Multiplicities of Fission Neutrons

A large liquid scintillator with approximately 80 percent efficiency for detection of neutrons has been used to obtain data on numbers of neutrons emitted per fission for several fissioning nuclides. Reported here are the average number of neutrons per fission, $\overline{\nu }$, and the respective probabilities of 0, 1, 2,... neutrons per fission, for spontaneous fission of ${\mathrm{Pu}}^{240}$, ${\mathrm{Cm}}^{244}$, and ${\mathrm{Cf}}^{252}$, and for fission induced in ${\mathrm{U}}^{233}$, ${\mathrm{U}}^{235}$, and ${\mathrm{Pu}}^{239}$ by 80-kev neutrons. The values of $\overline{\nu }$ for these cases are, respectively, 2.257±0.045, 2.810±0.059, 3.869±0.078, 2.585±0.062, 2.47±0.03, and 3.048±0.079. The thermal neutron value for ${\mathrm{U}}^{235}$ (2.46±0.03), obtained by other methods, was used here as a standard for calibrating the detector efficiency. The efficiency was also measured by another method, involving scattering neutrons into the scintillator, with results in good agreement. The probabilities of 0, 1, 2,... neutrons per fission approach closely a binomial type distribution, with the maximum number of neutrons equal to 5, 6, or 7, depending on $\overline{\nu }$.