Shell Effects in Highly Excited Nuclei

The boron absorption method has been used to obtain the following estimates of the energy of the lowest neutron resonance for each of the following target nuclides: ${\mathrm{Rb}}^{85}$, 0.97 kev; ${\mathrm{Rb}}^{87}$, 0.42 kev; ${\mathrm{Y}}^{89}$, 5.0 kev; ${\mathrm{La}}^{139}$, 0.08 kev; ${\mathrm{Pr}}^{141}$, 0.38 kev; and ${\mathrm{Tl}}^{205}$, 10 kev. The corrections were made in such a way that the figure found is a lower limit for the energy and consequently a lower limit for the order of magnitude of the spacing of $s$-wave resonances. These figures were added to data in the literature, and a plot of $s$-level spacing was made which included most target nuclides of odd mass and odd charge. This curve showed roughly symmetric maxima at the neutron magic numbers 50, 82, and 126. The cases where the compound nucleus contains more neutrons than the magic numbers are qualitatively understandable, since these nuclides are in a low state of excitation compared to normal nuclei in their neighborhood. An unusually close level spacing, which would be expected on the basis of excitation energy when there are slightly less than a magic number of neutrons, was not observed. It appears (at least near 126 neutrons and 82 protons) that the shell structure is not broken up to any great extent in the compound nucleus.