Neutron beam shaping system for BNCT

Abstract

The invention discloses a neutron beam shaping system for a BNCT, and relates to the technical field of neutron regulation and control, the system comprises a moderation module which is used for decelerating high-energy fast neutrons from a target material into surface thermal neutrons or thermal neutrons, the moderation module is made of an aluminum fluoride material, and a value adding module which is arranged between the target material and the moderation module and is used for adding the surface thermal neutrons or the thermal neutrons into the surface thermal neutrons or the thermal neutrons. The value-added module is used for increasing the neutron yield, the value-added module is made of a metal molybdenum material with the thickness of 9 cm, the gamma absorption module is used for absorbing gamma rays generated in the moderation process, and the gamma absorption module is made of a bismuth material with the thickness of 2.5 cm; compared with an existing BSA system, the neutron beam shaping system for the BNCT is more efficient and reliable in the aspects of neutron yield, ray purification capacity and dose space control, and the dose precision and biological selectivity of BNCT treatment can be improved.

Description

Technical Field

[0001] The present invention relates to the technical field of neutron regulation, and in particular to a neutron beam shaping system for BNCT. Background Art

[0002] In accelerator-driven boron neutron capture therapy (BNCT), the beam shaping system (BSA) is a key device that connects neutron production from proton bombardment of the target to clinical irradiation. The BSA is designed to convert high-energy fast neutrons generated in the target into therapeutically suitable thermal neutrons or surface thermal neutrons while minimizing the removal of non-therapeutic impurities such as fast neutrons and gamma rays, thereby improving the dose accuracy and biological selectivity of BNCT treatment.

[0003] Currently, common BSA systems generally consist of the following components: a moderator, a gamma-ray absorbing layer, a neutron reflector, a thermal neutron absorber, and a beam collimator. The moderator reduces neutron energy to the thermal or surface thermal neutron range required for treatment; the gamma-ray absorbing layer removes high-energy gamma rays generated during neutron reactions or moderation; the neutron reflector reflects stray neutrons back into the beam direction to increase the flux; the thermal neutron absorber reduces the neutron dose to the skin surface; and the collimator forms a collimated neutron beam through spatial confinement, enabling precise irradiation. Although existing BSA structures meet the basic requirements for BNCT clinical application to a certain extent, several issues remain that require optimization. For example, current systems face a trade-off between increasing neutron flux and suppressing radiation impurities, and some materials cannot achieve both high flux and a good neutron energy spectrum. Furthermore, there is a lack of systematic optimization methods for the selection and thickness control of the value-added layer, which hinders maximizing neutron yield. Summary of the Invention

[0004] The object of the present invention is to provide a neutron beam shaping system for BNCT to solve the problems existing in the prior art.

[0005] To achieve the above objectives, the present invention provides the following technical solution: a neutron beam shaping system for BNCT, the system comprising:

[0006] A moderator module is used to decelerate high-energy fast neutrons from the target material into surface thermal neutrons or thermal neutrons, and the moderator module is made of aluminum fluoride material;

[0007] The value-added module is placed between the target and the moderator module to increase the neutron yield. The value-added module is made of 9cm thick molybdenum metal.

[0008] A gamma absorption module, used to absorb gamma rays generated during the slowing down process, wherein the gamma absorption module is made of bismuth material with a thickness of 2.5 cm;

[0009] A reflector module, used to reflect scattered neutrons to enhance flux, is made of lead material, has an axial thickness and a radial thickness of 30 cm, and is installed at the periphery and end of the moderator module;

[0010] A thermal neutron absorption module, used to absorb excess thermal neutrons, wherein the thermal neutron absorption module is made of neodymium or boride material;

[0011] The collimation module is arranged at the outlet end of the shaping system and is used to form a neutron beam with uniform spatial distribution. The collimation module has a tapered channel structure and the inner wall is lined with boron-containing material to absorb scattered neutrons.

[0012] Preferably, the moderator module is cylindrical in structure, with an axial thickness of 30 cm and a radius of 20 cm. The maximum epithermal neutron flux with an axial thickness of 30 cm and a radius of 20 cm is obtained through Monte Carlo simulation verification.

[0013] Preferably, the bismuth material in the gamma absorption module can effectively absorb gamma rays with energy greater than 0.5 MeV and has a thickness of 2.5 cm.

[0014] Preferably, the reflection module is made of lead material, is arranged radially and downstream around the moderation module, and can reflect escaping neutrons toward the central axis.

[0015] Preferably, the absorption material used in the thermal neutron absorption module is cadmium, boride or lithium fluoride, which is used to adjust the neutron energy spectrum according to the depth of the treatment target area.

