An activity with a half-life of 18.0±0.2 hours and a weaker one of 5.5±0.2 days have been produced by neutron bombardment of gadolinium, resulting in a saturation intensity ratio of 25. A deuteron bombardment of gadolinium gave a ratio of 3 and produced 72-day ${\mathrm{Tb}}^{160}$ with a saturation intensity equal to that of the 5.5 day. The 18.0-hour activity decays by the emission of negative $\beta$-particles of 0.85 Mev and a $\gamma$-ray of 0.3 Mev. The 5.5-day activity decays by the emission of negative $\beta$-particles of 0.5 Mev and $\gamma$-rays of 1.28 Mev. Neither the 18.0-hour nor the 5.5-day substance is produced by fast neutron bombardment of Tb, but there is some evidence that both are produced by fast neutron bombardment of dysprosium. In accordance with the above data both the 18.0-hour and the 5.5-day activities are assigned mass 161. An activity with a minimum half-life of 20 years and one with a minimum half-life of 75 days have been produced by a deuteron bombardment of europium. The decay of the 20-year activity is by the emission of $\beta$-particles of 1.4 Mev and a $\gamma$-ray. The 75-day activity is probably attributable to ${\mathrm{Gd}}^{153}$ and the 20-year activity to ${\mathrm{Eu}}^{154}$.

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