In order to enable the measurement of nuclear structure effects on the emission of x rays following ($\alpha ,xn$) reactions, thin samples of Pb isotopes have been irradiated with $\alpha$ particles in 2-MeV steps between 30 and 110 MeV. X-ray excitation-yield curves have been measured on ${}_{}{}^{206}{}_{}{}^{}\mathrm{Pb}$, ${}_{}{}^{207}{}_{}{}^{}\mathrm{Pb}$, ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$, and ${}_{}{}^{\mathrm{nat}}{}_{}{}^{}\mathrm{Pb}$ and show large variations with energy and the presence of maxima and minima. For each separated target isotope it is observed that the cross-section maxima and minima are correlated with the formation of a residual nucleus with odd and even neutron numbers, respectively. This effect is attributed to the level density in the residual nucleus which affects the transition energies and the conversion coefficients. In the case of ${}_{}{}^{206}{}_{}{}^{}\mathrm{Pb}$ and ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ the nuclear cross sections are available and it is then possible to calculate the average number ${\overline{n}}_K$ of $K$ ionizations per nuclear reaction; this parameter is found to increase with bombarding energy as it is also correlated with the neutron number. These results clearly show that the x-ray emission yield is strongly dependent on the neutron number of the residual nucleus.

• Received 8 April 1982

DOI:https://doi.org/10.1103/PhysRevA.27.833