Scattering of 100 and 172 MeV $\alpha$ particles has been used to study giant resonances in ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}$ and ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$. First, experiments have been performed at scattering angles between 8 and 17° using a 1 m scattering chamber. Consistent fits to these data at both incident energies indicate that the giant monopole resonance in these nuclei splits into two parts separated by more than 4 MeV. This is confirmed by new magnetic spectrograph results which have been obtained in the angular region 3°-5°. A splitting of the monopole strength as deduced from our data can be explained by mixing of giant monopole and quadrupole resonances in these deformed nuclei. From the angular dependence of the differential cross sections evidence for the existence of odd parity ($L=1,3\mathrm{}$) giant resonances at high excitation energies is obtained. From the small angle data the giant octupole resonance was located at $E_x=19.6\mathrm{}\pm 1$ MeV with a width $\Gamma \left(\mathrm{FWHM}\right)=6.5\mathrm{}\pm 1$ MeV.

NUCLEAR REACTIONS ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}$, ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$($\alpha$,${\alpha }^{\prime }$); $E_{\alpha }=100\mathrm{} \mathrm{and} 172$ MeV; measured $\sigma (\ominus )$. Deduced excitation strengths for multipolarities $L=0-4\mathrm{}$, splitting of the $L=0\mathrm{}$ strength.

• Received 28 October 1981

DOI:https://doi.org/10.1103/PhysRevC.25.2939