Background: Near-threshold $\alpha$-clustered states in light nuclei have been postulated to have a structure consisting of a diffuse gas of $\alpha$ particles which condense into the $0s$ orbital. Experimental evidence for such a dramatic phase change in the structure of the nucleus has not yet been observed.

Purpose: To understand the role of $\alpha$ condensation in light nuclei experimentally.

Method: To examine signatures of this $\alpha$ condensation, a compound nucleus reaction using 160-, 280-, and 400-MeV ${}^{16}\mathrm{O}$ beams impinging on a carbon target was used to investigate the ${}^{12}\mathrm{C}({}^{16}\mathrm{O},7\alpha )$ reaction. This permits a search for near-threshold states in the $\alpha$-conjugate nuclei up to ${}^{24}\mathrm{Mg}$.

Results: Events up to an $\alpha$-particle multiplicity of seven were measured and the results were compared to both an extended Hauser-Feshbach calculation and the Fermi breakup model. The measured multiplicity distribution exceeded that predicted from a sequential decay mechanism and had a better agreement with the multiparticle Fermi breakup model. Examination of how these $7\alpha$ final states could be reconstructed to form ${}^8\mathrm{Be}$ and ${}^{12}\mathrm{C}\left({0_2}^{+}\right)$ showed a quantitative difference in which decay modes were dominant compared to the Fermi breakup model. No new states were observed in ${}^{16}\mathrm{O}, {}^{20}\mathrm{Ne}$, and ${}^{24}\mathrm{Mg}$ due to the effect of the $N-\alpha$ penetrability suppressing the total $\alpha$-particle dissociation decay mode.

Conclusion: The reaction mechanism for a high-energy compound nucleus reaction can only be described by a hybrid of sequential decay and multiparticle breakup. Highly $\alpha$-clustered states were seen which did not originate from simple binary reaction processes. Direct investigations of near-threshold states in $N-\alpha$ systems are inherently impeded by the Coulomb barrier prohibiting the observation of states in the $N-\alpha$ decay channel. No evidence of a highly clustered 15.1-MeV state in ${}^{16}\mathrm{O}$ was observed from $[{}^{28}\mathrm{Si}^{★},{}^{12}\mathrm{C}\left({0_2}^{+}\right)]{}^{16}\mathrm{O}\left({0_6}^{+}\right)$ when reconstructing the Hoyle state from three $\alpha$ particles. Therefore, no experimental signatures for $\alpha$ condensation were observed.

• Received 12 July 2019

DOI:https://doi.org/10.1103/PhysRevC.100.034320