Spontaneous fission properties of 2.9-s 256No.

Abstract

We have measured the mass and kinetic-energy distributions of fragments from the spontaneous fission of {sup 256}No produced via the {sup 248}Cm({sup 12}C,4{ital n}) reaction. The production cross section using 71-MeV {sup 12}C projectiles was found to be 250 nb. The total kinetic energy for spontaneous fission of {sup 256}No is 196{plus minus}3 MeV. The mass distribution is very broad (full width at half maximum of {similar to}50 mass units) with no appreciable decrease in yield for symmetric mass division. {sup 256}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 {sup 257}Fm, the isotope at which this transition occurs in the Fm isotopes, but the {sup 256}No mass distribution is broader than that for {sup 257}Fm, and its average total kinetic energy for symmetric mass division is about 15 MeV lower. We determined the half-life of {sup 256}No to be 2.91{plus minus}0.05 s by measuring its {alpha} decay. We measured a spontaneous fission to {alpha} ratio of 0.0053{sub {minus}0.0003}{sup +0.0006}, which gives a partial half-life for spontaneous fission of 550{submore » {minus}70}{sup +40} s. An energy of 8.448{plus minus}0.006 MeV was measured for the {alpha}-particle decay to the ground state of {sup 252}Fm, allowing us to calculate the mass excess for {sup 256}No as 87820{plus minus}8 keV. The energy of the 2{sup +} rotational level in the {sup 252}Fm daughter is 47{plus minus}5 keV, and the intensity of the 8.402-MeV {alpha} group populating this level is (13{plus minus}2)%.« less