Excitation of a Giant Isoscalar Resonance by α Particles

The inelastic scattering of 115-MeV $\alpha$ particles from ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$ shows an enhancement of the continuum at about 18.25 MeV excitation energy. This observation supports recent electron, proton, and ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ scattering analyses which ascribe similar enhancements at $E_x\approx \frac{63}{A^{\frac{1}{3}}}$ MeV to an isoscalar giant quadrupole resonance. Although contributions from a monopole excitation cannot be ruled out, attributing all observed yields to a giant quadrupole resonance exhausts only 32% of the energy-weighted sum rule.