Study of the production and decay properties of neutron-deficient nobelium isotopes

Abstract

The new neutron-deficient isotope \(^{249}\)No was synthesized for the first time in the fusion-evaporation reaction \(^{204}\)Pb(\(^{48}\)Ca,3n)\(^{249}\)No. 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 \(^{245}\)Fm and \(^{241}\)Cf. The alpha-decay activity of \(^{249}\)No 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 \(^{249}\)No. Based on the present data and recent information on the decay properties of \(^{253}\)Rf and aided by Geant4 simulations, the ground state of \(^{249}\)No is assigned the 5/2\(^+\)[622] neutron configuration and a partial decay scheme from \(^{253}\)Rf to \(^{245}\)Fm could be established. The production cross-section was found to be \(\sigma \)(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 \(^{249}\)No 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 \(\beta ^{+}\) branch of \(^{245}\)Fm to \(^{245}\)Es. The excitation function for the 1n, 2n and 3n evaporation channels was measured. In the case of the 2n-evaporation channel \(^{250}\)No, a strong variation of the ground state and isomeric state populations as a function of bombarding energy could be evidenced.