First observation of $^{20}$B and $^{21}$B
Résumé
The most neutron-rich boron isotopes B20 and B21 have been observed for the first time following proton removal from N22 and C22 at energies around 230 MeV/nucleon. Both nuclei were found to exist as resonances which were detected through their decay into B19 and one or two neutrons. Two-proton removal from N22 populated a prominent resonancelike structure in B20 at around 2.5 MeV above the one-neutron decay threshold, which is interpreted as arising from the closely spaced 1-,2- ground-state doublet predicted by the shell model. In the case of proton removal from C22, the B19 plus one- and two-neutron channels were consistent with the population of a resonance in B21 2.47±0.19 MeV above the two-neutron decay threshold, which is found to exhibit direct two-neutron decay. The ground-state mass excesses determined for B20,21 are found to be in agreement with mass surface extrapolations derived within the latest atomic-mass evaluations.
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