First Observation of Twenty-two Exotic Isotopes Using a 208Pb Primary Beam at RIBF

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 twenty-two new exotic isotopes: 118Ce, 120Pr, 179Ho, 181, 182Er, 184 − 186Tm, 188, 189Yb, 191Lu, 152, 153, 193, 194Hf, 154, 195, 196Ta, 198, 199W, and 200, 201Re. This discovery significantly extends the known boundaries of the nuclear chart, paving the way to investigate the properties of unstable nuclei far from the β-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.