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The distribution of mass in the spontaneous fission of 256Fm has been investigated by the radiochemical determination of the fission yields for 28 mass chains. The mass distribution is asymmetric with a peak-to-valley ratio of -12. The average masses of the light and heavy groups are 111. 9 and 141.0, respectively. The fission-yield data indicate a value of 3+ 1 for v, the average number of neutrons emitted per fission. A comparison of the characteristics of the mass distribution for low-energy fission of nuclides ranging from Th to 56Fm is given.

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The mass distribution for spontaneous fission of '"Fm was determined for both preand post-neutronemission fragments. The post-neutron-emission (final) mass distribution was deduced from radiochemical measurements of 24 mass yields. Starting with an assumed distribution of neutron yield versus fragment mass, the pre-neutron-emission (initial) mass distribution and neutron-yield distribution were derived from the kinetic-energy measurements of the fragments and the final-mass distribution by an iterative method. These distributions together with distributions of fragment kinetic energy, total kinetic energy of both fragments, and average kinetic energy and average total kinetic energy as functions of fragment mass are presented. The results are discussed in terms of a scission-point model of fission.NUCLEAR REACTIONS, FISSION +Fm{sf); measured radiochemical mass yields, fragment E; deduced neutron yields, fragment mass and energy distributions.