A neutron spectrometer for boron neutron capture therapy irradiation beam

Abstract

The present invention relates to a neutron spectrometer for a boron neutron capture therapy irradiation beam, belonging to the technical field of tumor treatment. The spectrometer includes a moderating absorber and a plurality of thermal neutron detection units disposed therein. The detection surface of each thermal neutron detection unit is parallel to the end face of the moderating absorber, and the depth of each thermal neutron detection unit in the moderating absorber is different; each thermal neutron detection unit includes a detector and a neutron-charged particle converter, and a limiting aperture is provided between the detector and the neutron-charged particle converter, and the whole is encapsulated in an aluminum shell. The spectrometer provided by the present invention can not only cover the full energy range of the BNCT irradiation beam, but also greatly improve the energy resolution rate of the spectrometer to reduce the measurement uncertainty of the spectral integral fluence. At the same time, it can work normally in a strong neutron and γ mixed field to realize the on-line and rapid measurement of the neutron energy spectrum under the treatment beam intensity.

Description

Technical Field

[0001] The present invention belongs to the technical field of spectrometers, and particularly relates to a neutron spectrometer for a neutron beam used in boron neutron capture therapy irradiation. Background Art

[0002] Boron neutron capture therapy (BNCT) is a binary targeted radiotherapy method. That is, a boron carrier drug that targets tumor cells is injected into the patient's blood. The boron-containing drug accumulates in tumor cells through metabolism, and then low-energy neutrons (thermal or epithermal neutrons - thermal neutron beam for treating superficial tumors, epithermal neutron beam for treating deep tumors) with a large 10 B capture reaction cross-section are used to irradiate the lesion area, generating α particles with a high linear energy transfer (LET) and 7 Li nuclei. The combined range of the two in tissue is about 12 - 13 μm, which is comparable to the size of a single cell, so as to selectively kill tumor cells and achieve the effect of precise treatment. Determined by the generation method of the irradiation beam and the needs of treatment, the BNCT irradiation beam has the following characteristics: 1) Wide energy range (neutrons: thermal energy ~ about 10 MeV, with thermal neutrons or epithermal neutrons as the main components; γ: dozens of keV ~ about 10 MeV); 2) High radiation intensity (neutron fluence rate: about 10 9 cm -2 s -1 ; γ fluence rate: about 10 7 cm -2 s -1 ; 3) Usually a diverging beam with conical collimation, so there may be a relatively strong room scatter background (caused by scattering from the walls, ceiling, and floor of the treatment room).

[0003] Due to the limitations of the characteristics of the BNCT irradiation beam, currently, the devices used for neutron energy spectrum measurement internationally mainly include threshold activation foil detectors and Bonner sphere spectrometers. Others such as bubble (superheated droplet) detectors, recoil proton proportional counters, organic scintillation detectors, etc., although having better energy resolution and can reduce the uncertainty of measurement results, the covered neutron energy range is outside the main energy region of the BNCT irradiation beam (above 10 keV), and they are not suitable for measurement in a strong radiation field (limited by the system dead time), and can only be used for verification measurements in the fast neutron energy region. The technical principles and characteristics of the threshold activation foil detector and Bonner sphere spectrometer related to the design concept of the present invention are described below.

[0004] The threshold activation foil detector is a type of threshold detector, consisting of a group of activation foils with different neutron activation reaction threshold energies. The basic principle of measuring the neutron energy spectrum is: The activation foils are irradiated in the neutron field to be measured. For example, Figure 1As shown in the figure, since each activation foil has a different energy response to epithermal neutrons (it has no response to sub-thermal neutrons), the saturation activities (i.e., reaction rates) of the radionuclides produced by the neutron activation reaction are different. And the saturation activity of each activation foil is proportional to the neutron fluence rate (the number of neutrons per unit area per unit time). Therefore, the distribution of the neutron fluence rate as a function of neutron energy, i.e., the neutron energy spectrum, can be obtained by inverting the saturation activity (i.e., spectrum deconvolution). The advantages of the threshold activation foil detector are: 1) It covers a wide energy range (thermal energy ~ about 15 MeV); 2) It is suitable for measuring high neutron fluence rate neutron fields accompanied by strong γ radiation. The disadvantages are: 1) The threshold energy is determined by the activation foil material and cannot be arbitrarily selected. Therefore, in some energy regions, the energy resolution is poor and the measurement uncertainty is large. Taking the epithermal beam of BNCT as an example, the relative standard uncertainties (typical values) of the spectral integral fluence of different neutron components are: thermal neutron energy region (below 0.5 eV): 20% - 25%, epithermal neutron energy region (0.5 eV - 10 keV): 5% - 10%, fast neutron energy region (above 10 keV): 25% - 30%; 2) It is necessary to measure the activity of each activation foil offline, and the process is cumbersome and time-consuming.

