Measurements of cross sections for the fusion-evaporation reactions 204,206,207,208Pb+48Ca and 207Pb+34S: Decay properties of the even-even nuclides 238Cf and 250No

In preparation for recent experiments on the synthesis of superheavy nuclei with Z=114 in the reaction ²⁴⁴Pu+⁴⁸Ca, we modified the Dubna gas-filled recoil separator and its detection system and carried out bombardments of lead targets with ⁴⁸Ca projectiles. We studied excitation functions of the reactions ²⁰⁶Pb(⁴⁸Ca,1-4n) and ^204,207,208Pb(⁴⁸Ca,2n). Maximum cross sections for the evaporation of 1-4 neutrons in the complete fusion reaction ²⁰⁶Pb+⁴⁸Ca were measured to be σ_1n=60 nb, σ_2n=500 nb, σ_3n=30 nb, and σ_4n=0.3 nb. In the bombardment of an enriched ²⁰⁴Pb target, we simultaneously obtained excitation functions of the ^{204,206,207,208}Pb(⁴⁸Ca,2n) reactions induced on the isotopic admixtures present in the target material. The maximum cross sections for the evaporation of two neutrons from the compound nuclei ²⁵⁶No, ²⁵⁵No, and ²⁵²No were measured to be 2.1 μb, 1.3 μb, and 10 nb, respectively. The spontaneously fissioning even-even isotope ²⁵⁰No, with a half-life T_1/2=36 μs, was identified for the first time in this experiment. In the reaction ²⁰⁷Pb+³⁴S, we measured the excitation function for the production of the 21-ms spontaneously fissioning isotope ²³⁸Cf, confirming our preliminary identification of this nuclide based on the results of cross bombardments.