A neutron-gamma ratio bidirectional adjusting device and method for a mixed radiation field

Abstract

The application relates to a mixed radiation field neutron gamma ratio bidirectional adjusting device and method, comprising a plurality of filter assembly and neutron reflectors arranged in sequence in the beam direction formed after the mixed radiation field is collimated, and the plurality of filter assembly and neutron reflectors can independently enter or move out of the beam area; through different combination modes of the filter assembly and neutron reflectors in the beam area, a composite structure capable of reducing the gamma background and enhancing the neutron flux or a composite structure capable of reducing the neutron flux and enhancing the gamma flux can be formed. The application can realize seamless switching of two working modes of "increasing the n / gamma ratio" and "reducing the n / gamma ratio" on a single device, solves the problem that the existing radiation field adjusting device has single function and can only realize one-way regulation, and significantly improves experimental flexibility and equipment reuse rate.

Description

Technical Field

[0001] This invention relates to the field of radiation field modulation technology, specifically to a bidirectional adjustment device and method for the neutron gamma ratio of a hybrid radiation field. Background Technology

[0002] In accelerator-based white-light neutron sources, high-energy protons (e.g., above 100 MeV) bombard a heavy metal target (e.g., a tungsten target), generating a mixed radiation field containing broad-spectrum neutrons and strong transient gamma rays through spallation reactions and evaporation processes. The neutron spectrum of a white-light neutron source covers a continuous energy range from thermal neutrons to high-energy neutrons with energies comparable to the incident protons, while the accompanying gamma rays mainly originate from de-excitation of nuclear reactions in the target material, inelastic neutron scattering, and neutron capture. In applications such as nuclear radiation detection experiments, single-event effects and displacement damage effects experiments in electronic devices, and biological irradiation experiments, it is often necessary to quantitatively adjust the n / γ ratio in the mixed radiation field according to specific experimental requirements. Some experiments require the purest possible neutron field to study neutron effects, while others require a significant increase in gamma composition to simulate specific irradiation environments.

[0003] Existing radiation field modulation methods are mostly single-function devices, such as using boron-containing polyethylene or boron carbide to absorb neutrons, or using lead plates to shield gamma rays. However, these devices are single-function and cannot achieve bidirectional adjustment of the n / γ ratio within the same device. Chinese patent application "A Neutron Flux Regulator" (ZL202222518213.6) only adjusts the neutron flux itself and does not involve the synergistic regulation of the gamma component. Chinese patent application "Beam Shaper for Neutron Capture Therapy" (CN108079443A) uses Pb or Ni reflectors, but its goal is neutron beam orientation shaping and shielding, and it does not provide a design scheme for bidirectional adjustment of the n / γ ratio. Chinese patent application "A Method and System for Generating a Neutron-Gamma Mixed Field Based on an Accelerator" (CN118354508A) uses a thin lithium target as the neutron target, through... 7 Li(p,n) 7 The Be reaction produces quasi-monoenergetic neutrons, which have a weak background of gamma rays. This scheme uses high atomic number metal materials as gamma yield gain materials and low atomic number organic materials as neutron yield reduction materials, but it can only achieve a unidirectional reduction of the n / γ ratio and cannot achieve bidirectional adjustment on the same device.

[0004] Therefore, there is an urgent need for a modular solution that can achieve bidirectional adjustment of the n / γ ratio on the same device to meet the different experimental requirements for the mixed field composition. Summary of the Invention

[0005] The purpose of this invention is to address the problem that existing radiation field adjustment devices have limited functionality and cannot flexibly achieve bidirectional large-range adjustment of the n / γ ratio on the same device. This invention provides a bidirectional adjustment device and method for the neutron gamma ratio of a mixed radiation field, thereby significantly improving experimental flexibility and equipment reuse rate.

[0006] To achieve the above objectives, in one aspect, a specific embodiment of the present invention provides a bidirectional adjustment device for the neutron gamma ratio of a mixed radiation field, comprising a plurality of filter assemblies and a neutron reflector sequentially arranged in the beam direction formed after the mixed radiation field is collimated. Each of the plurality of filter assemblies and the neutron reflector can individually enter or exit the beam region. By different combinations of the filter assemblies and the neutron reflector in the beam region, a composite structure that reduces the gamma background and enhances the neutron fluence, or a composite structure that reduces the neutron fluence and enhances the gamma fluence, can be formed.

[0007] Furthermore, in some specific embodiments, in the bidirectional neutron gamma ratio adjustment device of the mixed radiation field as described above, the neutron reflector has a cylindrical structure, the beam passes through the cylinder, and the neutron reflector slows down the neutrons scattered in the direction of the beam and partially reflects them back to the direction of the beam.

[0008] Furthermore, in some specific embodiments, the bidirectional adjustment device for the neutron gamma ratio of the mixed radiation field as described above, wherein the plurality of filter assemblies and neutron reflectors include Fe filter assemblies, boron-containing polyethylene filter assemblies, boron carbide filter assemblies, high-purity bismuth filter assemblies, and graphite neutron reflectors.

