Experimental evidence for subshell closure in $^{8}$He and indication of a resonant state in $^{7}$He below 1 MeV
Abstract
The spectroscopy of the unstable $^{8}$He and unbound $^{7}$He nuclei is investigated via the p($^{8}$He, d) transfer reaction with a 15.7A MeV $^{8}$He beam from the SPIRAL facility. The emitted deuterons were detected by the telescope array MUST. The results are analyzed within the coupled-channels Born approximation framework, and a spectroscopic factor $C^2$S=4.4±1.3 for neutron pickup to the $^{7}$He_g.s.$ is deduced. This value is consistent with a full p3/2 subshell for $^{8}$He. Tentative evidence for the first excited state of $^{7}$He is found at $E^*$=0.9±0.5 MeV (width $\Gamma$=1.0±0.9 MeV). The second one is observed at a position compatible with previous measurements, $E^*$=2.9±0.1 MeV. Both are in agreement with previous separate measurements. The reproduction of the first excited state below 1 MeV would be a challenge for the most sophisticated nuclear theories.
Domains
Nuclear Experiment [nucl-ex]
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