Electric monopole transition in 228Th

The 184.5-keV transition from the 1153.6-keV level of ²²⁸Th excited in the decay of ²²⁸Ac is shown to be (93±2)% E0 in character from K/L and $\frac{L_1}{L_{11}}$ conversion ratios. The half-life of the 1153.6-keV level is measured to be T_1/2=0.29±0.02 ns from β-γ delayed coincidence measurements. From this, the value of the monopole strength parameter is deduced to be ρ=0.11±0.02.[RADIOACTIVITY ²⁸⁸Ac measured $I_{ce}$; β-γ delay; levels in ²²⁸Th; deduced γ multipolarity T_1/2.]