Angular distributions indicate both stripping and compound nucleus processes even at low bombarding energies (1954PA1D). However, the p_{1} group, leading to ^{11}B*(2.1), shows no stripping even at E_{d} = 3 MeV; it is suggested that an orbital angular momentum selection rule is operative here (1956MA69: see ^{11}B). Absolute cross sections reported by (1954BU06, 1954PA28, 1956MA69) differ rather greatly. The yields of 6.8 and 7.3 MeV γrays have been measured at four bombarding energies between 0.8 and 2.2 MeV (1955SA1B). At E_{d} = 1.70 MeV, θ(lab) = 58°, the cross section for protons leading to the 6.76 MeV state is 6.1 (± 15%) mb/sr (1956KA1A).
See ^{10}B.
Excitation curves for ground state αparticles have been measured for E_{d} = 0.9 to 2.6 MeV at 45°, 90° and 150°. Broad maxima are observed at 1.0, (1.4) and 2.0 MeV. At E_{d} = 0.91 MeV, the angular distribution of α_{0} particles shows a peaking in the forward direction (1956MA69). See also ^{8}Be.
Not reported.
Proton groups reported by (1955BI26) and (1958MO99) are listed in Table 12.5 (in PDF or PS). A careful search, at E(^{3}He) = 1.25 MeV, reveals no other level in the range E_{x} = 4.4 to 7.7 MeV (1958MO99: region at E_{x} = 6.4 MeV obscured). At E(^{3}He) = 2.0 MeV, the proton group leading to the 15.11 MeV level was found to be in coincidence with a 15.10 MeV γray: Γ_{γ}/Γ for this level is 0.77 ± 0.20. The ratio of the width for γemission to the 4.4 MeV level to the ground state Γ_{γ} is ≈ 0.03. The 12.76 MeV level also emits γrays: Γ_{γ}/Γ ≈ 0.02, suggested J^{π} = 1^{+} ((1957GO1B) and H.E. Gove, private communication). Coincidence studies by (1958MO99) lead to Γ_{γ}/Γ < 0.9% for the 7.7 MeV level, Γ_{γ}/Γ = 3 ± 1 % for the 12.76 MeV level, and 50 ± 25 % for the 15 MeV level. See also ^{11}B(d, n)^{12}C, (1958BR1D, 1958SW63) and ^{13}N.
See also ^{14}N.
In the range E_{p} = 0 to 3 MeV, five principal resonances occur, at E_{p} = 0.16, 0.67, 1.4, 2.0 and 2.6 MeV (see Table 12.6 (in PDF or PS)). All except the second and fourth exhibit resonance for α_{0}, α_{1}, γ_{0} and γ_{1} (to ^{8}Be*(0, 2.9) and ^{12}C*(0, 4.4)); at E_{p} = 0.67 MeV, only α_{1}, γ_{1} are resonant. It follows from angular momentum selection rules that resonances for α_{0} must have the character J^{π} = 0^{+}, 1^{}, 2^{+}, 3^{}...; J = 0^{+} is excluded by observation of γ_{0}. The E_{p} = 0.16 MeV resonance (^{12}C*(16.11)) is well established as J = 2^{+}; probably the T = 1 analogue of ^{12}B*(0.95). The angular distribution of α_{0} particle is strongly anisotropic at resonance and shows a (cos θ) term varying with energy near resonance. The assumption J = 2^{+}, l_{p} = 1, with interference from an swave state at higher energy gives a good account of the observed angular distributions from E_{p} = 0.13 to 0.3 MeV. The channel spin ratio χ = 0.42 ± 0.02; the relative amplitude of the interfering J = 1^{} state is 0.022 ± 0.002 (1952TH1B). The angular correlation of α_{1} and the subsequent breakup of ^{8}Be*(2.9) also requires J = 2^{+}, with the ratio of reduced matrix elements for outgoing d to swaves, B = 0.80, phase difference cos β = 0.60 (1955GE1A). The angular distribution of γ_{1} and of the following 4.4 MeV radiation is consistent with the scheme 2^{+}(M1)2^{+}(E2)0^{+} with the channel spin ratio χ = 0.42 (1954GR1C); (1956CR1C) obtain χ = 0.51 ± 0.03. Angular distributions of the 16 MeV radiation, γ_{0}, require J = 2^{+}, with interference from a J = 1^{} level at E_{p} = 1.4 MeV (1954GR1C); (1956CR1C). (γ_{0} is not resonant at E_{p} = 0.67 MeV, so this state cannot be involved here.) The resonant energy is 163.1 ± 0.2 keV; Γ_{lab} = 6.5 ± 0.6 keV (see (1955AJ61)). The very small αwidth suggests T = 1 (1953BE61). For the E_{p} = 1.4 MeV state (^{12}C*(17.23)), the possible assignments are 1^{} (swave), 2^{+} (pwave), 1^{} or 3^{} (dwave); dwave formation would seem to be excluded by the observed width. As indicated above, J = 1^{} appears to be required to account for the interference at lower energies in α_{0} and γ_{0}; known higher resonances are probably too narrow to produce the observed effects. (1957DE11) find that the α_{0}dsitributions for E_{p} = 0.6 to 1.4 MeV are well accounted for by the assumption of swave formation of J = 1^{} through channel spin (χ = 0) with a relative dwave amplitude A = 0.5, and interference by the 2.6 MeV, J = 2^{+}, state, with