Quasifree (p,pα) Scattering onLi6,Mg24,Si28,Ca40,Ce140, andTh232at 156 MeV

Knockout of $\alpha$ particles by 156-MeV protons from ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$, ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$, ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$, ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$, ${}_{}{}^{140}{}_{}{}^{}\mathrm{Ce}$, and ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}$ has been used to investigate the $A$ dependence of $\alpha$ clustering in nuclei. Protons and $\alpha$ particles were detected in coincidence under conditions of quasifree scattering. Corresponding reaction cross sections were measured. Cross sections for ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$ and ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$ were found to be about 2 orders of magnitude lower than for ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$. The cross section for ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$ was found to be about 3 times lower than that for ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$ and ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$. Upper limits of the integrated cross sections for ${}_{}{}^{140}{}_{}{}^{}\mathrm{Ce}$ and ${}_{}{}^{232}{}_{}{}^{}\mathrm{U}$ are still compatible with a $A^{\frac{2}{3}}$ dependence of the $\alpha$ knockout process.