Radiative capture mechanisms in heavy nuclei

Abstract

The known cross section data for radiative capture in heavy nuclei of 5–50 MeV nucleons and composite projectiles are summarized and the probable reaction mechanisms involved are discussed. Almost all the (α, γ) and (d, γ) results are shown to be in reasonable agreement with statistical theory predictions, especially when corrections for multiple γ-ray cascades and angular momentum effects are included. In particular, the variation of over two orders of magnitude in the reported (α, γ) maximum cross-sections are correctly predicted by statistical theory. To allow for the different nuclear penetrabilities of the various projectiles, each of the experimental capture cross section values has been divided by the appropriate optical model total reaction cross-section. The results provide strong support for the proposal that nucleon capture proceeds predominantly by direct or collective reaction mechanisms. Evidence that non-statistical processes make important contributions in the case of 3He ion capture is also presented. It is concluded that there is no current evidence for mechanisms other than compound nucleos formation and decay in the radiative capture of deuterons and α-particles and that the projectile neutron/proton ratio has a marked effect on the probability of radiative capture.