We have measured the mass and kinetic-energy distributions of fragments from the spontaneous fission of ${}_{}{}^{256}{}_{}{}^{}\mathrm{No}$ produced via the ${}_{}{}^{248}{}_{}{}^{}\mathrm{Cm}$${(}^{12}$C,4n) reaction. The production cross section using 71-MeV ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ projectiles was found to be 250 nb. The total kinetic energy for spontaneous fission of ${}_{}{}^{256}{}_{}{}^{}\mathrm{No}$ 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. ${}_{}{}^{256}{}_{}{}^{}\mathrm{No}$ 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 ${}_{}{}^{257}{}_{}{}^{}\mathrm{Fm}$, the isotope at which this transition occurs in the Fm isotopes, but the ${}_{}{}^{256}{}_{}{}^{}\mathrm{No}$ mass distribution is broader than that for ${}_{}{}^{257}{}_{}{}^{}\mathrm{Fm}$, and its average total kinetic energy for symmetric mass division is about 15 MeV lower. We determined the half-life of ${}_{}{}^{256}{}_{}{}^{}\mathrm{No}$ to be 2.91±0.05 s by measuring its α decay. We measured a spontaneous fission to α ratio of 0.${0053}_{\mathrm{-}0.0003}^{+0.0006\mathrm{}}$, which gives a partial half-life for spontaneous fission of ${550}_{\mathrm{-}70}^{+40\mathrm{}}$ s. An energy of 8.448±0.006 MeV was measured for the α-particle decay to the ground state of ${}_{}{}^{252}{}_{}{}^{}\mathrm{Fm}$, allowing us to calculate the mass excess for ${}_{}{}^{256}{}_{}{}^{}\mathrm{No}$ as 87820±8 keV. The energy of the $2^{+}$ rotational level in the ${}_{}{}^{252}{}_{}{}^{}\mathrm{Fm}$ daughter is 47±5 keV, and the intensity of the 8.402-MeV α group populating this level is (13±2)%.

• Received 5 October 1989

DOI:https://doi.org/10.1103/PhysRevC.41.631