Prompt neutron emission and the second step of asymmetric nuclear fission

Summary

The apparent near-universality of the neutron yield dependence on fragment mass is a property of low-energy asymmetric nuclear fission of even-even trans-²⁰⁸Pb nuclei and can be explained in a two-step theory of fission. For nuclei in which a light cluster can be formed from the valence nucleons in the first step, the formation of fission fragments in a second step indeed results from a near-universal nucleon transfer mechanism. There is no reason why heavy fragments with mass around 132 should lose neutrons, as they are kinds of rumps of this nucleon transfer, and are stabilized by magic proton and neutron nymbers, whereas the complementary fragments, resulting from the capture of 76 nucleons, or fewer, by the light cluster, can lose part of their valence neutrons. For nuclei in which no stable cluster can be formed, as is the case for²¹³At, a different$\bar v(A)$ curve has to be expected.