Quality assurance validation of remote neutron spectrometer for boron neutron capture therapy.

Background: The neutron beam in a boron neutron capture therapy (BNCT) irradiation field comprises a range of energies with different relative biological effectiveness. The neutron energy spectrum can change over time due to variations in the neutron source. Current methods for measuring the neutron energy spectrum are impractical and have significant limitations, such as being time-consuming and posing radiation exposure risks; therefore, neutron energy spectrum measurement has not been incorporated into routine BNCT quality assurance (QA) procedures. To address these issues, we developed a cylindrical hemisphere accurate remote multilayer spectrometer (CHARMS) that integrates a liquid moderator supply and drainage system with real-time neutron detection for suitable use in the BNCT QA procedure.

Purpose: To validate CHARMS for QA procedures in BNCT irradiation field.

Methods: We conducted experimental validations of CHARMS at the Heavy Water Neutron Irradiation Facility of Kyoto University Reactor under two irradiation conditions (with and without a collimator), performing three separate measurement sessions over 3 months.

Results: The total measurement time required by CHARMS to achieve a target neutron count uncertainty below 1% was less than 10 min. Monitoring the neutron counts at ten uniformly spaced intervals during each measurement showed that most counts fell within a Poisson-derived standard deviation. The neutron energy spectrum under irradiation without collimator was successfully evaluated. However, because of the effects of the neutron beam intensity and angular distribution in BNCT, the neutron energy spectrum under irradiation with collimator could not be properly evaluated.

Conclusion: The validity of the CHARMS for QA procedures in BNCT was confirmed. The rapid measurements and stable operation of the liquid moderator injection and drainage system show that CHARMS is well-suited for routine BNCT QA, eliminating the need for moderator replacement and thereby minimizing radiation exposure. Future work will address the challenges related to neutron beam intensity and angular distribution to enable the evaluation of neutron energy spectrum unfolding under collimated irradiation conditions, which is essential for clinical BNCT.