Abstract
Dipole sequences in the 143Sm nucleus have been investigated via the 124Sn (24Mg, 5n) reaction at Elab = 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). Though, 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, the calculations fail to reproduce the sharp rise in the B(M1)/B(E2) ratio at the highest spins in DB I. Such rise in the B(M1)/B(E2) values is in well agreement with the theoretical calculations within the shears mechanism as prescribed by the Clark and Macchiavelli considering the smooth decrease of the core rotation along the sequence. The experimental observations along with 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.
Highlights
Dipole sequences in the 143Sm nucleus have been investigated via the 124Sn (24Mg, 5n) reaction at Elab = 107 MeV using the Indian National Gamma Array (INGA)
In the spherical or weakly deformed nuclei in the vicinity of the shell closures, the current distribution of few nucleons outside the core crucially contribute in the generation of angular momentum and several exotic phenomena have been observed in such systems [4]
Though the nuclei in the vicinity of the shell closures do not have a stable deformation in the ground state, the excited multi-quasiparticle configurations involving high-j deformation driving orbitals, associated with Magnetic Rotation (MR) bands, can extend a polarizing effect on the core leading to a weak deformation of the latter, manifested in the weak cross-over E2 transitions in these bands
Summary
In the previous study [13] the spin-parity assignments of the associated levels were tentative owing to the large uncertainities or impossibilities in the RDCO and polarization asymmetry measurements of the transitions, such as 126 keV, leading to the dipole band. The RDCO, polarization asymmetry and multipole mixing ratio of the first four member transitions of this band, 239.5, 339.6, 444.8 and 578.2 keV, determined in the present study establish their predominantly M1 nature in compliance with the earlier assignments and confirming the spin-parity of the sequence upto 51/2−.
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