[0005] The Bonner sphere spectrometer usually consists of a series of polyethylene neutron moderation spheres with different diameters, and a thermal neutron detector (such as: 3 3He proportional counter, BF3 proportional counter, 6 LiI(Eu) crystal detector, etc.) is placed in the center. The basic principle of measuring the neutron energy spectrum is similar to that of the threshold activation foil detector: The detector is irradiated in the field to be measured. As Figure 2 shown in the figure, since the energy responses of the neutron moderation sphere detectors with different diameters are different, the counting rates caused by neutrons are different. And the counting rate of each detector is proportional to the neutron fluence rate. Therefore, the distribution of the neutron fluence rate as a function of neutron energy, i.e., the neutron energy spectrum, can be obtained by inverting the counting rate. The advantages of the multi-sphere spectrometer are: 1) It covers a very wide energy range (theoretically from thermal energy to the GeV level); 2) The response is almost isotropic and is suitable for measuring neutron fields with unknown directional distributions; 3) It has good fluence response (detection efficiency) characteristics, and different central thermal neutron detectors can be selected to be suitable for measuring neutron fields with different intensities. The disadvantages are: The response function is a wide peak distribution and there is serious overlap (especially in the range of 100 eV - 100 keV), the energy resolution is poor, and the measurement uncertainty is large. Taking the epithermal beam as an example, the relative standard uncertainties (typical values) of the spectral integral fluence of different neutron components are: thermal neutron energy region: about 8%, epithermal neutron energy region: 10% - 20%, fast neutron energy region: 10% - 20%; 2) It is necessary to replace different moderation sphere detectors, and the measurement process is cumbersome. The Bonner sphere spectrometer can be an active detector (such as: 3Active ones centered around proportional counters (such as He proportional counters, BF3 proportional counters, etc.), or passive ones centered around passive detectors (such as gold foils, thermoluminescent (TLD) chips, etc.). The former can give instantaneous pulse signals for online measurement, but is limited by the system dead time and is mainly used for measuring the energy spectrum of low-intensity neutron fields, so it cannot be used for the treatment beam intensity; while the latter can be used for measuring the energy spectrum of high-intensity neutron fields, but cannot give instantaneous pulse signals and can only be measured offline.

[0006] The one closest to the inventive concept of the present invention internationally is a single moderator, multi-detector Bonner sphere spectrometer developed by J. Burian et al. from the Czech Institute of Nuclear Physics. As Figure 3 shown, this spectrometer contains a total of 7 3 He or BF3 proportional counters, which are located at different depths of a polyethylene moderator (cadmium-coated on the outside) to simulate different diameter polyethylene balls of a Bonner sphere spectrometer (the small ball near the bottom simulates the small ball, and the large ball near the top simulates the large ball). The advantages of this spectrometer are: 1) The spectrometer is designed with a single moderator, and all data can be obtained in one measurement, so the measurement process is simple and fast; 2) The moderator is a cube (or cylinder) and can be closely attached to the irradiation beam outlet for measurement, so no position correction is required; 3) There is a boron-containing polyethylene shielding layer on the side of the moderator to shield the room scattered neutron background. The existing deficiencies are: 1) The basic principle of the spectrometer is still a Bonner sphere spectrometer, and the response function is similar to that of a conventional Bonner sphere spectrometer (as Figure 4 shown). Although the energy resolution in the epithermal energy region is slightly improved by coating with cadmium, there is no essential improvement; 2) The detection efficiency of the thermal neutron detector used is too high and cannot be measured at the treatment beam intensity (high neutron fluence rate). Summary of the Invention

[0007] To solve the defects existing in the prior art, the purpose of the present invention is to provide a neutron spectrometer for boron neutron capture therapy irradiation beams, which can not only cover the full energy range (thermal energy ~ 10 MeV) of BNCT irradiation beams, but also greatly improve the energy resolution rate of the spectrometer to reduce the measurement uncertainty of the spectral integral fluence, and can work normally in a strong neutron and γ mixed field to achieve online and rapid measurement of the neutron energy spectrum at the treatment beam intensity.

