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 [1]. Missing mass and center-of-mass (c.m.) angular distributions of 7 H were reconstructed, as well as the momentum distribution of the 3 H fragment in the 7 H frame. In addition to the investigation reported in Ref.[1], we carried out another experiment with the same beam but a modified setup. In the second experiment the cross-check of the experimental calibration was performed by the studies of the 2 H( 10 Be, 3 He) 9 Li reaction. There is a solid experimental evidence of the population of two resonant states in 7 H at energies of 2.2(5) and 5.5(3) MeV relative to 3 H+4n threshold. Also, there are indications on the resonant states at 7.5(3) and 11.0(3) MeV. Based on the energy and angular distributions the weakly populated 2.2(5) MeV peak is assigned as 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 doublet, built on the 2 + configuration of valence neutrons. The 7.5 MeV state can be a member of this 5/2 + -3/2 + doublet of excited states, which could not be resolved in Ref. [1] and which consequently appeared in [1] as a 6.5 MeV state of 7 H.