Development of fiber-insertable ionization chamber for BNCT

The Boron Neutron Capture Therapy (BNCT) irradiation field comprises high-intensity neutrons and gamma-rays that need to be measured separately because of their different biological effects. To conduct appropriate quality assurance (QA), we should measure the neutrons and gamma-rays. In a previous study, an air-ionization chamber combined with a fiber-type neutron monitor was constructed for QA. However, the fibers attached to the chamber were perturbed by neutron scattering. To eliminate the effect of perturbations, we developed an air ionization chamber into which the fiber could be inserted. Irradiation tests in the BNCT irradiation field showed that the perturbation effect was reduced by less than half, and it was confirmed that the system could measure neutrons and gamma-rays simultaneously for weekly QA. The sensitivity of the chamber to neutron energy was evaluated via simulations, and it was found that corrections for sensitivity to high-energy neutrons are required.