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Background: Near-threshold α-clustered states in light nuclei have been postulated to have a structure consisting of a diffuse gas of α-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. Method:To examine signatures of this α-condensation, a compound nucleus reaction using 160, 280, and 400 MeV 16 O beams impinging on a carbon target was used to investigate the 12 C( 16 O, 7α) reaction. This permits a search for near-threshold states in the α-conjugate nuclei up to 24 Mg.Results: Events up to an α-particle multiplicity of 7 were measured and the results were compared to both an Extended Hauser-Feshbach calculation and the Fermi break-up model. The measured multiplicity distribution exceeded that predicted from a sequential decay mechanism and had a better agreement with the multi-particle Fermi break-up model. Examination of how these 7α final states could be reconstructed to form 8 Be and 12 C(0 + 2 ) showed a quantitative difference in which decay modes were dominant compared to the Fermi break-up model. No new states were observed in 16 O, 20 Ne, and 24 Mg due to the effect of the N-α penetrability suppressing the total α-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 multi-particle breakup. Highly α-clustered states were seen which did not originate from simple binary reaction processes. Direct investigations of near-threshold states in N-α systems are inherently impeded by the Coulomb barrier prohibiting the observation of states in the N-α decay channel. No evidence of a highly clustered 15.1 MeV state in 16 O was observed from ( 28 Si , 12 C(0 + 2 )) 16 O(0 + 6 ) when reconstructing the Hoyle state from 3 α-particles. Therefore, no experimental signatures for α-condensation were observed. arXiv:1907.05471v2 [nucl-ex]

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The TexAT (Texas Active Target) detector is a new active-target time projection chamber (TPC) that was built at the Cyclotron Institute Texas A&M University. The detector is designed to be of general use for nuclear structure and nuclear astrophysics experiments with rare isotope beams. TexAT combines a highly segmented Time Projection Chamber (TPC) with two layers of solid state detectors. It provides high efficiency and flexibility for experiments with low intensity exotic beams, allowing for the 3D track reconstruction of the incoming and outgoing particles involved in nuclear reactions and decays.

Two recent experiments have indicated that the break-up of the 12C Hoyle state is dominated by the sequential 8Be(g.s.) + α decay channel. The rare direct 3α decay was found to contribute with a branching ratio of less than 0.047% (95% C.L.). However, the ability of experimentalists to successfully disentangle these two competing decay channels relies on accurate theoretical predictions of how they each manifest in phase space distribution of the three break-up α-particles. The following paper reviews the current theoretical approaches to calculating the break-up of the Hoyle state and introduces a semi-classical WKB approach, which adequately reproduces the results of more sophisticated calculations. It is proposed that a more accurate upper limit on this branching ratio may be obtained if these new theoretical results are taken into account when analysing experimental data.

Our present understanding of the structure of the Hoyle state in 12 C and other near-threshold states in α-conjugate nuclei is reviewed in the framework of the α-condensate model. The 12 C Hoyle state, in particular, is a candidate for α-condensation, due to its large radius and α-cluster structure. The predicted features of nuclear α-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 α-conjugate nuclei. However, the commonly-held belief that the decay of a condensate state will result in N α-particles is challenged. We further conclude that unambiguously characterising excited states built on α-condensates is difficult, despite improvements in detector technology. This contribution has been presented during the ceremony of the Few-Body Systems Award for young professionals.

Background: The structure of the Hoyle state, a highly α-clustered state at 7.65 MeV in 12 C, has long been the subject of debate. Understanding if the system comprises of three weakly-interacting α-particles in the 0s orbital, known as an α-condensate state, is possible by studying the decay branches of the Hoyle state. Purpose: The direct decay of the Hoyle state into three α-particles, rather than through the 8 Be ground state, can be identified by studying the energy partition of the 3 α-particles arising from the decay. This paper provides details on the break-up mechanism of the Hoyle stating using a new experimental technique. Method: By using beta-delayed charged-particle spectroscopy of 12 N using the TexAT (Texas Active Target) TPC, a high-sensitivity measurement of the direct 3 α decay ratio can be performed without contributions from pileup events. Results: A Bayesian approach to understanding the contribution of the direct components via a likelihood function shows that the direct component is < 0.043% at the 95% confidence level (C.L.). This value is in agreement with several other studies and here we can demonstrate that a small non-sequential component with a decay fraction of about 10 −4 is most likely. Conclusion: The measurement of the non-sequential component of the Hoyle state decay is performed in an almost medium-free reaction for the first time. The derived upper-limit is in agreement with previous studies and demonstrates sensitivity to the absolute branching ratio. Further experimental studies would need to be combined with robust microscopic theoretical understanding of the decay dynamics to provide additional insight into the idea of the Hoyle state as an α-condensate.

The determination of absolute branching ratios for high-energy states in light nuclei is an important and useful tool for probing the underlying nuclear structure of individual resonances: for example, in establishing the tendency of an excited state towards α-cluster structure. Difficulty arises in measuring these branching ratios due to similarities in available decay channels, such as ( 18 O,n) and ( 18 O,2n), as well as differences in geometric efficiencies due to population of bound excited levels in daughter nuclei. Methods are presented using Monte Carlo techniques to overcome these issues.