The positive odd-even effects observed in cold fragmentation — are they real?

Abstract

The experimental fission-fragment-nuclide yield in the cold region of the spontaneous fission of 252Cf was evaluated for bins of constant total excitation energy TXE. The result showed that the previously seen undulatory structure of five to six mass units in the proton odd-even effect as a function of mass δ Z ( A L) disappeared. These undulatory structures which were found in many other investigations are artificial and depend on the way the evaluation cut through the nuclide-yield landscape is made with respect to the Q-value surface Q( A L, Z L). The TXE-bin-integrated odd-even effects for protons and neutrons δ Z and δ N became negative, where δ N has a 1.5 to 2.5 times larger negative value than δ Z . These negative values imply a preference for odd neutron and proton numbers compared with even ones for fragmentations with low total excitation energy. The reason for this behaviour can be the higher level density for odd-odd deformed nuclei compared to even-even nuclei. The preponderance of odd nucleon numbers in fragments with low TXE shows that for even-even compound systems pair breaking must occur in the very early stages of fission.