| Boron neutron capture therapy(BNCT) is a very promising cancer therapy technology. Neutron source is one of the key factors for the success of BNCT. For modern BNCT neutron sources, epithermal neutron(0.5 eV < En < 10 keV) flux is one of the basic physical characteristics. In recent years, higher energy fast neutrons(En > 10 keV) in BNCT neutron sources have been moderated as properly as possible, while fast neutrons from ten to several hundred keV still remain and they are difficult to completely remove. Such fast neutrons, whose energy are just slightly higher than epithermal neutron energy, have larger relative biological effectiveness, which can cause unwanted harm to normal tissues of human bodies. Thus, in order to evaluate the qualities of BNCT neutron sources and estimate the invasive neutron radiation doses against cancer patients during the treatments, it is necessary to accurately measure the fluxes of epithermal neutrons and neutrons from ten to several hundred keV in BNCT neutron sources, respectively.However, it is quite difficult to directly and accurately measure the fluxes of the above-mentioned two types of neutrons in BNCT neutron sources taking into account the neutron spectrum shapes, because there are no suitable spectrometers that can be used to directly measure the neutron spectra so far. Therefore, in this work, based on the basic principle of activation method to measure the neutron flux and using the 71Ga(n,?)72Ga activation reaction, the epithermal neutron flux monitor and neutron flux monitor from ten to several hundred keV with gallium nitride(GaN) wafers as activation material were designed by Monte Carlo simulations to accurately measure the fluxes of these two types of neutrons in BNCT neutron sources, respectively.In the presently designed epithermal neutron flux monitor, the activation material, i.e., GaN wafer, was positioned in the geometric center of a polyethylene sphere(neutron moderating material) covered with cadmium foil as the thermal neutron absorber material outside. The sensitivities of this monitor in the neutron energy range of 0.01 eV < En < 10 MeV were calculated by MCNP5 in this work. The simulation results showed that the monitor was very sensitive to epithermal neutrons and had flat sensitivity curve in epithermal neutron energy range, while its sensitivities to thermal(En < 0.5 eV) and higher energy fast neutrons were significantly low.The neutron flux monitor from ten to several hundred keV designed in this work consisted of two monitors, which were almost the same in shape and had an absorber/moderator/absorber/GaN wafer arrangement of structural materials from outside to inside. The differences between these two monitors were mainly the types of the used materials, the thicknesses of the neutron absorber materials and the diameter of the neutron moderating material. The sensitivities of these two monitors in the neutron energy range of 0.01 eV < En < 10 MeV were calculated by MCNP5 in this work, respectively. The simulation results indicated that by making difference of the sensitivities between these two monitors, the contributions of thermal, epithermal and higher energy fast neutrons to the monitor sensitivities were almost completely removed, and flat monitor sensitivity curve in the neutron energy range from ten to several hundred keV was extracted successfully.The experimental tests of the performances of the designed two different types of neutron flux monitors were carried out at the intense deuterium-tritium neutron source facility OKTAVIAN of Osaka University, Japan, respectively. The experimental results showed that these two different types of monitors can be employed to accurately measure the fluxes of epithermal neutrons and neutrons from ten to several hundred keV in BNCT neutron sources, and their measurement accuracies were estimated to be less than 5% and 10%, respectively.The two different types of neutron flux monitors designed in this work can accurately measure the fluxes of neutrons over wide energy ranges in BNCT neutron sources without measuring the neutron spectra, and this is exactly what the innovation of this work is. |
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