The atomic cross section is often measured in cm-squared per atom, the linear absorption coefficient is in inverse cm per 1/e attenuation, and the mass attenuation coefficient is in grams per cm-squared to 1/e. In Compton scattering, there is another: Energy attenuation coefficient, because primary photons create scattered photons of lower energy and additional penetration. Also, at very high energies, photons create high energy showers comprised mainly of electron, positrons, and photons (plus pions, etc.)