Attempts to produce new americium isotopes near N=126

Abstract

Searches for new americium isotopes with neutron number around 126 are made in bombardments of $^{191,193}\mathrm{Ir}$ targets with $^{40}\mathrm{Ar}$ ions by employing the gas-filled recoil separator SHANS and the recoil-$\ensuremath{\alpha}$ correlation technique. No evidence for nuclides of interest is obtained, and the upper limits of the cross sections producing the evaporation residues are estimated to be 31.4 pb and 5.4 pb for the reactions of $^{40}\mathrm{Ar}+^{193}\mathrm{Ir}$ and $^{40}\mathrm{Ar}+^{191}\mathrm{Ir}$, respectively. We have performed microscopic finite-temperature Skyrme Hartree-Fock$+$ (Bardeen- Cooper- Schripffer theory) BCS calculations, and found that the fission barriers of compound nuclei in this region decrease rapidly with increasing proton number. The non-observation of new americium isotopes would be attributed to small survival probabilities of the $^{231}\mathrm{Am}$ and $^{233}\mathrm{Am}$ compound nuclei due to their much reduced fission barriers at high excitations.