Radiation Widths of Nuclear Energy Levels

The highly excited energy levels formed by capture of slow neutrons can be studied by means of the resonances in neutron cross sections as functions of energy. In the present work the radiation widths of levels in heavy nuclei have been measured by means of total cross section curves obtained with the Brookhaven fast chopper. The "shape," "area," and "interference" methods of analyzing the neutron transmission data are described. The radiation widths obtained, together with results of similar measurements, show that radiation widths of levels in the same nuclide are nearly constant, the observed variations from level to level being of the order of the experimental error. The radiation widths decrease slowly with atomic weight, except for discontinuities at nuclear shells; these discontinuities can be satisfactorily correlated with variations in excitation energy and level spacing at the shells. The variation of radiation width with excitation energy and level spacing is consistent with theoretical calculations for electric dipole transitions; the absolute theoretical widths are too large by an order of magnitude, however.