Neutron energy spectrum measurement method of BNCT device

Abstract

The invention belongs to a novel tumor radiotherapy technology in the field of biological medicine, and relates to a neutron spectrum measurement method of a BNCT device. The method comprises the following steps: constructing a response function, constructing a geometric model based on a Monte Carlo program, and calculating a thermal neutron fluence rate to construct the response function; experimental measurement: measuring a thermal neutron fluence rate depth curve according to a cadmium difference gold wire activation principle; performing spectrum unfolding inversion, and performing inversion based on a plurality of algorithms by combining the two to obtain a neutron energy spectrum; the gold wire activation technology is adopted, the pulse accumulation risk is avoided, the method is insensitive to gamma rays, equipment is simple, low in cost, easy to popularize, high in measurement efficiency, good in accuracy and high in robustness, the corresponding relation is obtained for the first time, a theoretical basis is provided for inversion, and it is ensured that the result is reliable through cross check.

Description

Technical Field

[0001] This invention relates to the technical field of novel tumor radiotherapy technology in the biomedical field, focusing on the application of neutrons in the field of medical device performance evaluation technology, specifically a neutron energy spectrum measurement method for boron neutron capture therapy (BNCT) devices. Background Technology

[0002] Boron neutron capture therapy (BNCT), as a novel tumor radiotherapy technique, has demonstrated good efficacy in the treatment of head and neck tumors, gliomas, melanomas, and other cancers due to its unique principles and significant therapeutic advantages, showing broad application prospects. To ensure the effectiveness and safety of BNCT treatment, the International Atomic Energy Agency (IAEA) has recommended basic neutronics performance indicators for BNCT devices, emphasizing that a neutron source with a high flux of hyperthermal neutrons, low-thermal neutrons, and a high proportion of fast neutrons is a key prerequisite for ensuring the efficacy of BNCT.

[0003] Neutron energy spectrum, which describes the relationship between the number of neutrons and their energy, is a core parameter for evaluating neutron source performance. For accelerator-based BNCT devices, the neutron beam's energy spectrum characteristics are influenced by multiple factors, including proton energy, neutron production target, neutron shaper materials and configuration, equipment manufacturing quality, and installation technology. Key neutronics parameters recommended by the IAEA, such as ultrathermal neutron flux, fast neutron dose rate composition, gamma-ray dose composition, and the ratio of thermal to ultrathermal neutron flux, can all be derived directly or indirectly from neutron energy spectrum analysis.

[0004] However, due to the high flux and wide energy distribution of the neutron beam output from BNCT devices, a universal and standardized method for neutron energy spectrum measurement has not yet been established globally. While existing multi-foil neutron activation measurement techniques and multi-sphere neutron spectrometers have some applications, they suffer from problems such as sensitivity to gamma rays, low measurement accuracy, poor energy resolution, and susceptibility to pulse accumulation in high-flux fields, limiting their widespread use in BNCT devices.

[0005] Therefore, developing an efficient, accurate, and gamma-ray-insensitive neutron energy spectrum measurement technique suitable for BNCT devices is of great significance for evaluating the neutron beam quality in BNCT treatment, formulating reliable treatment plans, and achieving precise cancer radiotherapy. Summary of the Invention

[0006] To address the aforementioned problems, this invention aims to provide a neutron energy spectrum measurement method for boron neutron capture therapy (BNCT) devices. This method is particularly suitable for neutron energy spectrum measurement of high-flux-rate, wide-energy-distribution neutron beams in BNCT devices, and can provide an accurate, efficient, and gamma-ray-insensitive neutron energy spectrum measurement means to support the formulation and optimization of BNCT treatment plans.

