Laser neutron yield increasing method based on composite conversion target

Abstract

The invention discloses a laser neutron yield increasing method based on a composite conversion target, which comprises the following steps of: arranging a plurality of elements to form the composite conversion target, bombarding the composite conversion target by using a deuterium ion beam obtained by interaction of laser and a heavy water liquid film target, and realizing neutron yield increasing through nuclear reaction section optimization; elements in the composite conversion target comprise at least two of T, D, < 7 > Li and < 9 > Be. According to the laser neutron yield increasing method based on the composite conversion target, the neutron yield and the average flux of a repetition frequency laser neutron source method are greatly increased, and the problem of instability in the continuous production process of a heavy water liquid film target is solved.

Description

Technical Field

[0001] This invention relates to a method for increasing laser neutron yield based on a composite conversion target, belonging to the field of nuclear control technology. Background Technology

[0002] Neutrons play a vital role in scientific research, national production, weaponry, and healthcare. For example, their high penetrability has led to the development of boron neutron capture therapy (BNCT) for tumor treatment; their ease of being moderated by low-Z elements allows them to complement X-rays, enabling their application in non-destructive testing such as weapon flaw detection; and their magnetic moment characteristics allow for probing the material structure and dynamic properties of magnetic materials. Neutron sources, as devices that generate neutrons, can be categorized into radioactive nuclide neutron sources, reactor neutron sources, and accelerator neutron sources.

[0003] Neutron sources driven by the interaction of high-power femtosecond lasers and plasma, with beam pulse widths on the order of ps to ns and source sizes on the micrometer scale, hold great promise for ultrafast applications. For example, an anisotope identification method based on neutron time-of-flight can be established using ultrashort laser neutron sources, enabling the resolution of isotopes in materials through neutron resonance energy spectroscopy, and revealing the spatial distribution of isotopes in a sample through neutron resonance imaging. However, currently, the yield of laser-accelerated neutron sources is limited, and the average neutron flux density is low. (~10 6 cm -2 s -1 ) This is one of the main factors restricting the application of laser-accelerated neutron sources.

[0004] Therefore, it is necessary to conduct more in-depth research on laser-accelerated neutron source generation methods to solve the above problems. Summary of the Invention

[0005] To overcome the above problems, in-depth research was conducted, and a method for improving laser neutron yield based on a composite conversion target was proposed. The method is characterized by setting up a composite conversion target composed of multiple elements, and using a deuterium ion beam obtained by the interaction between a laser and a heavy water liquid film target to bombard the composite conversion target, thereby improving the neutron yield.

[0006] In a preferred embodiment, the elements in the composite conversion target include T, D, and 7 Li, 9 At least two of Be.

[0007] In a preferred embodiment, the composite conversion target is a T compound and 7 It is composed of layers of Li compounds.

[0008] In a preferred embodiment, the composite conversion target has a bilayer structure, with the first layer being a Li compound layer and the second layer being a T compound layer.

[0009] In a preferred embodiment, a composite conversion target core reaction cross-section model is established, and the thickness ratio of different elemental compounds in the composite conversion target is obtained based on the composite conversion target core reaction cross-section model.

[0010] In a preferred embodiment, the reaction cross-section model of the composite conversion target nucleus is represented as follows:

[0011]

[0012] Where, σ comp σ1 represents the cross section of the nuclear reaction of the composite conversion target, n1 represents the cross section corresponding to the first element compound in the composite conversion target, σ2 represents the cross section corresponding to the second element compound in the composite conversion target, n2 represents the volume atomic density of the second element compound in the composite conversion target, and m:k represents the thickness ratio of the first element compound and the second element compound.

[0013] In a preferred embodiment, the 7 The thickness ratio of the Li compound layer to the T compound layer is 4:6.

[0014] In a preferred embodiment, a heavy water liquid film target is provided, and a deuterium ion beam is obtained by irradiating the heavy water liquid film target with a laser.

[0015] In a preferred embodiment, the heavy water liquid film target is obtained by a collision device. A collection unit is provided below the collision device, a liquid nitrogen cold trap is provided below the collection unit, a waste liquid pipeline is provided below the liquid nitrogen cold trap, and a peristaltic pump is provided in the waste liquid pipeline. The waste liquid that has been fully condensed by the liquid nitrogen cold trap is collected into a waste liquid pool through the waste liquid pipeline.

