Resonant states in ${}^7$H. I. Experimental studies of the ${}^2$H(${}^8$He,${}^3$He) reaction

Abstract

The extremely neutron-rich system ${}^7$H was studied in the direct ${}^2$H(${}^8$He,${}^3$He)${}^7$H transfer reaction with a 26 AMeV secondary ${}^8$He beam [Bezbakh et al., Phys. Rev. Lett. 124 (2020) 022502]. The missing mass spectrum and center-of-mass (c.m.) angular distributions of ${}^7$H, as well as the momentum distribution of the ${}^3$H fragment in the ${}^7$H frame, were constructed. In addition to the investigation reported in Ref. [Bezbakh et al., Phys. Rev. Lett. 124 (2020) 022502], we carried out another experiment with the same beam but a modified setup, which was cross-checked by the study of the ${}^2$H(${}^{10}$Be,${}^3$He${)}^9$Li reaction. A solid experimental evidence is provided that two resonant states of ${}^7$H are located in its spectrum at 2.2(5) and 5.5(3) MeV relative to the ${}^3$H+4$n$ decay threshold. Also, there are indications that the resonant states at 7.5(3) and 11.0(3) MeV are present in the measured ${}^7$H spectrum. Based on the energy and angular distributions, obtained for the studied ${}^2$H(${}^8$He,${}^3$He)${}^7$H reaction, the weakly populated 2.2(5) MeV peak is ascribed to the ${}^7$H ground state. It is highly plausible that the firmly ascertained 5.5(3) MeV state is the $5/2^{+}$ member of the ${}^7$H excitation $5/2^{+}$-$3/2^{+}$ doublet, built on the $2^{+}$ configuration of valence neutrons. The supposed 7.5 MeV state can be another member of this doublet, which could not be resolved in Ref. [Bezbakh et al., Phys. Rev. Lett. 124 (2020) 022502]. Consequently, the two doublet members appeared in the spectrum of ${}^7$H in [Bezbakh et al., Phys. Rev. Lett. 124 (2020) 022502] as a single broad 6.5 MeV peak.

16 pages, 15 figures