High-Spin States in Neutron-Rich Tantalum Isotopes

Abstract

Deep-inelastic reactions between 6.2 MeV/A Xe-136 ions and a thick W-186 target have been used to investigate excited states in neutron-rich tantalum isotopes, establishing new structures in Ta-182 and Ta-183. Spins and parities were determined from angular correlations, internal conversion coefficients deduced from intensity balances, and consideration of transition strengths. The structure and configuration assignments have been established from the measured magnetic properties and alignments of the observed rotational bands, as well as comparison with multi- quasiparticle calculations. Multi-quasi particle calculations using adjusted Nilsson energies and Lipkin-Nogami pairing model were performed for the series of tantalum nuclei between A=181 and A=185.The previously known 1300 ns isomer in Ta-183 was fully characterised with its energy deduced as 1332 keV and a K = 19/2+ assignment. Several new multi-quasiparticle states, including two isomers with 41(5) ns mean-life, K = 29/2- and 70(4) ns (mean-life), K = 41/2-, have been further identified and found to be feeding the known 1300 ns isomer. From the multi-quasiparticle calculations, a three-quasiparticle configuration was assigned to both the K = 19/2+ and 29/2- isomers, while the upper-most isomer at 41/2- was established to have a five-quasiparticle nature.The out-of-beam gamma-ray coincidence data revealed a new level structure in coincidence with tantalum X-rays. Measurements of the magnetic properties of this structure from branching ratios, together with the yields from various data sets using W-186, Re-185, Re-187 and Os-192 targets, enabled an isotopic assignment of this structure to Ta-182. Three isomeric states were found to be feeding the known 10-> isomer in Ta-182. A rotational band built on the 10- isomer has been > observed and was found to be fed by a new 14+, 356 ns (mean-life) > isomer at 1950 keV. Several intrinsic states above this isomer are > also identified. Firm spins and parities have been established up to > the 14+ isomeric state. Due to limited spectroscopic information, the > spins and parities for the levels above this isomer are tentative.The structure of Ta-182 and Ta-183 are also investigated by comparing the measured transition strengths with similar transitions in the mass region. Most of the transition strengths measured are in good agreement with expectations in the region, except for a few cases of E1 decays that are anomalous. Possible explanations for some of these anomalous transition strengths have been discussed.Few cases has been observed where the predicted yrast states are not observed experimental in both Ta-182 and Ta-183. Possible explanations have been discussed as well.Both the calculations and experimental results suggest a high density of intrinsic states occurs along the yrast line at high excitation energy in both nuclei. This pattern seems to emerge in the more neutron-rich tantalums isotopes resulting in a more complex level scheme that is difficult to interpret.