First excited12+state in9B

Abstract

Evidence has been found for a broad unbound state near 1 MeV excitation in ${}^{9}$B that is a candidate for the long-disputed ${\frac{1}{2}}^{+}$ mirror of the unbound 1.68 MeV state in ${}^{9}$Be. Reactions of ${}^{6}$Li${+}^{6}$Li were studied with a 60 MeV beam incident on a 240 $\ensuremath{\mu}$g/cm${}^{2}$ ${}^{6}$LiF target. The breakup fragments from the decay of the reaction products were detected in five Si-Si-CsI telescope detectors and the breakup particles were reconstructed using the technique of resonant particle spectroscopy. It is shown that contrary to a previous study, the ${\frac{1}{2}}^{+}$ is not populated in the reaction ${}^{6}$Li(${}^{6}$Li,$t$)${}^{9}$B, but that it is populated via the ${}^{6}$Li(${}^{6}$Li,$d$)${}^{10}$B reaction. The sequential decays of ${}^{10}$B populated the channels ${}^{6}$Li(g.s.)$+\ensuremath{\alpha}$, ${}^{6}$Li(2.186 MeV)$+\ensuremath{\alpha}$, ${}^{8}$Be$+d$, and $pn\ensuremath{\alpha}\ensuremath{\alpha}$, with the latter including (${}^{9}$B$+n$ or ${}^{9}$Be$+p$) decay from ${}^{10}$B. Decays through ${}^{9}$B are identified and show the presence of the ${\frac{1}{2}}^{+}$ state as a broad asymmetric peak around 0.8--1.0 MeV with $\ensuremath{\Gamma}\ensuremath{\approx}1.5$ MeV.