[0016] Preferably, the collimation module is a multi-section tapered channel structure, and the inner wall of the multi-section tapered channel structure is evenly embedded with boron-containing polyethylene material to improve beam uniformity and reduce side scattered neutrons.

[0017] Preferably, a sealing structure is further included, which is arranged at the gaps between the modules to prevent scattered neutrons and gamma rays from leaking from the structural gaps. The sealing structure is filled with boron-containing polyethylene or lead rubber material.

[0018] Preferably, a terminal sealing module is provided between the collimation module and the external shielding body. The terminal sealing module is composed of a composite of multiple layers of boron-containing polyethylene and high-density polymer sheets and is used for absorbing side-scattered and edge-scattered neutrons.

[0019] It can be seen from the above technical solution that the present invention has the following beneficial effects:

[0020] This neutron beam shaping system for BNCT, utilizing a multi-module architecture optimized through Monte Carlo simulations, offers significant advantages in achieving neutron energy spectrum conversion, radiation impurity filtering, and flux enhancement. The system comprises a value-added module, a moderator module, a gamma absorption module, a reflector module, a thermal neutron absorption module, and a collimator module. The materials and geometric parameters of each module are precisely matched and verified. In particular, the value-added module utilizes 9 cm thick molybdenum, significantly increasing neutron yield while ensuring cost and processability. The moderator module is constructed of aluminum fluoride with a 30 cm axial thickness and a 20 cm radius, effectively controlling fast neutron composition and increasing epithermal neutron flux. The gamma absorption module utilizes 2.5 cm thick bismuth to effectively suppress gamma rays. The reflector module is surrounded by a 30 cm thick lead layer to enhance neutron flux. The thermal neutron absorption module utilizes neodymium or boride to control thermal neutron dose. The collimator module utilizes a tapered channel structure supplemented by a boron-containing liner to improve neutron beam collimation and uniformity. Compared with the existing BSA system, this solution is more efficient and reliable in terms of neutron yield, radiation purification capability and dose spatial control, which helps to improve the dose accuracy and biological selectivity of BNCT treatment and meet the beam technical indicators recommended by the International Atomic Energy Agency (IAEA). BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is a graph of the epithermal neutron flux of the present invention;

[0022] Figure 2 This is the epithermal neutron curve of the present invention. DETAILED DESCRIPTION

[0023] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0024] like Figure 1 and Figure 2 As shown, the present invention provides a technical solution: a neutron beam shaping system for BNCT, the system comprising:

[0025] A moderator module is used to decelerate high-energy fast neutrons from the target material into surface thermal neutrons or thermal neutrons, and the moderator module is made of aluminum fluoride material;

[0026] The value-added module is placed between the target and the moderator module to increase the neutron yield. The value-added module is made of 9cm thick molybdenum metal.

[0027] A gamma absorption module, used to absorb gamma rays generated during the slowing down process, wherein the gamma absorption module is made of bismuth material with a thickness of 2.5 cm;

[0028] A reflector module, used to reflect scattered neutrons to enhance flux, is made of lead material, has an axial thickness and a radial thickness of 30 cm, and is installed at the periphery and end of the moderator module;

[0029] A thermal neutron absorption module, used to absorb excess thermal neutrons, wherein the thermal neutron absorption module is made of neodymium or boride material;

[0030] The collimation module is arranged at the outlet end of the shaping system and is used to form a neutron beam with uniform spatial distribution. The collimation module has a tapered channel structure and the inner wall is lined with boron-containing material to absorb scattered neutrons.