[0007] The technical solution adopted in this invention is: a neutron energy spectrum measurement method for a BNCT device, comprising the following steps: Response function construction: Based on the Monte Carlo particle transport program, a geometric model is established, including the beam outlet of the BNCT device and the water phantom. The thermal neutron fluence rate at a specific depth and volume range within the geometric center of the beam in the water phantom when monoenergetic neutrons are incident on the water phantom is calculated, and the response function of the thermal neutron fluence rate depth curve is constructed. Experimental Measurement: Based on the principle of cadmium-gold wire activation, the thermal neutron fluence depth curve at the geometric center of the beam injection within the water phantom was measured. Spectral inversion: Combining the response function and experimentally measured thermal neutron flux depth curve, the neutron energy spectrum under free air conditions at the beam outlet of the BNCT device is obtained by inversion based on the maximum entropy principle, neural network method, or iterative method.

[0008] The specific steps for constructing the response function include: Establish a geometric model that includes the beam outlet of the BNCT device and a water phantom. The water phantom is made of acrylic or other bioequivalent wall materials and filled with water. A thermal neutron flux counter is set up on the central axis of the water phantom with a preset step size. The thermal neutron flux rate response induced by monoenergetic neutrons at a specific depth position is calculated to form a response matrix. The mapping relationship between the thermal neutron flux rate depth distribution curve and the incident neutron energy spectrum in the water phantom is established.

[0009] The experimental measurement steps specifically include: High-purity gold wires were arranged along the central axis of the water model and subjected to two rounds of irradiation: bare gold wires and cadmium-coated wires. After irradiation, the gold wire was segmented, each segment was weighed and the gamma spectrum was measured with a gamma spectrometer. After efficiency calibration, the characteristic peak count of each segment was obtained and the mononuclear saturation reaction rate was calculated. The thermal neutron flux rate depth curve within the water model was obtained by subtracting the contribution of superthermal neutrons using the cadmium difference method.

[0010] The spectral inversion step specifically includes: The Monte Carlo method was used to simulate and calculate the neutron energy spectrum of the BNCT device at the beam outlet location under free air conditions, and this spectrum was used as the initial spectrum (pre-formulated spectrum). The measured thermal neutron flux rate depth curve and response matrix are introduced into a spectrum decomposition algorithm based on the maximum entropy principle, neural network method, or iterative method. Under non-negativity constraints and convergence criteria, the neutron energy spectrum at the beam aperture of the BNCT device is obtained.

[0011] The neutron energy spectrum measurement method of the BNCT device further includes consistency verification and uncertainty assessment steps: The thermal neutron depth curve was calculated by convolving the expanded spectrum and the response matrix, and compared with the measured thermal neutron fluence rate depth curve, with the relative deviation used as the consistency evaluation. The uncertainty of the spectrum interpretation is evaluated by comprehensively taking into account error sources such as gamma spectrometer detection efficiency, gamma peak fitting, gold wire weighing, BNCT proton beam monitoring, and nuclear data.

[0012] The neutron energy spectrum measurement method of the BNCT device is applicable to the high fluence rate neutron-gamma mixed field of the BNCT device, avoiding the risk of pulse accumulation of the neutron detector under the high fluence rate neutron field conditions of BNCT.

[0013] The neutron energy spectrum measurement method for the BNCT device is applicable to the neutron-gamma mixed field of the BNCT device. Based on the gold wire activation measurement technology, it is completely insensitive to the gamma rays output by the BNCT device, thus improving the accuracy of neutron measurement.

[0014] The present invention proposes a method for measuring the neutron energy spectrum of a BNCT device based on the depth distribution curve of thermal neutron fluence within a water phantom, which has the following significant advantages compared to existing technologies: This invention effectively avoids the risk of pulse accumulation in neutron detectors under high flux rate neutron field conditions in BNCT by using gold wire activation measurement technology, thereby improving the stability and accuracy of the measurement.

[0015] Insensitive to gamma rays: Based on the principle of gold wire activation, this invention is completely insensitive to gamma rays output by the BNCT device, thereby eliminating the interference of gamma rays on neutron energy spectrum measurement and improving the accuracy of measurement.

