Background: The production of ${}^7\mathrm{Be}$ and ${}^7\mathrm{Li}$ nuclei plays an important role in primordial nucleosynthesis, nuclear astrophysics, and fusion energy generation. The ${}^3\mathrm{He}(\alpha ,\gamma ){}^7\mathrm{Be}$ and ${}^3\mathrm{H}(\alpha ,\gamma ){}^7\mathrm{Li}$ radiative-capture processes are important to determine the ${}^7\mathrm{Li}$ abundance in the early universe and to predict the correct fraction of $pp$-chain branches resulting in ${}^7\mathrm{Be}$ versus ${}^8\mathrm{B}$ neutrinos. The ${}^6\mathrm{Li}(p,\gamma ){}^7\mathrm{Be}$ has been investigated recently hinting at a possible cross section enhacement near the thershold. The ${}^6\mathrm{Li}(n,{}^3\mathrm{H}){}^4\mathrm{He}$ process can be utilized for tritium breeding in machines dedicated to fusion energy generation through the deuteron-tritium reaction, and is a neutron cross section standard used in the measurement and evaluation of fission cross sections.

Purpose: In this work we study the properties of ${}^7\mathrm{Be}$ and ${}^7\mathrm{Li}$ within the no-core shell model with continuum (NCSMC) method, using chiral nucleon-nucleon interactions as the only input, and analyze all the binary mass partitions involved in the formation of these systems.

Methods: The NCSMC is an ab initio method applicable to light nuclei that provides a unified description of bound and scattering states and thus is well suited to investigate systems with many resonances and pronounced clustering like ${}^7\mathrm{Be}$ and ${}^7\mathrm{Li}$.

Results: Our calculations reproduce all the experimentally known states of the two systems and provide predictions for several new resonances of both parities. Some of these new possible resonances are built on the ground states of ${}^6\mathrm{Li}$ and ${}^6\mathrm{He}$, and thus represent a robust prediction. We do not find any resonance in the $p+{}^6\mathrm{Li}$ mass partition near the threshold. On the other hand, in the $p+{}^6\mathrm{He}$ mass partition of ${}^7\mathrm{Li}$ we observe an $S$-wave resonance near the threshold producing a very pronounced peak in the calculated $S$ factor of the ${}^6\mathrm{He}(p,\gamma ){}^7\mathrm{Li}$ radiative-capture reaction.

Conclusions: While we do not find a resonance near the thershold in the $p+{}^6\mathrm{Li}$ channel, in the case of ${}^6\mathrm{He}+p$ reaction a resonant $S$-wave state is predicted at a very low energy above the reaction threshold, which could be relevant for astrophysics and its implications should be investigated. We note though that this state lies above the three-body breakup threshold not included in our method and may be influenced by three-body continuum correlations.

• Received 20 June 2019

DOI:https://doi.org/10.1103/PhysRevC.100.024304