Of the presently known nuclides, ∼300 are naturally occurring while another ∼3000 have been artificially produced, the heaviest being 294118Og (oganesson) [1]. Furthermore, according to modern theoretical calculations, more than 4000 further radioactive nuclei are expected to exist [2–4], mostly in the region of the neutron-rich isotopes, which are believed to be produced in the astrophysical r process [5]. A strong worldwide push across several existing and planned facilities is underway to expand the chart of nuclides, toward both the proton and neutron drip lines, with particular emphasis on the heaviest elements [6,7]. The discovery of new nuclides at the edges of stability often critically challenges the existing nuclear models, helping to refine them [3,7,8].

The stability or existence of an isotope depends on its binding energy, which in the simplest approximation, can be expressed as the sum of a bulk macroscopic part (the “liquid-drop contribution”) and a microscopic shell correction energy. In most nuclides, especially in the light region, the former contribution is dominant, but the shell corrections can play a substantial role and often result in dramatic nuclear structure changes, as documented in many cases across the chart of nuclei [9,10].

On the contrary, in the region of high atomic number ( Z≳100), the strong Coulomb repulsion between protons makes the macroscopic contribution relatively small such that it alone cannot bind superheavy nuclides (SHN) [3,11]. Because of this, the mere existence of SHN is defined by a shell correction energy which is manifested in the creation of nucleonic subshell closures of spherical or deformed nature, enhancing the stability of a nucleus [12,13]. For example, in the actinide and transactinide regions, the deformed subshell closures at Z=108 and N=162, and Z=100−102 and N=152 have long been predicted [14–17]. Figure 1 shows a plot of the ground state shell correction energy as calculated by a macroscopic-microscopic model (FRDM12 [18]) for the 90Th-102No nuclides around N=152. An island of strong shell corrections with values as low as about −5  MeV is centered around 100Fm-102No. Indeed, evidence of the deformed shell closure at N=152 was first established as early as the 1950s [19,20] and its nature has since been experimentally confirmed by various methods, e.g., through observation of rotational bands and high-spin K isomers [21,22], and measurements of masses [23] and spontaneous fission half-lives [24]. A primary reason for the richness of experimental data in this area is its accessibility via fusion reactions between partners close to doubly magic 20882Pb and 4820Ca, which result in enhanced production cross sections [25]...

The KEK Isotope Separation System (KISS) is a recently-commissioned facility for the nuclear spectroscopy of nuclides produced via MNT reactions, installed at the RIKEN Nishina Center [36,37]. KISS has successfully extracted neutron-rich isotopes of the refractory elements Pt, Ir, and Os, which were produced as targetlike fragments using the 13654Xe+19878Pt reaction system [38–41]. By stopping MNT products in a flowing gas, KISS has demonstrated an advantage for studying the neutron-rich isotopes of these elements, which are difficult to extract from traditional ISOL targets. In addition, a multireflection time-of-flight mass spectrograph (MRTOF MS), which was installed recently, provides high-precision mass measurements [42]. In this Letter, we report on the first systematic mass measurements of neutron-rich Pa-Pu isotopes produced as projectilelike fragments (PLF) via several MNT channels of the 23892U+19878Pt reaction at the KISS facility, resulting in the high-precision direct determination of the masses of 19 nuclides and the discovery of a new uranium isotope 24192U.

An overview of the KISS experimental setup is shown in Fig. 2. A primary beam of 23892U [ 10.75  MeV/nucleon, typical intensity of ∼30 particle nA (1 particle nA is 6.2×109  particles/s)], accelerated by the RIKEN Ring Cyclotron, impinged for four days upon a rotating 19878Pt (enriched to 91.63%) target [37] with a thickness of 12.5  mg/cm2. The isotopes of interest were produced as PLFs in the MNT reactions, having masses and velocities close to those of the primary beam particles, and being scattered around the grazing angle. The energy of these reaction products was attenuated by a 40-μm Ti energy degrader to maximize the fraction stopping in a doughnut-shaped gas cell [37] filled with 65-kPa argon gas...

FIG. 2.
Sketch of the KISS experimental setup. The blue- and yellow-colored areas are filled with Ar and He gases, respectively. Differential pumping systems are located after the doughnut-shaped gas cell as well as before and after the GCCB.