relative amplitude C = 0.25. A qualitative fit to the behavior of α_{1} can be obtained with the same assumptions (1957DE11). Angular distributions of γ_{0} at E_{p} = 1.4 MeV admit either J = 2^{+} or 1^{}; for the latter, however, formation in channel spin 2 (χ = ∞, dwaves) is required (1955GO10). The angular correlation of internal pairs indicates E1 for γ_{0} (1956GO1K, 1956GO1N, 1958AR1B). The large E1 width suggests T = 1 for this state (1953BE61). The E_{p} = 0.67 MeV state (^{12}C*(16.58)) may be formed by s or pwaves; dwaves are excluded by the width (1953BE61). The angular distribution of α_{1} at E_{p} = 0.64 and 0.93 MeV indicates swave formation: if J = 2^{} is assumed, the dwave admixture is < 10%. The correlation of α_{1} with the subsequent ^{8}Be*(2.9) breakup is consistent with J = 2^{} and excludes 1^{}; an appreciable fwave admixture in outgoing α_{1}particles is indicated (1957DE11). Correlation results at E_{p} = 270 keV can be accounted for by J = 2^{} with interference from the 1^{}, E_{p} = 1.4 MeV state (1955GE1A, 1957DE11). The angular distribution of γ_{1} is reported to require J = 2^{+}, with interference from the 1^{}, E_{p} = 1.4 MeV state (1954GR1B): according to (1957DE11), however, the distributions observed by (1954GR1C, 1955GO10) can equally well be ascribed to J = 2^{}, with interference from a broad, even parity state, possibly at E_{p} = 2.0 or 2.6 MeV (see, however, (1954GI1B)). The angular correlation of internal pairs indicates E1 for the γ_{1} radiation (1956GO1K, 1956GO1N, AR57). The relatively large E1 width suggest T = 1 for this state (1953BE61). The E_{p} = 2.0 MeV level is reported to be resonant for α_{0} and α_{1}; the relative weakness of α_{1} suggests J = 0^{+} (1953PA26). These seems to be no clear indication of resonance for γ_{0} or γ_{1} at this energy (1955GO10: see also (1953HU29)). At E_{p} = 2.65 MeV, resonance occurs for α_{0}, α_{1} (1953PA26) and, weakly, for γ_{0}, γ_{1} (1955BA22, 1955GO10). A large P_{2} coefficient in the angular distribution of γ_{0} suggests J = 2^{+} (1955GO10). (1955HO48) find E_{p} = 1.98 and 2.61 MeV for the resonant energies for α_{0}. Additional resonances for γ_{0} and γ_{1}, reported by (1955BA22) are listed in Table 12.6 (in PDF or PS). (1959GE33) have examined the excitation function for groundstate transitions from E_{p} = 4 to 7.7 MeV. The experiment locates the maximum of the ^{12}C giant resonance at E_{x} = 22.55 ± 0.1 MeV but does not resolve individual levels. Two additional peaks, at E_{x} = 21.4 and 22.1 MeV are suggested. The maximum value of σ(γ, p) is calculated to be 29 ± 5 mb. An upper limit for the total cross section (average value, E_{p} = 1.7 to 4.0 MeV) for the production of nuclear pairs with E_{π} = 6.5 to 9.5 MeV is 0.03 μb (1955BE62). See also (1955AJ61, 1956MA1T), (1957SI1B; theor.) and ^{8}Be.
Observed maxima in the (p, n) cross section are listed in Table 12.7 (in PDF or PS) (1951BL1A, 1955BA22, 1957KA1C, 1959GI47). The region covered is characterized by considerable overlapping of resonances (1959GI47). See also (1956KO1D, 1958MA1F, 1958TA03).
Absolute elastic scattering cross sections are reported for one angle for E_{p} = 0.6 to 2.0 MeV by (1956TA16), for four angles for E_{p} = 0.3 to 1.0 MeV by (1957DE11). A pronounced anomaly is observed near E_{p} = 0.67 MeV at all angles; the level is therefore formed by swaves. The 0.3 to 1.0 MeV results are well accounted for by two resonances: E_{p} = 0.67 MeV, swave, J = 2^{}, Γ = 0.33 MeV, Γ_{p}/Γ = 0.5, dwave < 10%, and E_{p} = 1.4 MeV, swave, J = 1^{}, Γ = 1.27 MeV, Γ_{p}/Γ = 0.05 (1957DE11). (The reported Γ_{p}/Γ for the 1.4 MeV resonance appears to be inconsistent with the values 0.8 or 0.2 derived by (1953BE61) from (p, γ) and (p, α) cross sections.) (1956TA16) find no rapid variation in cross section near E_{p} = 2.0 MeV. The absence of a detectable anomaly near E_{p} = 0.16 MeV confirms the small value of Γ_{p} assumed for this resonance; Γ_{p} < 200 eV (J.C. Overley, private communication). See also (1956KI54). Maxima in the yield of 2.1 MeV γradiation from ^{11}B*(2.1) are observed at E_{p} = 2.664 MeV, Γ = 48 keV: (1953HU29, 1955BA22) and at E_{p} = 3.15, 3.4, 3.78, 4.28, 4.68 and 5.13 MeV (1955BA22: see Table 12.7 (in PDF or PS)). (Judging form the width, the 2.66 MeV resonance is not that observed, e.g., in ^{11}B(p, γ)^{12}C.)
