Study of the production and decay properties of neutron-deficient nobelium isotopes
Abstract
The new neutron-deficient isotope 249No was synthesized for the first time in the fusion-evaporation reaction 204Pb(48Ca,3n)249No. After separation, using the kinematic separator SHELS, the new isotope was identified with the GABRIELA detection system through genetic correlations with the known daughter and granddaughter nuclei 245Fm and 241Cf. The alpha-decay activity of 249No has an energy of 9129(22)~keV and half-life 38.3(2.8) ms. An upper limit of 0.2\% was measured for the fission branch of 249No. Based on the present data and recent information on the decay properties of 253Rf and aided by Geant4 simulations, the ground state of 249No is assigned the 5/2+[622] neutron configuration and a partial decay scheme from 253Rf to 245Fm could be established. The production cross-section was found to be σ(3n)=0.47(4) nb at a mid-target beam energy of 225.4 MeV, which corresponds to the maximum of the calculated excitation function. Correlations of the 249No alpha activity with subsequent alpha decays of energy 7728(20) keV and half-life 1.2-0.4+1.0 min provided a firm measurement of the electron-capture or β+ branch of 245Fm to 245Es. The excitation function for the 1n, 2n and 3n evaporation channels was measured. In the case of the 2n-evaporation channel 250No, a strong variation of the ground state and isomeric state populations as a function of bombarding energy could be evidenced.
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