Air ionization chamber combined with LiCaAlF6 scintillator for y-ray dose evaluation in boron neutron capture therapy

The boron neutron capture therapy (BNCT) irradiation field includes neutrons with a wide energy range and undesirable high-intensity y-rays. Although several y-ray dose evaluation methods have been proposed, eliminating thermal neutron sensitivity is challenging; therefore, we focused on ionization chambers used in other radiotherapy centers. Since the y-ray dose evaluation with gas-flowing ionization chambers cannot be used in hospitals because of the requirement for treating activated gas and the setting complexity, a method with no gas-flowing ionization chamber is required. In this study, we developed a y-ray dose evaluation method and combined the air ionization chamber with a LiCAF scintillator. The LiCAF scintillator subtracts the neutron response of the chamber from the total response. In this study, irradiation tests at the BNCT field were conducted to obtain the conversion factor from the events of the LiCAF scintillator owing to the neutron response of the chamber via the conventional y-ray dose evaluation in the gas flow chambers. In addition, since the conversion factor varies for each irradiation field and cannot be evaluated experimentally in hospitals, we developed a method to calculate the factor by Monte Carlo simulation. Moreover, the sensitivity of the air ionization chamber as neutron energy range and the perturbation for the combined chamber and LiCAF scintillator were discussed.