Interpretation of delayed neutron phenomena

The Bohr-Wheeler mechanism of delayed neutron emission has been used, together with recent fission mass- and charge-distribution data, to interpret the observed characteristics of delayed neutrons from fission. This interpretation accounts satisfactorily for recently measured periods and relative abundances of the delayed neutrons from six fissionable nuclides: U²³³' U²³⁶, U²³⁸, Pu²³⁹, Pu²⁴⁰, and Th²³². A set of selection criteria are developed which lead to prediction of the “most probable” main precursors as well as possible contributors for each delayed neutron group. Assuming the most probable set of delayed neutron precursors, total delayed neutron yields have been calculated for each of the six fissionable nuclides studied. These calculated yields are in agreement with experiment, thus providing a reasonable explanation of the striking variation in observed total delayed neutron yields, which increase by an order of magnitude from Pu²³⁹ to Th²³² fission. Throughout the analysis, special attention is given to the dominant role of closed-shell effects in delayed neutron systematics.