Radiation Widths of Levels in Nuclei near Closed Shells

An experimental investigation has been made of the variations in the radiation widths of nuclear energy levels for isotopes in the regions of the neutron magic numbers, where fluctuations in level spacing and neutron binding energy are largest. Neutron transmission measurements were made using the Brookhaven fast chopper, and the Breit-Wigner level parameters were obtained for resonances in the following target isotopes: ${\mathrm{Sr}}^{87}$, ${\mathrm{Sb}}^{121}$, ${\mathrm{Sb}}^{123}$, ${\mathrm{Ba}}^{135}$, ${\mathrm{La}}^{139}$, ${\mathrm{Nd}}^{145}$, and ${\mathrm{Pt}}^{195}$. Other elements around closed neutron shells, namely, Rb, Zr, Nb, Ce, and Tl, were examined but no accurate measurements could be made of the radiation widths for these elements. Results show that the fluctuations in the measured radiation widths are small compared to the large fluctuations in neutron scattering widths, and that they are related to the level spacing and the effective level excitation energy. An effect arising from the closed shells at 82 and 126 neutrons is observed. Analysis of all the available "good" data indicates that the essential features of the theory of Blatt and Weisskopf are generally valid; that is, radiation widths are strongly dependent on the effective level excitation energy and weakly dependent on the level spacing. Experimental results are compared with predictions from semiempirical formulas.