Towards saturation of the electron-capture delayed fission probability: The new isotopes 240 Es and 236 Bk

Abstract

The new neutron-deficient nuclei 240 Es and 236 Bk were synthesised at the gas-filled recoil separator RITU. They were identified by their radioactive decay chains starting from 240 Es produced in the fusionevaporation reaction 209 Bi( 34 S,3n) 240 Es. Half-lives of 6(2) s and 22 +13 -6 s were obtained for 240 Es and 236 Bk, respectively. Two groups of particles with energies E = 8.19(3) MeV and 8.09(3) MeV were unambiguously assigned to 240 Es. Electron-capture delayed fission branches with probabilities of 0.16(6) and 0.04(2) were measured for 240 Es and 236 Bk, respectively. These new data show a continuation of the exponential increase of ECDF probabilities in more neutron-deficient isotopes.