## Abstract

The mechanism of the $^{12}$C($\gamma $, 3$\alpha $) reaction, for $\gamma $-ray energies, E$_{\gamma}$, 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 $\alpha $-particles. The reaction is initiated mainly by electric-dipole and electric-quadrupole $\gamma $-ray interaction, the former being unexpectedly strong when E$_{\gamma}$ < 20 MeV. For E$_{\gamma}$ < 25 MeV the reaction proceeds mainly by transitions to the ground-state of $^{8}$Be (spin J = 0), and to 2$\cdot $95 $\pm $ 0$\cdot $10 MeV (J = 2) and 4$\cdot $0 $\pm $ 0$\cdot $1 MeV (J = 2 or 4) levels of $^{8}$Be. Transitions to levels near 6, 10 and 15 MeV (all J = 0, 2 or 4) become predominant when 25 MeV $\leq $ E$_{\gamma}$ < 26 MeV. For E$_{\gamma}\geq $ 26 MeV, most transitions lead to 16$\cdot $8 $\pm $ 0$\cdot $2 MeV (J = 2) and 17$\cdot $6 $\pm $ 0$\cdot $2 MeV (J = 2, possibly 0) levels, and possibly to a further 16$\cdot $4 $\pm $ 0$\cdot $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 $^{12}$C and $^{8}$Be involved. For example, the 2$^{+}$ levels of $^{12}$C involved when 16 MeV $\leq $ E$_{\gamma}$ <20 MeV are (unexpectedly) found to have T = 1, and the 16$\cdot $8 and 17$\cdot $6 MeV levels of $^{8}$Be are also found to have T = 1. The relationship of the $^{12}$C($\gamma $, 3$\alpha $) reaction to other $^{12}$C photodisintegration reactions (including some new reactions established during the present experiments) is discussed.