[0016] In a preferred embodiment, the collider includes a molecular pump, a first forepump, and a second forepump.

[0017] The molecular pump is connected to the vacuum chamber via a flat gate valve;

[0018] The first fore-pump is connected to the molecular pump and the vacuum chamber via a three-way pipe, wherein a first shut-off valve is installed on the pipe connected to the molecular pump and a second shut-off valve is installed on the pipe connected to the vacuum chamber.

[0019] The second fore-pump is connected to the molecular pump and the collection unit via a three-way pipe. A third shut-off valve is installed on the pipe connected to the molecular pump, and a fourth shut-off valve is installed on the pipe connected to the collection unit.

[0020] The beneficial effects of this invention include:

[0021] (1) A significant increase in neutron yield and average flux of the high-repetition-rate laser neutron source method has been achieved;

[0022] (2) Obtain a laser neutron source with continuous output capability;

[0023] (3) The instability problem in the continuous generation process of heavy water liquid film target was solved. Attached Figure Description

[0024] Figure 1 A comparative diagram of the nuclear reaction cross sections of different elements is shown in the laser neutron yield enhancement method based on the composite conversion target according to the present invention.

[0025] Figure 2 The reaction cross-sectional curve of the composite target according to a preferred embodiment of the present invention is shown;

[0026] Figure 3 Showing different thickness ratios 7 Nuclear reaction cross-sectional curves of a composite conversion target consisting of Li and T compound layers;

[0027] Figure 4 A schematic diagram of the layout of an experimental device for improving neutron yield by obtaining deuterium ion bombardment of a composite conversion target through the interaction of a high-repetition-rate laser and a heavy water liquid film target is shown.

[0028] Figure 5 The diagram shows a heavy water liquid film target generation device in a laser neutron yield enhancement method based on a composite conversion target according to a preferred embodiment of the present invention.

[0029] Figure 6 A diagram of a conventional heavy water liquid film target generation device is shown.

[0030] Explanation of icon numbers:

[0031] 1-Collider;

[0032] 2-Collection Unit;

[0033] 3-Liquid nitrogen cold trap;

[0034] 4- Waste liquid pipeline;

[0035] 5- Waste liquid tank;

[0036] 6-Peristaltic pump;

[0037] 8-Vacuum chamber;

[0038] 9-Second cold trap;

[0039] 21-Small funnel;

[0040] 22-Large Funnel;

[0041] 23-Heating resistor;

[0042] 71-Molecular pump;

[0043] 72 - First backing pump;

[0044] 73 - Second back pump;

[0045] 711-Flat gate valve;

[0046] 721 - First shut-off valve;

[0047] 722 - Second shut-off valve;

[0048] 731 - Third shut-off valve;

[0049] 732 - Fourth shut-off valve. Detailed Implementation

[0050] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Through these descriptions, the features and advantages of the present invention will become clearer and more apparent.

[0051] The term “exemplary” as used herein means “serving as an example, embodiment, or illustration.” Any embodiment illustrated herein as “exemplary” is not necessarily to be construed as superior to or better than other embodiments. Although various aspects of embodiments are shown in the accompanying drawings, the drawings are not necessarily drawn to scale unless specifically indicated otherwise.

[0052] According to the present invention, a method for improving laser neutron yield based on a composite conversion target is provided. A composite conversion target composed of multiple elements is set up, and a deuterium ion beam is used to bombard the composite conversion target to obtain a neutron source, thereby improving the neutron yield.

[0053] Existing high-power femtosecond laser-plasma interaction-driven neutron sources all employ plasma to bombard a single-material target. This method results in low neutron yield and a low average neutron flux density (~10). 6 cm -2 s -1 ).

[0054] Research has found that targets of different elements have different nuclear reaction probabilities. By fully utilizing the advantages of different nuclear reaction probabilities at different energy levels and stacking targets of different elements, the matching between the corresponding nuclear reaction probability and the incident ion energy spectrum can be maximized under a specific incident ion energy spectrum, thereby improving the laser-neutron conversion efficiency, obtaining a high average flux laser-accelerated neutron source, and solving the problem of low average neutron flux caused by traditional target materials.