[0031] The neutron beam shaping system described in this invention utilizes multiple functional modules working in concert to convert high-energy fast neutrons generated by the target into thermal or surface thermal neutrons suitable for boron neutron capture therapy. First, protons are accelerated to an energy of 30 MeV in an accelerator. After striking a beryllium target, they produce fast neutrons with an average energy of 14 MeV. These fast neutrons first enter a 9-cm-thick molybdenum multiplication module positioned behind the target. This module significantly boosts the neutron flux through a neutron multiplication reaction. The neutron multiplication yield has been calculated to reach a magnitude of 10^16 at a given neutron flux. The neutron flux then enters a moderation module composed of aluminum fluoride, which has a low neutron absorption cross-section and excellent moderation properties. Through multiple elastic scattering, the fast neutron energy is gradually reduced to the thermal neutron energy range of less than 0.5 eV. Based on calculations of the initial energy of fast neutrons and the average energy loss ratio per collision, approximately 410 scatterings are required to complete the effective moderation process. The density and neutron propagation path of aluminum fluoride have been optimized, allowing energy conversion with a thickness of only 2 to 3 cm. The gamma rays generated during the moderation process are absorbed by a 2.5 cm thick bismuth module. Its linear attenuation coefficient is designed to reduce the gamma ray intensity to less than 1% of the initial value, effectively suppressing radiation leakage. To enhance neutron flux, 30 cm thick lead reflector modules are installed at the periphery and end of the system. These modules reflect over 95% of stray neutrons back to the reaction area, improving neutron utilization efficiency. To control the leakage of excess thermal neutrons, thermal neutron absorption modules are installed. Neodymium or boride materials are used. Borides, in particular, have an absorption cross-section of up to 767 barns, effectively intercepting excess thermal neutrons. Ultimately, to achieve a neutron beam with uniform spatial distribution and good directionality, the system features a conical collimation module at the exit. The inner wall of the channel is lined with a boron-containing polymer material with a boron content of between 5% and 10%, which absorbs scattered neutrons and limits the divergence angle. The channel length to exit diameter ratio is designed to be 2.8:1, ensuring that the neutron beam collimation performance meets treatment requirements. Through these precise calculations and parameter design, the shaping system achieves high efficiency, high safety, and high adaptability, providing a stable and reliable neutron beam source for BNCT treatment.

[0032] This structure can significantly improve the quality and adaptability of the neutron beam. By using aluminum fluoride as a moderator material, it not only has good moderation performance, but also reduces the probability of neutron absorption and improves the efficiency of thermal neutron output. The metal molybdenum value-added layer further increases the system's neutron yield. The bismuth gamma absorption layer effectively shields gamma rays, ensuring the safe operation of the system. The lead reflector can efficiently recover scattered neutrons, improving the overall neutron utilization rate. The neodymium or boride thermal neutron absorption layer enhances the ability to control neutron energy. The conical collimation module combined with the boron lining further suppresses scattered neutrons, significantly optimizing the directionality and uniformity of the neutron beam, and helping to improve the treatment accuracy and safety of BNCT.

[0033] The moderator module is cylindrical in structure, with an axial thickness of 30 cm and a radius of 20 cm. The maximum epithermal neutron flux with an axial thickness of 30 cm and a radius of 20 cm was obtained through Monte Carlo simulation.

[0034] The design principle of this moderator module is based on the energy loss distribution and spatial propagation characteristics of the neutron moderation process. After high-energy fast neutrons enter the moderator module from the multiplication module, they undergo multiple elastic scatterings within the aluminum fluoride material. The moderation efficiency is closely related to the number of scatterings and is also limited by the geometric boundaries of the material. Monte Carlo simulations compared the energy spectra of the outgoing neutrons for different combinations of radius (10 cm, 15 cm, 20 cm, and 25 cm) and thickness (10 cm, 20 cm, 30 cm, and 40 cm). It was found that when the thickness was 30 cm and the radius was 20 cm, the outgoing thermal neutron energy density distribution was most concentrated, and the thermal neutron flux per unit area reached its optimal peak. This size design ensures that the neutron path within the material is long enough for sufficient moderation while avoiding flux diffusion and reduced output directionality caused by exceeding the critical size.

[0035] The bismuth material in the gamma absorption module can effectively absorb gamma rays with energy greater than 0.5 MeV and has a thickness of 2.5 cm.

[0036] The module's operating principle is that during the neutron moderation process, some neutrons release gamma rays due to inelastic scattering or capture reactions. High-energy gamma rays, in particular, with energies exceeding 0.5 MeV, pose a potential threat to BNCT treatment accuracy and system safety. Bismuth has a high linear attenuation coefficient for gamma rays in this energy range, and its high atomic number increases the probability of photoelectric effect and Compton scattering, making it suitable for use as a gamma shielding material. The gamma-ray transmission equation was used for calculations during the design process, with the goal of attenuating the gamma-ray intensity to within 1% of its initial value after penetrating 2.5 cm of bismuth. Monte Carlo simulations further verified that this thickness maintained stable absorption capacity even under maximum gamma flux conditions, ensuring that subsequent neutron beam shaping and the clinical irradiation environment were not disrupted.

[0037] The reflection module is made of lead material and is arranged radially and downstream around the moderator module, and can reflect escaping neutrons toward the central axis.

