Engineering Design of a Spallation Reaction-Based Neutron Generator for Boron Neutron Capture Therapy

Abstract

An engineering design of an epithermal neutron generator for boron neutron capture therapy (BNCT) has been completed, which utilizes the spallation reaction by protons accelerated to 50 MeV. The critical issues for realization of the neutron generator are the mechanical structure of a target with cooling capability and its integrity under operating conditions with powers as high as 50 MeVÂ300 mA.The integrity of a target structure design has been confirmed by thermal and stress analyses with a finite element method code ANSYS. Moreover, a target replacement strategy is also studied based on a radioactivity evaluation performed by the IRACM code system.In addition to the target structure design, the neutronics design has been optimized with the Monte Carlo code MCNPX. A high epithermal neutron flux of 1:8Â10 9 cm À2 Ás À1 has been achieved at the aperture of the collimator, which allows a RBE dose of over 30 Gy-eq to be delivered to a brain tumor within 5.9 cm in phantom depth for a therapeutic time of 31 min.

Increase in Irradiation Beam Intensity by Using a Hybrid Target System in Cyclotron-Based Neutron Capture Therapy

“…12) By using the additional reflector, the flux shown in Fig. 8 calculated without the reflector was successfully enhanced by a factor of about 3.…”

“…The beam-shaping assembly used to generate epithermal neutrons from the sources was designed based on Ta target system studies. 6,9,12) The epithermal neutron flux and in-air neutron kerma rate were calculated at an exit of a collimator placed at the end of the assembly. The energy ranges of the epithermal neutrons and fast neutrons were defined to be 0.5 eV to 10 keV and over 10 keV, respectively, according to the recommendation by the IAEA.…”

“…Tahara et al indicated that for the system using a Ta target and 30 MeV protons, a filter thickness less than 40 cm resulted in a somewhat low ratio of tumor dose to normal tissue dose, and that of 60 cm was too thick, resulting in a dose decrease to a deep-seated tumor. 12) In addition, Yonai et al showed that systems using the Ta target at proton energy from 50 to 600 MeV with a filter of 40 cm thickness have differences of a few Gy-Eq (absorbed dose unit equivalent to gamma ray dose) in the maximum tumor dose despite the difference in the neutron spectra at the target. 6) We chose an approach of adjusting the iron thickness without varying the filter thickness in order to have an equal harmful effect of fast neutron dose.…”

“…6) In some design studies using protons of tens of MeV, the moderator/filter was surrounded by a lead reflector to prevent neutron leakage from the moderator side. 6,7,12) In this study, as we discussed the dependence of the moderator thickness on the ratio of high-energy neutron production to total neutrons, the reflector was placed behind the moderator as viewed from the patient position without surrounding the moderator/filter, as shown in Fig. 4.…”

Exploration of Adiabatic Resonance Crossing Through Neutron Activator Design for Thermal and Epithermal Neutron Formation in 99Mo Production and BNCT Applications

“…Yonai et al 14 and Tahara et al 15 investigated a neutron source using spallation reactions that occur for 30-50 MeV protons incident on a tantalum target. Tahara et al showed that the epithermal neutron field required by BNCT is 1 mA for 30 MeV incident protons.…”