Establishment of Neutron Shell Effects in (n, 2n) Cross-Sections at 14 MeV

Abstract

The existence of shell effects in ( n , 2 n ) cross-sections at 14 MeV neutron energy has been controversial for more than a decade. In the present work, recently available systematised data on ( n , 2 n ) cross-sections at 14.7 MeV has been reinvestigated critically in the vicinity of closed shells. These plots of σ( n , 2 n ) versus N and ( N - Z )/ A show definite shell effects at N =20,28 and 50 which have never been revealed so distinctly by earlier investigations. The disappearance of shell effect in heavy nuclei N =82 has been explained. It has been demonstrated that shell closure effect is perceptible clearly only in comparison to neighbouring nuclei of same N - Z and pairing effects. The distinct sharp rise of σ ( n , 2 n ) with ( N - Z )/ A >∼0.1, has been noted to be a characteristic of closed shell or sub-shell isotones only. An empirical relation for σ( n , 2 n ) is devised, for A ≧110.