VI. RNC ACTIVITIESRIKEN Accel. Prog. Rep. 54 (2021)

Nuclear Science and Transmutation Research Division
Superheavy Element Research Group
1. Abstract

The elements with their atomic number Z > 103 are called as trans-actinide or superheavy elements. This group has been studying
the physical and chemical properties of superheavy elements. They must be produced by artificially for the scientific study utilizing
the accelerators in RIBF. Two teams lead the study of the superheavy elements. Superheavy Element Production Team studies various
methods of eﬃcient production of the superheavy elements and their physical and chemical properties. Superheavy Element Device
Development Team develops the main experimental device, i.e., the gas-filled recoil ion separator, GARIS.
The synthesis of elements having atomic numbers over 119 will be attempted with the aim of establishing nuclear synthesis
technology that reaches the “island of stability” where the lifetime of atomic nuclei is expected be prolonged significantly. With theaim of constructing an ultimate nuclear model, maximum utilization will be made of key experimental devices which become fully
operational in order to conduct research for the syntheses of element 119 and 120.

2. Major Research Subjects

Superheavy Element Production Team
(1) Searching for new elements(2) Spectroscopic study of the nucleus of heavy elements(3) Chemistry of superheavy elements(4) Study of a reaction mechanism for fusion process Superheavy Element Device Development Team
(5) Maintenance of GARIS, GARIS-II and development of new gas-filled recoil ion separator GARIS-III(6) Maintenance and development of detector and DAQ system for GARIS, GARIS-II and GARIS-III(7) Maintenance and development of target system for GARIS, GARIS-II and GARIS-III

3. Summary of Research Activity(1) Searching for new elements
To expand the periodic table of elements and the nuclear chart, we will search for new elements.
(2) Spectroscopic study of the nucleus of heavy elements
Using the high sensitivity system for detecting the heaviest element, we plan to perform a spectroscopic study of nuclei of theheavy elements.
(3) Chemistry of superheavy elements
Study of chemistry of the trans-actinide (superheavy element) has just started world-wide, making it a new frontier in the field ofchemistry. Relativistic eﬀects in chemical property are predicted by many theoretical studies. We will try to develop this new field.
(4) Study of a reaction mechanism for fusion process
Superheavy elements have been produced by complete fusion reaction of two heavy nuclei. However, the reaction mechanism of
the fusion process is still not well understood theoretically. When we design an experiment to synthesize nuclei of the superheavy
elements, we need to determine a beam-target combination and the most appropriate reaction energy. This is when the theory becomes
important. We will try to develop a reaction theory useful in designing an experiment by collaborating with the theorists.
(5) Research highlight
The discovery of a new element is one of the exciting topics both for nuclear physicists and nuclear chemists. The elements with
their atomic number Z > 103 are called as trans-actinides or superheavy elements. The chemical properties of those elements have notyet been studied in detail. Since those elements do not exist in nature, they must be produced by artificially, by using nuclear reactions
for the study of those elements. Because the production rate of atoms of those elements is extremely small, an eﬃcient production andcollection are key issues of the superheavy research. In our laboratory, we have been trying to produce new elements, studying thephysical and chemical properties of the superheavy elements utilizing the accelerators in RIKEN.
Although the Research Group for Superheavy element has started at April 2013, the Group is a renewal of the Superheay Element
Laboratory started at April 2006, based on a research group which belonged to the RIKEN accelerator research facility (RARF), andhad studied the productions of the heaviest elements. The main experimental apparatus is a gas-filled recoil ion separator GARIS. The
heaviest elements with their atomic numbers, 107 (Bohrium), 108 (Hassium), 109 (Meitnerium), 110 (Darmstadtium), 111 (Roentogenium), and 112 (Copernicium) were discovered as new elements at Helmholtzzentrum fur Schwerionenforschung GmbH (GSI),
Germany by using 208 Pb or 209 Bi based complete fusion reactions, so called “cold fusion” reactions. We have made independentconfirmations of the productions of isotopes of 108th, 110th, 111th, and 112th elements by using the same reactions performed at GSI.
After these work, we observed an isotope of the 113th element, 278 113, in July 2004, in April, 2005, and in August 2012. The isotope,278
113, has both the largest atomic number, (Z = 113) and atomic mass number (A = 278) which have determined experimentally
among the isotopes which have been produced by cold fusion reactions. We could show the world highest sensitivity for production
and detection of the superheavy elements by these observations. Our results that related to 278 113 has been recognized as a discoveryof new element by a Joint Working Party of the International Union of Pure and Applied Chemistry (IUPAC) and International Union- 238 -VI. RNC ACTIVITIESRIKEN Accel. Prog. Rep. 54 (2021)of Pure and Applied Physics (IUPAP). Finally, we named the 113th element as “Nihonium.”
We decided to make one more recoil separator GARIS-II, which has an acceptance twice as large as existing GARIS, in order torealize higher sensitivity. The design of GARIS-II has finished in 2008. All fabrication of the separator will be finished at the end offiscal year 2008. It has been ready for operation after some commissioning works.
Preparatory work for the study of the chemical properties of the superheavy elements has started by using the gas-jet transportsystem coupled to GARIS. The experiment was quite successful. The background radioactivity of unwanted reaction products has beenhighly suppressed. Without using the recoil separator upstream the gas-jet transport system, large amount of unwanted radioactivity
strongly prevents the unique identification of the event of our interest. This new technique makes clean and clear studies of chemistryof the heaviest elements promising.
The spectroscopic study of the heaviest elements has started by using alpha spectrometry. New isotope, 263 Hs (Z = 108), which
has the smallest atomic mass number ever observed among the Hassium isotopes, had discovered in the study. New spectroscopic
information for 264 Hs and its daughters have obtained also. The spectroscopic study of Rutherfordium isotope 261 Rf (Z = 104) has
done and 1.9-s isomeric state has directly produced for the first time.
Preparatory works for the study of the new superheavy elements with atomic number 119 and 120 have started in 2013. We
measured the reaction products of the 248 Cm (48 Ca, xn)296−x Lv (Z = 116) previously studied by Frelov Laboratory of Nuclear Reaction,Russia, and GSI. We observed 5 isotopes in total which tentatively assigned to 293 Lv, and 292 Lv.

MemberDirector
Kosuke MORITA- 239 -