Investigation of the elusive 1/2+ state in 9B

The existence and excitation energy of the 9B1⁄2+ state has long been contested. The state exists in 9Be but appears missing in the mirror nucleus 9B, although there are several published (inconclusive) claims. Different theoretical approaches (single-particle potential, R-matrix and microscopic cluster models) have produced a range of excitation energies from 0.9 MeV to 1.8 MeV but agree a width (1-2MeV). States in 9B are unbound, and most are broad and overlapping creating difficult experimental conditions. The most convincing evidence for this state comes from a study of 6Li(6Li,t)9B, performed at Florida State University by the CHARISSA collaboration, published in 1995. The experiment suffered from poor statistics but indicated new structure in 9B. In 2001 new results were reported that highlighted the need for re-analysis of the FSU data. However, this would be severely limited by the poor statistics. Thus the CHARISSA collaboration repeated the 6Li(6Li,t)9B reaction in 2003 at the Australian National University, which offered greater detection efficiency and a data acquisition system better equipped to deal with many channels and high trigger rates. A 60 MeV 6Li3+ beam was impinged on a 240 μg cm-2 6LiF target. The breakup fragments from the decay of the resonant nuclei were detected in six ΔE-E telescopes, consisting of three stages: Si quadrants (70μm), Si strip (500μm), and Csl (1cm). The breakup particles were reconstructed using the technique of Resonant Particle Spectroscopy. This experiment conclusively showed that the 6Li(6Li,t)9B reaction does not populate the 9B 1/2 + state. However, the 6Li(6Li,d)10B reaction was also reconstructed in this analysis and showed 6Li(g.s.)+ɑ, 6Li(2.186 MeV)+ɑ, 8Be+d, and pnɑɑ (9B+n or 9Be+p) decay from 10B. Whilst the a decay channels were found to be most intensely populated, 9B spectra were obtained and showed the presence of the 1⁄2+ state with a broad asymmetric peak around 0.8-1.0MeV (Γ ≈ 1.5 MeV).