Decay ofTb156(5-day)

Radioactive ${\mathrm{Tb}}^{156}$ (∼5-day) was produced by the bombardment of ${\mathrm{Eu}}^{153}$ with $\alpha$ particles. It was found to decay by $K$ capture to ${\mathrm{Gd}}^{156}$. A decay scheme has been proposed which includes the following excited states of ${\mathrm{Gd}}^{156}$: 89 kev (2+), 289 kev (4+), 581 kev (6+), 1134 kev (2+), 1229 kev (3+), 1489 kev (4+), 1604 kev, 1904 kev (3), and 2024 kev (4-). The assignments of spins of levels and multipolarities of gamma-ray transitions are based on the results of measurements of conversion coefficients and gamma-gamma angular correlations. A value of (1.5±0.3)×${10}^{-10\mathrm{}}$ sec was found for the mean life of the 289-kev level of ${\mathrm{Gd}}^{156}$. The level scheme is discussed in the light of the unified model. The results support the suggestion that the 1134-kev and 1229-kev levels should be interpreted as collective vibrational levels.