[0055] In a preferred embodiment, the elements in the composite conversion target include T, D, and 7 Li, 9 At least two of Be.

[0056] T(TiT tritium), D(deuterium) 7 Li (lithium) and Be (beryllium) have different nuclear reaction pathways. The challenge of this invention lies in selecting which element combinations to use as composite conversion targets to obtain a high average flux laser-accelerated neutron source.

[0057] In this invention, the deuterium ion beam in a laser-driven neutron source is analyzed. Its energy spectrum conforms to the Maxwell-Boltzmann distribution, with the energy mainly distributed in the 0–20 MeV range, and a high concentration of low-energy ions compared to a relatively low concentration of high-energy ions. This invention also introduces the concept of a nuclear reaction cross section, and based on this cross section, a comprehensive analysis of different elements is performed to determine the optimal combination of composite conversion targets.

[0058] The nuclear reaction cross section is a physical quantity representing the probability of a specific nuclear reaction occurring between an incident particle and a target nucleus. For a single target nucleus, if I incident particles (beam particles) bombard the target nucleus per unit time, and if N' nuclear reaction events are observed to occur simultaneously, then the nuclear reaction cross section is expressed as:

[0059]

[0060] Where, N s It represents the total number of atomic nuclei per unit area that can interact with the incident particle.

[0061] Furthermore, based on the results of the nuclear reaction cross-sections of each element's target nucleus, a comparative diagram of nuclear reaction cross-sections for different elements can be obtained, such as... Figure 1 As shown in the figure. Here, T(d, n) represents the number of neutrons n produced by tritium element T under bombardment by a deuterium ion beam d. 7 Li(d,n), D(d,n), 9 Be(d,n) has a similar meaning, where d represents the deuterium ion beam. The nuclear reaction cross section can be simply understood as the probability of a nuclear reaction occurring. Therefore, the more incident deuterium ions fall into the region with a high nuclear reaction cross section, the easier it is for a nuclear reaction to occur, thus increasing the conversion efficiency of incident ions to neutrons. From Figure 1 As can be seen, a single type of target nucleus reaction cannot effectively utilize deuterium ions in various energy spectrum segments. In this invention, based on the comparison diagram of the nuclear reaction cross-sections, the element combination corresponding to the largest nuclear reaction cross-section is selected to form a composite conversion target, namely, the T compound and 7 Li compounds.

[0062] In a preferred embodiment, the composite conversion target is a T compound and 7 The Li compounds are stacked together. The reaction cross-section curve of the resulting composite target is theoretically as follows: Figure 2 The area below the mid-shade covers both low and high energy ranges, allowing for full utilization of deuterium ions in different energy zones.

[0063] In a preferred embodiment, the composite conversion target has a two-layer structure, the first layer being... 7 The first compound layer is a Li compound layer, and the second compound layer is a T compound layer. The first compound layer refers to the compound layer that is first contacted by the deuterium ion beam, and the second compound layer refers to the compound layer that is contacted by the deuterium ion beam later.

[0064] Studies have found that the stacking order of different elemental targets has a significant impact on neutron yield, especially when using front-end placement. 7 When a stacked architecture of a lithium target and a toluene target is configured at the back end, high-energy deuterium ions first react with... 7 Li layer formation 7 The Li(d,n) reaction, with its remaining energy decaying to a low-energy region after ionization loss, then generates secondary neutrons through the T(d,n) reaction. This sequence design, through a segmented energy spectrum matching mechanism, achieves multi-stage energy utilization of incident deuterium ions. At this point, the T(d,n) nuclear reaction cross-section in the low-energy range is relatively large, effectively utilizing a large number of low-energy deuterium ions below 500 keV in the exponential energy spectrum distribution; higher-energy ions react with the first layer... 7 The Li target undergoes a nuclear reaction that produces neutrons, and 7 The Li(d,n) nuclear reaction cross section is relatively large. This scheme maximizes the matching between the incident ion energy spectrum and the conversion target, thereby effectively improving the neutron conversion efficiency and significantly increasing the neutron yield. However, in the stacked architecture with a T compound layer in front, although low-energy deuterium ions can be effectively captured in the T compound layer and generate neutrons through the T(d,n) reaction, high-energy deuterium ions penetrate the T compound layer and, due to significant energy loss, enter... 7 When the Li compound layer was already in the state 7 The cross-sectional trough region of the Li(d,n) reaction. At this time... 7 The gain effect of the Li compound layer is limited, while the relative reduction in the content of T compounds in the composite target actually results in a lower total yield than that of the monolayer T compound target.