[0008] To achieve the above purpose, a technical solution adopted by the present invention is:

[0009] A neutron spectrometer for boron neutron capture therapy irradiation beam, comprising a moderating absorber and a plurality of thermal neutron detection units arranged in the moderating absorber. The detection surface of each thermal neutron detection unit is parallel to the end face of the moderating absorber, and the depth of each thermal neutron detection unit in the moderating absorber is different. Each thermal neutron detection unit is connected to a signal output head located at the end face of the moderating absorber through a signal lead wire;

[0010] Each thermal neutron detection unit includes a detector and a neutron-charged particle converter, and a limiting aperture is arranged between the detector and the neutron-charged particle converter, and the whole is encapsulated in an aluminum shell.

[0011] Furthermore, for the neutron spectrometer for boron neutron capture therapy irradiation beam as described above, the material of the moderating absorber is a uniform mixture of high-density polyethylene and 70% high-purity boron powder, which has a dual function of moderating and absorbing neutrons simultaneously.

[0012] Furthermore, for the neutron spectrometer for boron neutron capture therapy irradiation beam as described above, the shape of the moderating absorber is a cylinder, a cube, a prism or other shapes.

[0013] Furthermore, for the neutron spectrometer for boron neutron capture therapy irradiation beam as described above, the thermal neutron detection unit adopts an active detection mode, and realizes online fast measurement by giving an instantaneous pulse signal.

[0014] Furthermore, for the neutron spectrometer for boron neutron capture therapy irradiation beam as described above, the detector adopts a chemical vapor deposition electron-grade high-purity single-crystal diamond semiconductor material.

[0015] Furthermore, for the neutron spectrometer for boron neutron capture therapy irradiation beam as described above, the effective size of the detector is 4.5mm×4.5mm×0.1mm.

[0016] Furthermore, for the neutron spectrometer for boron neutron capture therapy irradiation beam as described above, the neutron-charged particle converter is a 6 LiF thin film prepared by evaporation method, which is used to convert neutrons into detectable charged particles.

[0017] Furthermore, for the neutron spectrometer for boron neutron capture therapy irradiation beam as described above, the surface of the detector is provided with a ceramic substrate package, and the surface of the neutron-charged particle converter is provided with a high-purity aluminum bottom substrate.

[0018] Furthermore, for the neutron spectrometer for boron neutron capture therapy irradiation beam as described above, the limiting aperture is used to control the counting rate of the detector to avoid overloading of the electronics system.

[0019] Furthermore, the neutron spectrometer for boron neutron capture therapy irradiation beam as described above optimizes the depths of multiple thermal neutron detection units in the moderator absorber, so that the new neutron spectrometer covers the full energy range of the BNCT irradiation beam and improves its energy resolution.

[0020] Using the neutron spectrometer of the present invention has the following remarkable technical effects:

[0021] 1) This spectrometer can cover the full energy range (thermal energy ~ 10 MeV) of the BNCT irradiation beam;

[0022] 2) The energy resolution of this spectrometer is greatly improved compared with that of the multi-sphere spectrometer, so as to reduce the measurement uncertainty of the spectral integral fluence. Taking the epithermal beam as an example, the relative standard uncertainty (typical value) of the spectral integral fluence of different neutron components can reach: thermal neutron energy region: 5%, epithermal neutron energy region: 5%, fast neutron energy region: 10%;

[0023] 3) This spectrometer is in an active detection mode and can work normally in a strong neutron and γ mixed field to achieve on-line and rapid measurement of the neutron energy spectrum under the treatment beam intensity;

[0024] 4) The moderator absorber uses boron-containing polyethylene material, which can effectively shield the scattered neutron background of the chamber by itself, so that no additional background measurement is required, and the operation process is more simple and fast. Description of the Drawings

[0025] Figure 1 is the energy response of some activation reactions mentioned in the background art;

[0026] Figure 2 is the response function of the conventional Bonner sphere spectrometer (not cadmium-covered) mentioned in the background art;

[0027] Figure 3 is the response function of the conventional Bonner sphere spectrometer (cadmium-covered) mentioned in the background art;

