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

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.
7 pages, 4 figures