Facility upgrade for superheavy-element research at RIKEN

The RIKEN Nishina Center (RNC) executed an accelerator upgrade project for the heavy-ion linac (called RILAC). A superconducting RIKEN linear accelerator (SRILAC) and a new superconducting electron-cyclotron-resonance ion source (SC-ECRIS) to boost the final energy and intensity were constructed, aimed at synthesizing a new superheavy element, 119, through a hot fusion reaction. The project included the construction of a gas-filled recoil ion separator (GARIS-III) suitable for detecting the residues of the hot-fusion reaction. To avoid research interruption during the SRILAC construction period (2017-2019) and gain experience in hot-fusion reaction processes, GARIS-II located in the GARIS experimental hall in LINAC building was moved to the E6 experimental hall in Nishina building. Certain exploratory measurements were performed employing the beams accelerated by RILAC2 and the RIKEN ring cyclotron (RRC), which is a part of the existing accelerator complex of the radioactive isotope beam factory (RIBF). Further, commissioning experiments with the upgraded facility (SRILAC and GARIS-III) were performed. The upgrade project and its commissioning results are chronologically described in this article.

Extraction of carrier-free 99Mo by ionic liquids from acid solutions: A model of seaborgium (Sg) experiment

A new approach for extraction of ⁹⁹Mo tracer as a lighter homolog of Seaborgium (Sg) by three different the ionic liquids are studied. Aliquat-336 [Aliq-336.Cl^-] as anion exchange has been used for the preparation of three ionic liquid, namely: ([Aliq-336]⁺ [SCN]^-), ([Aliq-336]⁺ [S]^- ) and ([Aliq-336]⁺ [Fe(CN)₆]^-). Their potential extraction of carrier free ⁹⁹Mo from HNO₃ solutions has been evaluated. The obtained results demonstrated that successful extraction of carrier free ⁹⁹Mo from HNO₃ solutions is achieved. The ([Aliq-336]⁺ [Fe(CN)₆]^-) is found to give the highest extraction affinity for ⁹⁹Mo than the others ionic liquids investigated. The preliminary results could be useful for the upcoming aqueous experiments of Seaborgium.

Chemistry of the superheavy elements

The quest for superheavy elements (SHEs) is driven by the desire to find and explore one of the extreme limits of existence of matter. These elements exist solely due to their nuclear shell stabilization. All 15 presently 'known' SHEs (11 are officially 'discovered' and named) up to element 118 are short-lived and are man-made atom-at-a-time in heavy ion induced nuclear reactions. They are identical to the transactinide elements located in the seventh period of the periodic table beginning with rutherfordium (element 104), dubnium (element 105) and seaborgium (element 106) in groups 4, 5 and 6, respectively. Their chemical properties are often surprising and unexpected from simple extrapolations. After hassium (element 108), chemistry has now reached copernicium (element 112) and flerovium (element 114). For the later ones, the focus is on questions of their metallic or possibly noble gas-like character originating from interplay of most pronounced relativistic effects and electron-shell effects. SHEs provide unique opportunities to get insights into the influence of strong relativistic effects on the atomic electrons and to probe 'relativistically' influenced chemical properties and the architecture of the periodic table at its farthest reach. In addition, they establish a test bench to challenge the validity and predictive power of modern fully relativistic quantum chemical models.

Superheavy element studies with preseparated isotopes

In recent years, significant progress in the field of superheavy element research has been achieved thanks to a novel combination of techniques from different fields. This “physical preseparation” approach includes the coupling of an ancillary setup – typically a chemistry apparatus or a counting setup – to a physical recoil separator. This latter preseparator removes unwanted nuclear reaction products as well as the intense heavy-ion beam associated with superheavy element experiments and thus isolates the evaporation residues of the nuclear fusion reactions. These are guided to the separators's focal plane, where they are extracted and available for further transport to external setups, e.g., by a gas-jet. In this overview, the development of physical preseparation is described, and experimental results from nuclear chemistry and physics that were achieved with “preseparated” isotopes are summarized, with an emphasis on results relevant for superheavy element research. The covered topics range from chemical studies in the liquid as well as in the gas phase, the measurement of nuclear decay properties and of atomic masses. Preseparation was already shown to be a very powerful approach in these studies and promises to allow further progress in superheavy element research.