A coincidence apparatus has been developed which permits the measurement of energies and intensities of rare alpha groups leading to excited states of the product nucleus. It has proved possible to measure transitions with intensities as low as ${10}^{-8\mathrm{}}$, and the technique has been used in the present study, principally to characterize vibrational states. The energies, relative abundances, and multipole orders of the gamma-ray and conversion electron transitions which de-excite the levels populated by these weak alpha branchings can be determined, and these lead to assignments of spin and parity for the levels. Excited states with spin and parity 0+ (beta vibrations) were confirmed in the energy level spectrum of ${\mathrm{Th}}^{230}$, ${\mathrm{U}}^{234}$, ${\mathrm{Pu}}^{238}$, and ${\mathrm{Pu}}^{240}$. A corresponding state was observed at 780 keV in the energy level spectrum of ${\mathrm{U}}^{235}$ and was given the assignment 1/2+. The reduced alpha transition probabilities to states of this type were found generally to be about an order of magnitude smaller than the theoretical value for unhindered alpha decay. The characterization of these levels simply as beta vibrations appears to be inadequate in that there are distinct differences in the modes of de-excitation for the different nuclides.

• Received 18 October 1962

DOI:https://doi.org/10.1103/PhysRev.130.2000