See ^{10}B.
Reported neutron groups are listed in Table 12.8 (in PDF or PS). The group corresponding to the 7.6 MeV state is weak, relative to neighboring groups, at all bombarding energies investigated, and the stripping pattern is poorly developed. The relative weakness of the 7.6 MeV state in this reaction and in the ^{12}C(e, e')^{12}C* and ^{12}C(p, p')^{12}C* reactions is attributed to lack of parentage overlap with the ground state of ^{12}C (1955LA1C). At E_{d} = 0.92 MeV, there is no indication of a state in the range E_{x} = 5.1  6.6 MeV: the upper limit of the intensity of the corresponding neutron group is ≲ 1% of the intensity of the group corresponding to the 4.4 MeV state (1957BI78). For E_{d} = 1.1 to 2.0 MeV only the groups corresponding to ^{12}C*(4.4, 12.76) are accompanied by γradiation; an upper limit for (n, γ) coincidences from ^{12}C*(7.6) is 0.2% of ^{12}C*(4.4) (1958DA11, 1958NE38, 1959NE1A). Angular distributions of the neutrons to the first four states of ^{12}C have been reported for a number of energies in the range E_{d} = 0.5 to 10 MeV. At the higher energies, the distributions are understood in terms of simple stripping theory (except for the 7.6 MeV state). At the lower energies, E_{d} = 0.5 to 5 MeV, a good account of the angular distributions of groundstate neutrons is obtained with the theory of (1957OW03) which includes not only stripping of the deuteron but also the possibility of stripping a neutron from ^{11}B, and the interference between the two processes. The relative probability of the exchange stripping increases with energy until the Coulomb barrier is surmounted. The exchange process seems to involve swave deuteron capture by a ^{10}B core with J = 1^{+} (1956PR1B, 1957AM48, 1957OW03: see also (1959NE1A)). Angular distributions at E_{d} = 9 MeV indicate odd parity for the 9.6 MeV state (1956MA83: see also (1954GR53) and Table 12.8 (in PDF or PS)). For other work on angular distributions, see (1957AM48, 1958AM13: E_{d} = 0.50 to 1.15 MeV), (1955WA30: E_{d} = 0.6 MeV), (1955IH1B: E_{d} = 0.69 MeV), (1954GR53: E_{d} = 0.85 MeV), (1957BI78: E_{d} = 0.92 MeV), (1956PR1B: E_{d} = 1.5 to 5 MeV), (1953GI05: E_{d} = 8.1 MeV), and (1957ZE1A: E_{d} = 10 MeV). See also (1954BU06, 1956BO1F, 1956BO43, 1956KO1E, 1957RA1A) and (1955MA1J, 1958ED1C; theor.). In the range E_{d} = 1.0 to 5.5 MeV, two slow neutron thresholds are observed at 1.627 ± 0.004 MeV (E_{x} = 15.11 ± 0.01 MeV) and near 4.1 MeV (broad; E_{x} = 17.23 MeV) (1955MA76). Gamma rays are observed with E_{γ} = 4.44 ± 0.05 (1951RU1A) and 12.8 ± 0.3 MeV (1958KA31). If the latter γray is properly attributed to decay of the 12.8 MeV level, it is not clear why the level should so decay in view of its instability with respect to ^{8}Be and ^{8}Be*(2.9) (1958KA31: see also ^{10}B(^{3}He, p)^{12}C). A 15.1 MeV γray is observed with a threshold of E_{d} = 1633 ± 3 keV, attributed to the first T = 1 state of ^{12}C at 15.11 MeV. The observed width is < 2 keV. A search for αparticles to ^{8}Be and ^{8}Be* gives Γ_{α}/Γ_{γ} < 1.5. At E_{d} = 2.96 MeV the cross section for production of the 16.1 MeV T = 1 state is < 1 mb/sr (1958KA31). See also (1955AJ61) and (1957WA04).
At E(^{3}He) = 4.5 MeV, the ground state deuteron group is strongly peaked in the forward direction (1957HO61).
Not reported.
