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

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.