The ${}^6$H states studied in the ${}^2\text{He}{(}^8\text{He}{,}^4\text{He})$ reaction and evidence of extremely correlated character of the ${}^5$H ground state

Abstract

The extremely neutron-rich system ${}^6$H was studied in the direct ${}^2\text{H}{(}^8\text{He},{}^4\text{He}{)}^6$H transfer reaction with a $26A$ MeV secondary ${}^8$He beam. The measured missing mass spectrum shows a broad bump at $\sim 4-8$ MeV above the ${}^3$H+$3n$ decay threshold. This bump can be interpreted as a broad resonant state in ${}^6$H at $6.8(5)$ MeV. The population cross section of such a presumably $p$-wave state (or may be few overlapping states) in the energy range from 4 to 8 MeV is $d\sigma /d{\Omega }_{\text{c.m.}}\simeq 190_{-80}^{+40}$ $\mu$b/sr in the angular range $5^{\circ }<{\theta }_{\text{c.m.}}<16^{\circ }$. The obtained missing mass spectrum is practically free of the ${}^6$H events below 3.5 MeV ($d\sigma /d{\Omega }_{\text{c.m.}}\lesssim 5$ $\mu$b/sr in the same angular range). The steep rise of the ${}^6$H missing mass spectrum at $\sim 3$ MeV allows to derive the lower limit for the possible resonant-state energy in ${}^6$H to be $4.5(3)$ MeV. According to the paring energy estimates, such a $4.5(3)$ MeV resonance is a realistic candidate for the ${}^6$H ground state (g.s.). The obtained results confirm that the decay mechanism of the ${}^7$H g.s.\ (located at 2.2 MeV above the ${}^3$H+$4n$ threshold) is the "true" (or simultaneous) $4n$ emission. The resonance energy profiles and the momentum distributions of fragments of the sequential ${}^6$H$\to {}^5$H(g.s.)+$n\to {}^3$H+$3n$ decay were analyzed by the theoretically-updated direct four-body-decay and sequential-emission mechanisms. The measured momentum distributions of the ${}^3$H fragments in the ${}^6$H rest frame indicate very strong "dineutron-type" correlations in the ${}^5$H ground state decay.

Comments: 16 pages, 19 figures