Speaker
Description
The study of the fast neutron (En = 3-100 MeV) induced fission of actinide nuclei allows us to study the relative role of Coulomb and dissipative forces in a large scale nuclear collective motion. One is aware of the practical applications of knowing the energy release and its dependence on neutron energy and fragment mass. We have been engaged in studies of the total kinetic energy release in the fast neutron induced fission of 232Th, 233U, 235U, 237Np and 239Pu, which we will summarize. We have prepared the targets for these studies by vapor deposition which leads to very uniform, high purity targets, an essential component of such studies.
Our experiments were done at the LANSCE facility at LANL using the WNR “white” neutron spectrum beams, with typical intensities of 105 n/s. The neutron energies were measured by time of flight with typical uncertainties of 5%. Fission fragments were detected by 9 pairs of Si PIN diode detectors. Corrections were applied for the pulse height defect of the detectors and the fragment energy loss in the target and backing. Our apparatus, targets, etc. were benchmarked by measuring the TKE release in the thermal neutron induced fission of 233U, 235U and 239Pu. No normalizations of the data were needed and thus our measurements are absolute measurements.
The TKE distributions for each system studied were Gaussian in shape. In Figure 1, we show the measured TKE values (as a function of neutron energy) for several of the systems we have studied. The data are in good agreement with previous work with some deviations at higher energies. The data can be parameterized in terms of Viola scaling. The variances of the TKE distributions show jumps at neutron energies corresponding to 2nd, 3rd, etc, chance fission.
The fission mass distributions show the expected evolution from asymmetric fission at low neutron energies to symmetric fission at higher neutron energies. The data clearly show the importance of shell closures near A=134 reflecting the extra stability of Z=56, due to octupole deformation. The data are in general agreement with the predictions of the GEF model but with smaller jumps in the TKE release at nth chance fission energies.