Fragment velocities, energies, and masses from fast neutron induced fission of U235

We report on a complete (2E,2v) experiment for fast neutron induced fission on U235. The energy dependence of fragment properties so far known only for thermal neutron induced fission is studied. Experimental problems as well as difficulties in data analysis are considered in detail in order to obtain clean and unbiased results. In particular, a self-consistent determination of the fragment total kinetic energy (EK,tot) was achieved by comparing the results obtained via the respective velocities and pulse heights. We find systematic discrepancies of 2 MeV if EK,tot is determined from the observed pulse heights with the calibration scheme of Schmitt et al. Therefore, refined calibration constants were deduced by comparison with accurate radiochemical mass yields. Measurements were performed at neutron energies of 0.50 and 5.55 MeV. Our results include mean values of fragment properties before and after neutron evaporation, e.g., of fragment velocities and masses, total kinetic energies, and the respective variances. We also show the distributions of fragment mass, of EK,tot, and of the variance of EK,tot. In addition, the number of prompt fission neutrons is given as a function of fragment mass. Our resolution of 2.1 mass units reveals fine structure not only in the fragment mass distribution but also in EK,tot(A*) and (A*). For the lower neutron energy of 0.50 MeV the present results compare reasonably well with similar measurements performed with thermal neutrons. Apparently the 0.5 MeV increase in saddle point excitation does not alter the results significantly. The improved accuracy of this measurement is demonstrated by comparison of our neutron emission data with direct measurements of fission neutrons. At the higher neutron energy of 5.55 MeV we observe the expected decrease of shell and pairing effects which indicates an increase in nuclear temperature. These results are in qualitative agreement with the model of Wilkins, Chasman, and Steinberg. However, a striking discrepancy exists for the number of fission neutrons, where we find that the increase in the total number of fission neutrons is totally accounted for by heavy fragments alone.