[0016] The measuring equipment required for this invention includes only common instruments such as acrylic water phantoms, gold wires, and gamma spectrometers. The equipment is simple in composition, low in cost, and easy to implement and promote. Compared with the multi-foil activation method, the operation process of this invention is standardized and simple, and it is more feasible in the measurement field, which greatly improves the measurement efficiency.

[0017] Verification experiments demonstrate that the relative deviation between the neutron energy spectrum of the BNCT device obtained using the proposed measurement method and the measured thermal neutron depth curve is no higher than ±5%, significantly improving measurement accuracy. The measurement of the thermal neutron fluence depth distribution curve within a water phantom based on the gold wire activation method exhibits high repeatability, low dependence on the initial spectrum during spectral interpretation, and stable spectral results under parameter perturbations, enhancing measurement robustness.

[0018] This invention, based on the Monte Carlo method, for the first time obtained the correspondence between the thermal neutron fluence rate depth distribution curve and the neutron energy spectrum of the water phantom at the beam outlet position of the BNCT device. A response function for the thermal neutron fluence rate depth distribution was established, providing a theoretical basis for the accurate inversion of the neutron energy spectrum. This invention employs different types of spectral decomposition algorithms to achieve the inversion of the neutron energy spectrum of the BNCT device, and cross-validation ensures the accuracy of the neutron energy spectrum, improving the reliability of the measurement results. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the overall process of the present invention.

[0020] Figure 2 This is a schematic diagram of the sub-process of experimental measurement in this invention.

[0021] Figure 3 This is a schematic diagram of the spectrum inversion process in this invention. Detailed Implementation

[0022] The specific embodiments of the present invention are described in detail below with reference to the accompanying drawings and specific examples: This embodiment describes a neutron energy spectrum measurement method for a BNCT (Boron Neutron Capture Therapy) device. This method constructs a response function of the thermal neutron flux rate depth distribution curve, combines it with the cadmium-difference gold wire activation principle for experimental measurement, and obtains the neutron energy spectrum under free air conditions at the beam aperture based on the maximum entropy principle, neural network method, or iterative method. This method effectively solves problems such as pulse accumulation and associated gamma-ray interference caused by high flux rate neutron beams in BNCT devices, improving the accuracy and reliability of neutron energy spectrum measurement. Furthermore, the measurement equipment in this embodiment is simple, consisting only of an acrylic water phantom, gold wire, and gamma spectrometer, reducing costs. It also boasts high measurement efficiency; compared to the multi-foil activation method, the operation process is standardized and simple, making it highly feasible in the measurement field. Importantly, the neutron energy spectrum measurement accuracy is high. Verification experiments have shown that the relative deviation between the neutron energy spectrum obtained by this method and the measured thermal neutron depth curve is no higher than ±5%, and the spectral results are stable with low dependence on the initial spectrum.

[0023] like Figure 1-3 As shown, Figure 1 This is a schematic diagram of the overall process of this embodiment, illustrating the overall flow of the method, including key steps such as response function construction, experimental measurement, spectral inversion, and consistency verification and uncertainty assessment. The diagram visually presents the logical relationships and execution order between each step. Figure 2This diagram illustrates the sub-process flow of the experimental measurement in this embodiment, detailing the specific operational procedures of the experimental measurement stage, including the arrangement of high-purity gold wire, irradiation of bare gold wire and cadmium-coated gold wire, segmented weighing of gold wire, gamma spectral measurement, and calculation of the thermal neutron fluence rate depth curve. This diagram ensures the standardization and repeatability of the experimental process. Figure 3 This diagram illustrates the sub-process of spectral inversion in this embodiment, showing the inversion process from experimental measurement data to the neutron energy spectrum. It includes setting the initial spectrum, substituting the response matrix with the measured curve, and spectral calculations based on the maximum entropy principle, neural network methods, or iterative methods. This diagram helps to understand the core ideas and execution steps of the spectral inversion algorithm. The specific implementation steps of this embodiment are as follows: S1. Response Function Construction Geometric model establishment: A geometric model including the beam outlet of the BNCT device and the water phantom was established based on the Monte Carlo particle transport program. The water phantom uses acrylic as the wall material, with internal dimensions of 20 cm × 20 cm × 20 cm and a wall thickness of approximately 4 mm. It is filled with deionized water (density 1.0 g / cm³).

