The ¹²C(γ, 3α) reaction energy levels of ⁸Be and ¹²C

The mechanism of the ¹²C(γ, 3α) reaction, for γ-ray energies, E_γ, up to about 40 MeV, has been determined from a study of over 2500 stars in nuclear emulsions. The study includes investigation of the angular distributions and correlations of the α-particles. The reaction is initiated mainly by electric-dipole and electric-quadrupole γ-ray interaction, the former being unexpectedly strong when E_γ < 20 MeV. For E_γ < 25 MeV the reaction proceeds mainly by transitions to the ground-state of ⁸Be (spin J = 0), and to 2⋅95 ± 0⋅10 MeV (J = 2) and 4⋅0 ± 0⋅1 MeV (J = 2 or 4) levels of ⁸Be. Transitions to levels near 6, 10 and 15 MeV (all J = 0, 2 or 4) become predominant when 25 MeV ≤ E_γ <26 MeV. For E_γ ≥ 26 MeV, most transitions lead to 16⋅8 ± 0⋅2 MeV (J = 2) and 17⋅6 ± 0⋅2 MeV (J = 2, possibly 0) levels, and possibly to a further 16⋅4 ± 0⋅2 MeV (J = 0 or 2) level, levels which have not been detected in other reactions. The reaction mechanism is interpreted in terms of competing modes of decay of a compound nucleus, demonstrating the strong influence of the isotopic spins (T) of the levels of ¹²C and ⁸Be involved. For example, the 2⁺ levels of ¹²C involved when 16 MeV ≤ E_γ <20 MeV are (unexpectedly) found to have T = 1, and the 16⋅8 and 17⋅6 MeV levels of ⁸Be are also found to have T = 1. The relationship of the ¹²C (γ, 3α) reaction to other ¹²C photodisintegration reactions (including some new reactions established during the present experiments) is discussed.