Study of the nuclear structure of 128Xe by means of the (α,n) reaction at threshold

With the reaction 125Te(α,n)128Xe near the Coulomb threshold 81 excited nuclear levels far from the Yrast line were populated. The determination of the level-specific quantities (J,π,ELevel) was performed via following measuring methods: γ-excitation functions between 12.5 and 20 MeV beam energy; γ-angular distribution at 14.5 MeV α energy; γ-γ coincidence at 14.5 and 15.5 MeV projectile energy; conversion electron measurement at 16.5 MeV with transmission range between 400 und 1500 keV electron energy. 40 of the 81 populated states were not yet known. All levels hitherto only known from beta-decay data below 2.5 MeV were detected in the beam experiment. The ground-state band and the quasi-γ band could be populated up to the 10+ respectively 7+ and the level parameters verified. The non-selective character of the (α,n) reaction permitted to apply the direct population of a level (side feeding) to the spin determination. This was extensively discussed and an empirical fit formula given for the behaviour of the direct feeding as function of the beam energy, the level spin, and the level energy. In a comparison with the O(6) limit of the IBA-1 the collective states of the ground-state and the quasi-γ band were described. Furthermore the so-called τ=4 multiplet could be completely identified. (HSI)