This paper reports that two low-lying “octupole” states in the nucleus 154Gd have much weaker electric dipole links than expected. The authors measured how long the negative-parity states labeled 1− (at 1414 keV) and 2− (at 1398 keV) live before they decay by emitting γ rays. From those lifetimes they extracted the strengths of the electric dipole (E1) transitions and found them to be strongly hindered when compared with some other transitions.

The experiment used a γ–γ fast-timing method with the VENTURE detector array at the VECC laboratory in Kolkata. The states of interest were fed by the β decay of 154Tb, which the team produced in proton-induced reactions at the K130 cyclotron. By recording pairs of γ rays and measuring their time differences, the researchers obtained lifetimes for the two negative-parity levels and then converted those lifetimes into absolute B(E1) values. B(E1) is the standard number that describes how likely an electric dipole transition is.

The measured B(E1) values were compared with data from neighboring gadolinium isotopes and with theoretical calculations that combine a microscopic mean-field method (Gogny Hartree–Fock–Bogoliubov, HFB) and a collective model called the s,d,f interacting-boson model (sdf-IBM). The comparisons show that the E1 strengths linking these octupole states are much smaller than the strengths seen in transitions with no change in the quantum number K (ΔK=0). Here ΔK denotes the change in the projection K of the nucleus’s angular momentum on its symmetry axis; a ΔK=1 transition changes that projection by one unit.

Why this matters: the size of E1 transition strengths is a direct clue about the shape and internal motion of the nucleus. Finding strongly hindered ΔK=1 dipole strength in 154Gd means that the ways these octupole states connect to other states are weaker than models or comparisons might otherwise suggest. That has implications for how well theoretical descriptions capture the detailed structure of nuclei around neutron number N=90.