[0065] In a preferred embodiment, a composite conversion target cross-sectional model is established, and the thickness ratio of different elemental compounds in the composite conversion target is obtained based on the composite conversion target cross-sectional model.

[0066] The composite conversion target cross-section model was obtained through the following method:

[0067] The nuclear reaction cross section of the composite conversion target is obtained and represented as follows:

[0068]

[0069] N s,i =n i ×d i

[0070] Where, σ comp For the nuclear reaction cross section of the composite conversion target, Ns i Let σ be the number of target nuclei per unit area of ​​the i-th element compound in the composite conversion target. i Let n be the cross section corresponding to the i-th element compound in the composite conversion target. i d represents the volume atomic density of the i-th element compound in the composite conversion target. i This represents the effective thickness of the i-th element compound in the composite conversion target.

[0071] For a composite conversion target composed of two elements, when the thickness ratio of the first element compound to the second element compound is m:k, the nuclear reaction cross section of the composite conversion target can be expressed as:

[0072]

[0073] Set a threshold for the nuclear reaction cross section of the composite conversion target. When the nuclear reaction cross section of the composite conversion target is higher than the threshold, the thickness ratio of the first element compound and the second element compound can be obtained in the range of m:k.

[0074] In a preferred embodiment, when the first elemental compound is 7 When the Li compound and the second element compound is a T compound, the following is true. 7 The thickness ratio of the Li compound layer to the T compound layer is 4:6.

[0075] The thickness ratio is 4:6 7 Normalized nuclear reaction neutron yield of the Li compound layer and T compound layer composite conversion target is as follows: Figure 3 As shown in the figure, 7 When the thickness ratio of the Li compound layer to the T compound layer is 4:6, and the first layer is... 7 When the first layer is a Li compound layer and the second layer is a T compound layer, it is more... 7 The Li-based single-type conversion target yields a 2-fold increase in neutron yield, compared to a 2.7-fold increase in T-type conversion targets. This result demonstrates that, with a fixed target thickness, placing a target material with excellent high-energy reaction cross-sections in the first layer is beneficial for utilizing the deuterium ion spectrum at different energy levels; and it also proves the feasibility and rationality of the composite neutron conversion target scheme.

[0076] In a preferred embodiment, a heavy water liquid film target is set up, and a high-repetition-rate laser is used to focus and irradiate the heavy water liquid film target to accelerate the generation of a high-repetition-rate deuterium ion beam. This beam then bombards the recombination conversion target to obtain neutrons, thereby increasing the average neutron flux. The schematic diagram of the experimental setup is shown below. Figure 4 As shown.

[0077] According to the present invention, the heavy water liquid film target is acquired by a collision device 1, such as... Figure 5As shown, a collection unit 2 is provided below the collider 1, and a liquid nitrogen cold trap 3 is provided below the collection unit 2.

[0078] Traditional collection units such as Figure 6 As shown, the waste liquid pipe 4 is located on the side of the collection unit 2, and the waste liquid is pumped into the waste liquid pool 5 through the waste liquid pipe 4.

[0079] Unlike traditional liquid collection units, in this invention, a waste liquid pipe 4 is provided below the liquid nitrogen cold trap 3, and a peristaltic pump 6 is provided in the waste liquid pipe 4 to collect the waste liquid after it has been fully condensed by the liquid nitrogen cold trap 3 into the waste liquid pool 5.

