Abstract
The decay of radiochemically-separated /sup 228/Ac sources has been reinvestigated using high-resolution coaxial HPGe detectors and ..gamma..-..gamma.. coincidence measurements. Energies and intensities of more than 240 photons were accurately measured, 80 of them for the first time. A /sup 228/Th level scheme, built on the bases of good energy fits and on ..gamma..-..gamma.. coincidence data, involves 58 excited states, of which 17 are new, up to 2159 keV with spin 0less than or equal toIless than or equal to6. Collective properties of low-energy states belonging to K/sup ..pi../ = 0/sup -/,2/sup +/,2/sup -/ bands are discussed in the framework of the Bohr-Mottelson model. Decay modes of a K/sup ..pi../ = 0/sup +/ excited band at 831.82 keV favor strong two octupole phonon K/sup ..pi../ = 0/sup -/ components. The K/sup ..pi../ = 1/sup -/ band is expected to have I = 2 and 3 members at 938.5 and 968.4 keV. Coriolis coupling calculations involving the four K/sup ..pi../ = 0/sup -/, 1/sup -/, 2/sup -/, and 3/sup -/ bands were carried out. Tentative K/sup ..pi../ = 2/sup +/ two-quasiparticle bands at 1638.23 and 1899.99 keV are discussed in terms of Nilsson orbitals.
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