Schematic diagram of the Massachusetts Institute of Technology Reactor (MITR). The fission converter based epithermal neutron irradiation (FCB) facility is housed in the experimental hall of the MITR and operates in parallel with other user applications. The FCB contains an array of 11 MITR-II fuel elements cooled by forced convection of heavy water coolant. The converter power is 120 kW at 6 MW reactor power. A shielded horizontal beam line contains an aluminum and Teflon® filter-moderator to tailor the neutron energy spectrum into the desired epithermal energy range. A patient collimator defines the beam aperture and extends into the shielded medical room to provide circular apertures ranging from 16 to 8 cm in diameter. The in-air epithermal flux is 6.2 × 109 n/cm2s at the patient position with the 12 cm collimator. The measured specific absorbed doses are constant for all field sizes and are well below the inherent background of 2.8 × 10-12 Gywcm2/n produced by epithermal neutrons in tissue. The dose distributions achieved with the FCB approach the theoretical optimum for BNCT. This facility is useful for clinical studies of superficial cancers and small animal studies.