RU2282909C2 - Neutron production method - Google Patents

Abstract

FIELD: neutron entrapping therapy.

SUBSTANCE: proposed method includes irradiation of stream of solid active-material rarefied particles moved perpendicular to mentioned beam by accelerated charged particles, such as protons, to conduct neutron-producing reaction in mentioned particles. Active material particles can be moved using gravitational forces or pressure difference between volumes of active-material particles being fed and received to form particle suspension in rarefied light gas. Size, volume concentration, and thickness of active particle stream are chosen so as to optimize nuclear reaction conditions. Heat energy released in the process is uniformly distributed throughout entire volume of active particle stream in working zone thereby simplifying target cooling. Neutron-producing assemblies implementing this method are described in invention specification.

EFFECT: facilitated procedure, reduced cost of neutron production for medicine.

17 cl, 10 dwg

Description

Technical field

The invention relates to nuclear physics and medicine and can be used in neutron sources based on charged particle accelerators. Such sources are mainly intended for use in medical equipment used for neutron therapy, mainly in neutron capture therapy (NRT).

State of the art

The concept of neutron capture therapy in oncology was proposed in 1936, 4 years after the discovery of the neutron. Its physical principle is as follows. A solution containing the stable boron-10 isotope is introduced into human blood and after some time, boron is sorbed in the cells. Then the tumor is irradiated with a stream of epithermal neutrons. As a result of neutron absorption by the stable ¹⁰ V isotope, a nuclear reaction occurs, and the resulting α energy particle and ⁷ Li ion are rapidly inhibited along the length of the tumor cell, releasing an energy of ~ 2.3 MeV within the exact cell that contained the boron nucleus, which leads to her defeat. Thus, if a higher concentration of ¹⁰ V in the cancer cell is achieved compared to a healthy one, then boron neutron capture therapy (BRNT) will allow for selective damage to malignant tumor cells.

Over the past period of time, the concept of NRT and, in particular, BNCT has experienced falls and rises. However, in the late 60s, interest in NRT increased significantly. In this, the research of the Japanese scientist Hatanaki and the progress in the synthesis of ¹⁰ V isotope-containing pharmaceutical preparations played a large role. Preparations were obtained that create a concentration of ¹⁰ V isotope in tumor tissue up to 40 μg / g, which is 3.5 times more than in healthy tissue. This concentration makes it possible to selectively damage a cancerous tumor, which was not possible in early studies due to the lack of such pharmaceuticals. To obtain the neutron beam, special medical reactors built for this purpose in Brookhaven and Massachusetts (USA) were used.

Today, interest in BNCT is shown in many countries of the world. At the same time, work is underway aimed at obtaining neutrons of the energies necessary for BNCT without the use of reactor facilities.

Various principles are proposed for constructing BNCT systems based on an accelerator with a particle beam interacting with a neutron generation target.

A promising achievement is the concept of a proton beam - a lithium target, according to which a beam of low-energy protons (with an energy of the order of 2 MeV) hits a lithium target, generating neutrons by (p, n) reaction. Attractive features of the concept are:

- a relatively high neutron yield (about 10 ^-4 ),

- the ability to provide low maximum energy of the generated neutrons (not more than 30 keV),

- a relatively simple low-energy proton accelerator,

- the ability to work in the "open" geometry, when the distance from the target to the patient is minimal (10-20 cm),

- the minimum required shielding and acceptable residual radioactivity.

In Russian concept elaborated proton beam - lithium target, comprising the use of a tandem proton accelerator to influence the lithium target with energy exceeding the threshold of 1.88 MeV for the production reaction ⁷ Li (p, n) ⁷ Be, similar to that described in U.S. Patent №5976066, publ. 11/02/99, Yanch J.C. et al. Patentee Massachusetts Institute of Technology, Newton Scientific, Inc. Preliminary studies have shown that this proposal is quite acceptable, while the required proton beam current should be 10 mA or more to provide the necessary dose in the treatment area.

