Scaling relative neutron depth-dose distributions from one phantom material to another: a comparison of experimental...

To correct percentage depth-dose data from one phantom material to another, experimental and theoretical scaling factors (SF) are compared for different neutron beam qualities. Differences of up to 10% were observed for different phantom materials relative to water. The ratio SF/rho was plotted as a function of H concentration by mass where rho is the mass density of the phantom material. A nearly linear relationship resulted at all energies for the theoretical scaling factors, while, for the experimental points, important deviations appeared at high energies for materials with relatively low H and high C concentrations. It can be shown that a single linear relationship for all compounds composed of H, C and O can only be valid if the ratio of total cross sections of carbon to oxygen is equal to 3/4. Experimental scaling factors will be more accurate than calculated values because of the uncertainty in the average total cross sections. If these factors for tissue equivalent (TE) liquid relative to water are converted to those of ICRU muscle by correcting to a mass density of 1.04 g cm-3, then the scaling factors are, within experimental uncertainty, equal to one.