Fission properties of 253Rf and the stability of neutron-deficient Rf isotopes

Background: An analysis of recent experimental data [J. Khuyagbaatar et al., Phys. Rev. C 104, L031303 (2021)] has established the existence of two fissioning states in 253Rf: The ground state anda low-lying isomeric state, most likely involving the same neutron single-particle configurations as in the lighter isotone 251No. The ratio of fission half-lives measured in 253Rf was used to predict the fission properties of the 1/2+ isomeric state in 251No and draw conclusions as to the stability against fission of even lighter Rf systems. Purpose: This paper focuses again on the fission properties of 253Rf and their impact on the stability of other neutron-deficient isotopes, using new and improved data collected from two experiments performed at the Flerov Laboratory of Nuclear Reactions in Dubna, Russia. Methods: 253Rf and 251No nuclei were produced in fusion-evaporation reactions between 50Ti and 48Ca ions and the atoms of isotopically enriched 204Pb targets. The nuclei of interest were separated from the background of other reaction products and implanted into a Si detector where their characteristic radioactive decays were observed through position and time correlations between detected signals. Results: Two fission activities with half-lives of 52.8(4.4) mu s and 9.9(1.2) ms were measured in the case of 253Rf, confirming the results of J. Kkuyagbaatar et al. A third state, at much higher excitation energy, was also observed through the detection of its electromagnetic decay to the 52.8-mu s state. This observation leads to the opposite quantum-configuration assignments for the fissioning states as compared to the ones established by Khuyagbaatar et al., namely, that the higher-spin state has the shortest fission half-life. This inversion of the ratio of fission hindrances between the low- and high-spin states is corroborated in the isotone 251No by the nonobservation of any substantial fission branch from the low-spin isomer. Conclusions: In going from 251No to 253Rf, the fission half-life of a specific quantum state is found to decrease by close to seven orders of magnitude. Large reductions of more than five and six orders of magnitude are also found between the fission half-lives of the ground states of 252No and 254Rf and between those of 254No and 256Rf, respectively, pointing to a similar rate of decrease inthe fission barrier as one removes neutrons from both systems. Following this trend to the N = 148 isotones, our results suggest that the fission half-life of the ground state of the next even-even Rf isotope 252Rf will be extremely short, possibly at the limit of existence of an atom.