Abstract
This study aims to well characterize the BNCT epithermal neutron beam at THOR through measurements. The foil activation method and the indirect neutron radiography were applied to measure the neutron intensity and the 2D spatial distributions. The measurements were not only performed free-in-air but also inside the PMMA phantoms. In addition to the original beam design for the brain tumor treatment, an extended PE collimator was recently designed for the head and neck tumor. Hence, the comparison of the neutron characteristics including intensity and 2D spatial distribution between these two conditions, with and without the extended collimator, was also carried out by measurements. The triple-foil sets consisting of gold, copper, and manganese as well as the foil sets with different arrangements of copper and cadmium were utilized as the activation detectors to determine the reaction rates. Regarding the neutron flux mapping, three major components were utilized in the indirect neutron radiography: IP as a radiation area detector, copper foil as activation converters, and cadmium plates as filters. According to the results of measurements, the angular distribution for epithermal neutrons is quite straight-forward, while the angular distribution for thermal neutrons is more close to isotropic, which is consistent with the original beam design. The contribution of the room-scattered neutrons was found to be only a few percent inside the irradiation field within 20 cm away from the beam exit. Once the PE extended collimator is mounted on the beam, owing to the scattering effect, the thermal neutron intensity will be built up and hence increase the ratio of the delivered neutron fluence at phantom surface to the maximum value, which benefit the treatment for head and neck tumors. It is also demonstrated that, for a beam characterization, the indirect neutron radiography coupling with the foil activation method can precisely determine the neutron intensity and the 2D spatial distributions in an effort-saving way. Furthermore, the measurements can provide essential information for the source validation.