[0009] Furthermore, in some specific embodiments, the bidirectional adjustment device for the neutron-gamma ratio of the mixed radiation field as described above, wherein the composite structure for reducing the gamma background and enhancing the neutron flux includes a combination of a high-purity bismuth filter assembly and a graphite neutron reflector, wherein the graphite neutron reflector is located after the high-purity bismuth filter assembly, the beam first passes through the high-purity bismuth filter assembly to attenuate the gamma component, and then passes through the cylindrical graphite neutron reflector, the graphite neutron reflector slows down the neutrons scattered in the beam direction by the high-purity bismuth filter assembly and partially reflects them back to the beam direction.

[0010] Furthermore, in some specific embodiments, the bidirectional adjustment device for the neutron-gamma ratio of the mixed radiation field as described above, wherein the composite structure for reducing neutron flux and enhancing gamma flux includes an Fe filter assembly, a boron-containing polyethylene filter assembly, and a boron carbide filter assembly arranged sequentially along the beam direction. The beam first passes through the Fe filter assembly to de-energize high-energy neutrons and excite secondary gamma rays. The boron-containing polyethylene filter assembly further thermalizes the de-energized neutrons and the medium-energy neutrons in the original beam, and captures thermal neutrons while releasing gamma rays. The boron carbide filter assembly intercepts residual low-energy neutrons.

[0011] Furthermore, in some specific embodiments, in the bidirectional adjustment device for the neutron gamma ratio of the mixed radiation field as described above, the Fe filter assembly, the boron-containing polyethylene filter assembly, the boron carbide filter assembly, and the high-purity bismuth filter assembly all adopt a multi-layer thin-sheet stacked structure, and the effective thickness of each filter assembly can be independently adjusted by increasing or decreasing the number of stacked layers of each filter assembly.

[0012] Furthermore, in some specific embodiments, in the bidirectional adjustment device for neutron gamma ratio of the mixed radiation field as described above, the plurality of filter assemblies and neutron reflectors are all mounted on independent moving platforms, and can enter or exit the beam region in a direction perpendicular to the beam.

[0013] Furthermore, the mobile platform is equipped with limit switches and position sensors at both ends.

[0014] Furthermore, the supporting frame and mobile platform base of the plurality of filter assemblies and neutron reflectors are made of aluminum alloy.

[0015] On the other hand, specific embodiments of the present invention provide a method for bidirectional adjustment of the neutron gamma ratio in a mixed radiation field using the above-mentioned device, comprising:

[0016] 1) Through simulation and experimental measurement, establish the correspondence between the thickness of the filter assembly and the n / γ ratio adjustment factor;

[0017] 2) Based on experimental requirements, different filter components and neutron reflectors are combined in the beam region to form a composite structure that reduces the gamma background and enhances the neutron fluence, or a composite structure that reduces the neutron fluence and enhances the gamma fluence.

[0018] 3) The required n / γ ratio can be adjusted by changing the thickness of each filter assembly.

[0019] Furthermore, in some specific embodiments, in the bidirectional adjustment method of neutron gamma ratio of the mixed radiation field as described above, in step 1), the reference mixed field n / γ ratio values ​​under the conditions of no filter assembly and neutron reflector are compared, and the n / γ ratio enhancement factor and n / γ ratio reduction factor of the filter assembly and neutron reflector are respectively calibrated to establish the correspondence between the thickness of each filter assembly and the n / γ ratio adjustment multiple.

[0020] Furthermore, in some specific embodiments, in the bidirectional adjustment method of neutron-gamma ratio in the mixed radiation field as described above, in step 2), the method for forming the composite structure that reduces the gamma background and enhances the neutron flux is to place a high-purity bismuth filter assembly and a graphite neutron reflector in the beam region. The beam first passes through the high-purity bismuth filter assembly to attenuate the gamma component, and then passes through the cylindrical graphite neutron reflector. The graphite neutron reflector slows down the neutrons scattered from the high-purity bismuth filter assembly in the beam direction and partially reflects them back to the beam direction. In step 3), the increase in the n / γ ratio is quantitatively adjusted by changing the thickness of the high-purity bismuth filter assembly and the radial thickness and axial length of the graphite neutron reflector.

[0021] Furthermore, in some specific embodiments, in the bidirectional adjustment method of neutron-gamma ratio in the mixed radiation field as described above, in step 2), the method for forming the composite structure that reduces neutron flux and enhances gamma flux is to place the Fe filter assembly, the boron-containing polyethylene filter assembly, and the boron carbide filter assembly in the beam region. The beam first passes through the Fe filter assembly to de-energize high-energy neutrons and excite secondary gamma rays. The boron-containing polyethylene filter assembly further thermalizes the de-energized neutrons and the medium-energy neutrons in the original beam, and captures thermal neutrons while releasing gamma rays. The boron carbide filter assembly intercepts residual low-energy neutrons. In step 3), the reduction in the n / γ ratio is quantitatively adjusted by adjusting the thickness of the Fe filter assembly, the thickness and boron content of the boron-containing polyethylene filter assembly, and the thickness of the boron carbide filter assembly.

