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

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.