The Variation of Neutron Dose with Depth in a Tissue-Equivalent Phantom

The fast and slow neutron fluxes through a tissue-equivalent phantom have been measured for several incident neutron energies. The derivation of the components of dose to tissue, from 1 H( n,γ ) 2 D and 14 N( n,p ) 14 C reactions and proton recoils, based on these measurements, is given in detail. The work was directed towards experimental confirmation of the calculated values of Snyder and Neufeld and Kogan et al. for doses to a 30 cm slab phantom irradiated by monoenergetic neutrons. Fair agreement is shown with Snyder and Neufeld, but it is suggested that the gamma dose from 1 H( n,γ ) 2 D reactions is overestimated by the use of such a phantom. The results for 14 N( n,p ) 14 C doses are not in agreement with the calculated values of Kogan et al. It is suggested that the surface recoil proton doses obtained by them are incorrect, as they are considerably lower than the first collision recoil proton doses to tissue for neutrons of the energies considered. Experimental work was extended from the 30 cm slab phantom to a 20 cm thick elliptical phantom irradiated by monoenergetic and complex neutron spectra.