Abstract
Neutron beams adopted in radiobiology and radiotherapy always accompany secondary photons and charged particlesm which lead to a need of mixed radiation dosimetry. Paired ionization chambers method is usually used for dose measurement in mixed neutron/photon fields to separate the photon and neutron dose components which have different relative biological effectiveness. Dosimetry as one of beam characteristics is important because of the correlation with patient safety and radiation protection. Although this technique has been widely used for about 50 years, parameters and corrections involved in the dose derivation are still problematic and need a further and thorough study. Therefore, the purpose of this study is to establish a more accurate, complete and high quality paired ionization chambers detection system, and used in the boron neutron capture therapy (BNCT) beam at the Tsing Hua Open-pool Reactor (THOR). A magnesium chamber with argon gas (Mg(Ar)) and the other A150 tissue-equivalent plastic chamber with tissue-equivalent gas (TE(TE)) were used to determine the photon and neutron doses. The contents of this Ph. D. thesis work are consist of the following items: (A) establishment of verified chamber models and detailed response analysis including energy, angular and thickness dependent dose responses; (B) calibration and calculation of the lack parameters such as beam conversion factor according to the dosimetry protocol principle; (C) modification of dose derivation, activation contamination correction and determination of neutron and gamma-ray sensitivities; (D) neutron source verification based on activation detector reaction rates; (E) neutron and gamma-ray spectra calculation; and (F) measurements and verifications of the neutron and gamma-ray dose rates and corresponding uncertainty evaluation for measured values. In this study, the MCNP based TE(TE) chamber model, comparing to EGSnrc, FLUKA, GEANT4 and measurement of 7 realistic photon fields (60Co, keV and MeV level X-rays) as well as two MeV level electron fields, had perfect outcome. In the low energy region, the MCNP based Mg(Ar) chamber underestimated the detector responses, but it is still the most ideal candidate regarding the application of BNCT dosimetry where the hydrogen capture 2.2 MeV photons are dominant.The energy dependent neutron and photon sensitivities of two chambers by using the modified dose derivation were also investigated. The differences of neutron doses between calculations and measurements reduced ~10 percents based on the Monte Carlo calculated sensitivities while compared to those from paper citions. Finally, results of this study were applied to beam photon spectrum adjustment from measurements of Mg(Ar) chamber with different thickness build up caps, inter-center dose comparison between BNCT facilities at the THOR and High Flux Reactor, and beam quality control & quality assurance in the of the THOR BNCT clinical trials.