The halflife is 20.34 ± 0.5 msec (weighted mean of (1955AJ61, 1956NO1A, 1957CO57, 1958KR65, 1958VE20, 1959KR1B), excluding (1948JE03)). E_{β}(max) = 13.40 ± 0.05 MeV (1958VE20). Branches are observed leading to ^{12}C*(0, 4.4, 7.65, 10.1): see Table 12.9 (in PDF or PS). The fact that the transition to the 0^{+} ground state is allowed establishes J = 1^{+} for ^{12}B. This assignment is confirmed by the allowed character of the transition to ^{12}C*(4.4), J = 2^{+}. The 7.7 MeV level decays mainly by αemission to ^{8}Be(0) with Q = 278 ± 4 keV; E_{x} = 7.653 ± 0.008 MeV (1957CO59). Gamma transitions with E_{γ} > 6 MeV accompany < 10^{4} of all βdecays (1956KA1A, 1958KA31); an upper limit of 3 × 10^{5} is obtained in a search for βγ (7.6) coincidences (1958KA14). Upper limits for (3.2 + 4.4) MeV cascades are given as (0.4 ± 2) × 10^{3} and 10^{5} of all decays by (1956TA07) and (1958KA14) respectively. It follows that the relative partial width of the 7.6 MeV level is < 3 × 10^{3} for 7.6 MeV γrays and < 10^{3} for 3.2 MeV γrays (see also ^{9}Be(α, n)^{12}C). Since the βtransition is allowed, the 7.7 MeV state has J = 0^{+}, 1^{+} or 2^{+}; J = 1^{+} is ruled out by the αdecay. The preponderance of αdecay over γdecay speaks for J = 0^{+} (1957CO59). The 10.1 MeV level decays mainly via αemission to ^{8}Be(0); transitions to ^{8}Be*(2.9) amount to < 4%. The c.m. width is about 2.5 MeV (after removing the E_{β}^{5} factor): the best account of the observed αspectrum is obtained with J = 0^{+}, E_{λ} = 1 0.4 MeV, θ^{2}_{α} = 1.5, R = 5.21 × 10^{13} cm (1958CO66). The βspectrum has been reported by (1950HO01, 1958VE20). (1958GE1C) discusses a possible distortion of highenergy βspectra in axialvector coupling due to a "weakmagnetic" interaction. See also (1957CH25), (1955JA1C, 1957FE1C, 1959SC1B; theor.).
The lifetime of the 4.4 MeV state has been determined by resonance scattering and resonant absorption (of γradiation from ^{15}N(p, α)^{12}C*) as τ_{mean} = (6.5 ± 1.2) × 10^{14} sec (1958RA14). Excitation of the 15.1 MeV level by bremsstrahlung is reported by (GA57B, 1957HA13, 1959GA09). From measurement of the yield of resonance scattered radiation and of the selfabsorption coefficient, the integrated scattering cross section ∫ σ_{s}dE, and the peak absorption cross section σ^{0}_{n} are deduced. The resulting values for partial widths are given in Table 12.10 (in PDF or PS). It is noted the γray width for the ground state transition is near the singleparticle M1 value of 65 eV, and that the very small αwidth strongly suggests T = 1 for this level (1957HA13). The scattering angular distribution indicates dipole radiation (1956LE1E, GA57B, 1959GA09). The strength of the M1 radiation also indicates T = 1: see (1958MO17). Inelastic scattering in the giantresonance region has been studied by (1959GA09). See also (1957GO1F, 1958AX1A, 1958MC1D).
The cross section for production of ^{11}C exhibits a broad peak at E_{γ} = 22.5 MeV, Γ ≈ 4 MeV, σ_{max} = 8.3 mb (1955BA63). Other reported values for σ_{max} are summarized by (1957CO57: note a 10% correction in this work). See also (1957CA1D). AT high energies, the cross section exhibits a long tail, falling off approximately as E_{γ}^{3}. The integrated cross section to E_{γ} = 250 MeV is 80 MeVmb, accounting for about 1/3 of the sumrule limit for all absorption processes. It is noted that the relative prominence of the highenergy tail is not a general feature of (γ, n) reactions in heavy elements (1955BA63, 1957CO57). Comparison of (γ, n) and (e, n) cross section for 28 to 145 MeV are consistent with the assumption that the transitions are predominantly E1 (1958BA60). The angular distribution of photoneutrons at the giant resonance is W(θ) = 1 + (1.35 ± 0.88)sin^{2}θ, indicating considerable emission of neutrons with l > 0 (1956FA30). See ^{12}C(γ, p)^{11}B and (1954TE1A, 1955MO1B, 1957BA1K; theor.). Discontinuities in the yield function are reported to indicate levels at 19.3, 19.8, 20.1, 20.5, 20.7, 21.1, 21.6, 22.4, and 22.8 MeV (1954GO39, 1954KA1A). The first two are given as 19.09 ± 0.05 and 19.55 ± 0.05 MeV by (1955SP1A). Eighteen discontinuities observed between E_{γ} = 18.90 and 22.88 MeV are tabulated by (1958KA1D). A search for resonance absorption near E_{γ} = 22.8 MeV indicates a width Γ > 580 keV, in apparent contradiction of the activation results (1956TZ1A). (1958WO1B), using monochromatic gamma rays, find no evidence of fine structure in the total cross section from E_{γ} = 20.3 to 20.8 MeV. The upper limit is, however, not in conflict with the recent report of (1958KA1D). See also (1955JO1B, 1955SA1F, 1958BA1K, 1958SM1A).
