Abrupt phase change of the core rotation in the 143 Sm nucleus

TL;DR

This study investigates the abrupt phase change in core rotation in 143 Sm, revealing a transition from magnetic rotation to core rotation as the main angular momentum source, supported by experimental data and theoretical modeling.

Contribution

It provides the first evidence of a sudden phase change in core deformation causing forking of the shears band in 143 Sm.

Findings

Core rotation dominates angular momentum at high spins.

Sharp rise in B(M1)/B(E2) ratio indicates phase change.

For the first time, forking of a shears band linked to core deformation.

Abstract

Dipole sequences in the 143 Sm nucleus have been investigated via the 124 Sn ( 24 Mg, 5n) reaction at E lab = 107 MeV using the Indian National Gamma Array (INGA). The spin-parity of the associated levels have been firmly established from the spectroscopic measurement. Level lifetimes of several levels in the dipole bands have been measured using the Doppler Shift Attenuation Method. The decreasing trend of the measured B(M1) and B(E2) transition strengths in one of the sequence (DB I) spells out its origin as Magnetic Rotation (MR). The trends of B(M1) and B(E2) in DB I are reproduced well in the theoretical calculations using the Shears mechanism with the Principal Axis Cranking (SPAC) model. However, the calculations fail to reproduce the sharp rise in the B(M1)/B(E2) ratio at the highest spins in DB I and the same has been interpreted from the decreasing of the core rotation along the sequence. The experimental observations along with the the theoretical calculations for the second dipole band (DB II), indicate that the core rotation, rather than the shears mechanism, is being favored for angular momentum generation. This represents a unique observation of forking of the shears band DB I from an abrupt phase change of the core from spherical into the deformed one.