α decay of the new isotope 204Ac

A new proton-unbound isotope ²⁰⁴Ac was produced in the fusion-evaporation reaction Tm(40169Ca,5n)204Ac and identified via the recoil-α correlation at the gas-filled recoiled separators SHANS2 and SHANS. The excitation function for producing ²⁰⁴Ac was measured and the optimal beam energy was evaluated to be 212 MeV. Nineteen ER - α1 - α2 - α3 decay chains were observed for this isotope. Based on these chains, the α-particle energy and half-life of ²⁰⁴Ac were determined to be 7948(15) keV and 7.4-1.4+2.2 ms, respectively. Both are consistent with the theoretical prediction. The reduced α-decay width was deduced to be 49-15+10 keV, which indicates a favored α decay for ²⁰⁴Ac. In addition, more precise decay properties of Eα = 7883(15) keV and T1/2 = 7.7-1.6+2.7 ms were obtained for ²⁰⁵Ac.