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The $^{8}$Be nucleus and the Hoyle state in dissociation of relativistic nuclei

D. A. Artemenkov, A. A. Zaitsev, P. I. Zarubin

TL;DR

This review synthesizes a decade of BECQUEREL results on alpha-cluster states in relativistic nuclear fragmentation studied with nuclear emulsions, enabling invariant-mass reconstruction of near-threshold decays such as $^8$Be$(0^+)$, $^8$Be$(2^+)$, and $^{12}$C$(0^+_2)$ (the Hoyle state). By tracking relativistic fragments with sub-micron precision and analyzing the angular correlations within tight fragmentation cones, the study reveals the coalescence-like formation of alpha clusters and demonstrates universality across a wide range of projectile masses and energies, including high-midelity identifications of decays down to $Q$ values of a few hundred keV. The work connects cluster physics to nuclear astrophysics, showing how the Hoyle state and related states emerge in relativistic fragmentation and how increasing alpha-multiplicity enhances the production of unstable cluster states, consistent with an $ ext{α}$BEC-inspired picture. Looking forward, automated microscopy and Xe-beam exposures at facilities like NICA promise to extend these observations to even heavier systems and richer $ ext{H}$–$ ext{He}$ ensembles, potentially informing both nuclear structure and stellar nucleosynthesis models.

Abstract

The 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 calculated on the basis of precise data of emission angles in the extremely narrow fragmentation cone, the contributions of the decays of $^8$Be(0$^+$), $^8$Be(2$^+$), $^9$Be(1.7), $^9$B, $^6$Be, $^{12}$C(0$^+_2$) or the Hoyle state and $^{12}$C(3$^-$) have been identified now. The increase in the contribution of $^8$Be(0$^+$) with the multiplicity of accompanying $α$-particles, followed by $^9$B and $^{12}$C(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. 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.

The $^{8}$Be nucleus and the Hoyle state in dissociation of relativistic nuclei

TL;DR

This review synthesizes a decade of BECQUEREL results on alpha-cluster states in relativistic nuclear fragmentation studied with nuclear emulsions, enabling invariant-mass reconstruction of near-threshold decays such as Be, Be, and C (the Hoyle state). By tracking relativistic fragments with sub-micron precision and analyzing the angular correlations within tight fragmentation cones, the study reveals the coalescence-like formation of alpha clusters and demonstrates universality across a wide range of projectile masses and energies, including high-midelity identifications of decays down to values of a few hundred keV. The work connects cluster physics to nuclear astrophysics, showing how the Hoyle state and related states emerge in relativistic fragmentation and how increasing alpha-multiplicity enhances the production of unstable cluster states, consistent with an BEC-inspired picture. Looking forward, automated microscopy and Xe-beam exposures at facilities like NICA promise to extend these observations to even heavier systems and richer – ensembles, potentially informing both nuclear structure and stellar nucleosynthesis models.

Abstract

The 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 calculated on the basis of precise data of emission angles in the extremely narrow fragmentation cone, the contributions of the decays of Be(0), Be(2), Be(1.7), B, Be, C(0) or the Hoyle state and C(3) have been identified now. The increase in the contribution of Be(0) with the multiplicity of accompanying -particles, followed by B and C(0), 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 -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. 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.
Paper Structure (22 sections, 4 equations, 47 figures, 1 table)

This paper contains 22 sections, 4 equations, 47 figures, 1 table.

Table of Contents

  1. Introduction
  2. Preface
  3. Historical notes
  4. Introductory remarks on nuclear clustering
  5. Development of clustering research and its current motivation
  6. From the $^8$Be nucleus to the Hoyle state
  7. Development of clustering concepts
  8. Combination of cluster and shell aspects
  9. Influence of nucleon structure
  10. Some experiments at low energy
  11. Search for the Hoyle state in the dissociation of light nuclei
  12. $^8$Be and $^9$B in relativistic dissociation of $^9$Be and $^{10}$C
  13. Dissociation of $^{12}$C
  14. Dissociation of $^{16}$O
  15. Dissociation of $^{16}$O on protons
  16. ...and 7 more sections

Figures (47)

  • Figure 1.1: Illustration of the NE resolution. The background is a photograph of the interaction of the $^{32}$S nucleus at 200 GeV per nucleon, superimposed at the same resolution on a 60 $\mu$m thick hair.
  • Figure 1.2: Macro photograph of the dissociation of a $^{24}$Mg nucleus at 3.65 GeV per nucleon in NE into six He nuclei.
  • Figure 1.3: (a) Fragmentation of Mg-Si nucleus group of cosmic origin at several GeV per nucleon 15. (b) Fragmentation of a cosmic origin nucleus $^{14}$N 16. (c) Fragmentation of the cosmic origin nucleus $^{12}$C into a triple of $\alpha$-particles accompanied by a recoil proton 14.
  • Figure 1.4: (a) The event of coherent dissociation of a proton at 10 GeV with the formation of eight relativistic particles 20. (b) Event of complete destruction of a heavy nucleus by a proton at 10 GeV.
  • Figure 1.5: NE exposure at the JINR Synchrophasotron to $^{12}$C nuclei at 3.65 GeV per nucleon (1974): complete destruction (left) and coherent dissociation into the $\alpha$-particle triple.
  • ...and 42 more figures