Radioactive Decay ofTb157andTb158

Dysprosium, enriched to 13.8% in the mass number 156 and to 0.9% in the mass number 158, was irradiated with neutrons and a terbium chemical fraction was obtained. The mass number of 158 was assigned to the gamma activity in the chemical fraction. The assignment was confirmed by a comparison of the scintillation spectrum of the terbium chemical fraction with that obtained from gadolinium enriched in the mass numbers 157 and 158, respectively, when bombarded with 6-MeV protons. ${\mathrm{Tb}}^{158}$ decays mostly by electron capture and partly by beta emission. In the electron capture decay of ${\mathrm{Tb}}^{158}$, gamma rays of energy 79.7, 182.7, 770 and 950 keV are emitted. A gamma ray of energy 99.3 keV is emitted in the beta decay of ${\mathrm{Tb}}^{158}$. Coincidence studies were carried out and the relative intensities of the gamma rays determined. A decay scheme consistent with the data obtained is given. The enriched samples of gadolinium bombarded with protons did not show any gamma rays that could be associated with a long-lived activity of ${\mathrm{Tb}}^{157}$. Experimental results indicate a long-lived activity of ${\mathrm{Tb}}^{157}$ which decays by $L$ and $K$ capture.