High-precision mass measurement of superheavy nuclides

An international collaborative research group led by the High Energy Accelerator Research Organization (KEK) Wako Nuclear Science Center, RIKEN Nishina Center for Accelerator-Based Science, and Kyushu University have succeeded in precisely measuring the mass of superheavy element dubnium isotope ²⁵⁷Db with the atomic number 105 by using a gas-filled recoil ion separator (GARIS-II) and a multi-reflection-type time-of-flight mass spectrometer (MRTOF) at RIKEN's Heavy Ion Accelerator Facility "RI Beam Factory (RIBF)". Superheavy elements with a large atomic number (i.e., number of protons) have large and unstable repulsive force between the positively charged protons, and their mass measurement is the key to clarifying why they exist. The experiment was carried out using GARIS-II, which is known for the discovery of nihonium in the RIKEN laboratory, and the Superheavy Element Mass Measuring Device (SHE-MASS-II) developed by the research group.

In particular, as the newly developed αTOF detector can detect α-rays as well as mass signals, it was able to accurately identify superheavy elements, of which only 2 could otherwise be detected per day. The mass of ²⁵⁷Db was determined to be 257.10742(25)u with a higher relative accuracy of one millionth. The binding energy was derived as 1892.1(2) MeV. The research results will be the first step for clarifying the reason for the existence of superheavy elements, and it is expected that the atomic numbers of heavier superheavy elements can be reliably identified from the mass measurement of nihonium and moscovium isotopes scheduled for the future. In addition, although only superheavy element isotopes with short life and continuous α decay could be detected to date, with this result, it was demonstrated that with only separation from background events, precision mass measurement could be a new detection and identification method.

Professor Michiharu Wada of KEK Wako Nuclear Science Center said, "For clarifying the more fundamental question of why superheavy elements can exist, we have carried out a comprehensive measurement of mass, an indicator of the strength of nuclear bonds, from possible isotopes and our measurement of dubnium isotopes is pioneering. In the future, we would like to measure the mass of more superheavy elements, clarify the reason for their existence and attempt to develop a reliable identification method for new elements."