(γ, n) Cross Sections of Nuclides near Neutron Number 50

Measurement of the ($\gamma , n$) cross sections of the nuclides ${\mathrm{Sr}}^{86}$, ${\mathrm{Sr}}^{87}$, ${\mathrm{Sr}}^{88}$, ${\mathrm{Y}}^{89}$, ${\mathrm{Zr}}^{92}$, together with earlier measurements for the nuclides ${\mathrm{Zr}}^{90}$, ${\mathrm{Zr}}^{91}$, and ${\mathrm{Nb}}^{93}$, show a pronounced narrowing of the giant dipole resonances in the region of neutron number 50, the narrowest resonances being those for the 50-neutron nuclides. Possible confusion of the interpretation of the data due to ($\gamma , 2n$) and ($\gamma , \mathrm{pn}$) processes is shown to be inadequate to explain the observed effects, and the narrowing is concluded to be real, in agreement with the prediction of the single-particle model of Wilkinson. The changes in widths are observed to occur entirely on the high-energy sides of the curves, the low-energy sides being remarkably similar for all 8 nuclides. A regularity in the behavior of the peak values of the cross sections is also observed. The ($\gamma , n$) threshold of ${\mathrm{Sr}}^{86}$ is observed to be at about 11.5 Mev, in agreement with other reaction data, but in disagreement with an earlier tentative assignment based on neutron yield measurements on natural Sr.