Electric Transition Strengths in Octupole Nuclei Measured by Coulomb Excitation

There is now a wealth of experimental and theoretical evidence, e.g.1,2, that nuclei with Z ≈ 60, N ≈ 88 and Z ≈ 88, N ≈ 134 are reflection-asymmetric in the intrinsic frame. The usual experimental signature for such pear-shaped nuclei is the observation of low-lying negative parity states which become interleaved with the yrast positive parity states at medium values of angular momentum. These data have been interpreted using geometric models within the mean field approach as arising from the presence of static octupole deformation, i.e. $${\bar \beta _3} \ne 0$$ As with quadrupole degrees of freedom, the most reliable measurements of collective octupole strength are the Eλ matrix elements connecting the rotational states in the octupole and ground state bands. While many E1 transition strengths have been determined in ‘octupole’ nudei, they are strongly affected by fluctuating single particle contributions and are not a reliable indicator of octupole strength. While observation of real photon E3 emission is not yet technically possible, the determination of E3 matrix elements from Coulomb excitation (Coulex) is feasible. Results of such measurements of E1, E2 and E3 matrix elements in 148Nd are presented here, and are used to extract deformation parameters which can be compared with theoretical predictions. Recent measurements in other nudei are also briefly discussed.