Bi-209; when the last captures another neutron, it becomes Po-210 (or Bi-210m) then alpha decays back to mass 206. Should another neutron be captured, alpha decay from 211 to 207 is faster yet. The net result of the cycle is that 4 neutrons are fused into one alpha particle with two beta decays.

In fact, this cycle does not reach equilibrium because the neutron cross section of the doubly magic ²⁰⁸Pb