Discovery of Proton-Rich Radioactive Isotopes in the <i>Z</i> = 60 Region Produced by the Projectile Fragmentation of a 345-MeV/Nucleon 238U Beam

Abstract We report proton-rich new isotopes obtained in the region of atomic number $Z$ = 60 produced by the projectile fragmentation of a 345-MeV/nucleon $^{238}$U beam at the RI Beam Factory, RIKEN. Based on the evaluation shown in the National Nuclear Data Center as of June 2025, the following 13 new isotopes have been discovered: $^{118,119}_{\qquad \! 57}$La, $^{119,120}_{\qquad \! 58}$Ce, $^{122,123}_{\qquad \! 59}$Pr, $^{123,124,126}_{\qquad \quad \, \, 60}$Nd, $^{125,126,127}_{\qquad \quad \, \, 61}$Pm, and $^{128}_{\ 62}$Sm. These highly proton-rich radioactive isotopes were separated and identified using BigRIPS, the large-acceptance two-stage separator at RIKEN. Hydrogen-like ions of the isotopes of interest were transported between the production target and the first degrader, following which the ions were fully stripped to increase the purity of the new isotopes in the secondary beams. The cross sections of the highly proton-rich isotopes were deduced and compared with the predictions using semi-empirical formulas.