Deeply bound hole states and isobaric analog states in 89Zr

Abstract

The 90Zr(τ, α) 89Zr reaction, at 39 MeV incident energy, was used to populate hole states up to 13.5 MeV excitation energy in 89Zr. A split-pole spectrometer was used for particle analysis and detection. In the excitation energy range between 0 and 3.5 MeV, about twenty well-resolved states were observed in 89Zr, many for the first time. These states contain almost all of the spectroscopic strengths of the 1g 9 2 , 2p 1 2 , 2p 3 2 and 1f 5 2 neutron Subshells. The 30–35 keV energy resolution of the present study revealed a fine structure in the bump previously observed around 4.5 MeV and tentatively identified as the main part of the 1f 7 2 . hole orbital. About thirty levels or groups of levels were excited between 3.5 and 7.0 MeV excitation energy and angular distributions have been extracted for each individual group as well as for the gross structure itself. A DWBA analysis of the data was carried out and shows that the main part of the observed fine structure is populated by l = 3 angular momentum transfer. The full sum rule strengths obtained for the states below 3.5 MeV for the pick-up of the 1g 9 2 , 2p 1 2 . 2p 3 2 and 1f 5 2 nucleon lead to the identification of this highly fragmented bump with the 1f 7 2 shell. We found between 50 to 70 % of the 1f 7 2 hole strength in this region. Four sharp and strongly excited peaks are observed at higher excitation energy (8–10 MeV) above a continuous background. They are identified with the known isobaric analog states of the first four excited levels in the 89Y parent nucleus. Around 13 MeV a weakly excited peak is observed which has an l = 3 angular distribution. This level is proposed as the IAS of the first component of the 1f 7 2 . proton hole orbital located around 5 MeV in 89Y. In order to compare the spectroscopic strengths of the IAS and of their parent states, the neutron form factor has been calculated by solving the Lane coupled-channel equations. This method is shown to produce an overall agreement between the strengths of the parent analog pairs of levels.