(See the Energy Level Diagram for 14C)
The mean of reported end points is 156 ± 1 keV (1957LI51). The weighted mean half-life is 5568 ± 30 years (1955LI1D: see (1958ST50)): log ft = 9.03. The spectrum does not deviate from the allowed shape down to 3 keV (1954MO84); see also (1954FO29, 1955PO04).
It appears that the long lifetime of 14C may be due to chance cancellation in the matrix element for the decay, brought about by a small tensor force (1954JA1A, 1955SH84, 1956ST1D, 1957VI1A); see, however, (1958BA1A). See also (1955SM1A, 1957ST1G).
At E(6Li) = 2 MeV, the ground state proton group, and proton groups corresponding to the known levels between Ex = 6.1 and 8.5 MeV have been observed (1958LI42).
The thermal capture cross section is 0.5 ± 0.2 mb, 0.9 ± 0.3 mb (1955HU1B).
The thermal scattering cross section is 4.7 ± 0.09 b (1955HU1B: free atoms); see also (1952KO1A). In the range En = 0.2 to 3 MeV, resonances are observed at En = 1.75 MeV (Γ = 25 keV), 2.43 and 2.45 MeV. The latter levels, whose widths are of the order of 15 keV, interfere strongly. The non-resonant cross section decreases smoothly from about 3 b at 0.2 MeV to 1 b at 3 MeV (1958WI01).
Proton groups reported by (1954SP01) and (1955MC75) are displayed in Table 14.2 (in PDF or PS). No other groups have been observed for Ex = 0 to 8.1 MeV with an intensity greater than 0.2 of the group corresponding to the 6.1 MeV state (1954SP01: θ = 90°). See also (1953KO42, 1956EL1A, 1956KO26, 1956VA17).
Observed γ-radiation assigned to 14C is exhibited in Table 14.3 (in PDF or PS). The internally-formed positron distribution suggests that the 6.1 MeV line is E1 and hence that the level has J = 1- and is the analog to the 8.06 MeV level of 14N (1952TH24, 1958CH1A, 1958GO81). The 0.81 MeV cascade transition from the 6.89 MeV state shows a Doppler shift (τm < 3 × 10-13 sec) and hence is predominantly dipole. The angular correlation of 6.1 and 0.8 MeV γ-rays is consistent with J = 0 and excludes J = 1 or 2. J = 0 is also suggested by the absence of the direct ground state transition for the 6.89 MeV level (1958WA02). The 6.89 MeV level is probably the analog of the J = 0- level of 14N at 8.71 MeV. Assuming that the 6.1 MeV radiation is E1, the relative intensities of external and internal pairs for the 6.1 and 6.7 MeV γ-rays are consistent with E2, E1, M1 or E3 for the 6.7 MeV transition (1955BE1G). The mean lifetime of the 6.72 MeV state is greater than 3 × 10-13 sec, suggesting E3 or M2 (1958WA02). The 0.61 MeV γ-ray is in coincidence with the 6.7 MeV γ-ray and has an intensity roughly equal to that of the 7.35 MeV γ-ray (within a factor of 5). The strength of the cascade then suggests J = 2- or 3- for the 7.35 MeV state (1958WA02): see also (1955AU1A; theor.). Comparison of reduced widths and calculation of level shifts suggests the following associations of 14C and 14N levels: 6.09 - 8.06, 6.72 - 8.91, 6.89 - 8.71 and 7.35 - 9.50 (1959WA04).
Reduced widths derived from pick-up angular distributions at Ed = 14.8 MeV are listed in Table 14.4 (in PDF or PS) (1958MO97). The observation of strong contributions from 13C excited states indicates considerable configuration mixing in the ground state of 14C. An admixture of 2s2 and 2d2 consistent with these results also permits a satisfactory account of the cancellation in the 14C-14N beta decay matrix element without recourse to tensor forces (1958BA1A). See also 14C(β-)14N.