Ratio of Asymmetric to Symmetric Fission ofPu239andPu241as a Function of Neutron Energy

Further radiochemical experiments have been carried out to test the theoretical suggestion that in low-energy neutron-induced fission, the asymmetric to symmetric fission ratio should depend on the spin of the fissioning nucleus. In the present work two kinds of experiments were done. In the first, with both ${\mathrm{Pu}}^{239}$ and ${\mathrm{Pu}}^{241}$, comparisons were made of the asymmetric/symmetric yield ratios for thermal fission with the corresponding ratios for gross resonance fission with epi-Sm neutrons. In the second kind, monoenergetic neutrons were employed to study variations of ${\mathrm{Pu}}^{239}$ fission yields with neutron energy over the 0.297-ev resonance. If the assumption is made that spin difference is the principal cause of change in the asymmetric/symmetric ratio in this energy region, the results indicate that this ratio differs by a factor of at least 5.3 between the two spin states of ${\mathrm{Pu}}^{239}$. This is the largest effect of this kind observed to date.