[0038] The module operates according to the following principle: During the neutron moderation process, some neutrons tend to escape radially or downstream due to energy changes or multiple scattering. If left uncontrolled, this can result in neutron loss and reduced system efficiency. Lead, with its high atomic number and density, exhibits excellent scattering and reflection capabilities for neutrons, particularly in the neutron energy range below 1 MeV, where the reflectivity can exceed 50%. By wrapping the lead reflector module around the moderator module in a cylindrical structure and sealing it at its end to form a reflector cavity, the escaping neutrons undergo inelastic or elastic scattering with lead nuclei, thereby redirecting their motion. The reflector module's structural design uses a lead material with an axial and radial thickness of 30 cm. The optimal structural dimensions were calculated based on the neutron mean free path and reflection probability. This configuration ensures that escaping neutrons are refocused to the central axial region after one or two reflections, increasing the axial neutron flux density and reducing edge radiation leakage, ultimately achieving a concentrated and stable beam output.

[0039] The absorption material used in the thermal neutron absorption module is cadmium, boride or lithium fluoride, which is used to adjust the neutron energy spectrum according to the depth of the treatment target area.

[0040] The module operates as follows: BNCT therapy relies on the capture reaction of thermal neutrons with boron-10 in tumor tissue, producing high linear energy deposition. The depth of this reaction is significantly affected by the neutron energy distribution. If the thermal neutron ratio is too high, the absorbed dose to superficial tissues increases while the dose to deeper tissues is insufficient. Conversely, this may cause radiation spillover. This system utilizes optional materials in the thermal neutron absorption module to selectively absorb excess low-energy thermal neutrons in the system, thereby adjusting the energy spectrum of the final neutron beam. Cadmium has an extremely high thermal neutron absorption cross-section (approximately 2520 barn), making it suitable for rapid attenuation of shallow thermal neutrons. Borides (such as B₄C) have an absorption cross-section of 767 barn, making them suitable for medium-range control needs. Lithium fluoride exhibits excellent selective absorption of thermal neutrons in the energy range and a low probability of gamma emission, making it suitable for high-precision fine-tuning of the thermal neutron flux. Based on the target depth and dose distribution requirements provided by the treatment planning system, the above materials can be flexibly selected and their layer thickness or layout can be controlled to achieve directional optimization of the neutron beam energy spectrum, ensuring that the target dose meets the standard and reducing the risk of normal tissue irradiation.

[0041] The collimation module has a multi-section tapered channel structure, and the inner wall of the multi-section tapered channel structure is evenly embedded with boron-containing polyethylene material to improve beam uniformity and reduce side-scattered neutrons.

[0042] The module works as follows: Although neutrons are essentially oriented before the exit of the shaping system, there is still a certain degree of angular divergence and scattering, especially in the periphery and edge areas of the system where scattered neutrons accumulate. To further improve the collimation and energy spectrum purity of the outgoing neutron beam, a multi-segment conical channel is set up. The first segment has a larger cone angle to guide the initial convergence of the neutrons, and the cone angles of subsequent segments decrease successively, so that the neutrons gradually tend to be axially concentrated. The smooth transition between each segment helps to suppress the interference caused by multiple reflections of neutrons on the inner wall of the channel. To effectively absorb scattered neutrons with large angles to the central axis, the inner wall of each conical channel is uniformly embedded with a boron-containing polyethylene material of uniform thickness. The mass fraction of boron in this material is controlled in the range of 5% to 10%, which can efficiently capture thermal neutrons and surface thermal neutrons. At the same time, the polyethylene substrate has good neutron moderation properties, which enhances the ability to control the energy spectrum of incident scattered neutrons. In the overall design, the cone length and inner diameter are calculated and determined based on the spatial requirements and dose distribution characteristics of the usage scenario, ensuring that the emitted neutron beam has good directionality, consistency and spatial uniformity, meeting the strict requirements of BNCT treatment for high-quality beam.

[0043] The system also includes a sealing structure, which is arranged at the gaps between the modules to prevent scattered neutrons and gamma rays from leaking from the structural gaps. The sealing structure is filled with boron-containing polyethylene or lead rubber material.

[0044] In a neutron beam-shaping system, multiple functional modules must be arranged in series or around each other, inevitably creating connections, seams, or structural interfaces. These areas, due to structural discontinuities, can become channels for scattered neutrons or gamma rays to leak. Once scattered neutrons escape through these gaps, they pose a safety hazard to the treatment environment and radiation control, while gamma-ray leakage can also impact the radiation protection compliance of the system's periphery. Therefore, dedicated sealing structures are installed at each module's contact areas and structural seams. These sealing structures are filled with boron-containing polyethylene or lead rubber. Boron-containing polyethylene absorbs thermal and surface thermal neutrons through its boron content, while the polyethylene matrix exhibits excellent moderation, further slowing neutron penetration. Lead rubber, with its flexibility, sealing properties, and excellent gamma-ray shielding capabilities, is particularly suitable for sealing irregularly shaped interfaces. The sealing material is pressed into sheets or strips using precision molds and fixed to each gap position by embedding, press-fitting or bonding during the system assembly process, ensuring that the neutron and gamma ray shielding performance of the joint area is consistent with that of the main structure, thereby building a closed, continuous and efficient radiation protection barrier.

