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 $\ddot{I}ƒ$(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 $β^{+}$ 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.