Compound states of high excitation in ${\mathrm{Ne}}^{22}$ and ${\mathrm{Si}}^{30}$ have been observed in the total neutron yield from the reaction ${\mathrm{O}}^{18}(\alpha , n){\mathrm{Ne}}^{21}$ and ${\mathrm{Mg}}^{26}(\alpha , n){\mathrm{Si}}^{29}$. In the case of ${\mathrm{Ne}}^{22}$, twenty-five resonances were observed, varying in width from 5 to 150 keV for alpha bombarding energies from 2.5 to 5 MeV (excitation energy from 11.7 to 13.8 MeV). The reaction ${\mathrm{Mg}}^{26}(\alpha , n){\mathrm{Si}}^{29}$ showed forty resonances varying in width from less than 10 to 60 keV for alpha energies from 3 to 5.3 MeV (excitation energy from 13.3 to 15.3 MeV). Absolute cross sections were measured for both reactions. A statistical analysis of the area under the excitation curves gives an alpha-particle strength function ${\overline{S}}_{\alpha }={0.02}_6$ for ${\mathrm{O}}^{18}$+$\alpha$ and ${\overline{S}}_{\alpha }={0.01}_6$ for ${\mathrm{Mg}}^{26}$+$\alpha$. Analysis of the individual resonances in ${\mathrm{O}}^{18}$+$\alpha$ gives a value of $\frac{〈{{\gamma }_{\alpha }}^2〉}{D}\le 0.04$, in agreement with the ${\overline{S}}_{\alpha }$ obtained by the statistical analysis. The strength functions are probably reliable to better than a factor of 2.

• Received 6 April 1962

DOI:https://doi.org/10.1103/PhysRev.128.299