However, the accelerator-based lithium target concept still has unresolved critical problems, including high proton current requirements, excess energy neutrons, and the cooling of a lithium target with a low melting point. As an alternative, beryllium, which has a higher melting point than lithium, is more easily cooled and successfully used in clinical equipment for fast neutron therapy, is considered as the target material. For example, for the ⁹ Be (d, n) ¹⁰ Be reaction at E = 2.5 MeV and 1 = 10 mA, the neutron yield is S ~ 4 · 10 ¹³ n / s, which is 4 times greater than for the other ⁷ Li ( p, n) ⁷ Be for the same conditions. However, in this case, high-energy neutrons with an energy of 100-800 keV are observed in the spectrum, and moderators based on beryllium oxide, aluminum oxide, and heavy water are used to convert them into epithermal neutrons with an energy of 1-30 keV, which are needed for NRTs. In this case, distances from the target to the patient of at least 50 cm are required, which reduces the density of epithermal neutron flux in the tumor by an order of magnitude.

Using the reaction ⁷ Li (p, n) ⁷ Be in the near supercritical region of proton energy, when the difference between the reaction threshold Ecr = 1.88 MeV and the energy incident on the target is 0-30 keV, it allows:

- bring the patient closer to the target at a distance of 10-20 cm, i.e. work in "open geometry"

- use the so-called "kinematic collimation" when epithermal neutrons propagate directly within the spatial angle of 20-30 °.

In experiments, the last reaction was studied at currents of 1-2 mA. This corresponds to a beam power of 2-4 kW. The current level of development of NRT, taking into account the achievements of medicine, technology and pharmaceuticals, requires an increase in capacity by about an order of magnitude.

Known methods for producing neutrons for use in NRT can be divided into two types. The first method consists in irradiating a beam of charged particles from the output of the accelerator of a fixed target with active material. The second method involves irradiating with a similar beam a moving target containing the active material on which the neutron-producing reaction occurs.

The first of these methods is implemented, for example, in devices protected by US patents No. 4666651, publ. 05/19/1987, Barjon Robert, patent holder: Commissariat a l′Energie Atomique (Paris, Fr), Center Antoine-Lacassagne (Nice, Fr.) and No. 5920601, publ. 07/06/1999, Nigg David W. et al., Patent holder: Lockheed Martin Idaho Technologies Company (Idaho Falls, ID). The disadvantage of this method is the need to use intensive cooling of the active material of a static target. The second method is implemented in devices protected by US patent No. 4582673, publ. 04/15/1986, Gunter Bauer, patent holder: Kernforschungsanlage Juljich Gesellschaft, Fed. Rep. of Germany. With a moving target, the cooling system requirements are reduced, but special devices are required to drive the target.

The static and moving targets of the neutron producing site are made of solid or liquid active material.

As a prototype, a method for producing neutrons based on a nuclear reaction in the active material of a dynamic target irradiated by a stream of accelerated charged particles was selected. As a dynamic target in this solution, a liquid lithium stream is used, flowing under the influence of gravity from one container to another, forming a thin wall layer in which a nuclear reaction occurs (US patent No. 39993910, publ. 11.23.1976, Don M. Parkin , Norman D. Dudey, United States Energy Research & Development Administration patent holder).

When using this method, the problem of cooling the system is relatively easy to solve, but intractable problems arise associated with cleaning the system and lithium from radioactive contamination. This occurs, inter alia, from the intensive evaporation of the active material, lithium, into the accelerator volume.

Disclosure of invention

The present invention is directed to a method for eliminating or significantly reducing these disadvantages.

The problem is solved in that in a method for producing neutrons based on a nuclear reaction in the active material of a dynamic target irradiated by a stream of accelerated charged particles, according to the invention, a stream of rarefied solid particles of active material perpendicular to the beam of charged particles is used as a target.

Thus, it is proposed to use a different aggregate state of the active material - a suspension, which, in contrast to a thin coating with the active material in a liquid or solid state, takes up 10 ² -10 ⁴ times larger volume and provides a neutron-producing reaction in the interaction of charged particles with rarefied particles of the active material. In this case, thermal energy is released on a substantially larger surface than is the case in known targets and is fairly evenly distributed throughout the volume, practically without heating other elements of the device interacting with the active material. In addition, this method allows the use of an active material with a higher melting point compared to pure lithium, which significantly reduces the evaporation of the active material and the contamination of the elements of the system with radioactive vapors.

