The <sup>8</sup>Be nucleus and the Hoyle state in dissociation of relativistic nuclei

Having become observable since the pioneering era of cosmic ray physics fragmentation, the events of relativistic nuclei in nuclear emulsions highlight the potential of this method to study extremely cold ensembles of H and He nuclei, thereby advancing the physics of nuclear clustering and, potentially, expanding nuclear astrophysics. Following the presentation of the progress of this method and orientation to the current problems, this review presents the key results and generalizations of the BECQUEREL experiment at JINR, obtained in the study of unstable nuclear states in the relativistic dissociation of a wide variety of nuclei. The productivity of this method is ensured by record-breaking spatial resolution and full sensitivity to relativistic fragments. According to invariant masses based on the most accurate measurements of emission angles in the extremely narrow fragmentation cone, the contributions of the decays of 8Be(0+), 8Be(2+), 9Be(1.7), 9B, 6Be, 12С(0+2) or the Hoyle state and 12C(3–) have been identified now. The increase in the contribution of 8Be(0+) with the multiplicity of accompanying α-particles, followed by 9B and 12C(0+2), has been established. The structure of these states and the diversity of parent nuclei without the influence of the initial energy assume the coalescence of α-particles and nucleons which appear in dissociation. The initial density and duration of the secondary interaction of the latter may be sufficient up to the lowest-energy fusion reactions. Such a scenario requires low-energy physics concepts to interpret the relativistic fragmentation. The usage of automated microscopy for the analysis of irradiation beams from the JINR NICA accelerator complex becomes a modern basis to apply the nuclear emulsion method which has become fundamental in the physics of the micro-world.