Fine structure in the $\alpha$ decay of $^{223}$U and structure evolution in the $N$ = 129 and 131 isotones.

Fine structure in the $\alpha$ decay of $^{223}$U was observed in the fusion-evaporation reaction $^{187}$Re($^{40}$Ar, p3n) by using fast digital pulse processing technique. Three $\alpha$-decay branches of $^{223}$U were identified by establishing the decay chain $^{223}$U $\xrightarrow{\alpha_{1}}$ $^{219}$Th $\xrightarrow{\alpha_{2}}$ $^{215}$Ra $\xrightarrow{\alpha_{3}}$ $^{211}$Rn. The $\alpha$-particle energy for the ground-state to ground-state transition of $^{223}$U was determined to be 8993(17) keV, 213 keV higher than the previous value. Quadrupole-octupole deformation in the ground states of $^{223}$U and its lighter isotones $^{219}$Ra and $^{221}$Th was suggested by comparison of the experimental and calculated masses. Properties of $\alpha$ decays ($\alpha$-particle energies, branching ratios and hindrance factors) of $^{223}$U, feeding several low-lying levels of daughter nucleus $^{219}$Th, were compared with those of its lighter $N$ = 131 isotones $^{219}$Ra and $^{221}$Th. Evolution of nuclear structure for $N$ = 129 and 131 even-$Z$ isotones from Po to U was discussed in the framework of reduced $\alpha$-decay width.