Direct observation of first-chance fission of 258No

The multiplicities, angular correlations, and energy spectra of the neutrons associated with fission were measured for the reactions of Mg-25,Mg-26 + Th-232 at Mg-25 and Mg-26 projectile energies of 132 and 148 MeV, respectively. From these data, we extracted a value of Gamma(n)/Gamma(f) for the first-chance fission of No-258 at an excitation energy of 61 MeV. The implications of this result for understanding the survival probabilities of excited heavy nuclei formed in hot fusion reactions are discussed.