Here we perform the matching of neutron counts in two detector gasses through capture reactions and radiation transport–optimized moderating materials. One of our detectors uses helium-3 (3He) gas and has been widely used as a neutron detection material in proportional detector tube designs. This study examines boron trifluoride (BF3) as a potential gas for neutron detection in place of 3He based on a previously studied “spectrally matched” design derived from deterministic adjoint analyses that closely mimic the spectral response of 3He. The integrated spectral response of each tube, i.e., the count rate, is calculated and measured at various distances from an isotropic neutron source where similar “total sources” are achieved in either detection system. Our results show the integrated spectral response of a dual BF3 tube detector was within 10% of a single 3He tube when exposed to a similar source. GEANT4 Monte Carlo simulations were used to calculate the total source for each detector and showed count rates within 5% of those produced by MCNP, providing a strong confidence in its behavior in the thermal energy regime. We provide results in this study to partially validate the replacement based on the spectrally matched design, which will lead to further validation through the utilization of multiple neutron spectra via simulated and experimental studies.