The cross section exhibits a giant resonance at E_{γ} = 21.5 ± 0.5 MeV, Γ = 1.7 ± 0.5 MeV (1951HA1C). The peak cross section is 22 mb, and the integrated cross section to 24 MeV is 56 MeVmb (1956CO59): compare ^{11}B(p, γ)^{12}C (1959GE33). The photoproton spectrum shows the general features of the inverse reaction, ^{11}B(p, γ)^{12}C, and suggests resonances at E_{γ} = 17.3, (20.8), 22.6, and (23.1) MeV (1956CO59: see, however, (1958WO1B)). (1957LI1A) finds indications of peaks at E_{γ} = 21.8, 22.6, 23.3 and 25.8 MeV. An absolute measurement, based on σ(^{12}C*(γ, 3α)) for E_{γ} = 17.6 MeV yields σ(γ, p) = 1.19 ± 0.21 mb, in good agreement with the value 1.09 ± 0.16 mb calculated from the inverse reaction (1956MA1T). See also (1956GO1G, 1958CH31, 1958PE1A, 1958WH35). Angular distributions of photoprotons show a pronounced 90° peaking, somewhat skewed in the forward direction (1952HA1B, 1953HE1B, 1955JO1B, 1956KL19, 1957DO1A, 1957LI1A, 1957MI1A). Such distributions are inconsistent with swave proton emission from J = 1^{}; ^{12}C compound states formed by E1 absorption and suggest a direct interaction involving independentparticle states: LS coupling seems to be favored (1955MA1H). The angular distributions of ^{12}C(γ, n) and (γ, p) are evidently quite similar, as expected on the assumption of charge independence; the difference of about a factor of 2 in total cross section is ascribed to a 1% T = 0 admixture in the intermediate state (1957BA1K). See also (1953HE1B, 1957CH24, 1958BA1M, 1958BA30, 1958PA1B, 1958PE1B, 1958SM1A) and (1955MO1B, 1957SI1B; theor.).
Maxima in the yield of 3prong stars are reported at E_{γ} = 17.3, 18.3, 21.9, 24.3 and 29.4 MeV; some evidence of fine structure is also found. The integrated cross section is 1.21 ± 0.16 MeVmb for E_{γ} < 20.5 MeV, 2.8 ± 0.4 MeVmb for 20.5 ≤ E_{γ} < 42 MeV, and < 0.2 MeVmb for 42 ≤ E_{γ} < 60 MeV (1953GO13, 1955GO59). (1955CA19) summarize cross section measurements for the ^{7}Li(p, γ) radiation and find evidence for a resonance near E_{γ} = 12.3 MeV and possibly others at 15 and 16 MeV. According to (1955JO1C), peaks occur at E_{γ} = 14.7, 15.8, 16.6, 18.3, 24.3, and > 29 MeV, in fair agreement with (1953GO13). Absolute cross sections are reported for E_{γ} = 13 to 30 MeV, and integrated cross sections agree well with (1953GO13, 1955JO1C). See also (1953DA1A, 1953GU1A, 1953MI31, 1955HA1D). According to (1955GO59), the threebody reaction is not involved for E_{γ} < 40 MeV (see, however, (1953MI31, 1954CH1B)). The reaction ^{12}C(γ, α)^{8}Be*(p)^{7}Li is reported by (1956LI05). Studies of angular distributions indicate that for E_{γ} = 12 to 15.6 MeV, the reaction involves mainly E2 absorption (^{12}C*: J = 2^{+}; T = 0); from 15.6 to 20 MeV both E1 (J = 1^{}; T = 1) and E2 (J = 2^{+}; T = 1), and for E_{γ} > 20 MeV, mainly E1 (J = 1^{}; T = 1). Significant E2 absorption (J = 2^{+}; T = 0) also occurs for E_{γ} = 20 to 25 MeV (1955GO59: see, however, (1951TE1A, 1953GE1B)). See also (1953LI1C, 1954GR1B, 1955CO1A, 1955SO1B, 1955TI1A, 1956MA1T, 1957MU1C).
