Abstract
Our present understanding of the structure of the Hoyle state in ^{12}C and other near-threshold states in alpha -conjugate nuclei is reviewed in the framework of the alpha -condensate model. The ^{12}C Hoyle state, in particular, is a candidate for alpha -condensation, due to its large radius and alpha -cluster structure. The predicted features of nuclear alpha -particle condensates are reviewed along with a discussion of their experimental indicators, with a focus on precision break-up measurements. Two experiments are discussed in detail, firstly concerning the break-up of ^{12}C and then the decays of heavier nuclei. With more theoretical input, and increasingly complex detector setups, precision break-up measurements can, in principle, provide insight into the structures of states in alpha -conjugate nuclei. However, the commonly-held belief that the decay of a condensate state will result in Nalpha -particles is challenged. We further conclude that unambiguously characterising excited states built on alpha -condensates is difficult, despite improvements in detector technology.
Highlights
The Hoyle state in carbon-12 is considered royalty in the world of nuclear physics
The case of finite nuclear systems was approached in a flagship 2001 paper by Tohsaki, Horiuchi, Schuck and Röpke (THSR) [9], who concluded that such a condensate state could exist in light α-conjugate nuclei at energies around the α-decay threshold
Open questions include understanding the nature of further, higher energy, excited states in these nuclei, that could correspond to excitations of condensate states
Summary
The Hoyle state in carbon-12 is considered royalty in the world of nuclear physics. This prestige originates from the crucial role it plays during helium burning, facilitating the production of 12C through the triple-α process [1]. The Hoyle state lies just beyond the 7.27 MeV 3α threshold These two theoretical approches are oversimplifications and we have experimental evidence of, and theoretical descriptions for, α-clustering in both the ground and excited states of nuclei. The case of finite nuclear systems was approached in a flagship 2001 paper by Tohsaki, Horiuchi, Schuck and Röpke (THSR) [9], who concluded that such a condensate state could exist in light α-conjugate nuclei at energies around the α-decay threshold. This theoretical approach has played a leading role in the description of near-threshold states in α-conjugate nuclei for nearly 20 years. Open questions include understanding the nature of further, higher energy, excited states in these nuclei, that could correspond to excitations of condensate states
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