Photon-Nucleus Total Cross Sections

Abstract

Quantitative predictions for the energy and $A$ dependence of the cross sections for nuclear photoabsorption and inelastic electron-nucleus scattering are given. In general, the nucleons do not contribute equally to the total photon-nucleus cross section when coherent contributions of photoproduced hadrons are taken into account. At low energies ($E_{\gamma }\sim 1$ BeV), the cross sections are proportional to nuclear number $A$, but at high energies, they become proportional to the number of surface nucleons—provided that the photon interactions are mediated by hadrons of sufficiently low mass. The condition on the masses is that the momentum transfers in forward photoproduction of these states should be small compared to the reciprocals of their mean free paths in nuclear matter. In the case of $\rho$ dominance, the real-photon photoabsorption cross section has the same $A$ dependence as hadron-nucleus total cross sections for photon energies above $\approx 10$ BeV, whereas the cross section for virtual photon absorption at that energy, obtained from inelastic electron scattering is nearly proportional to $A$ for spacelike momentum transfer $\left|Q^2\right|\gtrsim 5$ Be${\mathrm{V}}^2$. We then generalize to an arbitrary spectrum of intermediate particles, and discuss the sensitivity of feasible experiments to various models in which the spectrum contains important structure beyond the $\rho$. Measurements of the photon-nucleus cross sections will provide a fundamental test of "hadron dominance" in general, and of $\rho -\omega -\phi$ dominance in particular, as well as help to determine the basic parameters of photon-nucleon and $\rho$-nucleon interactions. We also calculate the photon-deuteron cross section and discuss the multiple scattering approach to photon-nucleus interactions. This discussion provides insight into the many-body processes which underlie the eikonal, optical-model calculations; it is also relevant to the determination of ${\sigma }_{\gamma n}$ at high energies.