The photon-fluence scaling theorem for Compton-scattered radiation

This paper concerns a method of scaling photon fluence from one scattering material to another when the photon energies are such that the dominant mode of interaction is Compton scattering. The theorem establishes a one-to-one correspondence between points in the two scattering media where the spectra of primary and scattered photons have the same distribution in energy and angle, and where the fluence ratio equals the square of the electron density ratio. Experimental tests were made with cobalt-60 gamma radiation using ionization-chamber measurements in graphite, acrylic plastic, polystyrene, and water phantoms. The experimental results are consistent with the equality of photon spectral shapes and angular distributions at corresponding points. The fluence ratios may differ by a few percent from the predicted values, depending on distance from the source.