Experimental investigation of α condensation in light nuclei

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Abstract

Background: Near-threshold $\ensuremath{\alpha}$-clustered states in light nuclei have been postulated to have a structure consisting of a diffuse gas of $\ensuremath{\alpha}$ 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 $\ensuremath{\alpha}$ condensation in light nuclei experimentally.Method: To examine signatures of this $\ensuremath{\alpha}$ 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 $^{12}\mathrm{C}(^{16}\mathrm{O},7\ensuremath{\alpha})$ reaction. This permits a search for near-threshold states in the $\ensuremath{\alpha}$-conjugate nuclei up to $^{24}\mathrm{Mg}$.Results: Events up to an $\ensuremath{\alpha}$-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 $7\ensuremath{\alpha}$ final states could be reconstructed to form $^{8}\mathrm{Be}$ and $^{12}\mathrm{C}({{0}_{2}}^{+})$ 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 $N\ensuremath{-}\ensuremath{\alpha}$ penetrability suppressing the total $\ensuremath{\alpha}$-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 $\ensuremath{\alpha}$-clustered states were seen which did not originate from simple binary reaction processes. Direct investigations of near-threshold states in $N\ensuremath{-}\ensuremath{\alpha}$ systems are inherently impeded by the Coulomb barrier prohibiting the observation of states in the $N\ensuremath{-}\ensuremath{\alpha}$ decay channel. No evidence of a highly clustered 15.1-MeV state in $^{16}\mathrm{O}$ was observed from $[^{28}\mathrm{Si}^{★},^{12}\mathrm{C}({{0}_{2}}^{+})]^{16}\mathrm{O}({{0}_{6}}^{+})$ when reconstructing the Hoyle state from three $\ensuremath{\alpha}$ particles. Therefore, no experimental signatures for $\ensuremath{\alpha}$ condensation were observed.

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