[0080] The existing liquid collection unit 2 is connected to a liquid nitrogen cold trap 3 at its lower part. It utilizes the properties of liquid nitrogen to condense volatile waste liquid vapor. The condensed waste liquid flows into the drain port, reducing the amount of waste liquid volatilization and thus its impact on the vacuum level. Ideally, to fully utilize the cold trap, the collected waste liquid should approach the liquid nitrogen cold trap as quickly as possible to rapidly reduce volatilization. However, due to the significant pressure difference between the vacuum chamber and the cold trap, the waste liquid cannot descend quickly enough to approach the cold trap by gravity alone, let alone enter the cold trap for sufficient cooling. Therefore, traditional liquid collection units have a waste liquid pipe 4 on the side of the liquid collection unit 2, preventing the waste liquid from passing through the liquid nitrogen cold trap. This results in low condensation of the waste liquid, and after long-term operation, the vacuum level stability of the vacuum chamber deteriorates, leading to a decrease in the stability of continuous liquid film formation.

[0081] In this invention, a waste liquid pipe 4 is installed below the liquid nitrogen cold trap 3, and a peristaltic pump 6 is installed in the waste liquid pipe 4. The peristaltic pump periodically squeezes and releases the elastic hose through rollers, forming alternating negative and positive pressure zones. When the rollers squeeze the hose, local fluid is forced out; after the rollers leave, the hose recovers its elasticity and forms a negative pressure, drawing in subsequent fluid. This dynamic pressure change can overcome the pressure difference limitation between the external atmospheric pressure and the vacuum chamber, allowing the waste liquid to quickly approach the cold trap 3 and be discharged from below the cold trap through the liquid nitrogen cold trap 3, achieving faster and more intense condensation, thereby reducing the volatility of the waste liquid and improving the vacuum stability within the vacuum chamber. In addition, using a peristaltic pump as the extraction pump ensures that the liquid only contacts the pump pipe and not the pump itself during operation, effectively protecting the pump when transporting heavy water; and the closed pipeline avoids the risk of contamination caused by seal wear and lubricant leakage.

[0082] In a preferred embodiment, the liquid collection unit 2 consists of two funnels. The inner, inverted small funnel 21 is aligned with the tail end of the liquid film and plays a primary role in collection. The inverted design is intended to prevent the collected liquid from evaporating and diffusing into the cavity, thus affecting the vacuum level. The outer funnel is an upright large funnel 22, used to collect liquid that flows out due to misalignment with the orifice during debugging or splashed out during operation, preventing it from affecting other instruments within the cavity. To prevent the liquid from freezing due to low temperatures during vacuuming and clogging the orifice, preferably, multiple turns of heating resistor 23 are wound inside the collection unit. This heating resistor ensures that the flow path will not be blocked by liquid freezing in a vacuum environment.

[0083] In a preferred embodiment, the collider includes a molecular pump 71, a first fore-pump 72, and a second fore-pump 73.

[0084] The molecular pump 71 is connected to the vacuum chamber 8 via a flat gate valve 711;

[0085] The first fore-pump 72 is connected to the molecular pump 71 and the vacuum chamber 8 through a three-way pipe. A first shut-off valve 721 is provided on the pipe connected to the molecular pump, and a second shut-off valve 722 is provided on the pipe connected to the vacuum chamber.

[0086] The second fore-pump 73 is connected to the molecular pump 71 and the collection unit 2 via a three-way pipe. A third shut-off valve 731 is installed on the pipe connected to the molecular pump, and a fourth shut-off valve 732 is installed on the pipe connected to the collection unit.

[0087] In a more preferred embodiment, a second cold trap 9 is also provided on the side of the vacuum cavity, near the heavy water liquid film target, to achieve local cooling and condensation and reduce the amount of liquid evaporation.

[0088] In existing colliders, the vacuum stability is poor. This is because under vacuum conditions, the boiling point of liquids decreases, leading to the generation of a large amount of water vapor. As water vapor is generated, the vacuum level in the device gradually decreases.

[0089] In this invention, a multi-stage pumping structure is formed by combining a molecular pump, a first forepump, and a second forepump. By using differential vacuuming, the opening of the top plate gate valve is reduced. At the same time, the second forepump is used to create a vacuum from below the cold trap, which can form a large negative pressure below, reduce the water vapor flowing back into the cavity, and liquefy the water vapor as much as possible so that it flows into the drain port in liquid form.

[0090] Specifically, the differential vacuum method includes:

[0091] In the initial pre-evacuation stage, two back pumps are started simultaneously, and the second and fourth shut-off valves are opened to perform initial vacuum evacuation of the vacuum chamber and the collection unit, respectively, in order to quickly reduce the initial pressure of the system to the effective operating threshold of the molecular pump.

