Measurements of lineal energy spectra for the BNCT test beam of THOR

A 2.5 cm tissue-equivalent Rossi type proportional counter was fabricated at St Andrews University and used in this study. Boron-doped and non-boron chambers were performed to measure the lineal energy spectra of a test epithermal neutron beam, built for the research of boron neutron capture therapy (BNCT) at Tsing Hua Open-pool Reactor (THOR). Measurements were made using standard microdosimetry equipment, including a low pressure gas flow system, low noise charge sensitive preamplifier, linear amplifier, multi-channel analyzer, and americium-241 calibration alpha source. Chambers were positioned at different depths in an acrylic phantom. Lineal energy spectra were determined for several gas pressures to simulate various cell sizes. Spectra of the boron-doped chamber are dominated by a peak at ∼200 keV/μm corresponding to the traversal of alpha particles and lithium recoils through the chamber. Peaks in the non-boron spectra correspond to gamma-rays, recoil protons, and fast neutrons. (author)