Compton scattering of 87 MeV photons by 4He

Differential cross sections for Compton scattering by 4He have been measured at photon energies ranging from 83.7 to 93.8 MeV with a weighted average of 87 MeV using the bremsstrahlung beam of the MAX-LAB (Lund) nuclear-physics facility. The experimental differential cross sections show a pronounced increase at backward angles, strongly differing from predictions based on zero-angle dispersion relations and on the standard method used to calculate the differential cross section at nonzero angles. It is shown that the attempt to explain this effect in terms of quadrupole strength leads to unreasonable properties of the photoabsorption cross section. A parametrization in terms of a shift in the multipolarity ratio of the electromagnetic polarizabilities of the bound nucleon leads to Δβ = − Δα = 7 × 10−4 fm3.