Neutron spectra in bimodal fission of 226Th

The neutron spectra associated with the two fission modes of 226Th have been measured in coincidence with the fragment yields of the 18O+208Pb reaction at Elab=78 MeV. The experimental neutron energy and angular distribution are analyzed in terms of multiple-source parameterizations, assuming three emitters. This analysis allows the determination of the mean multiplicities of neutrons originating from the compound nucleus (νpre) and from the fission fragments (νpost1+νpost2). For the first time, neutron multiplicities are extracted for the symmetric and asymmetric mode in bimodal fission. Pre-fission multiplicity for asymmetric mode is larger than one for symmetric, and this can be explained by the fact that time for asymmetric fission mode is longer than time for symmetric mode. (authors)