Both elastic and inelastic scattering angular distributions have been studied at E_{e} = 80, 150 and 187 MeV by (1955FR1G, 1956FR27) and at 420 MeV by (1958EH1B). The elastic data are well accounted for by a modified Gaussian charge distribution of r.m.s. radius 2.50 × 10^{13} cm, derived from a harmonic well with a characteristic length parameter of 1.68 × 10^{13} cm (1956FR27, 1958EH1B). See also (1953HO79, 1956FE1B, 1956HO93, 1957HO1E, 1958EH1A, 1958RA43). Inelastic peaks corresponding to ^{12}C*(4.4, 7.7, 9.6) are observed, in addition to some unresolved structure near 11 MeV. There is no indication of the 15.1 MeV level. The observed angular distributions agree well with shellmodel calculations of (1955RA1D, 1956MO1E, 1956TA1C, 1957TA1B) with a harmonic well of r.m.s. radius 2.40 × 10^{13} cm. Predicted absolute cross sections are low by a factor of 2 in LS coupling, 6 in jj coupling; it is presumed that some collective modes of excitation are involved. Excitation of the 4.4 MeV level is electric; a width of (12.5 ± 2.5) × 10^{3} eV, τ_{m} = (0.53 ± 0.11) × 10^{13} sec, is obtained (1956HE83). The 7.7 and 9.6 MeV levels are also electrically excited; the angular distributions indicate either monopole or quadrupole transitions, J = 0^{+}, 2^{+}. The matrix element for the 7.7 MeV E0 transition is 50 mb, in good agreement with that observed for the ^{16}O monopole transition (1955SC1B, 1956FR27). A shell model calculation in intermediate coupling indicates that configuration mixing is required to give a nonzero matrix element for the 0^{+}0^{+} transition and suggests that a semicollective model is indicated (1955SC1B, 1956SH1F, 1957TA1B: see also (1955LA1C)). According to (1956EL1C), satisfactory agreement is obtained with a 50% admixture of 1s^{3}1p^{8}2s and 1s^{4}1p^{7}2p. (1956RE1C) also finds reasonable agreement using the 1s^{1}2s configuration and suggests that such a "core" excitation might appear quite generally in the light nuclei (see ^{16}O, ^{14}C and (1954CH1A)). Calculations using an independentparticle approach to a collective description give a good account of the form factors for both the 4.4 and 7.7 MeV excitations. The form factor for the 9.6 MeV level is consistent with J = 1^{} (1956FE1B: see also (1957PA1B)). See also (1958EL48; theor.). Neutron production with E_{e} = 35 to 150 MeV has been studied by (1956GE1B). Comparison of σ(γ, n) and σ(e, e'n) for E_{e} = 24 to 145 MeV indicates that the transitions are largely E1 (1958BA60).
For E_{n} ≳ 14 MeV, elastic scattering angular distributions show pronounced opticalmodel effects: see (1956BU95, 1956CU1A, 1958NA09) and ^{13}C. A gamma ray of energy 4.42 ± 0.03 MeV is observed at E_{n} = 6.58 MeV (1956DA23: see also (1954TH42, 1955BA95, 1955BE1H)). Production of 15.1 MeV γrays is observed at E_{n} = 90 MeV (1957WA04, 1957WA1F). At E_{n} = 14 MeV, inelastic neutron groups corresponding to ^{12}C*(4.4, 9.6) are reported by (1956WO1B: see also (1953WH1A, 1956BE1F, 1956CA1E, 1958AN32)). The angular distributions for neutrons corresponding to ^{12}C*(4.4) agree well with (p, p') distributions and with direct interaction theory (1958AN32: E_{n} = 14 MeV). Inelastic excitation leading to αparticle states has been studied by (1955FR35); levels of ^{12}C at 7.7 and 9.6 MeV are involved. (1953JA1C) find that for E_{n} < 20 MeV, most events proceed through a level at 10 ± 0.8 MeV, Γ_{obs} = 1.6 MeV, to ^{8}Be(0) (See (1955FR35) and ^{12}B(β^{})^{12}C). See also ^{13}C, (1953LI1C) and (1956LA1D; theor.).
Elastic scattering differential cross sections have been determined by (1953BU72, 1957GI14, 1957GR53: E_{p} = 9.4 to 9.6 MeV), (1954FI1B: E_{p} = 10 MeV), (1956SH1C: E_{p} = 12 MeV), (1957PE14: E_{p} = 14 to 19.4 MeV), (1956DA03: E_{p} = 17.0 MeV, c.m.), (1955KI43, 1956KI54: E_{p} = 14.5, 20 and 31.5 MeV), (1952BR52, 1953WRZZ: E_{p} = 30.6 MeV), (1957HI1C: E_{p} = 40 MeV) and (1957GE08: E_{p} = 96 MeV). Calculations based on the diffusesurface optical model give a good account of the data at higher energies (1957GL58, 1957ME21, 1958GL11). At E_{p} = 9.5 MeV, some evidence for compound elastic scattering is seen in the marked energy variation at back angles (1957GI14). See also ^{13}N. Polarization of elastic and inelastic protons has been studied by (1958BR24) for E_{p} = 16 to 18 MeV. Polarization of elastic protons for E_{p} = 5  7 MeV is reported by (1958WA1D, 1958WA1E). See also (1957LE1G, 1958KO03, 1958SQ53; theor.). Polarization studies at high energies are discussed by (1956ER1A, 1956NI1B, 1957AL79, 1957HE51, 1957HI98, 1957TY1B, 1958NI1B, 1958NI26). Excitation of the 4.4 MeV level is commonly observed for all energies above E_{p} ≈ 5 MeV: see (1955AJ61) and (1956RE39: E_{p} = 5 to 6 MeV), (1957GI14, 1957HO1H: E_{p} = 9.5 MeV), (1957CO53: E_{p} = 12 MeV), (1957PE14: E_{p} = 14 to 19 MeV), (1952BR52: E_{p} = 31.5 MeV), (1958CH26: E_{p} = 40 MeV), (1956ST65: E_{p} = 96 MeV), (1957DI28: E_{p} = 95 and 135 MeV) and (1957AL39, 1957TY36, 1957TY37, 1958MA1B, 1958TY49: E_{p} = 182 MeV). E_{x} = 4431 ± 8 keV (1957BU36). The 7.7 and 9.6 MeV levels are reported by (1957CO53, 1957HO1H, 1957PE14, 1957TY37, 1958CH26). Levels at 12.6, 15.0, and ≈ 20 MeV are also reported by (1957TY36, 1957TY37); the last appears strongly in the work of (1956ST65) (E_{x} = 20.8 MeV) and is there associated with the giant resonance seen in ^{12}C(γ, n) and ^{12}C(γ, p). For E_{p} ≲ 30 MeV, the angular distribution of the proton group corresponding to the 4.4 MeV level exhibits a minimum near 90°  100° (c.m.) and a definite foreandaft asymmetry. Neither the compound nucleus model nor the direct interaction theory appears to give a satisfactory account of these distributions (1957CO53, 1957GI14, 1957PE14). A direct interaction calculation, using distorted waves, does reproduce the general features of the distributions and also fits the observed (p'γ) correlation of (1956SH1E) and (1958LE06). See also (1956BE1G, 1957BA1L, 1957BU52, 1957MA58, 1958BR83, 1958CH26, 1958MO98). The angular distribution of protons corresponding to the 7.7 MeV state depends strongly on energy in the range E_{p} = 14 to 19 MeV, but consistently shows a strong forward peak, indicative of l = 0 and hence J = 0^{+} (1957PE14). See also (1955LA1C). An attempt to observe γdecay of this level yields an upper limit of 3% for Γ_{γ}/Γ_{α} (1956HO1D). The angular distribution of protons leading to the 9.6 MeV level is consistent with l = 1, J = 0^{}, 1^{}, 2^{} (1957PE14). The angular distribution of pickup deuterons at E_{p} = 95 MeV indicates significant contributions of high momentum components in the bound, 1p neutron wave function, suggesting strong interactions, (≈ 200 MeV), for close distances, R ≈ 1 × 10^{13} cm (1955SE1C, 1956SE1A). See also (1956GR1E). Emission of (15.1 ± 0.2) MeV γradiation, ascribed to the first T = 1 level, has been studied in the range E_{p} = 15 to 340 MeV by (1957WA04, 1957WA1F). The general shape of the excitation function indicates direct nucleonnucleon interaction for the higher energies; near threshold, emission of swave protons is indicated. Estimates of the αparticle width, and comparison with isobaric spin forbidden reactions (e.g. ^{12}C(α, α')^{12}C*, ^{12}C(d, d')^{12}C*) indicate a T = 0 admixture a^{2} ≈ 10^{3}. At 31 MeV, θ_{lab} = 80°, γrays of energy 15.1, 12.8 and 10.7 MeV are observed, with relative intensities 1/0.090 ± 0.015/0.095 ± 0.014. The 12.8 MeV radiation is ascribed to excitation of a ^{12}C level of that energy, while the 10.7 MeV radiation represents a cascade from ^{12}C*(15.1) to ^{12}C*(4.4) (1957WA1F: see also ^{10}B(^{3}He, p)^{12}C). A study of reaction (c) at an energy of 29 MeV shows no indication of direct 4body decay of ^{13}N*; 1/4 of the events proceed via ^{8}Be(0), and > 1/2 via ^{8}Be*(2.9). Evidence is found for the participation of ^{12}C*(9.6, ≈ 12, 16, 20, 25) (1955NE18). See also (1955RE16), (1957JA1B) and (1955CU1C, 1956SA1C, 1956SA1D). See also (1956ST30, 1957AL39, 1957GO1D) and (1957KA1D, 1958EL48; theor.).
The angular distribution of elastic scattering has been studied at E_{d} = 19 MeV by (1954FR24); several diffraction peaks appear. See also (1958WA07). Inelastic groups corresponding to ^{12}C*(4.4, 9.6) are reported by (1951KE02, 1954FR24, 1956GR37, 1956HA90: see also (1956CA65)). The 7.7 MeV level has not been observed. The angular distribution of the Q = 4.4 MeV group at E_{d} = 15 MeV has been analyzed in terms of direct nuclear interaction theory and in terms of electric interaction theory by (1956HA90); neither appears to give a satisfactory account of the observations. A search for 15 MeV γradiation at E_{d} = 85 MeV yielded a negative result; a T = 0 admixture of < 4 × 10^{2} is indicated (1957WA04, 1957WA1F). See also (1955KH31, 1955KH35).
