Highly purified ytterbium, on irradiation in the pile, yields radioactivities whose half-lives are found to be 32.4 days, 4.2 days, and 6.7 days, in addition to the 2.4-hour activity known to exist. The 32.4-day activity is assigned to Yb 169 which decays by $K$-capture to excited Tm 169, yielding no general beta-radiation, but about 30 electron conversion lines which are interpreted as due to 10 gamma-rays. Many of the gamma-rays fit into a simple level scheme. The 4.2-day activity is assigned to Yb 175 and in addition to a strong beta-radiation shows conversion electron lines indicative of 4 gamma-rays. It is shown that the 6.7-day activity is a daughter product in lutecium 177, growing from a radioactive ytterbium 177 which is presumably the well-known 2.4-hour activity.

A re-interpretation of the approximately 40 electron conversion lines in radioactive tantalum 182 leads to 18 gamma-rays, all of which fit well a proposed energy level scheme.

• Received 8 December 1949

DOI:https://doi.org/10.1103/PhysRev.78.95