Shiro Goda

The Half Life of Tb

A sample of Gd/sub 2/O/sub 3/ was borabarded for four hours with 38.5- Mev alpha particles and the activity of Dy/sup 157/ produced by the ( alpha ,3n) reaction was separated from the target material. Terbium was separated from dysprosium by an ionexchange method. The decay rate of Tb/sup 157/ nuclei was determined from the counting rate of the K x rays in a well type crystal, and the K and L electron capture ratio was measured using a NaI(Tl) crystal covered with Al, corrected for absorption by the Al cover, self absorption, photopeak efficiency, and fluorescence yield. The half life of Tb/sup 157/ was estimated at 160 ction in DDT-C/ 40 yr; a decay mode was proposed. (C.E.S.)

Decay of 157Tb

A sample of Gd2O3 enriched to 97.01% in the mass number 156 was bombarded with 38.5 MeV alpha-particles. The activity of 157Dy produced by the (α, 3n) reaction was isolated chemically from the target material. The number of nuclei of 157Dy was determined from the counting rate on the 330 keV photopeak. After the 157Dy radioactivity (T12 = 8.2 h) had died out, terbium was separated from dysprosium. The scintillation spectrum of the terbium fraction showed only one photopeak at the energy of 43 keV X-rays of gadolinium when the source was situated inside a well-type crystal. The number of 157Tb nuclei was determined from the counting rate of the K X-rays in the well-type crystal and by the measurement of the K and L orbital electron capture ratio with a thin NaI(Tl) crystal. From the number of nuclei of the parent and the daughter, the half life of 157Tb was estimated to be 150±30 y. A decay scheme of 157Tb is proposed.