Properties of 15 2^- states in ²¹⁵Ra and ²¹⁷Th; evaluation of the 15 2^- to 9 2⁺ E3 strength in N = 127 isotones

Abstract

The lifetime of the yrast 15 2- state in 215Ra was measured using pulsed beams and γ-ray and electron techniques. Transition multipolarities were established from measured conversion coefficients. The B(E3) of the 15 2-→ 9 2+ transition is found to be considerably larger than previously reported. A candidate for the corresponding transition in 217Th was also observed. The E3 strength of the 15 2-→ 9 2+ transition in the N = 127 isotones is evaluated in the light of these and other recent results. Interpretation in the framework of particle-octupole vibration coupling requires a systematic lowering of the core 3- vibration as proton pairs are added to 208Pb.