U(铀)-238 → Th(钍)-234 → Pa(镤)-234 → U(铀)-234 → Th(钍)-230 → Ra(镭)-226 → Rn(氡)-222 → Po(钋)-218 → Pb(铅)-214 或 At(砈)-218 → Bi(铋)-214 → Po(钋)-214 或 Tl(铊)-210 → Pb(铅)-210 → Bi(铋)-210 → Po(钋)-210 或 Tl(铊)-206 → Pb(铅)-206.