First Observation of 22 Exotic Isotopes Using a 208Pb Primary Beam at the RI Beam Factory

Abstract This letter reports the results from the first experimental study utilizing a 345-MeV/nucleon 208Pb beam at the RI Beam Factory at RIKEN Nishina Center. This experiment aimed at searching for new proton- and neutron-rich isotopes beyond $Z=50$. The BigRIPS in-flight separator and state-of-the-art radiation detectors were used to efficiently separate and reliably identify the produced isotopes. We confirmed a total of 22 new exotic isotopes: 118Ce, 120Pr, 179Ho, $^{181,182}$Er, $^{184-186}$Tm, $^{188,189}$Yb, 191Lu, $^{152,153,193,194}$Hf, $^{154,195,196}$Ta, $^{198,199}$W, and $^{200,201}$Re. This discovery significantly extends the known boundaries of the nuclear chart, paving the way for investigating the properties of unstable nuclei far from the $\beta$-stability line in the $Z=58$–75 region. This region is of particular importance for exploring phenomena such as exotic decay modes appearing on the proton-rich side and $r$-process nucleosynthesis associated with neutron-rich nuclei.