The first excited 1/2+ state in 9Be and 9B

Abstract

Nuclear states observed around threshold energies provide us with interesting problems associated with
the nuclear cluster structure [1, 2, 3, 4]. The first excited Jπ = 1/2+ state of ⁹Be [5], which is an α+ α + n
Borromean nucleus, is one of the typical examples in light nuclei. This state of ⁹Be has been observed as a
sharp peak above the ⁸Be + n threshold energy in the photo-disintegration cross section of γ+⁹Be→ α + α
+ n [6, 7]. The strength of the peak has a strong influence on the reaction rate of the ⁹Be synthesis. We
performed the calculations using an α + α + n three-body model [8, 9] and the complex scaling method
(CSM), which well reproduces the observed photo-disintegration cross section. However, the result
indicates that the 1/2+ state shows the s-wave virtual-state character of ⁸Be+n. In addition to this problem,
we discuss a mirror state problem of the first excite 1/2+ state in ⁹B.