Octupole excitations in 141,144Cs and the pronounced decrease of dipole moments with neutron number in odd-Z neutron-rich 141,143,144Cs

Abstract The level scheme of odd- Z neutron-rich 141 Cs ( Z = 55 , N = 86 ) was extended and expanded and that of 144 Cs ( N = 89 ) was identified for the first time by means of γ – γ – γ coincidence measurements of prompt γ rays in the spontaneous fission of 252 Cf with Gammasphere. Spin/parity was assigned to the levels based on angular correlations and level systematics in 141,143 Cs. Parity doublets characteristic of both simplex quantum number s = + i and s = − i were proposed in 141 Cs. The tests by using rotational frequency ratio ω − ( I ) / ω + ( I ) imply octupole vibrations in 141 Cs and 143 Cs. B ( E 1 ) / B ( E 2 ) values and electric dipole moments D 0 were calculated for 141 Cs, and re-determined for 143 Cs. It was found that B ( E 1 ) / B ( E 2 ) values of 141 Cs are simplex-dependent and the average value is one order of magnitude larger than that of 143 Cs, and the deduced dipole moment D 0 of 141 Cs is considerably larger than that of 143 Cs, and comparable to the N = 86 isotone 142 Ba. For 144 Cs the yrast sequence looks like a well-deformed rotational band, but no octupole band structure was identified in this nucleus. The overall variations of D 0 in 141,143,144 Cs exhibit a pronounced drop of dipole moment with increasing neutron number in this odd- Z isotopic chain, which may be analogous in nature to the quenching of D 0 observed in even–even 146 Ba ( Z = 56 , N = 90 ) and 224 Ra ( Z = 88 , N = 136 ), and to the drop of D 0 in the odd- Z neutron-rich 147 La ( Z = 57 , N = 90 ) reported by our collaboration.