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 10 spent MITR-II fuel elements cooled by
forced convection of heavy water coolant. 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 for the available
field sizes ranges from 3.2 to 4.6 x 109 n /cm2
s at the patient position. The measured specific absorbed doses
are constant for all field sizes and are well below the inherent background
of 2.8x10 -12 RBE Gy cm 2/n produced by epithermal
neutrons in tissue. The dose distributions achieved with the FCB
approach the theoretical optimum for BNCT.