[0022] Furthermore, in some specific embodiments, in the bidirectional adjustment method of neutron gamma ratio of the mixed radiation field as described above, in step 3), the thickness of each filter assembly is changed by changing the number of stacked layers of each filter assembly.

[0023] The beneficial effects of this invention are as follows:

[0024] (1) The present invention can achieve seamless switching between the two working modes of “increasing the n / γ ratio” and “decreasing the n / γ ratio” on a single device, solving the problem that the existing radiation field adjustment device has a single function and can only achieve unidirectional control, which significantly improves the experimental flexibility and equipment reuse rate.

[0025] (2) The present invention adopts a composite structure of “Bi filter + graphite reflector”, which can significantly reduce the gamma background while maintaining the axial neutron fluence and achieve a significant increase in the n / γ ratio.

[0026] (3) The present invention adopts a three-layer series filter structure of “Fe + boron-containing polyethylene + B4C”. Through the synergistic effect of the three-layer structure, the emitted gamma fluence can be significantly enhanced while the neutron fluence is reduced to an extremely low level, thereby achieving a significant reduction in the n / γ ratio.

[0027] (4) The Fe filter, boron-containing polyethylene filter, B4C filter and Bi filter of the present invention all adopt a multi-layer thin sheet stacking structure. By increasing or decreasing the number of stacking layers of each filter component, the effective thickness of each functional layer can be adjusted independently. This design can achieve multi-level or continuous adjustment of the n / γ ratio without replacing the components, meeting the differentiated and precise requirements of different experimental scenarios for radiation field composition.

[0028] (5) All filter components and graphite reflectors of the present invention adopt independent transverse moving platforms with the same structure and are uniformly fixed along the beam line, eliminating the need for two independent subsystems: a complex "peripheral opening and closing system" and an "axial translation system". Mode switching only involves simple "moving in / out" operations of each component, simplifying the mechanical structure, reducing manufacturing costs, clarifying the control timing, ensuring high operational reliability, and facilitating maintenance.

[0029] (6) This invention provides a set of experimental methods for quantitative evaluation of n / γ ratio based on neutron time-of-flight method and pulse shape discrimination technology. It can measure the neutron and gamma energy spectra in two modes respectively, accurately calibrate the n / γ ratio enhancement factor of mode A and the n / γ ratio reduction factor of mode B, establish the correspondence between filter thickness parameters and n / γ ratio adjustment multiple, and guide users to quickly select or optimize adjustment parameters according to the target radiation field requirements.

[0030] (7) This invention is not only applicable to accelerator white light neutron sources (such as spallation neutron sources and white light neutron source devices), but also applicable to various neutron-gamma mixed radiation field devices such as reactor neutron beam lines and isotope neutron sources after parameter adaptation. It has a significant ability to adjust the n / γ ratio of the radiation field and has broad application prospects in the fields of electronic device radiation effect experiments, detector calibration, and radiation biological effect research. Attached Figure Description

[0031] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0032] Figure 1 This is a schematic diagram of the structure of the bidirectional adjustment device for the neutron gamma ratio of the mixed radiation field in a specific embodiment of the present invention;

[0033] Figure 2 This is a schematic diagram of the working state of the bidirectional adjustment device for neutron gamma ratio of mixed radiation field to improve the n / γ ratio in a specific embodiment of the present invention.

[0034] Figure 3This is a schematic diagram of the working state of the bidirectional adjustment device for neutron gamma ratio of mixed radiation field reducing the n / γ ratio in a specific embodiment of the present invention.

[0035] Figure 4 This is an installation example diagram of the neutron gamma ratio bidirectional adjustment device for a mixed radiation field, as described in a specific embodiment of the present invention. Detailed Implementation

[0036] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0037] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning as understood by one of ordinary skill in the art to which this invention pertains.

[0038] The terms “comprising”, “including”, etc., as used herein indicate the presence of the steps, features, operations, or components, but do not preclude the addition of one or more other steps, features, operations, or components.

[0039] This invention provides a bidirectional adjustment device and method for the neutron gamma ratio of a hybrid radiation field, to solve the problem that existing radiation field adjustment devices have limited functionality and cannot flexibly achieve bidirectional large-range adjustment of the n / γ ratio on the same device.

[0040] In some specific embodiments, the present invention provides a modularly designed bidirectional adjustment device, comprising several filter assemblies and a neutron reflector. The mixed radiation field is collimated to form a beam. Each of the filter assemblies and the neutron reflector can individually enter or exit the beam region. Through different combinations of the filter assemblies and the neutron reflector within the beam region, a composite structure that reduces the gamma background and enhances the neutron fluence, or a composite structure that reduces the neutron fluence and enhances the gamma fluence, can be formed. The present invention achieves rapid and seamless switching between two operating modes—"increasing the n / γ ratio" (Mode A) and "reducing the n / γ ratio" (Mode B)—on a single device by combining movable neutron reflectors and filter assemblies.

