Decay times for second-chance fission of 239U studied by crystal blocking

Neutron-induced fission of ²³⁸U has been studied by the crystal-blocking technique for neutron energies just below and above the threshold for second-chance fission. In agreement with earlier measurements, in this energy range the lifetime for first-chance fission is found to be too short to have an observable effect on the blocking dips. Above the threshold, however, an appreciable filling-in of the dips is observed. The results are analyzed in terms of a two-component lifetime distribution and then indicate an average lifetime of a few fsec for second-chance fission at a neutron energy of E_n ≈ 7.2 MeV, in agreement with results from a simple calculation. It is shown that in this analysis it is important to take into account the anisotropy of the fission-fragment distribution and, in particular, the difference between the angular distributions for first- and second-chance fission.