[0045] A terminal sealing module is arranged between the collimation module and the external shielding body. The terminal sealing module is composed of a composite of multiple layers of boron-containing polyethylene and high-density polymer sheets and is used to absorb side and edge scattered neutrons.

[0046] Although the neutron beam has developed strong directionality during the shaping process in the collimator module, some neutrons still reflect off the inner walls of the collimator channel and propagate along marginal or non-axial paths due to scattering and geometric boundary effects. This tendency is particularly pronounced at the beam exit, where there is a tendency for lateral expansion. If unchecked, these neutrons can escape from the transition zone between the collimator module and the shielding structure, causing dose overshoot and potentially compromising the treatment environment. To address this issue, an integrated terminal enclosure module is installed at this location. This laminated structure combines boron-containing polyethylene with high-density polymer sheet materials, which synergistically absorb multi-level neutrons. The boron element in the boron-containing polyethylene has a high absorption capacity for thermal and surface thermal neutrons, while the polyethylene matrix is efficient in moderating neutron kinetic energy. High-density polymer sheet materials, such as high-density polyethylene or polyoxymethylene, offer excellent mechanical strength and neutron scattering resistance. The module is precisely machined and modularly assembled, nesting between the collimator exit and the external shielding interface. This not only enhances scattered neutron absorption but also ensures structural integrity and installation compatibility, providing a stable, sealed, and effective protective structure for the system output.

[0047] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A neutron beam shaping system for BNCT, characterized in that: The system comprises: A moderator module is used to decelerate high-energy fast neutrons from the target material into surface thermal neutrons or thermal neutrons, and the moderator module is made of aluminum fluoride material; The value-added module is placed between the target and the moderator module to increase the neutron yield. The value-added module is made of 9cm thick molybdenum metal. A gamma absorption module, used to absorb gamma rays generated during the slowing down process, wherein the gamma absorption module is made of bismuth material with a thickness of 2.5 cm; A reflector module, used to reflect scattered neutrons to enhance flux, is made of lead material, has an axial thickness and a radial thickness of 30 cm, and is installed at the periphery and end of the moderator module; A thermal neutron absorption module, used to absorb excess thermal neutrons, wherein the thermal neutron absorption module is made of neodymium or boride material; The collimation module is arranged at the outlet end of the shaping system and is used to form a neutron beam with uniform spatial distribution. The collimation module has a tapered channel structure and the inner wall is lined with boron-containing material to absorb scattered neutrons.

2. A neutron beam shaping system for BNCT according to claim 1, characterized in that: The moderator module is cylindrical in structure, with an axial thickness of 30 cm and a radius of 20 cm. The maximum epithermal neutron flux with an axial thickness of 30 cm and a radius of 20 cm is obtained through Monte Carlo simulation verification.

3. The neutron beam shaping system for BNCT according to claim 1, characterized in that: The bismuth material in the gamma absorption module can effectively absorb gamma rays with energy greater than 0.5 MeV and has a thickness of 2.5 cm.

4. The neutron beam shaping system for BNCT according to claim 1, characterized in that: The reflection module is made of lead material, is arranged radially and downstream around the moderation module, and can reflect escaping neutrons toward the central axis.

5. The neutron beam shaping system for BNCT according to claim 1, characterized in that: The absorption material used in the thermal neutron absorption module is cadmium, boride or lithium fluoride, which is used to adjust the neutron energy spectrum according to the depth of the treatment target area.

6. The neutron beam shaping system for BNCT according to claim 1, characterized in that: The collimation module is a multi-section tapered channel structure, and the inner wall of the multi-section tapered channel structure is evenly embedded with boron-containing polyethylene material, which improves beam uniformity and reduces side-scattered neutrons.

7. A neutron beam shaping system for BNCT according to claim 1, characterized in that: It also includes a sealing structure, which is arranged at the gaps between the modules to prevent scattered neutrons and gamma rays from leaking from the structural gaps. The sealing structure is filled with boron-containing polyethylene or lead rubber material.

8. The neutron beam shaping system for BNCT according to claim 1, characterized in that: A terminal sealing module is provided between the collimation module and the external shielding body. The terminal sealing module is composed of a composite of multiple layers of boron-containing polyethylene and high-density polymer plates and is used for absorbing side and edge scattered neutrons.

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