AUTHOR=Kahl Daid , Yamaguchi Hidetoshi , Hayakawa Seiya 

TITLE=Alpha clustering in nuclear astrophysics and topology

JOURNAL=Frontiers in Physics

VOLUME=Volume 11 - 2023

YEAR=2023

URL=https://www.frontiersin.org/journals/physics/articles/10.3389/fphy.2023.1189040

DOI=10.3389/fphy.2023.1189040

ISSN=2296-424X

ABSTRACT=When we think of clustering in nuclear physics, the astrophysical importance within light nuclei
and the structural manifestations with classical analogs immediately come to mind. 4 He, also
known as the alpha particle, is the most abundant nucleus in the Universe, being quite tightly
bound for its mass, with a first excited state over 20 MeV. The nature of the alpha particle places
it in a unique position within nuclear astrophysics and structure (including geometry). The plurality
of energy release from stellar hydrogen fusion – whether quiescent or explosive – comes from the
conversion of hydrogen to helium. Within more complex nuclei, alpha particles are continuously
arranged, leading to fascinating phenomena such as excited rotational bands, Borromean ring
ground states, linear structures, etc. Nuclei with an equal and even number of protons and
neutrons are colloquially referred to as “alpha conjugate nuclei” where such special properties
are the most pronounced and easiest to spot. However, when a single nucleon or a pair of
nucleons are added to the system, the alpha clustering not only remains evident, but it may even
be enhanced. Excited states with large alpha partial widths are a signature of clustering behavior,
and these states can have a profound effect on the reaction rates in astrophysical systems when
the excitation energy aligns with the so-called Gamow energy – the preferential thermal energy
to statistically overcome the Coulomb barrier. In this article, we will consider in detail the specific
ramifications of alpha clustering in selected scenarios for both nuclear astrophysics and topology.
In particular, we discuss the astrophysical reactions of 7 Li(α, γ), 7 Be+α, 11 C(α, p), and 30 S(α, p),
where α-clusters may increase the reaction rates from 10% to an order of magnitude; large α
resonances makes the astrophysical rate of 18 F(p, α) quite uncertain. We also focus on the α
rotational bands, of both positive and negative parity, of 11 B and 11 C, and finally on the strongest
evidence for the linear chain cluster state observed in 14 C.