Characterization of the relationship between neutron production and thermal load on a target material in an accelerator-based boron neutron capture therapy system employing a solid-state Li target

Abstract

An accelerator-based boron neutron capture therapy (BNCT) system that employs a solid-state Li target can achieve sufficient neutron flux derived from the 7Li(p,n) reaction. However, neutron production is complicated by the large thermal load expected on the target. The relationship between neutron production and thermal load was examined under various conditions. A target structure for neutron production consists of a Li target and a target basement. Four proton beam profiles were examined to vary the local thermal load on the target structure while maintaining a constant total thermal load. The efficiency of neutron production was evaluated with respect to the total number of protons delivered to the target structure. The target structure was also evaluated by observing its surface after certain numbers of protons were delivered. The yield of the sputtering effect was calculated via a Monte Carlo simulation to investigate whether it caused complications in neutron production. The efficiency of neutron production and the amount of damage done depended on the proton profile. A more focused proton profile resulted in greater damage. The efficiency decreased as the total number of protons delivered to the target structure increased, and the rate of decrease depended on the proton profile. The sputtering effect was not sufficiently large to be a main factor in the reduction in neutron production. The proton beam profile on the target structure was found to be important to the stable operation of the system with a solid-state Li target. The main factor in the rate of reduction in neutron production was found to be the local thermal load induced by proton irradiation of the target.

Neutron flux evaluation model provided in the accelerator-based boron neutron capture therapy system employing a solid-state lithium target

“…Recent studies have indicated that accelerator-based neutron sources can achieve sufficient neutron flux for use in BNCT 20 – 24 . Two main types of accelerator-based neutron sources are generally considered to acquire neutrons for accelerator-based BNCT systems.…”

“…Two main types of accelerator-based neutron sources are generally considered to acquire neutrons for accelerator-based BNCT systems. To generate the neutrons, one source utilizes the reaction of 7 Li(p, n) 7 Be, whereas the other source utilizes the reaction of 9 Be(p, n) 9 B 20 – 26 . In the former system, the maximum generated neutron energy is below 1 meV and the incident proton energy of around 2.5 meV is generally considered, whereas that for the latter system is higher by a few MeV (i.e., the incident proton energy is higher than 8 meV) 20 – 24 .…”

“…To generate the neutrons, one source utilizes the reaction of 7 Li(p, n) 7 Be, whereas the other source utilizes the reaction of 9 Be(p, n) 9 B 20 – 26 . In the former system, the maximum generated neutron energy is below 1 meV and the incident proton energy of around 2.5 meV is generally considered, whereas that for the latter system is higher by a few MeV (i.e., the incident proton energy is higher than 8 meV) 20 – 24 . Therefore, the advantage of the former system is that the lower neutron energy facilitates a more compact BNCT system as the generated neutrons can be easily moderated to produce the ideal neutron energy (approximately 10 keV).…”

“…This is related to reduced shielding requirements and moderator for the system. However, a disadvantage is that the melting point of Li is lower than that of Be as a high thermal loading is required on the Li target 20 , 27 , 28 . Considering these properties, the National Cancer Center Hospital (NCCH), Tokyo, Japan, is conducting a clinical study using an accelerator-based BNCT system employing the Li target to evaluate its efficacy in clinical oncology 20 , 23 , 24 .…”

“…In an accelerator-based BNCT system employing a Li target, a huge number of the 7 Li(p, n) 7 Be reaction is required to acquire a sufficient number of neutrons, which can result in a high thermal loading 20 , 27 , 28 . Previous studies suggested that the degradation of the Li target, such as thinning and damage, could be expected owing to ion-impacts, high operating temperatures, and other factors resulting from proton bombardment.…”