[0028] Figure 4 is the schematic structural diagram of the Bonner sphere spectrometer developed by J. Burian et al. mentioned in the background art;

[0029] Figure 5 is the front view of the schematic structural diagram of an embodiment of the neutron spectrometer for boron neutron capture therapy irradiation beam provided in the specific embodiment of the present invention;

[0030] Figure 6 is Figure 5 the side view of the schematic structural diagram of the embodiment;

[0031] Figure 7 is Figure 5Schematic diagram of the structure of the detection unit in the described embodiment;

[0032] Figure 8 is the response function diagram of the neutron spectrometer provided by the present invention. Detailed implementation manners

[0033] The present invention will be further described below in conjunction with specific embodiments and the accompanying drawings of the specification.

[0034] Figure 5 and Figure 6 shows a schematic diagram of the structure of the neutron spectrometer for boron neutron capture therapy irradiation beam according to the present invention. The spectrometer includes a moderator absorber 1 and a plurality of thermal neutron detection units 2 disposed in the moderator absorber. The detection surface of each thermal neutron detection unit 2 is parallel to the end face of the moderator absorber 1, and the depth of each thermal neutron detection unit 2 in the moderator absorber is different. Each thermal neutron detection unit 2 is connected to a signal output head 4 located at the end face of the moderator absorber 1 through a signal lead 3.

[0035] The material of the moderator absorber 1 is a homogeneous mixture of high-density polyethylene and 70% high-purity boron powder ( 10 the B content is natural abundance), and is made into a cylinder, cube, prism or other shapes by machining. This material simultaneously moderates and absorbs neutrons. In addition, the moderator absorber 1 itself can effectively shield the scattered neutron background of the chamber, so that no additional background measurement is required, and the operation process is more simple and fast.

[0036] Each thermal neutron detection unit 2 is composed of a detector 21 and a neutron-charged particle converter 22, and a limiting aperture is provided between the detector 21 and the neutron-charged particle converter 22. The whole is encapsulated in an aluminum shell 23 with a wall thickness of 0.5 mm, and its structural schematic diagram is as Figure 7 shown.

[0037] In this embodiment, the detector 21 uses a chemical vapor deposition (CVD) electron-grade high-purity single-crystal diamond semiconductor material, with an effective size of 4.5 mm × 4.5 mm × 0.1 mm, and is encapsulated by a ceramic substrate 24. This diamond semiconductor material has radiation resistance, fast response speed and low detection efficiency.

[0038] The neutron-charged particle converter 22 is a 6 LiF thin film prepared by evaporation method, and is protected by a high-purity aluminum bottom substrate 25 on the outside, and is used to convert neutrons into detectable charged particles.

[0039] The limiting aperture is used to control the counting rate of the detector to an acceptable level (the electronics system is not overloaded).

[0040] The thermal neutron detection unit 2 can reduce the detection efficiency of the spectrometer, enabling it to operate normally in a strong neutron and γ mixed field; it can achieve the measurement of the neutron energy spectrum under a high neutron fluence rate (treatment beam intensity); meanwhile, the detection system is in an active mode, which can give instantaneous pulse signals to achieve online rapid measurement.

[0041] By optimizing the design of the depth of the internal thermal neutron detection unit 2 in the moderator absorber 1, a threshold detector spectrometer similar to the threshold activation foil detector and the bubble detector is formed, and the threshold energy can be arbitrarily selected according to actual needs, so that the spectrometer can cover the full energy range of the BNCT irradiation beam and improve its energy resolution.

[0042] Figure 8 The response function of the spectrometer described in the present invention is shown, and the average core width ΔL of the energy resolution of the spectrometer is calculated and evaluated, as shown in Table 1, and is compared with the traditional Bonner sphere spectrometer and Figure 4 The calculation results of the shown spectrometer are compared. It can be seen that the spectrometer described in the present invention can greatly improve the energy resolution rate of the spectrometer, especially in the epithermal energy region, thereby reducing the measurement uncertainty of the spectral integral fluence.