[0041] In some specific embodiments, the filter assemblies and neutron reflectors used in this invention include Fe filter assemblies, boron-containing polyethylene filter assemblies, boron carbide filter assemblies, high-purity bismuth filter assemblies, and graphite neutron reflectors. One arrangement of these filter assemblies and graphite neutron reflectors is as follows: Figure 1As shown, when high-energy protons bombard the neutron target, the resulting mixed radiation field of neutrons and gamma rays, after passing through the front-end shielding and collimation system, forms a quasi-parallel beam with a certain divergence angle, exiting along the 0° direction. Fe filter assemblies, boron-containing polyethylene filter assemblies, boron carbide filter assemblies, high-purity bismuth filter assemblies, and graphite neutron reflectors are arranged sequentially along the beam direction. Each assembly is mounted on its own transverse moving platform, allowing it to independently move into or completely out of the beam path. By combining different assemblies for entry / exit, bidirectional switching between Mode A (increasing the n / γ ratio) and Mode B (decreasing the n / γ ratio) is achieved.

[0042] Figure 1 The device structure shown is only a relatively reasonable arrangement of filter components and neutron reflectors. Adjusting the front and rear positions of different components will not affect the purpose and function to be achieved by the present invention. Therefore, the arrangement of the Fe filter components, boron-containing polyethylene filter components, boron carbide filter components, high-purity bismuth filter components and graphite neutron reflectors along the beam direction should not be regarded as the only implementation of the present invention, nor should it be regarded as a limitation on the scope of the present invention.

[0043] In some specific implementations, Mode A (increasing the n / γ ratio) employs a composite structure of "high-purity bismuth (Bi) filter assembly + nuclear-grade graphite neutron reflector," such as... Figure 2 As shown, the high-purity bismuth filter assembly and the graphite neutron reflector are positioned within the beam region, with the graphite neutron reflector located after the high-purity bismuth filter assembly. The Fe filter assembly, boron-containing polyethylene filter assembly, and boron carbide filter assembly are all moved out of the beam region.

[0044] Mode A utilizes the highly efficient attenuation capability of high-purity bismuth for the transient gamma emitted from the target source. Simultaneously, bismuth exhibits relative transparency to thermal neutrons, with an extremely low thermal neutron capture cross section (approximately 0.034 barn) and a minimal captured gamma yield. This effectively shields primary gamma while preventing the introduction of a new secondary gamma background due to neutron capture. The graphite neutron reflector, in a cylindrical structure, is positioned after the high-purity bismuth filter assembly, enclosing this section of the beamline. The enclosed graphite reflector reflects lateral and rearward leaked neutrons back to the 0° axis, and some of the thermal neutrons, moderated by graphite, can effectively reach the experimental terminal. This structure significantly enhances the axial neutron flux while suppressing the gamma background. Furthermore, by varying the thickness of the bismuth filter and the radial thickness and axial length of the graphite neutron reflector, the increase in the n / γ ratio can be quantitatively adjusted, outputting high n / γ ratio mixing fields with different purity requirements.

[0045] In some specific implementations, Mode B (reduced n / γ ratio) employs a three-layer tandem filter structure consisting of an iron (Fe) filter assembly, a boron-containing polyethylene filter assembly, and a boron carbide (B4C) filter assembly, such as... Figure 3As shown, the Fe filter assembly, boron-containing polyethylene filter assembly, and boron carbide filter assembly are sequentially arranged within the beam region along the beam direction. The high-purity bismuth filter assembly and the graphite neutron reflector are both moved out of the beam region.

[0046] Mode B utilizes the inelastic scattering effect of iron on fast neutrons to rapidly de-energize high-energy neutrons and excite secondary gamma rays, achieving primary neutron energy decay and initial enhancement of gamma composition. It further thermalizes the iron-de-energized neutrons and unmoderated medium-energy neutrons in the primordial mixing field using the efficient moderation effect of hydrogen in boron-containing polyethylene. Boron-10 in boron-containing polyethylene has an extremely high absorption cross section (approximately 3837 barn) for thermal neutrons, simultaneously releasing a characteristic gamma ray of 478 keV, contributing further gamma rays while reducing neutron energy. Finally, a boron carbide layer intercepts any residual low-energy neutrons that may leak out from the first two layers, further suppressing neutron intensity. Through the synergistic effect of these three layers, the emitted gamma ray is significantly enhanced while reducing the neutron fluence to an extremely low level, resulting in a substantial reduction in the n / γ ratio. Furthermore, by adjusting the thickness of the iron layer (to control the energy reduction depth of high-energy neutrons and the secondary gamma yield), the thickness and boron content of the boron-containing polyethylene layer (to control the moderation of neutrons in the medium energy region and the thermal neutron absorption efficiency), and the thickness of the boron carbide layer (to control the residual neutron cutoff effect), the synergistic regulation of the neutron attenuation degree and the gamma output intensity can be flexibly achieved, outputting low n / γ ratio mixed fields with different n / γ ratio requirements.