[0092] During the preheating transition phase of the molecular pump, the molecular pump is turned on and both back pumps are kept on. The second shut-off valve is closed to isolate the vacuum chamber, and the first shut-off valve is opened to maintain the back pressure of the molecular pump. Once the rotor reaches the preset speed threshold, it enters a stable operating state. This operation can avoid the risk of dynamic imbalance of the rotor caused by the direct impact of atmospheric pressure on the molecular pump, and effectively prevent mechanical failures such as blade plastic deformation and bearing overload.

[0093] During the coordinated pumping phase, keep the molecular pump and the two fore-pumps running, close the fourth shut-off valve, and open the flat gate valve and the third shut-off valve. Utilize the high pumping speed characteristics of the molecular pump to pump the vacuum chamber and the collection unit in parallel.

[0094] The present invention has been described above with reference to preferred embodiments; however, these embodiments are merely exemplary and illustrative. Various substitutions and modifications can be made to the present invention based on these embodiments, all of which fall within the scope of protection of the present invention.

Claims

1. A method for increasing laser neutron yield based on a composite conversion target, characterized in that, By setting up a composite conversion target composed of multiple elements and bombarding the composite conversion target with a deuterium ion beam, the neutron yield can be increased.

2. The method for increasing laser neutron yield based on a composite conversion target according to claim 1, characterized in that, The elements in the composite conversion target include T, D, and 7 Li, 9 At least two of Be.

3. The method for increasing laser neutron yield based on a composite conversion target according to claim 1, characterized in that, The composite conversion target is a T compound and 7 It is composed of layers of Li compounds.

4. The method for increasing laser neutron yield based on a composite conversion target according to claim 3, characterized in that, The composite conversion target has a bilayer structure, with the first layer being a Li compound layer and the second layer being a T compound layer.

5. The method for increasing laser neutron yield based on a composite conversion target according to claim 1, characterized in that, A cross-sectional model of the nuclear reaction of the composite conversion target was established, and the thickness ratio of different elemental compounds in the composite conversion target was obtained based on the cross-sectional model of the nuclear reaction of the composite conversion target.

6. The method for increasing laser neutron yield based on a composite conversion target according to claim 5, characterized in that, The composite conversion target nucleus reaction cross section model is represented as follows: Where, σ comp σ1 represents the cross section of the nuclear reaction of the composite conversion target, n1 represents the cross section corresponding to the first element compound in the composite conversion target, σ2 represents the cross section corresponding to the second element compound in the composite conversion target, n2 represents the volume atomic density of the second element compound in the composite conversion target, and m:k represents the thickness ratio of the first element compound and the second element compound.

7. The method for increasing laser neutron yield based on a composite conversion target according to claim 4, characterized in that, The 7 The thickness ratio of the Li compound layer to the T compound layer is 4:

6.

8. The method for increasing laser neutron yield based on a composite conversion target according to claim 1, characterized in that, A heavy water liquid film target is set up, and a deuterium ion beam is obtained by irradiating the heavy water liquid film target with a laser.

9. The method for increasing laser neutron yield based on a composite conversion target according to claim 8, characterized in that, The heavy water liquid film target is obtained through a collision device. A collection unit is set below the collision device, a liquid nitrogen cold trap is set below the collection unit, a waste liquid pipeline is set below the liquid nitrogen cold trap, and a peristaltic pump is set in the waste liquid pipeline. The waste liquid after being fully condensed by the liquid nitrogen cold trap is collected into the waste liquid pool through the waste liquid pipeline.

10. The method for increasing laser neutron yield based on a composite conversion target according to claim 8, characterized in that, The collider includes a molecular pump, a first forepump, and a second forepump. The molecular pump is connected to the vacuum chamber via a flat gate valve; The first forepump is connected to the molecular pump and the vacuum chamber via a three-way pipe, wherein A first shut-off valve is installed on the pipeline connecting to the molecular pump, and a second shut-off valve is installed on the pipeline connecting to the vacuum chamber; The second fore-pump is connected to the molecular pump and the collection unit via a three-way pipe. A third shut-off valve is installed on the pipe connected to the molecular pump, and a fourth shut-off valve is installed on the pipe connected to the collection unit.

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