Level structure of 172Yb from the 171Yb(n, γ) reaction

Abstract

The energy-level structure of the nucleus 172Yb has been studied using the 171Yb(n,γ) reaction with both thermal the 2 keV neutrons. Gamma-ray energy and intensity measurements were made using Ge(Li) detectors. From these data, a neutron separation energy of 8020.1±0.5 keV has been determined for 172Yb. In addition, a level scheme is proposed for 172Yb. Where possible, these states have been grouped into rotational bands. A total of fourteen such rotational bands with band-head energies below 2.2 MeV are contained in the proposed level scheme. Because of the averaging feature of the 2 keV neutron-capture data, it is believed that all the positive-parity states with I = 0, 1 and 2 with energies below 2.3 MeV in 172Yb have been identified in this study. Other noteworthy features of the level scheme are the following. Octupole-vibrational bands with K π =0 −,1 −, 2 − and are 3 − are observed, with band-head energies of 1599, 1154, 1757 and 2030 keV, respectively. Four excited K π = 0 + bands, with band-head energies of 1042, 1404, 1794 and 1894 keV, have been located; and it is proposed that these constitute all the excited 0 + bands below 2.3 MeV in 172Yb. Two bands having K π = 1 + are established, with band-head energies of 2010 and 2194 keV. A Coriolis-mixing analysis has been made of the energy-level structure of the K π = 0 −−3 − octupole bands, and good fits to the observed level energies have been obtained. The E1 γ-ray branching to the ground-state band of the I π = 1 − and 3 − members of the K π = 1 − band is observed to deviate markedly from the Alaga-rule predictions. These branching ratios can be accounted for quite well in this analysis assuming a value of 0.028±0.001 for Z, the ratio of the intrinsic E1 matrix element with | ΔK| = 1 to that with ΔK = 0.