[0024] Thermal neutron flux rate calculation: A thermal neutron flux (En < 0.4 eV) counter was set up on the central axis of the water phantom with a step size of 5 mm. The thermal neutron flux rate response R(E,d) induced by the monoenergetic neutron E at depth d was calculated, and the response matrix R was formed. This step established the mapping relationship between the depth distribution curve of the thermal neutron flux rate in the water phantom and the incident neutron energy spectrum.

[0025] S2 Experimental Measurement Gold wire arrangement and irradiation: High-purity gold wire (purity >99.99%) with a diameter of about 0.25 mm and a length of about 20 cm is arranged along the central axis of the water model. Two rounds of irradiation are performed, one with bare gold wire and the other with cadmium-coated wire (fine cadmium tube: purity >99.95%, inner diameter about 1.27 mm, wall thickness about 0.51 mm).

[0026] Gold wire segmentation and weighing: After irradiation, the gold wire is segmented into approximately 5 mm segments, and each segment is weighed to ensure the accurate quality of each segment.

[0027] Gamma Spectroscopy Measurement: The gamma spectrum of each gold wire segment was measured using a gamma spectrometer (such as GEM20), and the efficiency of each segment was determined. 198 Au characteristic peaks were counted, and the mononuclear saturation reaction rate was calculated.

[0028] Calculation of thermal neutron flux rate depth curve: The thermal neutron flux rate depth curve M(d) in the water model is obtained by subtracting the contribution of hyperthermal neutrons using the cadmium difference method.

[0029] S3 Despectral Inversion Initial spectrum setting: The neutron energy spectrum under free air conditions at the beam opening was obtained by performing simulation calculations on the BNCT device using the Monte Carlo simulation program and used as the initial spectrum Φ0(E).

[0030] Spectral calculation: Substitute the measured thermal neutron flux rate depth curve M(d) and response matrix R into the calculation based on the maximum entropy principle, neural network method or iterative method, and perform the calculation under non-negativity constraints and convergence criteria to obtain the neutron energy spectrum Φ(E) at the beam aperture of the BNCT device.

[0031] S4 Consistency Check and Uncertainty Assessment Consistency verification: The thermal neutron depth curve is calculated by convolving the expanded spectrum with the response matrix R and compared with the measured M(d). The relative deviation is used as the consistency evaluation standard.

[0032] Uncertainty assessment: Taking into account error sources such as gamma detection efficiency, beam monitoring, energy peak fitting, weighing, and nuclear data, the uncertainty assessment results of the method are given.

[0033] This embodiment uses the Monte Carlo method to obtain the correspondence between the thermal neutron fluence depth distribution curve and the neutron spectrum of the water phantom at the beam outlet of the BNCT device treatment head. The thermal neutron fluence depth distribution curve in the water phantom is experimentally measured using the gold wire cadmium difference neutron activation measurement method. Different types of spectrum resolution algorithms are used to realize the inverse solution of the neutron spectrum of the BNCT device, and cross-validation is used to ensure the accuracy of the neutron spectrum.

[0034] The method described in this embodiment is applicable to high-flux-rate neutron fields and neutron-gamma mixed fields in BNCT devices, avoiding the risks of pulse accumulation and gamma-ray interference, and improving the accuracy and reliability of neutron measurements. Simultaneously, the measurement equipment is simple in composition, reducing costs, and the operation procedure is standardized and straightforward, making it highly feasible for on-site measurement. Verification experiments have shown that the relative deviation between the neutron energy spectrum obtained using this method and the measured thermal neutron depth curve is no higher than ±5%, and the spectral resolution results are stable with low dependence on the initial spectrum.

