Octupole strength in the neutron-rich calcium isotopes

Low-lying excited states of the neutron-rich calcium isotopes ${}^{48\text{–}52}\mathrm{Ca}$ have been studied via $\gamma$-ray spectroscopy following inverse-kinematics proton scattering on a liquid hydrogen target using the GRETINA $\gamma$-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.

Figure 1

Projectile-frame $\gamma$-ray spectra measured in coincidence with incoming and outgoing ${}^{49,51,52}\mathrm{Ca}$ particle-identification gates. The solid curves are the geant4 fits. The shaded region is the background, consisting of nonprompt (dark gray) and prompt (light gray) components.

Figure 2

Partial level schemes of ${}^{48\text{–}52}\mathrm{Ca}$ showing levels populated in the present work. Arrow widths are proportional to the measured $\gamma$-ray intensities.

Figure 3

(a) Energies, (b) proton-scattering cross sections, and (c) proton-scattering deformation lengths of octupole excitations in ${}^{48\text{–}52}\mathrm{Ca}$.