Improving the performance of the FiR 1 BNCT facility

The option to improve the penetration of the epithermal neutron field of the FiR 1 BNCT facility using a filter, which hardens the spectrum, has been studied. The effect of a 6Li-filter to the thermal neutron flux as well as Au- and Mn-reaction rates in a PMMA phantom has been calculated and measured. The measured distribution using a 9 mm thick filter agreed well with the calculations. According to the calculations a close to optimal situation is achieved using a 5 mm thick 6Li-filter at the beam exit. That will bring the 50 % boron isodose 5 mm deeper and will increase the weighted tumour dose at the depth of 7 cm by 16%, and will attenuate the peak boron dose rate by only a factor of 2. The real benefit of the filter concept is demonstrated using the filtered beam model in the dose planning system with historical patient cases irradiated without the filter. The attenuation of the beam intensity due to the filter can be compensated for by increasing the reactor power. The feasibility of a power increase is studied in an ongoing project. Both the technical and licensing aspects are considered. There seem to be no technical obstacles for increasing the power from the currently licensed 250 kW to 500 kW.