Parameters and gamma-ray spectra of neutron resonances in the isotopes of mercury

Abstract

The Argonne fast chopper has been used to study the neutron resonances of mercury. The neutron transmissions of five samples of normal mercury and six isotopically enriched samples were measured in the energy interval from 10 to 30 000 eV. Twenty-eight resonances were isotopically identified and parameters, including the spin, were determined for most of them. Average values of level spacings, radiations widths, and neutron strength functions were derived from the individual parameters. The spacings and radiation widths exhibit the expected rapid increase as the doubly closed shell is approached. Unexpectedly, the total radiation widths of different resonances of Hg 201 are found to fluctuate by a larger factor than has been previously observed in any other nuclear species. The gamma-ray spectra and the multiplicity of gamma rays from capture in these resonances were measured and both quantities were found to differ from resonance to resonance in a way that is consistent with the fluctuations in the widths. Seven high-energy gamma rays were resolved in the capture gamma-ray spectra of three resonances in Hg 201. The absolute intensities of the transitions were determined by a least-squares fit of the data to an experimentally determined line shape. It was found that these high-energy transitions are on the average enhanced, relative to those of lower energy, by about a factor of 15 beyond the usual E γ 3 law. The remainders obtained by subtracting the partial widths of the seven high-energy transitions from the total radiation widths are constant, within errors, for the three resonances investigated. This indicates that the fluctuations in the total radiation width are the result of fluctuations in the intensities of the transitions of highest energy. The observed magnitude of the fluctuations is shown to be consistent with that expected from random fluctuations of the partial radiation widths.