RADIOACTIVE DECAY OF Tb$sup 157$ AND Tb$sup 15$$sup 8$

Dysprosium, enriched to 13.8% in Dy/sup 156/ and to 0.9% in Dy/sup 158/, is irradiated with neutrons, and a terbium chemical fraction is obtained. The mass number of 155 is assigned to the gamma activity in the chemical fraction. The assignment is confirmed by a comparison of the scintillation spectrum of the terbium chemical fraction with that obtained from gadolinium enriched in Gd/sup 157/ and Gd/sup 158/ respectively, when bombarded with 6-Mev protons. Tb/sup 158/ decays mostly by electron capture and partly by beta emission. In the electron capture decay of Tb/sup 158/, gamma rays of energy 79.7, 182.7, 770, and 950 kev are emitted. A 99.3-kev gamma ray is emitted in the beta decay of Tb/sup 158/. Coincidence studies are carried out and the relative intensities of the gamma rays are determined. A decay scheme consistent with the data obtained is given. The enriched samples of gadolinium bombarded with protons do not show any gamma rays that can be associated with a long-lived activity of Tb/sup 157/. Experimental results indicate a long-lived activity of Tb/sup 157/ that decays by L and K capture. (auth)