Particle movement can occur under the influence of gravitational forces.

Such a movement can be arranged by placing a volume supplying particles of the active material to the working area above the receiving volume.

In another embodiment, the movement of particles of the active material can be carried out in a stream of light rarefied gas under the action of a pressure differential between the volumes of supply and reception of particles of the active material. In this case, it is advisable to choose a pressure value in the feed volume of particles of the active material in the range of 10-100 mm Hg, and in the intake volume less than 1 mm Hg.

The active material located in the feed volume of the particles of the active material can be affected by vibration, which will provide a more uniform movement and uniformity of the suspension.

Using a pressure differential, it is possible to mix the particles of the active material in the feed volume of the particles of the active material, which will also ensure a uniform suspension.

To ensure a given volume concentration and speed of movement of particles of the active material in the working area at its inlet, controlled throttling and formation of a cross section of the particle stream of active material are carried out. By changing the size of the inlet that connects the supply tank to the channel of the working area, provide the channel with a given number of particles of active material per unit time.

The particle size of the active material, the thickness of the layer of particles and their concentration should be chosen from the condition of ensuring the full range δ of charged particles in such a layer. In other words, the particle layer thickness should not be less than the mean free path of charged particles in the active material of the target.

One of the solid lithium-containing materials can be used as the active material, and protons as charged particles.

In this case, the concentration of active particles in the working zone can be selected equal to 10 ^-2 -10 ^-3 , and the average particle size is chosen at least an order of magnitude less than the mileage δ. At this concentration, neutron production occurs in a relatively thin upper layer of the cross section of the target. In the rest of the working zone, flow particles will be heated due to the braking energy of charged particles in the active material.

In another embodiment, the thickness of the layer of particles of the active material and its concentration are selected from the condition of ensuring the path Δ of charged particles only in the supercritical reaction. In this case, it is necessary to cool the wall of the working zone in the neutron exit region. Such cooling is necessary because protons release their energy over the entire range and are most noticeable in the so-called Bragg peak.

In this case, the concentration of particles of the active material is advisable to choose ~ 10 ^{-4 of the} initial density of the solid material, and the average particle size is approximately an order of magnitude less than the supercritical range Δ.

In the next embodiment, the particle layer thickness should be chosen K times greater, where K = 2-10, but at the same time, an accelerating voltage U = (K-1) · (1.91-1 , 88) MeV. This is due to the fact that in the enlarged layer there will be K times more collisions of particles with energies above 1.88 MeV with particles of the active material and, as a result, their large energy losses, which will be compensated by the acceleration of charged particles during their passage through the working zone .

Tight separation of the channel of the active material particle stream from the feed channel of charged accelerated particles can be ensured by a thin tape of light metal with high heat capacity, which is moved perpendicular to the axis of the feed channel into the working zone of accelerated charged particles.

The cooling of the wall of the working zone in the neutron exit region can be carried out in different ways. The most traditional method is cooling with a stream of liquid or gas medium. In another embodiment, the cooling can be carried out by moving relative to the specified wall of the tape made of a material with high thermal conductivity, while the movement is perpendicular to the axis of the feed channel of accelerated charged particles. Using such a tape, the heat generated is recycled.

The flow rate of particles of the active material in the working area, it is advisable to choose in the range of 0.2-2.0 m / s

Brief Description of the Drawings

The inventive method is illustrated by figures 1-10.

Figure 1 shows a diagram of a target site that implements a method using gravitational force, which shows:

1 - the supply volume of the active material;

2 - finely dispersed active material (powder LiH, Li ₂ O, LiF, etc.);

3 - a beam of charged particles from the accelerator, for example, protons P;

4 - channel for the arrival of charged particles in the working area;

5 - receiving volume;

6 - flow of rarefied particles of the active material;

7 - throttle valve;

8 - node vibration of the feed volume;

v is the velocity of the suspension particles;

n are the produced neutrons.

