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KEK Isotope Separation System (KISS)

Vol. 28, No. 4, 2018, Nuclear Physics News 25 Introduction Where and how are the heavy elements such as gold and platinum synthesized in our universe? It is one of nuclear physics’ original questions, which the KEK Isotope Separation System (KISS) was constructed to attempt to answer. Burbidge, Burbidge, Fowler, and Hoyle proposed the synthesis of the elements in stars via nine astrophysical processes in 1957 [1]. Among those processes, the rapid neutron capture process (r-process) is considered to be responsible for half of the abundance of the elements heavier than iron. It occurs under an environment with a high neutron density and a high temperature. There are several candidates for the astrophysical site of the r-process, such as type-II supernovae and binary neutron star mergers. The recent fi rst observation of gravitational waves and electromagnetic radiation from a binary neutron star merger [2] makes such mergers a hot topic now that they are considered a promising candidate for the astrophysical site of the r-process. Although energetic efforts have been poured into experimental and theoretical research to determine the astrophysical environment, so far, the astrophysical site for the r-process remains a mystery. It is partially because of a lack of information for properties of many nuclei, such as a lifetime and a mass, which have crucial roles in the r-process. The most crucially important nuclei are unstable, neutron-rich nuclei around neutron magic numbers 50, 82, and 126. The r-process goes through extremely neutron-rich nuclei under a high neutron density environment. The enhanced neutron separation energy of neutron magic numbers results in waiting points along the path, and the waiting point nuclei are considered to be origins for peaks in the observed r-process contribution to the solar abundance distribution. Particularly, gold and platinum are part of a peak in the solar abundance distribution near A = 195, which is presumed to originate from r-process waiting nuclei with the neutron magic number 126. The nuclear properties of these nuclei as yet remain largely unstudied experimentally due to the diffi culty in producing them by traditional nuclear production methods such as fi ssion, fusion, and fragmentation. Those isotopes are also diffi cult to extract in existing Isotope Separator On-Line (ISOL) facilities, because they are refractory elements. The KISS project was launched in 2011 to produce these refractory elements, extract them, and explore their nuclear properties for determination of the astrophysical environment of the r-process. The project is executed under a collaboration between the Institute of Particle and Nuclear Studies (IPNS), High Energy Accelerator Research Organization (KEK), and RIKEN Nishina Center for Accelerator-Based Science. Wako Nuclear Science Center was founded under IPNS at RIKEN to promote the KISS project in April 2015. The element-selective isotope separator KISS was constructed at the RI Beam Factory in RIKEN [3, 4] (Figure 1). Multi-nucleon transfer (MNT) reactions within a target of enriched 198Pt are used to produce neutron-rich nuclei around the neutron magic number 126 at KISS. The MNT reactions are induced by the beam of 136Xe accelerated up to 10.75 MeV/nucleon by RIKEN Linear Accelerator 2 and RIKEN Ring Cyclotron. All reaction products are collected in an argon gas cell and separated by the laser resonance ionization and ISOL technique, which make it possible to selectively extract refractory elements and perform nuclear spectroscopy studied on them.