Discovery of Proton-Rich Radioactive Isotopes in the Z = 63–70 Region Produced by the Projectile Fragmentation of a 345-MeV/Nucleon 238U Beam

We report 14 proton-rich new isotopes with atomic numbers (Z) of 63–70 produced via projectile fragmentation of a 345-MeV/nucleon ²³⁸U beam at the RI Beam Factory, RIKEN, based on the new-isotope evaluation shown in the National Nuclear Data Center as of October 2025: 132,133\! 63Eu, 133,134,136\, \, 64Gd, 136,137,138\, \, 65Tb, ¹³⁸\ 66Dy, ¹⁴³\ 67Ho, ¹⁴³\ 68Er, ¹⁴⁴\ 69Tm, and 147,148\! 70Yb. These highly proton-rich radioactive isotopes were separated and identified using the large-acceptance two-stage separator, BigRIPS. A 10-µm Ta charge stripper was used in the first stage of the separator to efficiently remove less-exotic lower-Z contaminants than the isotopes of interest: Hydrogen-like and fully stripped ions were transported before and after the charge stripper, respectively. Production cross sections of the new isotopes and their neighboring nuclei were determined and compared with semi-empirical formulas, among which EPAX2.15 showed the best agreement with the experimental data.