Gamma-ray spectroscopic studies of 155Ho and 156Er

• Main Author: Rees, Joseph
• Other Authors: Paul, E. S.
• Published: University of Liverpool 2014
• Subjects: 530
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.617509

Excited states in 155Ho and 156Er extending to high spin have been populated as part of two experiments performed at the atlas facility of Argonne National Laboratory. In both experiments high-fold gamma-ray coincidence data were recorded in a high-statistics experiment with the Gammasphere spectrometer. In 156Er, weakly populated band structures have been established at low to medium spins, following the 114Cd(48Ca,6n) reaction at 215 MeV. Bands built on the second 0+ state and 2+ (gamma-vibrational) states have been established. A large staggering between the even- and odd-spin members of the gamma-vibrational band suggests a gamma-soft nature of this nucleus. An additional band is discussed as being based on a rotationally aligned �h9/2/f7/2 structure, coexisting with the systematically observed, more favourable (�i13/2)2 alignment seen in this mass region. At high spin, new transitions above the 42+ terminating state have been observed, establishing the presence of a further excited state, which is interpreted as the predicted terminating 46+ state built on a �particle-hole excitation (d5/2/g7/2 to d3/2) across the Z = 64 shell gap. New states in 155Ho have been observed following the 124Sn(37Cl,6n) reaction at 180 MeV. At low spin, a positive-parity �g7/2 rotational band, observed for the first time in this nucleus, has been established up to (33/2+). The �h11/2 band has been extended to higher spins, and the large signature splitting it exhibits interpreted as evidence of triaxiality at low spin. In addition, a positive-parity rotational structure has been extended to I� = 57/2− and discussed as being based on a �(i13/2h9/2) structure coupled to the h11/2 proton. B(M1)/B(E2) ratios of reduced transition probabilities have been calculated from observed branching ratios for all three bands and compared with theoretical calculations for specific particle configurations. Above spin � 59/2 the behaviour of the nucleus is discussed in the context of band termination. An energetically favoured state at I� = 79/2− has been established, which corresponds to valence-space band termination in 155Ho. Weak, high-energy core-breaking excitations feeding this state have been observed. New states with I� = (87/2+), (89/2+) are thought to be further predicted favoured terminating states at oblate shape, built on �(d5/2/g7/2 ! h11/2) particle-hole excitations across the Z = 64 shell gap. The newly observed high-spin states are compared to Cranked Nilsson Strutinsky calculations, and presented in the context of known band terminating states in the region.