[0043] Table 1 The average core width ΔL and the improvement ratio of energy resolution in each energy region of the spectrometer of the present invention, the spectrometer invented by J. Burian and the traditional Bonner sphere spectrometer

[0044]

[0045] The neutron spectrometer for the BNCT irradiation beam provided by the present invention is similar in geometric structure to the spectrometer developed by J. Burian et al., and is also designed with a single moderator and multiple detectors. However, its technical idea is different from theirs. In principle, it no longer belongs to the Bonner sphere spectrometer, but a threshold detector spectrometer, that is, it combines the common characteristics of the threshold detector spectrometer (in principle) and the Bonner sphere spectrometer (in structure), so as to both have Figure 4 the advantages of the shown spectrometer and overcome its disadvantages and deficiencies, and has the following beneficial effects:

[0046] 1) The spectrometer can cover the full energy range (thermal energy ~ 10 MeV) of the BNCT irradiation beam;

[0047] 2) The energy resolution of the spectrometer is greatly improved compared with the multi-sphere spectrometer to reduce the measurement uncertainty of the spectral integral fluence. Taking the epithermal beam as an example, the relative standard uncertainty (typical value) of the spectral integral fluence of different neutron components can reach: thermal neutron energy region: 5%, epithermal neutron energy region: 5%, fast neutron energy region: 10%;

[0048] 3) The energy spectrometer is in an active detection mode and can operate normally in a strong neutron and γ mixed field to achieve on-line and rapid measurement of the neutron energy spectrum at the therapeutic beam intensity.

[0049] 4) The moderating absorber uses boron-containing polyethylene material, which can effectively shield the scattered neutron background in the chamber by itself, thus eliminating the need for additional background measurement and making the operation process more simple and fast.

[0050] The above embodiments are only illustrative examples of the present invention. The present invention can also be implemented in other specific ways or other specific forms without departing from the gist or essential features of the present invention. Therefore, the described embodiments should be regarded as illustrative rather than restrictive in any aspect. The scope of the present invention should be defined by the appended claims, and any equivalent changes to the intent and scope of the claims should also be included within the scope of the present invention.

Claims

1. A neutron spectrometer for a neutron beam used in boron neutron capture therapy irradiation, characterized in that, It includes a moderating absorber (1) and a plurality of thermal neutron detection units (2) arranged in the moderating absorber. The outer shape of the moderating absorber (1) is a cylinder, a cube or a prism. The detection surface of each thermal neutron detection unit (2) is parallel to the end face of the moderating absorber (1). The arrangement form of the plurality of thermal neutron detection units (2) in the moderating absorber (1) is as follows: taking the central axis of the moderating absorber (1) as the axis, a plurality of channels with different depths are uniformly opened on the circumferences with different radii on one end face of the moderating absorber (1). Different thermal neutron detection units (2) are respectively arranged in different channels, so that the depths of each thermal neutron detection unit (2) in the moderating absorber are different. Each thermal neutron detection unit (2) is connected to a signal output head (4) located on the end face of the moderating absorber (1) through a signal lead (3); by optimizing the depths of the plurality of thermal neutron detection units (2) in the moderating absorber (1), the neutron spectrometer can cover the full energy range of the BNCT irradiation beam and improve its energy resolution; The material of the moderating absorber (1) is a uniform mixture of high-density polyethylene and 70% high-purity boron powder, which has a dual function of moderating and absorbing neutrons; Each thermal neutron detection unit (2) includes a detector (21) and a neutron-charged particle converter (22), and a limiting aperture is arranged between the detector (21) and the neutron-charged particle converter (22). The limiting aperture is used to control the counting rate of the detector to avoid overloading of the electronics system; it is integrally encapsulated in an aluminum shell (23); the surface of the detector (21) is encapsulated with a ceramic substrate (24), and the surface of the neutron-charged particle converter (22) is provided with a high-purity aluminum bottom substrate (25); The detector (21) uses a chemically vapor deposited electronic-grade high-purity single-crystal diamond semiconductor material.

2. The neutron energy spectrometer for boron neutron capture therapy irradiation beam according to claim 1, characterized in that, The thermal neutron detection unit (2) adopts an active detection mode, and realizes online fast measurement by giving an instantaneous pulse signal.

3. The neutron energy spectrometer for boron neutron capture therapy irradiation beam according to claim 1 or 2, characterized in that, The effective size of the detector (21) is 4.5mm×4.5mm×0.1mm.

4. The neutron energy spectrometer for boron neutron capture therapy irradiation beam according to claim 1, characterized in that, The neutron-charged particle converter (22) is prepared by vapor deposition method 6 LiF thin film, which is used to convert neutrons into detectable charged particles.

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