High Spin States In Odd-odd Nucleus ¹⁵⁸Tb

The A≈160 rare earth region, which contain a lot of Z=63-73 elements, consist plenty of information of nuclear structure, such as the signature inversion, triaxial deformation, band crossing, band termination and so on. The signature inversion of the πh_11/2 νi_13/2 band in this region has been study a lot, but the πd_5/2 νi_13/2 band did not. In this article, high spin states of ¹⁵⁸Tb in A≈160 mass region has been studied, the aims are to enrich the level structure information of ¹⁵⁸Tb, investigate for the existence of signature inversion in ¹⁵⁸Tb and compare it to the band in ¹⁵⁴Tb and ¹⁵⁶ Tb with same configuration, and summarize the systematics of signature splitting and inversion phenomenon in Tb isotopes.This work is done in China Institute of Atomic Energy（CIAE） in Beijing. Excited states in ¹⁵⁸Tb were populated using the fusion-evaporation reaction 154Sm（7Li, 3n）¹⁵⁸Tb at a beam energy of 27 Me V. A 0.67 mg/cm^{2 154}Sm foil, backed on a 1.49 mg/cm² gold foil, was used as the target. Nine HPGe detectors and two plane HPGe detectors were used for the experiment. About 16 million two folds γ-γ coincidence events were collected during 70 hours beam time. After careful energy calibration and gain matching of each detector, the γ-γ coincidence data were sorted offline into symmetrical and asymmetric matrices. These matrices were analyzed by using the RADWARE software based on a Linux-PC system. For the positive parity rotational band, 10 new levels and 17 new gamma transitions were added to the new level scheme, and the spin was completed up from 7h to 17h; for the negative parity rotational band, 13 new levels and 22 new gamma transitions were found, and the spin was completed up to 13h. The configuration assignments π_5/2 νi_13/2 was made for the positive parity band based on the single particle Nilsson diagram, systematics comparison and the comparison of the theoretical calculation and experimental measurement of B（M1）/B（E2）. The level structure and the alignment of the positive-parity rotational band were reproduced well by CNSB model calculation. The signature inversion is found firstly in the new level scheme. By comparing with the CSM（cranked shell model） calculation, the relationship between the triaxial deformation and signature inversion phenomenon in ¹⁵⁸Tb nucleus was investigated. In addition, the new level structures of ¹⁵⁸Tb can also extend our understanding about systematics of signature splitting and inversion phenomenon in Tb isotopes.