In boron neutron capture therapy (BNCT), the proportion of the fast neutron in the tumor (PFN) must be no more than 3%. If
a D–T neutron generator is used as a thermal neutron source in BNCT, the moderator must be optimized to decrease the PFN.
Based on the analysis of the theory, water, heavy water, polythene, graphite, lead, and tungsten were used to moderate the
fast neutrons. If the three-layer material is composed of a 4 cm thickness layer of tungsten, a 13 cm thickness layer of lead,
and a 23 cm thickness layer of heavy water, its thermalization efficiency (TE) is highest, which is increased by 191.5% than
the maximum TE moderated by single-layer materials and by 19.3% than the maximum TE moderated by double-layer materials.