Figure 2 shows a diagram of a target site that implements a method for producing neutrons using a suspension of particles in a light gas, where in addition to the above positions are shown:

9 - the volume of formation of the suspension;

10 - device for forming a suspension.

Figure 3 shows a diagram illustrating the effect of increasing the active surface due to the inclined wall, where:

S ₀ is the area of the wall perpendicular to the flow of charged particles;

S ₁ = S ₀ / sinα is the area of the inclined wall.

Figure 4 is a diagram illustrating the effect of an inclined wall in the volume of a dust suspension, where:

Δ is the particle size of the suspension 11;

and - the distance between the particles.

Figure 5 shows a conditional example of the flight of a proton with an energy of 1.91 MeV through three particles of active material in a supercritical reaction.

Figure 6 - diagram of the complete braking of the flow of charged particles in the stream 6 of rarefied particles of the active material, where:

δ is the total range of protons;

m is the number of conditional layers in the full range of protons δ;

and m is the proton mean free path in the supercritical reaction.

Figure 7 shows a diagram of the working area with a depth of the channel for moving the suspension, slightly larger than the supercritical range of charged particles, where:

12 - channel for moving a rarefied suspension;

13 - cooling channel;

Vzh - the speed of movement of the coolant.

Fig. 8 schematically shows the overlap of the suspension displacement channel from the volume of the channel for the arrival of charged particles with a thin tape moving perpendicular to the axis of the specified channel, where 14 is a light metal foil tape (Al, Be) rewound from drum to drum.

Figure 9 shows a diagram of the suspension moving channel with heating of an additional tape, accumulating residual thermal energy of the flow of charged particles, where 15 is a tape of material with high heat capacity, rewound from drum to drum.

Figure 10 shows a diagram of the channel for moving the suspension flow, the depth of which is several supercritical ranges K a · m, and an accelerating voltage equal to (K-1) · ΔЕ is applied in depth, where ΔЕ is the supercritical loss of proton energy (ΔЕ = 1 , 91 MeV - 1.88 MeV = 0.03 MeV).

Embodiments of the invention

The method is implemented as follows.

In the volume 1 (Fig. 1) or 6 (Fig. 2) of the active material supply, a certain amount of active material is placed in the form of a fine powder of solid material. It can be a lithium-containing material: lithium hydride LiH, lithium oxide Li ₂ O or lithium fluoride LiF. In this case, with proton bombardment, a nuclear reaction of ⁷ Li (p, n) ⁷ Be occurs. In another case, beryllium can be used as the active material (reaction ⁹ Be (p, n) ¹⁰ Be). It is also possible to use other active materials.

From volume 1 or 6, the free-flowing active material in the form of a flat rarefied wall of a certain thickness is transferred to a receiving volume at a given speed. In the simplest case, such a device can be performed by analogy with an hourglass, when the movement of the active material occurs under the influence of gravitational forces (figure 1). The valve 7 is opened with a throttle and a shaper of a particle stream cross section, after which a charged particle stream P is supplied. For example, for NCT, the following regime is required: proton energy E ₀ = 1.91 MeV, beam diameter 50 mm, current 10 mA, duration 500 s. For utilization of ~ 10 mJ of thermal energy arising from heating at 500 ° C, 5-6 kg of lithium hydride powder is sufficient. The powder consumption in this case will be about 10 g / s. If you set the suspension channel depth δ = 5 mm, then the concentration of particles of the active material in the center of the spot will be 10 ^-2 , and the speed of their movement ~ 0.4 m / s. With an average particle size of 1 μm, the particle stream will consist of 50 layers.

For a reliable and repeatable procedure, it is advisable to influence the volume of supply of the active material by vibration from a simple external device.

