Calculation of the thermal neutron flux in a 14.1 MeV neutron beam facility for application in BNCT

Abstract A small ionisation chamber with a magnesium wall, internally coated with 10 B (Mg/B-chamber) was used to determine the spatial distributions of thermal neutrons in a water phantom at a DT-14.1 MeV neutron facility. The measured distributions of thermal neutron flux can be described with a comparatively simple model: By superimposing single Gaussian distributions of thermal neutrons, each created by a “pencil beam” of fast neutrons, we have found parameters to fit the measured flux of thermal neutrons with an accuracy of 20 % (in the field of fast neutrons) to 40 % (outside the field). This model can be used as a basis to calculate the photon contamination (which is mostly due to the capture reaction 1 H(n,γ) 2 H) and the flux of thermal neutrons with respect to Boron Neutron Capture Therapy (BNCT) in a fast neutron beam.