First excited ${\frac{1}{2}}^{+}$ state in ${}^9$B

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 $\mu$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.)$+\alpha$, ${}^6$Li(2.186 MeV)$+\alpha$, ${}^8$Be$+d$, and $pn\alpha \alpha$, 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 $\Gamma \approx 1.5$ MeV.

Figure 1

Plan view of the detector arrangement relative to the target. The four forward detectors were placed symmetrically about the beam axis at $\pm$17${}^{\circ }$ and $\pm$47${}^{\circ }$, 140.2 mm from the target, whilst the rear detector was placed at 127.5${}^{\circ }$ and 55.2 mm from the target.

Figure 2

(a) Plot of reconstructed ${}^9$B excitation energy for stopped $\alpha \alpha p$ events. In comparison to the upper solid (black) line, the middle (red) line corresponds to stopped $p$ data with additional requirements on ${\theta }_p$ as described in the text; there is a 57$\%$ reduction in number of counts but efficiency becomes much less dependent on excitation energy. The lower dashed (blue) line additionally includes subtraction of the background beneath the relevant peak in the $Q$-value spectrum (b).

Figure 3

Data from ${}^6$Li(${}^6$Li,$d$)${}^{10}$B$\to pn\alpha \alpha$: ${}^{10}$B, ${}^9$Be, and ${}^9$B obtained for data from the reaction ${}^6$Li(${}^6$Li,$d$)${}^{10}$B$\to pn\alpha \alpha$ for the stopped proton data [spectra (a), (b), and (c)] and the punched through proton data [spectra (d), (e), and (f)].

Figure 4

Final ${}^9$B experimental spectra (black/lower) overlaid with the calculated efficiencies (red/upper) as described in the text: (a) and (c) data from the ${}^6$Li(${}^6$Li,$t$)${}^9$B reaction; (b) and (d) data from the ${}^6$Li(${}^6$Li,$d$)${}^{10}$B reaction. The protons either stopped in the strip detector [(a) and (b)] or punched through to the CsI [(c) and (d)].

Figure 5

Excitation energy spectrum for ${}^9$B from the ${}^6$Li(${}^6$Li,$d$)${}^{10}$B reaction, showing the stopped and punched through $p$ data after correction for detection efficiency.

Figure 6

Normalized and efficiency corrected ${}^9$B excitation energy spectrum from the stopped $p$ data. The vertical axis shows relative counts per 100 keV.