Octupole strength in the neutron-rich calcium isotopes

Low-lying excited states of the neutron-rich calcium isotopes $^{48-52}$Ca have been studied via $γ$-ray spectroscopy following inverse-kinematics proton scattering on a liquid hydrogen target using the GRETINA $γ$-ray tracking array. The energies and strengths of the octupole states in these isotopes are remarkably constant, indicating that these states are dominated by proton excitations.