[0047] In some specific embodiments, the Fe filter assemblies, boron-containing polyethylene filter assemblies, B4C filter assemblies, and Bi filter assemblies all employ a multi-layered sheet stacking structure. Each layer of sheets is mounted on a support frame, and the total thickness of each filter can be flexibly adjusted by increasing or decreasing the number of sheets. This design has the following advantages:

[0048] Parametrically adjustable: By changing the number of stacked layers of each filter element, the n / γ ratio can be continuously or incrementally adjusted without replacing the elements.

[0049] It is easy to optimize: the optimal thickness combination can be pre-calculated through Monte Carlo simulation according to the actual neutron energy spectrum and experimental requirements, and can be quickly configured in the experiment;

[0050] Easy to maintain: The single-layer thin sheet is lightweight and easy to replace manually or add or remove automatically.

[0051] In some specific embodiments, each filter assembly and graphite neutron reflector is mounted on its own independent transverse moving platform, which is arranged perpendicular to the beam direction.

[0052] like Figure 4As shown, in some specific embodiments, several moving platforms 1, each mounting a filter assembly and a graphite neutron reflector, are sequentially arranged on a platform support 3. Each moving platform consists of a high-rigidity linear guide, a ball screw drive mechanism, and a servo motor, and carries the corresponding filter assembly or graphite neutron reflector. The platform can move laterally between the position above the beam axis and the position where it is completely removed. Each assembly is driven by a servo motor with an absolute encoder, achieving a repeatability accuracy better than ±0.5 mm. Hardware limit switches and position sensors 2 are installed at both ends of each platform to ensure that the assembly is accurately moved into the beam centerline or completely removed from the beam path.

[0053] In some specific embodiments, the present invention further provides a method for quantitatively evaluating the neutron gamma ratio of a mixed radiation field using the above-described device, comprising the following steps:

[0054] 1) Through simulation and experimental measurement, the reference mixed field n / γ ratio under the conditions of no filter assembly and neutron reflector is compared. The n / γ ratio improvement factor of filter assembly and neutron reflector under mode A and the n / γ ratio reduction factor under mode B are respectively calibrated. The correspondence between filter thickness parameter and n / γ ratio adjustment factor is established to guide users to quickly select adjustment parameters according to the target radiation field requirements.

[0055] 2) Based on experimental requirements, different filter components and neutron reflectors are combined in the beam region to form a composite structure of mode A that reduces gamma background and enhances neutron fluence, or a composite structure of mode B that reduces neutron fluence and enhances gamma fluence.

[0056] 3) In Mode A, the increase in the n / γ ratio is quantitatively adjusted by changing the thickness of the high-purity bismuth filter assembly and the radial thickness and axial length of the graphite neutron reflector; in Mode B, the decrease in the n / γ ratio is quantitatively adjusted by adjusting the thickness of the Fe filter assembly, the thickness and boron content of the boron-containing polyethylene layer filter assembly, and the thickness of the boron carbide filter assembly.

[0057] Example

[0058] This embodiment uses a typical white-light neutron source bombarding a thick tungsten target with 100 MeV protons as an example to specifically describe the bidirectional adjustment device and method for the neutron gamma ratio of the mixed radiation field of the present invention.

[0059] High-energy protons bombard a thick tungsten target (proton total stopping), generating white-light neutrons through spallation and evaporation processes, along with a large number of transient gamma rays. This mixed radiation field, after passing through a front-end shielding and collimation system, forms a quasi-parallel beam with a certain divergence angle, exiting along the 0° direction. This embodiment provides a bidirectional neutron gamma-ratio adjustment device based on an accelerator-based white-light neutron source, installed before the 0° beamline experimental terminal after the front-end shielding and collimation system of the white-light neutron source. The main structure of the device is as follows... Figure 1 As shown, the following components are fixedly arranged sequentially along the beam propagation direction: Fe filter assembly, boron-containing polyethylene filter assembly, B4C filter assembly, Bi filter assembly, and a graphite reflector (cylindrical). Each component is mounted on its own independent transverse platform, which can move laterally perpendicular to the beam direction to move into or out of the beam axis. By sequentially setting multiple independent transversely movable filter assemblies and neutron reflectors, a bidirectional n / γ ratio adjustment device is constructed. Specifically, it includes bidirectional switching between Mode A (increasing the n / γ ratio) and Mode B (decreasing the n / γ ratio).

[0060] 1. Mode A – Increasing the n / γ ratio

[0061] In Mode A, the Fe filter assembly, boron-containing polyethylene filter assembly, and B4C filter assembly are all moved out of the beam path, leaving only the Bi filter assembly and the graphite reflector on the beam axis. (See...) Figure 2 .

[0062] Beam path: collimated beam (n+γ mixed field) → Bi (multilayer) → graphite reflector (cylindrical) → experimental terminal.

[0063] Bi filter assembly: Located before the graphite reflector, it is composed of multiple layers of high-purity bismuth sheets stacked together (each layer thickness is approximately 1 cm, the total thickness is adjustable from 1 to 10 cm, and the purity is not less than 99.9%), with its diameter completely covering the beam aperture. Its function is to utilize the high atomic number material's efficient attenuation capability for gamma rays, preferentially attenuating the gamma component in the collimated beam; at the same time, bismuth has relative transparency to thermal neutrons, and its thermal neutron radiation trapping cross section is extremely low (approximately 0.034 barn), resulting in a very small gamma trapping yield. This effectively shields primary gamma rays while avoiding the introduction of new secondary gamma background due to neutron trapping.

