Impact of breakup coupling in nuclear reactions on light elements

Abstract

The resurgence of interest in nuclear reactions involving weakly bound nuclei spans nuclear astrophysics, neutron production, and fast reactor dynamics. These reactions offer insights into complex couplings within nuclear reaction systems, especially in direct reactions. This affects the excitation functions and particle spectra. Notably, residue breakup couplings in reactions like 7 Li(p,n) and 6 Li(n,γ) are studied, complementing experimental analyses with Continuum Discretized Coupled-Channels (CDCC) and Coupled-Reaction-Channels (CRC) theoretical calculations.Continuum neutron spectra resulting from 7 Li(p,n) were estimated by measuring double differential cross sections for both continuum and resonant breakup of 7 Be via the 7 Li(p,n) 7 Be ∗reaction at 21 MeV proton energy. The K nn based machine learning algorithm facilitated precise event identification. The analysis unveiled continuum neutron distributions, attributed to coupling of 7 Be→ 3 He + α breakup levels with the final state, validated through experimental measurements and comparison with 3 He gated neutron spectra. Additionally, 6 Li(n,γ) cross sections were measured, utilizing Direct Capture (DC) formalism. Cross sections for 7 Li(n,γ) 8 Li were also measured, identifying non-resonant and 3 + resonant contributions to the excitation function. Reproduction of these components via fresco Direct Capture calculations highlighted the impact of resonant states above α + t breakup threshold of 7 Li in 6 Li neutron capture. Comparing 6 Li(n,γ) and 7 Li(n,γ) reactions revealed distinctive resonant dip in the excitation function due to breakup coupling. This demonstrates electromagnetic coupling from entrance to unbound resonant states 7/2 − , 5/2 − , and 3/2 − . However, no branching from unbound to bound states was observed. This suggests that the unbound resonant states lead to breakup rather than radiative capture. This study underscores the crucial role of electromagnetic coupling in populating resonant states within the exit channel, regardless of their bound status, and highlights the sequential formation of residual breakups primarily through spin transfer coupling mechanisms.