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Description
Ensembles of He and H isotopes can be studied with unique completeness and resolution in nuclear emulsion layers longitudinally exposed to relativistic nuclei [1,2]. Determination of the invariant mass of their pairs or triplets by emission angles in the velocity conservation approximation is sufficient to identify a number of unstable states – $^8$Be(0$^+$), $^8$Be(2$^+$), $^9$B, $^{12}$C(0$^+_2$), $^{12}$C(3$^-$), $^6$Be.
The BECQUEREL experiment [3,4], using this approach, is aimed at searching for the α-particle Bose-Einstein condensate (αBEC), an unstable of S-wave α-particle state. $^8$Be(0$^+$) is associated with 2αBEC, and $^{12}$C(0$^+_2$) or the Hoyle state with 3αBEC. In the relativistic fragmentation of heavy nuclei, an enhancement of $^8$Be, $^9$B and $^{12}$C(0$^+_2$) is detected, suggesting their synthesis in the fusion of associated α-particles. The focus of the search is the 4αBEC state of $^{16}$O(0$^+_6$) at 660 keV above the 4α threshold, decaying into α$^{12}$C(0$^+_2$) or 2$^8$Be. In this context, the status of the analysis of α-particle fragmentation in a nuclear emulsion exposed to $^{84}$Kr nuclei at 950 MeV per nucleon is presented. Secondary stars produced by relativistic neutrons are observed in the nucleus fragmentation cone [4]. The neutron average energy in the parent nucleus system is estimated to be 1.3 MeV [5].
The αBEC search leads to the study of nuclear matter in the region of temperature and density from red giants to supernova. It is characterized by the ratios of $^{1,2,3}$H and $^{3,4}$He. Nuclear emulsion layers exposed to heavy nuclei of several GeV per nucleon at the NICA accelerator complex are optimal for identifying H and He isotopes by multiple scattering, searching for unstable states, and assessing neutron accompaniment. An exposure to $^{124}$Xe nuclei of 3.8 GeV per nucleon, performed at the NICA/Nuclotron accelerator complex, allows the use of proven approaches. Parameters of the beam are determined using the CR-39 track detector by direct crater counting on the Olympus BX63 microscope.
[1] P.I. Zarubin, Lect. Notes in Phys. 875, Clusters in Nuclei, Volume 3. Springer Int. Publ., 51 (2013); arXiv: 1309.4881.
[2] D.A. Artemenkov et al., Eur. Phys. J. A 56 250 (2020); arXiv: 2004.10277.
[3] A.A. Zaitsev et al., Phys. Lett. B 820 136460 (2021); arXiv: 2102.09541.
[4] D.A. Artemenkov et al., Phys. At. Nucl. 85, 528 (2022); arXiv: 2206.096.
[5] A.A. Zaitsev et al., Phys. At. Nucl. 86, 1101 (2023); arXiv: 2307.16465.
