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The isotopic distribution of nuclei produced in the 50Ti + 249Cf reaction has been studied at the gas-filled recoil separator TASCA at GSI Darmstadt, which separates ions according to differences in magnetic rigidity. The bombardment was performed at an energy around the Bass barrier and with the TASCA magnetic fields set for collecting fusion-evaporation reaction products. Fifty-three isotopes located “north-east” of 208Pb were identified as recoiling products formed in non-fusion channels of the reaction. These recoils were implanted with energies in two distinct ranges; besides one with higher energy, a significant low-energy contribution was identified. The latter observation was not expected to occur according to kinematics of the known types of reactions, namely quasi-elastic, multi-nucleon transfer, deep-inelastic collisions or quasifission. The present observations are discussed within the framework of two-body kinematics passing through the formation of a composite system.
Two isomers decaying by electromagnetic transitions with half-lives of 4.7(1.1) and 247ð73Þ μs have been discovered in the heavy 254Rf nucleus. The observation of the shorter-lived isomer was made possible by a novel application of a digital data acquisition system. The isomers were interpreted as the Kπ ¼ 8−, ν2ð7=2þ½624; 9=2−½734Þ two-quasineutron and the Kπ ¼ 16þ, 8−ν2ð7=2þ½624; 9=2−½734Þ ⊗ 8−π2ð7=2−½514; 9=2þ½624Þ four-quasiparticle configurations, respectively. Surprisingly, the lifetime of the two-quasiparticle isomer is more than 4 orders of magnitude shorter than what has been observed for analogous isomers in the lighter N ¼ 150 isotones. The four-quasiparticle isomer is longer lived than the 254Rf ground state that decays exclusively by spontaneous fission with a half-life of 23.2ð1.1Þ μs. The absence of sizable fission branches from either of the isomers implies unprecedented fission hindrance relative to the ground state.