The angular distribution, relative to the direction of motion of the fragments, of the prompt fast neutrons emitted in the thermal neutron fission of ${\mathrm{U}}^{233}$, ${\mathrm{U}}^{235}$, and ${\mathrm{Pu}}^{239}$ has been measured. Collimated fission fragments were selected in energy in a gridded ionization chamber and coincident prompt neutrons in a given direction were counted by proton recoils in an electron collecting chamber filled with methane. The distributions obtained selecting only light fragments have been compared with curves computed on the basis of the evaporation of the neutrons from the moving fragments of the most probable mode. Reasonably good agreement is obtained if one postulates that in the fission of each of the three nuclides studied, the neutron emission probability is about thirty percent greater for the light fragment than for the heavy one. An upper limit of 4×${10}^{-14\mathrm{}}$ sec following fission may be placed on the time of emission of the neutrons.

• Received 9 July 1952

DOI:https://doi.org/10.1103/PhysRev.88.536