[0064] Graphite reflector: Located after the Bi filter assembly, it is a cylindrical structure made of nuclear-grade graphite (with an inner diameter of approximately 30 cm, a radial thickness of approximately 3 cm, and a length of approximately 30 cm) that encloses this section of the beamline. Its function is to slow down the neutrons scattered by the Bi filter in the beam direction and partially reflect them back to the 0° axis, reducing neutron loss; the thermal neutrons slowed down by graphite can continue to be transmitted to the experimental terminal.

[0065] Overall effect: The Bi filter attenuates the gamma background, while the graphite reflector recovers the neutrons scattered by the Bi filter and allows some of them to continue propagating as thermal neutrons. This results in a significant increase in the n / γ ratio while suppressing the gamma background. The increase in the n / γ ratio can be quantitatively adjusted by modifying the number of stacked layers of the Bi filter (to control the degree of gamma attenuation) and the size parameters of the graphite reflector.

[0066] 2. Mode B – Reduce the n / γ ratio

[0067] In Mode B, the Bi filter assembly and graphite reflector are all moved out of the beam path, leaving only the Fe filter assembly, boron-containing polyethylene filter assembly, and B4C filter assembly on the beam axis. (See...) Figure 3 .

[0068] Beam path: Collimated beam (n+γ mixed field) → Fe (multilayer) → Boron-containing polyethylene (multilayer) → B4C (multilayer) → Experimental terminal.

[0069] The Fe filter assembly, located closest to the upstream of the collimated beam, is composed of multiple layers of stacked iron sheets (each layer is approximately 0.1 cm thick, with a total thickness adjustable from 0.1 to 1 cm). Utilizing the inelastic scattering effect of iron on fast neutrons, high-energy neutrons are rapidly de-energized and secondary gamma rays are excited, achieving initial attenuation of neutron energy and preliminary enhancement of the gamma component; the existing gamma portion in the collimated beam continues to propagate through the Fe layer.

[0070] Boron-containing polyethylene filter assembly: Located after the Fe filter assembly, it is composed of multiple layers of boron-containing polyethylene sheets stacked together (each layer is approximately 1 cm thick, the total thickness is adjustable from 1 to 10 cm, and the boron mass fraction is approximately 5%). Utilizing the efficient moderation effect of hydrogen in polyethylene, the neutrons de-energized by Fe and the existing medium-energy neutrons in the collimated beam are further thermalized; the diffused boron-10 efficiently captures these thermal neutrons through its extremely high absorption cross-section (approximately 3837 barn), while simultaneously releasing a characteristic gamma of 478 keV, further contributing to the gamma field while reducing neutron levels.

[0071] B4C filter assembly: Located after the boron-containing polyethylene filter assembly, it is composed of multiple layers of boron carbide sheets stacked together (each layer is approximately 0.1 cm thick, with a total thickness adjustable from 0.1 to 2 cm). It intercepts any residual low-energy neutrons that may leak out from the first two layers, further reducing the neutron intensity.

[0072] Overall effect: The Fe layer excites secondary gamma through inelastic scattering, the boron-containing polyethylene layer releases a characteristic 478 keV gamma by trapping neutrons with boron-10, and the B4C layer removes residual neutrons. Through the synergistic effect of the three-layer structure, the emitted gamma fluence is significantly enhanced while reducing the neutron fluence, resulting in a substantial reduction in the n / γ ratio. By adjusting the stacking number of each filter assembly, the high-energy neutron de-energization depth and secondary gamma yield, the medium-energy neutron moderation and thermal neutron absorption efficiency, and the residual neutron cutoff effect can be independently controlled, achieving coordinated regulation of neutron attenuation and gamma output intensity.

[0073] In this embodiment, each filter assembly and graphite reflector is mounted on its own independent lateral moving platform, which is arranged perpendicular to the beam direction. Each moving platform consists of a high-rigidity linear guide, a ball screw drive mechanism, and a servo motor, and carries the corresponding filter assembly or graphite reflector. The platform can move laterally between the position above the beam axis and the position where it is completely removed. Each assembly is driven by a servo motor with an absolute encoder, achieving a repeatability accuracy better than ±0.5 mm. Hardware limit switches and position sensors are installed at both ends of each platform to ensure that the assembly is accurately moved into the beam centerline or completely removed from the beam path. The supporting frame and platform substrate of each filter assembly and graphite reflector are made of lightweight aluminum alloy to reduce activation and parasitic absorption / scattering in the neutron field.

[0074] This embodiment uses Monte Carlo simulation (MCNP / FLUKA) to quantitatively evaluate the n / γ ratio adjustment performance of the device. Using a baseline mixing field without filters as a reference, typical adjustment effects are measured as follows:

[0075] Mode A: The gamma fluence rate drops to below 5% of the reference field, while the neutron fluence rate remains above 60% of the reference field, and the n / γ ratio increases by more than 10.

