INVESTIGATION OF VIRTUAL STATE OF 8Be+n SYSTEM USING THE COMPLEX SCALING METHOD

Nuclear states observed around threshold energies provide us with interesting problems associatedwith the nuclear cluster structure. Most of them are also interesting astrophysically from the viewpoint ofnucleosynthesis.The first excited 1/2+ state in 9Be has been observed as a sharp peak above the 8Be+n threshold energy in thephoto-disintegration cross section of γ+9Be->α+α+n. Since the size of the peak has a strong influence on the reactionrate of the 9Be synthesis, new experimental data have been investigated. Recently, we performed calculations usingan α+α+n three-cluster model together with the complex scaling method (CSM), which well reproduces the recentlyobservedphoto-disintegration cross section. The results indicate that the virtual-state character of the 1/2+ state playsan important role in formation of the peak structure appearing in the cross section observed above the 8Be+nthreshold. From these results, we discuss that the first excited 1/2+ state in 9Be is a 8Be+n virtual state but notresonant one. However, the virtual state cannot be directly obtained as an isolated pole solution in the CSM, becausethe scaling angle in the CSM cannot be increased over the position of the virtual state pole on the negative imaginaryaxis of the complex momentum plane.In our previous work, the cross section form of the photodisintegration and the phase shift behavior of a virtualstate assuming a simple two-body model are discussed. In the CSM, the virtual state cannot be obtained as anisolated solution, but the continuum solutions are considered to include the effect of the virtual state. There is noprevious study that the CSM can be applied successfully to virtual state. Applying the CSM to the simple schematicpotential model, we have shown that the sharp peak of the photodisintegration cross section calculated just above thethreshold which does not correspond to a usual Breit-Wigner type pole. A new approach for the CSM to describe thevirtual state was proposed, and we discussed the pole position of the virtual state using the continuum level density(CLD), the scattering phase shift, and scattering length calculated in the CSM. The next problem is how todistinguish a virtual state from a resonant state and how to see the difference in the observed photo-disintegrationcross sections.The purpose of this work is to investigate the reliability of the virtual state solutions in the CSM as comparingwith the solutions of the Jost function method. To investigate the structure of the virtual state, we calculate theenergy eigenvalues, phase shifts and photodisintegration cross section of the two-body model with a two-rangeGaussian potential by changing the strength of the attractive potential.