In another embodiment (FIG. 2), the active material is preliminarily placed in a light gas medium, for example, in hydrogen or helium, with a small residual pressure of 10-100 mm Hg. Moreover, the supply volume 9 is an order of magnitude greater than the volume of active material 2 (if the active material, for example, is 5 l, then the supply volume should be> 50 l. Next, the device for forming suspension in volume 9 is turned on, in the simplest case, the fan. suspended in the supply volume 9 will be 10 ^-1 . If you open the valve 7 with a throttle and a section former, then under the influence of a pressure differential (in the supply volume P ₁ = 100 mm Hg, in the intake volume, which is much larger than the supply volume, P ₂ = 1 mm Hg) in the channel of the working zone 6, a stream of particles will be formed, of which protons P will begin to knock out epithermal neutrons n. The modes and parameters in this embodiment are similar to the example discussed above.

The use of an inclined working surface of the active material with respect to the direction of flow of charged particles is known. A simplified diagram of such a solution is shown in FIG. 3. If, for example, the cross section of the channel for the arrival of charged particles, in our case, the proton conduit, is 50 mm, and the working surface is ~ 20 cm ² , then the slope of the working surface by an angle α = 30 ° doubles the surface and, as a result, the specific thermal load decreases from 1 kW / cm ² to 0.5 kW / cm ² , and this is significant.

When used as a working material, the suspension increases the working surface of the active material many times.

It is easy to show (see Fig. 4) that at a concentration of 10 ⁻³ of the initial one, and / Δ = 10, the surface of the active material also increases by more than an order of magnitude. If you select a particle size of the suspension is less than the mileage (see figure 5), then this value should be multiplied by the number of layers that fit in the mileage (for figure 5 this number is 3).

In the simplest case for implementation and consideration, all the energy of a beam of charged particles is utilized in a stream of rarefied particles of active material. The flow cross section is shown in Fig.6. Protons are completely inhibited at a depth of δ. At the near-surface depth "a · m", a supercritical reaction occurs (neutrons are formed).

The table for the analysis of the reaction of ⁷ Li (p, n) ⁷ Be shows the characteristics of some active lithium-containing materials.

Characteristics Materials Li Lih Li ₂ O LiF Density, ρ, g / cm ² 0.534 0.78 2.01 2.63 Melting point, ° С 180 691 1727 849 Heat capacity, J / kg · K 3800 3600 2500 2200 Relative neutron yield one 0.72 0.47 0.33 Mileage in solid, μm 140 77 40 33 Supercritical mileage, microns 2.2 1.21 0.63 0.52

According to the complex of thermophysical, technological properties and cost, the most suitable material for implementing the method is lithium oxide.

Let us set the overheating of Li ₂ O equal to 500 ° C. At the same time, about 8 kg or about 4 l of powder will be required for the above NCT regimen.

The following requirements will need to be met:

- average grain size <0.63 μm, select 0.5 μm,

- powder consumption 8000/500 = 16 g / s,

- concentration 10 ^-2 (density 2.01 kg / m ³ ), with a channel depth of 5 mm this corresponds to an average velocity of the powder in the working volume of ~ 0.5 m / s.

You can use the known cooling system, for example, cooling the rear wall of the channel with a stream of liquid or gas (Fig.7).

In this case, one can drastically reduce the amount and consumption of the reaction material Li ₂ O and use it only for a supercritical reaction. We set the average grain size of the powder of 0.2 μm (three supercritical paths, m = 3), take the previous cross section of the flow channel 5 × 50 mm. Now, with the above modes, the powder consumption will be ~ 60 times less and only 125 g of powder (or ~ 60 cm ³ ) will be required.

The separation of the volumes of the suspension channel and the proton conduit can be realized using a thin tape of light metals (beryllium, aluminum) moving along the proton conduit window, rewound from drum to drum. In this case, it is necessary to raise the proton energy by the amount of loss, which in the beryllium tape with a thickness of 10 μm will be ~ 20 keV. Only 300 g of tape 60 mm wide is required.

A similar solution can also be used instead of traditional cooling of the opposite wall to store thermal energy. In this case, a tape with a thickness of 0.3-0.5 mm from beryllium or aluminum (Fig. 9) is rewound from drum to drum with a speed of V ₁ along the opposite wall of the suspension moving channel. Since the material of the tape has a high heat capacity, it will effectively accumulate the heat released during braking of charged particles. However, in this case, such an aluminum tape will need about 10 kg.

