Fine structure is observed in the velocity distribution of ${\mathrm{U}}^{235}$ fission fragments that have been slowed by aluminum or nickel, but none is observed for unslowed fragments. Identification of this velocity fine structure with the fine structure in the fission mass yield confirms the influence of the 82-neutron shell in the fission act, as distinguished from its influence in post-fission neutron boil-off. The production of velocity fine structure by aluminum or nickel slowing is shown, by means of the measured range-velocity relation of fragments in these metals, to be the result of isobaric velocity bunching. The measured range-velocity relation of fragments in gold indicates no velocity bunching and therefore is consistent with the absence of velocity fine structure in measured velocity distributions of ${\mathrm{U}}^{235}$ fragments slowed in gold or platinum.

The absence of velocity fine structure in the velocity distribution of ${\mathrm{U}}^{233}$ and ${\mathrm{Pu}}^{239}$ fragments slowed by aluminum is discussed.

• Received 13 August 1954

DOI:https://doi.org/10.1103/PhysRev.96.1366