[0076] Mode B: The neutron fluence rate is reduced to about 0.2% of the reference field, while the total gamma fluence rate remains above 40% of the reference field due to the contribution of secondary gamma, and the n / γ ratio is reduced to less than 1 / 200 of the reference field.

[0077] Furthermore, the n / γ ratio can be finely adjusted by changing the number of stacked layers of each filter assembly. The optimal thickness combination can be pre-calculated using Monte Carlo simulations, allowing for rapid configuration during experiments to meet the varying n / γ ratio requirements of different experimental scenarios.

[0078] In this embodiment, the neutron gamma ratio bidirectional adjustment device is automatically switched and controlled by a PLC or host computer with a single button.

[0079] Switch to Mode A (increase the n / γ ratio):

[0080] (1) Move the Fe filter assembly, the boron-containing polyethylene filter assembly, and the B4C filter assembly laterally out of the beam path in sequence;

[0081] (2) Confirm that the Bi filter assembly is on the beam axis (or move it in);

[0082] (3) Move the graphite reflector into the beamline so that it wraps around the beam section downstream of the Bi filter;

[0083] (4) Once the position sensors of each platform are confirmed to be in place, the system enters the high n / γ ratio irradiation ready state.

[0084] Switch to Mode B (reduce the n / γ ratio):

[0085] (1) Move the graphite reflector laterally out of the beam path;

[0086] (2) Move the Bi filter assembly laterally out of the beam path;

[0087] (3) Confirm (or move in) in sequence that the Fe filter assembly, boron-containing polyethylene filter assembly, and B4C filter assembly are on the beam axis;

[0088] (4) Once the position sensors of each platform are confirmed to be in place, the system enters the high gamma irradiation ready state.

[0089] The following points should be noted during the implementation of the bidirectional neutron gamma ratio adjustment device in this embodiment:

[0090] Appropriate mechanical clearances (1-2 cm recommended) should be reserved between each filter assembly. The impact of these clearances on the radiation field needs to be evaluated in a Monte Carlo simulation.

[0091] The servo motors, encoders, sensors, and other electronic components that drive the guide rail are all located on the side away from the main beam area, and are locally shielded with lead bricks and boron-containing polyethylene blocks to prevent failure caused by cumulative dose and single-event effects.

[0092] A reliable mechanical interlock must be installed between the graphite reflector and the adjacent filter assembly to prevent collisions caused by misoperation.

[0093] Boron-containing polyethylene filter sheets require regular checks on their hydrogen content and the uniformity of boron distribution during long-term use, and should be replaced when necessary to ensure the stability of neutron moderation and absorption performance.

[0094] Those skilled in the art, upon considering the specification and practicing the disclosure of this invention, will readily conceive of other embodiments of the invention. This application is intended to cover any variations, uses, or adaptations of the invention that follow the general principles of the invention and include common knowledge or customary techniques in the art not disclosed herein. Thus, if such variations and adaptations of the invention fall within the scope of the claims and their equivalents, the invention also intends to include these variations and adaptations.

[0095] The above embodiments are merely illustrative examples of the present invention. The present invention may also be implemented in other specific ways or forms without departing from its spirit or essential characteristics. Therefore, the described embodiments should be considered illustrative rather than limiting in any respect. The scope of protection of the present invention should be defined by the claims, and any variations equivalent to the intent and scope of the claims should also be included within the scope of the present invention.

Claims

1. A bidirectional adjustment device for the neutron gamma ratio in a hybrid radiation field, characterized in that, The system includes a plurality of filter assemblies and neutron reflectors arranged sequentially in the beam direction formed after the mixed radiation field is collimated. Each of the filter assemblies and neutron reflectors can enter or exit the beam region individually. Through different combinations of the filter assemblies and neutron reflectors in the beam region, a composite structure that reduces the gamma background and enhances the neutron fluence, or a composite structure that reduces the neutron fluence and enhances the gamma fluence, can be formed.

2. The bidirectional adjustment device for the neutron gamma ratio of the mixed radiation field as described in claim 1, characterized in that, The neutron reflector has a cylindrical structure, through which the beam passes. The neutron reflector slows down the neutrons scattered in the direction of the beam and partially reflects them back into the direction of the beam.

3. The bidirectional adjustment device for the neutron gamma ratio of the mixed radiation field as described in claim 1, characterized in that, The plurality of filter assemblies and neutron reflectors include Fe filter assemblies, boron-containing polyethylene filter assemblies, boron carbide filter assemblies, high-purity bismuth filter assemblies, and graphite neutron reflectors.

4. The bidirectional adjustment device for neutron gamma ratio in a mixed radiation field as described in claim 1, characterized in that, The composite structure for reducing gamma background and enhancing neutron flux includes a combination of a high-purity bismuth filter assembly and a graphite neutron reflector. The graphite neutron reflector is located after the high-purity bismuth filter assembly. The beam first passes through the high-purity bismuth filter assembly to attenuate the gamma component, and then passes through the cylindrical graphite neutron reflector. The graphite neutron reflector slows down the neutrons scattered in the beam direction by the high-purity bismuth filter assembly and partially reflects them back to the beam direction.

