Identification of multi-phonon γ -vibrational bands in odd- Z 105 Nb

Background: The odd-Z Nb-105 nucleus is located in the A = 100 neutron-rich region. The study of multi-phonon vibrational band structures is important for understanding nuclear structure in this region. Purpose: To search for multi-phonon gamma-vibrational bands in Nb-105. Methods: The high spin states of Nb-105 have been studied by measuring the prompt gamma rays emitted in the spontaneous fission of Cf-252. The data analysis is carried out using triple-and four-fold gamma coincidence methods. Results: A new level scheme of Nb-105 is established. The yrast band has been confirmed, and three new collective bands have been identified. Compared with previous results, a total of 14 new levels and 36 new gamma transitions are observed. Two bands built on 625.9 keV and 1231.9 keV levels are proposed as one-phonon- and two-phonon gamma-vibrational bands, respectively. The evidences for supporting assignments of the multi-phonon gamma-vibrational bands have been discussed. Triaxial projected shell model calculations for the gamma-vibrational band structures are found in good agreement with the experimental data, thus further supporting the gamma-vibrational interpretations for the experimental results in Nb-105. Conclusions: The one-phonon- and two-phonon gamma-vibrational bands have been identified in Nb-105. Our results provide new data to systematically understand the characteristics of the multi-phonon gamma-vibrational bands in the A = 100 neutron-rich region.