Postscript of the ACCULINNA-2 2018-2020 experimental campaign

During the first ACCULINNA-2 experimental campaign, two experiments were conducted to search for ⁷H in the ²H(⁸He,³Не)⁷H reaction. The key result was data on ⁷H and ⁶H – superheavy hydrogens – of exceptionally high quality (head-&-shoulders) compared to anything previously obtained. High universality of the detection system allowed for simultaneous acquisition of data on ⁹Li, ⁸Li (were used for calibrations), ⁵Н, ⁵He, and ⁷He (were used for data consistency checking).

In 2022, RIKEN yielded its first highly reliable result on the existence of a low-energy correlation in the spectrum of the 4n tetraneutron. Data analysis showed that similar low-energy correlation in tetraneutron spectrum is present in the authors’ data in two channels at once – ²H(⁸He,⁶Li)4n and ²H(⁸He,⁶Li*(18))4n, – which leaves little doubt about the reality of this observation.

Theoretical study of the obtained results demonstrated their interconnectedness. The lowest-energy structures (E<5 MeV) in ⁷H and 4n spectra may not be not the resonance states of these systems, but a reflection of certain spatially extended structures of the wave function of the initial ⁸He halo-nucleus (effects of reaction mechanism and initial structure).

From an experimental point of view, it was clear that a significant portion of issues in interpreting ⁷H, ⁶H, and 4n data is related to the choice of the kinematics of ²H(⁸He,³Не)⁷H reaction – registration of fast tritium and slow 3He. This problem can be solved in an improved experiment by modifying the charged particle telescopes to extend their dynamic range. The “luminosity” of such an experiment would increase 5-10 times with the same current integral as in work. Thus, ²H(⁸He,³Не)⁷H reaction in a new experimental formulation remains the best choice for an advanced understanding of the complex of issues surrounding ⁷H, ⁶H, and 4n.