5. The bidirectional adjustment device for the neutron gamma ratio of the mixed radiation field as described in claim 1, characterized in that, The composite structure for reducing neutron flux and enhancing gamma flux includes an Fe filter assembly, a boron-containing polyethylene filter assembly, and a boron carbide filter assembly arranged sequentially along the beam direction. The beam first passes through the Fe filter assembly to de-energize high-energy neutrons and excite secondary gamma rays. The boron-containing polyethylene filter assembly further thermalizes the de-energized neutrons and the medium-energy neutrons in the original beam, and captures thermal neutrons while releasing gamma rays. The boron carbide filter assembly intercepts residual low-energy neutrons.

6. The bidirectional adjustment device for the neutron gamma ratio of the mixed radiation field as described in claim 3, characterized in that, The Fe filter assembly, boron-containing polyethylene filter assembly, boron carbide filter assembly, and high-purity bismuth filter assembly all adopt a multi-layer thin-sheet stacked structure. The effective thickness of each filter assembly can be independently adjusted by increasing or decreasing the number of stacked layers.

7. The bidirectional adjustment device for the neutron gamma ratio of the mixed radiation field as described in claim 1, characterized in that, The filter assemblies and neutron reflectors are all mounted on independent moving platforms, and can enter or exit the beam region in a direction perpendicular to the beam.

8. The bidirectional adjustment device for neutron gamma ratio in a mixed radiation field as described in claim 7, characterized in that, The mobile platform is equipped with limit switches and position sensors at both ends.

9. The bidirectional adjustment device for the neutron gamma ratio of the mixed radiation field as described in claim 7, characterized in that, The supporting frame and moving platform base of the several filter components and neutron reflectors are made of aluminum alloy.

10. A method for bidirectional adjustment of the neutron gamma ratio in a mixed radiation field using the device described in any one of claims 1-9, characterized in that, include: 1) Through simulation and experimental measurement, establish the correspondence between the thickness of the filter assembly and the n / γ ratio adjustment factor; 2) Based on experimental requirements, different filter components and neutron reflectors are combined in the beam region to form a composite structure that reduces the gamma background and enhances the neutron fluence, or a composite structure that reduces the neutron fluence and enhances the gamma fluence. 3) The required n / γ ratio can be adjusted by changing the thickness of each filter assembly.

11. The method for bidirectional adjustment of neutron gamma ratio in a mixed radiation field as described in claim 10, characterized in that, In step 1), the reference mixing field n / γ ratio values ​​under the conditions of no filter assembly and neutron reflector are compared, and the n / γ ratio enhancement factor and n / γ ratio reduction factor of filter assembly and neutron reflector are calibrated respectively, and the correspondence between the thickness of each filter assembly and the n / γ ratio adjustment factor is established.

12. The method for bidirectional adjustment of neutron gamma ratio in a mixed radiation field as described in claim 10, characterized in that, In step 2), the method for forming the composite structure that reduces the gamma background and enhances the neutron flux is as follows: a high-purity bismuth filter assembly and a graphite neutron reflector are placed in the beam region. The beam first passes through the high-purity bismuth filter assembly to attenuate the gamma component, and then passes through the cylindrical graphite neutron reflector. The graphite neutron reflector slows down the neutrons scattered from the high-purity bismuth filter assembly in the beam direction and partially reflects them back to the beam direction.

13. The method for bidirectional adjustment of the neutron gamma ratio in a mixed radiation field as described in claim 12, characterized in that, In step 3), the increase in the n / γ ratio is quantitatively adjusted by changing the thickness of the high-purity bismuth filter assembly and the radial thickness and axial length of the graphite neutron reflector.

14. The method for bidirectional adjustment of neutron gamma ratio in a mixed radiation field as described in claim 10, characterized in that, In step 2), the method for forming the composite structure that reduces neutron flux and enhances gamma flux is as follows: Fe filter assembly, boron-containing polyethylene filter assembly, and boron carbide filter assembly are placed in the beam region. The beam first passes through the Fe filter assembly to de-energize high-energy neutrons and excite secondary gamma rays. The boron-containing polyethylene filter assembly further thermalizes the de-energized neutrons and the medium-energy neutrons in the original beam, and captures thermal neutrons while releasing gamma rays. The boron carbide filter assembly intercepts the remaining low-energy neutrons.

15. The method for bidirectional adjustment of the neutron gamma ratio in a mixed radiation field as described in claim 14, characterized in that, In step 3), the reduction rate of the n / γ ratio is quantitatively adjusted by adjusting the thickness of the Fe filter assembly, the thickness and boron content of the boron-containing polyethylene layer filter assembly, and the thickness of the boron carbide filter assembly.

16. The method for bidirectional adjustment of neutron gamma ratio in a mixed radiation field as described in claim 10, characterized in that, In step 3), the thickness of each filter element is changed by altering the number of stacked layers of each filter element.

Images