Systematic evidence for quasifission in 9Be-, 12C-, and 16O-induced reactions forming 258, 260No

Background: Cross sections for the formation of superheavy elements (SHE) by heavy ion fusion are suppressed by the competing quasifission process. This results in a fissionlike decay after capture but before formation of a compact compound nucleus. Fast quasifission is evident from very mass-asymmetric fission, focused in angle. In contrast, slow quasifission shows no significant mass-angle correlation, and a mass distribution peaked at symmetry. However, it shows angular distributions more anisotropic than those calculated for fission following fusion. Following fusion, low excitation energies should increase SHE survival through reduced competition from fission. However, in reactions with deformed actinide target nuclei, subbarrier fusion is highly suppressed by both fast and slow quasifission

Nuclear Structure Investigations of Es 253-255 by Laser Spectroscopy

Laser resonance ionization spectroscopy was performed on the rare einsteinium isotopes Es253-255 at the RISIKO mass separator in Mainz. With low sample sizes ranging down to femtograms, the prominent 352 nm-ground-state transition was measured in all three einsteinium isotopes, and four additional ground-state transitions were measured in Es254. Hyperfine-structure analysis resulted in assigned spin values of I(Es254)=7 and I(Es255)=7/2. From the extracted coupling constants, nuclear magnetic dipole moments of μI(Es254)=3.42(7)μN and μI(Es255)=4.14(10)μN as well as spectroscopic electric quadrupole moments of Qs(Es254)=9.6(1.2)eb and Qs(Es255)=5.1(1.7)eb were derived. Our value for Es254 deviates from the value of |μI(Es254)|=4.35(41)μN extracted from the angular anisotropy of α-radiation emitted by Es254. © 2022 authors. Published by the American Physical Society.Acknowledgments. This research was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Heavy Elements Chemistry Program, under Award DE-FG02-13ER16414. The isotopes used in this research were supplied by the U.S. DOE Isotope Program, managed by the Office of Science. This work has been supported by the Bundesministerium für Bildung und Forschung (BMBF, Germany) under Project No. 05P18UMCIA. This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 861198–LISA–H2020-MSCA-ITN-2019

Alpha-Photon Coincidence Spectroscopy Along Element 115 Decay Chains

Produced in the reaction 48Ca+243Am, thirty correlated α-decay chains were observed in an experiment conducted at the GSI Helmholzzentrum für Schwerionenforschung, Darmstadt, Germany. The decay chains are basically consistent with previous findings and are considered to originate from isotopes of element 115 with mass numbers 287, 288, and 289. A set-up aiming specifically for high-resolution charged particle and photon coincidence spectroscopy was placed behind the gas-filled separator TASCA. For the first time, γ rays as well as X-ray candidates were observed in prompt coincidence with the α-decay chains of element 115

Spectroscopy along flerovium decay chains. III. Details on experiment, analysis, 282Cn, and spontaneous fission branches

Flerovium isotopes (element Z = 114) were produced in the fusion-evaporation reactions 48Ca+242,244Pu and studied with an upgraded TASISpec decay station placed in the focal plane of the gas-filled separator TASCA at the GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt, Germany. Twenty-nine flerovium decay chains were identified by means of correlated implantation, α decay, and spontaneous fission events. Data analysis aspects and statistical assessments, primarily based on measured rates of various events, which laid the foundation for the comprehensive spectroscopic information on the flerovium decay chains, are presented in detail. Various decay scenarios of an excited state observed in 282Cn are examined in depth with the help of GEANT4 simulations and assessed by predictions of beyond mean-field calculations including triaxial shape degrees of freedom. Previous, revised, and newly derived fission probabilities of even-even superheavy nuclei are compared with various theoretical predictions

Spectroscopy along flerovium decay chains. II. Fine structure in odd-A 289Fl

Fifteen correlated α-decay chains starting from the odd-A superheavy nucleus 289Fl were observed following the fusion-evaporation reaction 48Ca+244Pu. The results call for at least two parallel α-decay sequences starting from at least two different states of 289Fl. This implies that close-lying levels in nuclei along these chains have quite different spin-parity assignments. Further, observed α-electron and α-photon coincidences, as well as the α-decay fine structure along the decay chains, suggest a change in the ground-state spin assignment between 285Cn and 281Ds. Our experimental results, on the excited level structure of the heaviest odd-N nuclei to date, provide a direct testing ground for theory. This is illustrated by comparison with new nuclear structure calculations based on the symmetry-conserving configuration mixing theory

Recoil-α-fission and recoil-α-α-fission events observed in the reaction 48Ca + 243Am

Products of the fusion-evaporation reaction 48Ca + 243Am were studied with the TASISpec set-up at the gas-filled separator TASCA at the GSI Helmholtzzentrum für Schwerionenforschung, Darmstadt, Germany. Amongst the detected thirty correlated α-decay chains associated with the production of element Z=115, two recoil-α-fission and five recoil-α-α-fission events were observed. The latter five chains are similar to four such events reported from experiments performed at the Dubna gas-filled separator, and three such events reported from an experiment at the Berkeley gas-filled separator. The four chains observed at the Dubna gas-filled separator were assigned to start from the 2n-evaporation channel 289115 due to the fact that these recoil-α-α-fission events were observed only at low excitation energies. Contrary to this interpretation, we suggest that some of these recoil-α-α-fission decay chains, as well as some of the recoil-α-α-fission and recoil-α-fission decay chains reported from Berkeley and in this article, start from the 3n-evaporation channel 288115