Decay of the IsomerEu152m2(96 min)

The structure of ${\mathrm{Eu}}^{152}$ is of great interest, since this nucleus is unique in being flanked by two even-even isobars which differ radically from each other in shape: ${\mathrm{Sm}}^{152}$ is strongly deformed while ${\mathrm{Gd}}^{152}$ is spherical. Only two states had previously been known in ${\mathrm{Eu}}^{152}$: the 13-year ground state (3-) and the 9.3-h isomeric state (0-). A detailed investigation of the decay of a new 96-min isomer, ${\mathrm{Eu}}^{152m_2}$, has been carried out. This isomer was first reported by Kirkby and Kavanagh, who found two transitions. We have detected a third transition of 18.25 keV ($99.9\% M1+0.1\% E2\mathrm{}$). The precise energies, and the spins and parities of three excited states of ${\mathrm{Eu}}^{152}$ were obtained: 89.83 keV (4+), 108.1 keV (5+), and 147.9 keV (8-) (96 min). The half-life of the 4+ state was found to be 4×${10}^{-7\mathrm{}}$ sec, that of the 5+ state ≲${10}^{-8\mathrm{}}$ sec. A renewed search for an isomeric transition accompanying the decay of the 9.3-h isomer ${\mathrm{Eu}}^{152m_1}$ (0-) was carried out, especially at lower electron energies (7-25 keV) than had hitherto been scanned. This search furnished an upper limit of 0.001% for a transition in the energy range $15\le E_{\mathrm{IT}}\le 68$ keV, and of ∼0.003% for $8\le E_{\mathrm{IT}}\le 15$ keV. The cross section for production of the 96-min isomer by pile neutrons is found to be $\frac{1}{(820\mathrm{}\pm 150)}$ of the production cross section of the 9.3-h isomer. Configurations of Nilsson orbitals are proposed for the ground state of ${\mathrm{Eu}}^{152}$ and for three excited states involved in the decay of ${\mathrm{Eu}}^{152m_2}$, as well as for ${\mathrm{Eu}}^{152m_1}\mathrm{}(0-)\mathrm{}$. These are consistent with the observed hindrance factors for the electromagnetic transitions.