Finally, it is possible to increase the neutron yield by a factor of K under the same initial conditions (Fig. 10) by increasing the supercritical path layer by a factor of K and applying an accelerating voltage of (K-1) · (1 , 91-1.88) MeV. In this case, the protons that have lost energy, having accelerated, will again be able to enter into a nuclear reaction, the cross section of which will increase by a factor of K.

Industrial applicability.

Using the proposed method can be created neutron-producing target sites for use in neutron capture therapy. The calculations and experiments showed that the creation of such a node can be carried out using existing technologies. The use of a different state of aggregation of the active material provides the simplification and cheapening of the production of neutrons for medical use.

Claims (17)

1. A method for producing neutrons based on a nuclear reaction in the active material of a dynamic target irradiated by a stream of accelerated charged particles, characterized in that the stream of rarefied particles of solid active material perpendicular to the beam of charged particles is used as the target. 2. The method of producing neutrons according to claim 1, characterized in that the movement of particles of the active material is carried out using gravitational forces. 3. The method of producing neutrons according to claim 1, characterized in that the movement of particles of the active material is carried out in a stream of light rarefied gas under the action of a pressure differential between the volumes of supply and reception of particles of the active material. 4. The method for producing neutrons according to claim 3, characterized in that the pressure in the feed volume of the particles of the active material is chosen equal to 10-100 mm Hg, and in the volume of reception less than 1 mm Hg. 5. The method of producing neutrons according to claim 2, characterized in that the active material in the supply volume of particles of the active material produce a vibration effect. 6. The method for producing neutrons according to claim 3 or 4, characterized in that the active material is mixed in the feed volume of the particles of active material. 7. The method of producing neutrons according to any one of claims 2 to 6, characterized in that the volume concentration and speed of movement of particles of the active material in the working area is provided by throttling and forming a cross-section of the flow of particles of active material at the entrance to the working area. 8. The method of producing neutrons according to any one of claims 3 to 7, characterized in that the particle size of the active material, the thickness of the layer of particles of the active material and their concentration are selected from the condition of ensuring the full range of δ charged particles in the specified layer. 9. The method of producing neutrons according to claim 8, characterized in that solid lithium-containing material is used as the active material, and protons are used as charged particles. 10. The method of producing neutrons according to claim 9, characterized in that the concentration of particles of the active material is chosen equal to 10 ^-2 -10 ^{-3 of the} original density of the solid material, and the average particle size is chosen at least an order of magnitude smaller than the mean free path δ. 11. The method of producing neutrons according to any one of claims 2 to 6, characterized in that the thickness of the layer of particles of the active material and its concentration are selected from the condition of ensuring the path Δ of charged particles in a supercritical reaction, while the wall of the working zone in the neutron exit region is cooled. 12. The method of producing neutrons according to claim 11, characterized in that the concentration of particles of the active material is chosen to be ~ 10 ^{-4 of the} initial density of the solid material, and the average particle size is about an order of magnitude less than the supercritical range. 13. The method of producing neutrons according to item 12, characterized in that the thickness of the layer of particles of the active material is chosen K · times greater, while the accelerating voltage U = (K-1) · (1 , 91-1.88) MeV, where K = 2-10. 14. The method of producing neutrons according to any one of claims 2 to 13, characterized in that the channel of the active material particle stream is sealed from the charged particle supply channel with a thin tape of light metal with high heat capacity, which is moved perpendicular to the axis of the accelerated charged particle supply channel. 15. The method of producing neutrons according to any one of paragraphs.10-14, characterized in that the cooling of the wall of the working zone in the neutron exit region is carried out by a stream of liquid or gas. 16. The method of producing neutrons according to any one of paragraphs.10-14, characterized in that the cooling of the wall of the working zone in the neutron exit region is carried out by moving relative to the specified wall of the tape made of a material with high thermal conductivity, while the movement is perpendicular to the axis of the accelerated feed channel charged particles. 17. The method of producing neutrons according to any one of claims 1 to 16, characterized in that the particle flow rate of the active material is chosen to be 0.2-2.0 m / s.

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