Direct measurement of Enhanced octupole collectivity in 148Dy

Excited states in ¹⁴⁸~66Dy were populated via β⁺/EC decay of ¹⁴⁸ᵐHo using the GRIFFIN spectrometer at the TRIUMF ISAC-I facility. A combined measurement of the mean lifetime of the 3₁⁻ level using the Generalized Centroid Difference (GCD) method and branching fraction of the 3₁⁻→0₁⁺ $γ$-ray decay has been performed. From these results, an enhanced electric octupole B(E3;3₁⁻→0₁⁺) transition strength of 46(3)~W.u. has been determined in ¹⁴⁸~66Dy. This is the largest measured value across the closed neutron shell at $N=82$ and provides direct evidence of enhanced octupole collectivity beyond $Z=64$. The evolution of the B(E3; 3⁻₁ → 0⁺₁) strength along the $N=82$ isotonic chain is compared with quasiparticle random-phase approximation (QRPA) calculations using the SkI3 and SkM^* Skyrme energy-density functionals, as well as with large-scale shell-model (SM) calculations. This result extends the boundaries of enhanced octupole collectivity far from the so-called `octupole magic numbers' $Z=56$ and $N=88$.