noun
: a radioactive series beginning with actinouranium, constituting the isotope of uranium of mass number 235, and ending with actinium D, constituting the nonradioactive isotope of lead of mass number 207: actinouranium, at. no. 92 (syn. uranium 235) → uranium Y, at. no. 90 (syn. thorium 231) → protactinium 231, at. no. 91 → actinium 227, at. no. 89 → radioactinium, at. no. 90 (syn. thorium 227) [or actinium K, at. no. 87 (syn. francium 223)] → actinium X, at. no. 88 (syn. radium 223) → actinon, at. no. 86 (syn. radon 219) → actinium A, at. no. 84 (syn. polonium 215) → actinium B, at. no. 82 (syn. lead 211) [or astatine 215, at. no. 85] → actinium C, at. no. 83 (syn. bismuth 211) → actinium C, at. no. 84 (syn. polonium 211) [or actinium C, at. no. 81 (syn. thallium 207)] → actinium D, at. no. 82 (syn. lead 207) — compare actinide series