Neutron transmutation doping is used to create high-quality silicon with a specific target resistivity. By implementing neutron absorbers, it is possible to obtain a broader range of postirradiation resistivities. To develop this method, the influence of neutron absorbers on the reactor spectrum in Belgian Reactor 1 was numerically simulated and experimentally verified. A comparison between the modeled reactor spectrum and the spectrum obtained through activation foils showed good agreement. These data were used to calculate the resistivity of silicon under cadmium and hafnium foils with different thicknesses after neutron irradiation. Experimental four-point probe measurements confirmed the calculated resistivities. Hence, the research shows that tailoring the reactor spectrum using neutron absorbers allows for a large range of final resistivities or doping concentrations in silicon during a single irradiation cycle.