Theoretical Investigation Of The High-Spin States In ^235,237Np

The exploration of the limit of nuclear mass and charge is one of the hotspots in physics.In the 1960s,nuclear physicists guessed that there might be stable or long-lived superheavy nuclei due to the quantum shell effects,and predicted that the island of superheavy nuclei would be located near Z=1 14 and N=184.The predicted results of different theoretical models are different,which is closely related to the single-particle level structure of the nucleus.Due to the low production cross section of the superheavy nuclei,it is difficult to synthesize them directly through experiments.Therefore,there is no enough experimental data to study their spectroscopic properties,and it is hard to get the single-particle structure close to the Fermi surface.Due to the deformation effects,the strongly downsloping orbitals originating from the spherical subshells and active in the vicinity of the superheavy island may come close to the Fermi surface in these deformed nuclei.We can obtain some information about superheavy nuclei by studying some lighter deformed nuclei(such as actinide nuclei).The actinides are the heaviest region in which the high-spin states can be populated,in which a series of rotational bands have been observed.In particularly,the first high-spin rotational band of the superheavy nuclei has been observed in 256 Rf in 2012.The rotational properties of the actinides are quite important for revealing the alignment mechanism,pairing correlations,level structure,etc.,of the nuclei in A≈250 mass region.On the one hand,investigation of the high-spin structures of these nuclei can provide benchmark for testing various theoretical models.On the other hand,it can help us to get more information about the superheavy nuclei.In this work,the particle-number conserving method based on the cranked shell model is adopted to investigate the rotational bands observed in 235Np and 237 Np.The moments of inertia,alignments,etc.,are reproduced by the calculations.Firstly,the ab formula which is used to investigate the rotational spectra,is adopted to determine the bandhead spin of the rotational bands observed in 235Np.Secondly,by comparing the experimental and calculated moments of inertia,the configuration for this signature partner band is assigned as π5/2-[523].Moreover,the influence of higher-order deformation ε6 on the delayed alignments of neutron j15/2 is discussed.The appearance of the neutron j15/2 alignment in the calculations,which is non-existent in experiment,is explored.The upbending mechanism in 235.237Np is investigated.Finally,the possible reason for the signature splitting of the rotational band π5/2-[523]in 237 Np is explored,which may be due to the different higher-order deformation ε6 in these two signature branches after upbending.