Evidence of the ⁹B(1/2⁺) state in the ¹²C(p,α)⁹B reaction

The low-lying excitation region in ⁹B has been studied through the ¹²C(p,α)⁹B reaction with a proton beam energy of 33 MeV at the Birmingham MC40 Cyclotron. Coincident recoil 𝛼-particle and ⁹B breakup products were detected by a silicon double-sided strip detector array and reconstructed to identify signatures of the elusive ⁹B(1/2⁺)  state. Data were collected for two distinct 𝛼-particle recoil angles, 31^∘ and 106^∘, with a total beam exposure of ≈ 0.52 mC. 𝑅-matrix analysis indicates the presence of a state at 𝐸_x=1.83±0.05 MeV with a width of Γ=850±¹¹⁰₈₈ keV with a confidence level of 99.1% and no signature of any lower-lying state. This result is shown to be consistent with four previous measurements over the last 25 years, signifying strong evidence that the first excited state in ⁹B lies at a weighted average excitation of 𝐸x=1.84±0.04 MeV with Γ=740±⁷⁷₆₆ keV.