See (1958WE1E).
Elastic scattering has been studied at E_{α} = 19 MeV by (1958PR65), at 31.5 MeV by (1956WA29), at 40 MeV by (1956IG02, 1956WE1C, 1957IG03) and at 48 MeV by (1955VA1A). The angular distributions show strong diffraction effects indicative of a direct interaction. Inelastic groups corresponding to levels at 4.4, 7.64 ± 0.07, 9.6 and possibly, 12.7 MeV are observed. ^{12}C recoils corresponding to the ground and 4.4 MeV states are also reported; the absence of recoils corresponding to the 7.7 MeV state is taken to indicate that this state disintegrates primarily (> 80%) by αemission (1955RA1B: see ^{11}B(d, n)^{12}C). From a similar experiment, (1958EC12) find that the chance is less than 0.1 for Γ_{γ}/Γ > 10^{3}. Angular distribution of the Q = 4.4 MeV inelastic group at E_{α} = 31.5 MeV are consistent with the direct surface interaction theory of (1953AU1A). A similar analysis of the Q = 7.7 MeV group gives good agreement for J = 0^{+} (1956WA29). At E_{α} = 42 MeV, the angular distribution of this group is well matched by the j^{2}_{0}(kr) or j^{2}_{2}(kr) functions, indicating J^{π} = 0^{+} or 2^{+}. The former is preferred in view of the small γwidth (1958EC12). See also (1956WE1C, 1957FI1C, 1958PR65, 1958SH65). A search for γradiation from the deexcitation of the 15 MeV, T = 1 level at E_{α} = 48 and 175 MeV gives an upper limit of ≈ 10^{3} for the T = 0 admixture (1957WA04, 1957WA1F). See also (1954JU1B). For reaction (b), see (1953LI28).
The decay is mainly to the ground state via an allowed transition. Transitions to ^{12}C*(4.4, 7.65) also are allowed. Branching ratios are 100/15/3; log ft = 4.17, 4.4 and 4.4, respectively (1958VE20). Delayed αparticles with a total energy of ≈ 4 MeV are also observed, suggesting that a state of ^{12}C in the region 11 to 12 MeV is involved (1950AL57). See ^{12}N.
See ^{13}C.
See (1955NE18) and (1957BE49).
Angular distributions of the ground state tritons have been measured at E_{d} = 2.2 and 3.3 MeV (1954HO48).
Angular distributions of the αparticle groups to the ground and 4.4 MeV states have been obtained at E(^{3}He) = 2 MeV (1957HO63) and 4.5 MeV (1957HO62). Some direct interaction appears to be involved at both energies. At E(^{3}He) = 2 MeV, a 15.1 MeV γray is observed (1957BR18, 1957GO1B, 1958BR1D).
Not observed.
Not observed.
Not observed.
For αgroups have been observed corresponding to ^{12}C*(0, 4.4, 7.7, 9.6): see Table 12.11 (in PDF or PS) and (1957HO1H). Alphagamma correlations give J = 2^{+} for the 4.4 MeV state (1954ST1C) while γγ correlations give J = 0 or > 2 for the 7.7 MeV state (1955SE03: see, however, ^{12}B(β^{})^{12}C). The width of the 7.7 MeV state is < 25 keV (1953DU23, 1956AH32) and that of the 9.6 MeV state is 30 ± 8 keV (c.m.). The J^{π} values for the 9.6 MeV state are limited to 0^{+}, 1^{}, 2^{+}, 3^{}, 4^{+} (1956DO41). Angular distributions of the αparticles to the ground, 4.4 and 9.6 MeV states have been measured at E_{d} = 20.9 MeV (1957FI1C). See also (1952GI01, 1958BO18, 1958BO71). A small yield of 15 MeV γradiation is observed at E_{d} = 10.8 MeV, presumably due to excitation of the 15.1 MeV, T = 1 state (1954RA35). See also (1956GR37, 1958RA13) and ^{16}O.
See ^{16}O.
Alpha particles have been observed to a state of ^{12}C at 4.432 ± 0.010 MeV (1952SC28). The γray energy after Doppler correction of 20 keV is 4.443 ± 0.020 MeV. The necessity for the correction implies a lifetime < 3 × 10^{13} sec (1952TH24); see ^{12}C(γ, γ')^{12}C*. The angular distributions of shortrange alpha particles and 4.4 MeV γradiation indicate that the 4.4 MeV state has J = 2^{+} or > 4 (1953KR1B: see also (1957GO1E)). See also ^{16}O and (1958RA14).
There is evidence for the involvement of ^{12}C states at 9.6 and ≈ 11 MeV which decay to the ground state of ^{8}Be, a state at 12  13 MeV, decaying mainly to the 2.9 MeV state of ^{8}Be, and T = 1 state at ≈ 16 and 18  19 MeV, again leading mainly to the 2.9 MeV ^{8}Be state. The 4.4 and 7.7 MeV states of ^{12}C seem to occur rarely, if at all (see ^{16}O).
See (1957ZA1A).