Claims

1. A method for measuring the neutron energy spectrum of a BNCT device, characterized in that, Includes the following steps: Response function construction: Based on the Monte Carlo particle transport program, a geometric model is established, including the beam outlet of the BNCT device and the water phantom. The thermal neutron fluence rate at a specific depth and volume range within the geometric center of the beam in the water phantom when monoenergetic neutrons are incident on the water phantom is calculated, and the response function of the thermal neutron fluence rate depth curve is constructed. Experimental Measurement: Based on the principle of cadmium-gold wire activation, the thermal neutron fluence depth curve at the geometric center of the beam injection within the water phantom was measured. Spectral inversion: Combining the response function and experimentally measured thermal neutron flux depth curve, the neutron energy spectrum under free air conditions at the beam outlet of the BNCT device is obtained by inversion based on the maximum entropy principle, neural network method, or iterative method.

2. The neutron energy spectrum measurement method of the BNCT device according to claim 1, characterized in that, The specific steps for constructing the response function include: Establish a geometric model that includes the beam outlet of the BNCT device and a water phantom. The water phantom is made of acrylic or other bioequivalent wall materials and filled with water. A thermal neutron flux counter is set up on the central axis of the water phantom with a preset step size. The thermal neutron flux rate response induced by monoenergetic neutrons at a specific depth position is calculated to form a response matrix. The mapping relationship between the thermal neutron flux rate depth distribution curve and the incident neutron energy spectrum in the water phantom is established.

3. The neutron energy spectrum measurement method of the BNCT device according to claim 1, characterized in that, The experimental measurement steps specifically include: High-purity gold wires were arranged along the central axis of the water model and subjected to two rounds of irradiation: bare gold wires and cadmium-coated wires. After irradiation, the gold wire was segmented, each segment was weighed and the gamma spectrum was measured with a gamma spectrometer. After efficiency calibration, the characteristic peak count of each segment was obtained and the mononuclear saturation reaction rate was calculated. The thermal neutron flux rate depth curve within the water model was obtained by subtracting the contribution of superthermal neutrons using the cadmium difference method.

4. The neutron energy spectrum measurement method of the BNCT device according to claim 1, characterized in that, The spectral inversion step specifically includes: The Monte Carlo method was used to simulate and calculate the neutron energy spectrum of the BNCT device at the beam outlet location under free air conditions, and this spectrum was used as the initial spectrum (pre-formulated spectrum). The measured thermal neutron flux rate depth curve and response matrix are introduced into a spectrum decomposition algorithm based on the maximum entropy principle, neural network method, or iterative method. Under non-negativity constraints and convergence criteria, the neutron energy spectrum at the beam aperture of the BNCT device is obtained.

5. The neutron energy spectrum measurement method of the BNCT device according to claim 1, characterized in that, It also includes consistency verification and uncertainty assessment steps: The thermal neutron depth curve was calculated by convolving the expanded spectrum and the response matrix, and compared with the measured thermal neutron fluence rate depth curve, with the relative deviation used as the consistency evaluation. The uncertainty of the spectrum interpretation is evaluated by comprehensively taking into account error sources such as gamma spectrometer detection efficiency, gamma peak fitting, gold wire weighing, BNCT proton beam monitoring, and nuclear data.

6. The neutron energy spectrum measurement method of the BNCT device according to claim 1, characterized in that, It is suitable for high-flux-rate neutron-gamma mixed fields in BNCT devices, avoiding the risk of pulse accumulation in neutron detectors under high-flux-rate neutron field conditions in BNCT.

7. The neutron energy spectrum measurement method of the BNCT device according to claim 1, characterized in that, Suitable for neutron-gamma mixed fields in BNCT devices, based on gold wire activation measurement technology, it is completely insensitive to gamma rays output by BNCT devices, thus improving the accuracy of neutron measurement.

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