SPONTANEOUS FISSION PROPERTIES OF 2.9-S NO-256

We have measured the mass and kinetic-energy distributions of fragments from the spontaneous fission of No256 produced via the Cm248(12C,4n) reaction. The production cross section using 71-MeV C12 projectiles was found to be 250 nb. The total kinetic energy for spontaneous fission of No256 is 196±3 MeV. The mass distribution is very broad (full width at half maximum of 50 mass units) with no appreciable decrease in yield for symmetric mass division. No256 seems to be the transition nucleus between the asymmetric mass division observed for spontaneous fission of the lighter No isotopes and the symmetric mass division observed for the heavier No isotopes. Its properties are similar to those of Fm257, the isotope at which this transition occurs in the Fm isotopes, but the No256 mass distribution is broader than that for Fm257, and its average total kinetic energy for symmetric mass division is about 15 MeV lower. We determined the half-life of No256 to be 2.91±0.05 s by measuring its ± decay. We measured a spontaneous fission to ± ratio of 0.0053-0.0003+0.0006, which gives a partial half-life for spontaneous fission of 550-70+40 s. An energy of 8.448±0.006 MeV was measured for the ±-particle decay to the ground state of Fm252, allowing us to calculate the mass excess for No256 as 87820±8 keV. The energy of the 2+ rotational level in the Fm252 daughter is 47±5 keV, and the intensity of the 8.402-MeV ± group populating this level is (13±2)%. © 1990 The American Physical Society.