Structure of A~130 nuclei in La--Ce region

TUMPA BHATTACHARJEE

Variable Energy Cyclotron Centre, 1/AF, Bidhan Nagar, Kolkata 700 064, India

E-mail: btumpa@veccal.erent.in

Abstract. The variety of shapes and structures, observed in light

rare earth $A$\,$\sim$\,130 nuclei, have been discussed in view of

different angular momentum coupling schemes and their interplay that

comes into effect at high spin. The $N = 79$ and 80 isotopes in La--Ce

region, produced via fusion evaporation reaction, have been studied

using the Indian National Gamma Array (INGA) consisting of 18

clover HPGe detectors. Two nearly degenerate $\Delta I = 1$ bands

have been observed at high spin of $^{137}$Ce and a triaxial

deformation of $\gamma = \pm 30^\circ$ has been assigned to the

bands, from the total Routhian surface (TRS) calculations. The

high-spin candidates of the yrast band of $^{138}$Ce show signature

splitting both in energy and $B(M1)/B(E2)$ values. The bandcrossing due

to the alignment of a pair of $h_{11/2}$ proton particles

has been conjectured at $\hbar\omega$ $\sim$ 0.3 MeV, from the

single-particle Routhians obtained from TRS calculations. Lifetime

measurements by Doppler shift attenuation method (DSAM) has been

carried out and from the estimated reduced transition probability

$B(M1)$, the $\Delta I = 1$ band in $^{138}$Ce has been characterized

as a magnetic rotation (MR) band. The rise in the values of $B(M1)$,

for the higher spin candidates of the band, has been conjectured as

the reopening of a different shear at the top of the Band B1. The

characteristic of the MR bands in $A \sim 130$ region has been

discussed in the light of a phenomenological calculation and

compared to the MR bands in other mass regions.

Keywords. Clover detector array; measured $E_\gamma$, $I_\gamma$,

DCO (directional correlation of oriented states), polarization;

lifetime measurement with Dopper shift attenuation method; reduced

transition probability $B(M1)$; shell model; Tilted Axis Cranking

calculation; total Routhian surface calculations; backbending;

magnetic rotation bands.

PACS Nos 21.10.-k; 23.20.-g; 27.60.+j