A new method, utilizing a modulated neutron source operated in antisynchronism with a long counter, has been used to measure the absolute and relative delayed-neutron yields of five fissionable isotopes for neutron-induced fission at 3.1 and 14.9 MeV. The results, in units of delayed neutrons per fission, for 14.9-MeV fission are:

239Pu0.0043 ± 0. 0004
233U0.0043 ± 0.0004
235U0.0095 ± 0. 0008
238U0.0286 ± 0. 0025
232Th0.031 ± 0.003
The measured ratios of delayed-neutron yields at 3.1-MeV to those at 14.9-MeV fission are:
239Pu1.60 ± 0.09
233U1.79 ± 0.10
235U1.89 ± 0.11
238U1.71 ± 0.10
232Th1.94 ± 0.11
These new delayed-neutron yield data are in accord with qualitative theoretical predictions but are in marked disagreement with earlier measurements.