O-16(GAMMA, 4-ALPHA) REACTION-MECHANISM INVESTIGATION

The energy dependence of the total cross section for the O-16(gamma, 4 alpha) reaction has been measured by the use of nuclear photoemulsion s exposed to bremsstrahlung of energy up to 300 MeV. Distributions of alpha-particles in their in-pair relative energies and in the opening- angle as well as excitation-energy distributions of intermediate Be-8 and C-12 nuclei were first measured. Besides, distributions in relativ e kinetic energies of alpha-particles were obtained. The experimental data are compared with the predictions of the statistical theory of mu ltiparticle nuclear reactions. It is shown for the reaction considered that at a near-threshold gamma-quanta energy the statistical decay of the O-16 is the major mechanism. As the gamma-quanta energy increases , its quasi-direct interaction with alpha-clusters associated in some complex structure within the nucleus starts to play an appreciable rol e.