Spectroscopy of neutron-rich nuclei produced in multinucleon transfer reactions at KISS

Multinucleon transfer (MNT) reactions have recently gained renewed interest as they provide access to heavy neutron-rich nuclei, particularly around $N=126$ and actinides, which are relevant to r-process nucleosynthesis. They produce a wide variety of nuclides around both the projectile and the target, with a wide distribution of angles and energies, requiring the development of experimental techniques to collect, separate, and identify the reaction products of interest. In particular, exotic nuclei far from the projectile or target are produced so infrequently that they can get buried among more abundant reaction products. We are developing the KEK Isotope Separation System (KISS) at the RIBF facility of RIKEN, which focuses on the extraction of MNT reaction products. We perform decay, mass, and laser spectroscopy of neutron-rich nuclei of refractory elements around the $N=126$ region. A multi-reflection time-of-flight mass spectrograph is used for high-precision mass measurements and unique identification of isobars. Recently, research has been extended to mass spectroscopy of neutron-rich actinides by using a ²³⁸U beam. The experimental methods and results of KISS as well as its next plan for further neutron-rich regions will be presented.