The decay of radiochemically-separated ${}_{}{}^{228}{}_{}{}^{}\mathrm{Ac}$ sources has been reinvestigated using high-resolution coaxial HPGe detectors and γ-γ coincidence measurements. Energies and intensities of more than 240 photons were accurately measured, 80 of them for the first time. A ${}_{}{}^{228}{}_{}{}^{}\mathrm{Th}$ level scheme, built on the bases of good energy fits and on γ-γ coincidence data, involves 58 excited states, of which 17 are new, up to 2159 keV with spin 0≤I≤6. Collective properties of low-energy states belonging to $K^{\pi }$${=0\mathrm{}}^{\mathrm{-}}$,$2^{+}$,$2^{\mathrm{-}}$ bands are discussed in the framework of the Bohr-Mottelson model. Decay modes of a $K^{\pi }$${=0\mathrm{}}^{+}$ excited band at 831.82 keV favor strong two octupole phonon $K^{\pi }$${=0\mathrm{}}^{\mathrm{-}}$ components. The $K^{\pi }$${=1\mathrm{}}^{\mathrm{-}}$ band is expected to have I=2 and 3 members at 938.5 and 968.4 keV. Coriolis coupling calculations involving the four $K^{\pi }$${=0\mathrm{}}^{\mathrm{-}}$, $1^{\mathrm{-}}$, $2^{\mathrm{-}}$, and $3^{\mathrm{-}}$ bands were carried out. Tentative $K^{\pi }$${=2\mathrm{}}^{+}$ two-quasiparticle bands at 1638.23 and 1899.99 keV are discussed in terms of Nilsson orbitals.

• Received 6 February 1987

DOI:https://doi.org/10.1103/PhysRevC.36.2510