First Observation of ^{20}B and ^{21}B.

The most neutron-rich boron isotopes B-20 and B-21 have been observed for the first time following proton removal from N-22 and C-22 at energies around 230 MeV/nucleon. Both nuclei were found to exist as resonances which were detected through their decay into B-19 and one or two neutrons. Two-proton removal from N-22 populated a prominent resonancelike structure in B-20 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 C-22, the B-19 plus one-and two-neutron channels were consistent with the population of a resonance in B-21 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 B-20,B-21 are found to be in agreement with mass surface extrapolations derived within the latest atomic-mass evaluations.