Observation of the νh11/2 sequence in the 97 Mo nucleus

The knowledge of the high-spin states in the A≅100 nuclei and especially in the Mo isotopes is more fragmentary than in many other nuclear regions. This report presents a first evidence of high spin states in the 97 Mo nucleus. The study has been performed with the reaction 82 Se(19 F,p3nγ) at 68 MeV. The 19 F8+ beam was delivered by the FN Tandem accelerator in Bucharest. The experimental setup consisted of two intrinsic Ge detectors of 20% efficiency, a 1 litre NE213 scintillator detector placed in the forward direction for the detection of the neutrons, and one ΔE-E Si detector telescope for the identification of the charged particles. Details of the measurement geometries are specified in a recent work. The level scheme evidenced in the experiment is shown. The main result is the observation of the h11/2 quasi-band structure up to the 31/2- state at Ex=5.5 MeV. In the positive-parity structure only the highest two transitions of 919 keV and 742 keV have been added. The level scheme has been constructed on the basis of coincidence relationships observed in a symmetric γ - γ matrix, and intensities observed in γ- and charged particle-gated spectra. Transition multipolarities have been determined on the basis of γ-ray angular distributions and DCO ratios determined from an asymmetric γ - γ matrix, with the two Ge detectors set at 90 angle and 45 angle, respectively. Presently, 97 Mo is the only Mo isotope with N > 50 in which the νh11/2 structure is known at spins higher than 19/2. We discussed the evolution of this structure in parallel to that of the even-even isotopes and have shown that it follows the sudden onset of deformation at N = 50. Both the positive- and the negative-parity states have been compared to the Interacting Boson-Fermion Model-1 (IBFM-1). This comparison is shown only for the high-spin structures (in the positive parity case, a large number of low-spin levels and their decays are explained too). One should emphasize that in these calculations the same Hamiltonian (same parameters of the boson-fermion interaction) has been used for calculation of both parities, and the calculations provide also the correct excitation energy of the 11/21+ state. The sequence of spins 13/2-, 17/2-,... is predicted unfavored in energy, and this explains why it could not be observed in the experiment. The structure above the 5/2+ state is dominated by the d5/2 orbital (coupled to the yrast core states) while the one above the 7/2+ state is dominated by the g7/2 orbital. At spins above 15/2, there is a change in the structure of the positive parity states, probably due to 3-qp states. (authors)