Effects of phonon transfer on near-thermal neutron fission cross sections

Abstract

For low energy neutron-induced fission in the thermal and epithermal range, the momentum transferred in forming the compound nucleus is small enough that the compound nucleus may remain bound in the lattice until it decays. Phonons, therefore, may be emitted or absorbed in the neutron absorption process changing the energy of the compound nucleus from that which would be derived simply from measuring the incoming neutron energy. The probability and influence of phonon transfer to and from the lattice is calculated at energies below 1 eV and is shown to have a small but significant effect on the observed cross section. The magnitude of the effect is temperature dependent and ranges in size from a few tenths of 1% for /sup 235/U fission at thermal to 5% for /sup 239/Pu fission at the 0.3 eV resonance. Some of the effect can be accounted for by applying the usual Doppler broadening approximation in the thermal range.