EP3091540B1 - Device for generating thermal neutron beams with high brilliance and method of manufacturing same - Google Patents

Description

The invention relates to a device for generating thermal neutrons for neutron scattering experiments and other applications of thermal neutron beams, e.g. Boron neutron capture therapy for tumor treatment and a method of making the device.

For the investigation of material properties such as the structure of a material u.a. Neutron radiation, used. When neutrons hit atoms, they are scattered by them. The directional distribution of the scattered neutrons is the desired quantity that can be used to determine the material structure or material properties. The generation of neutron beams for scattering experiments with high intensity and defined direction is a challenge.

In most of today's research neutron sources, neutron production is based on nuclear fission in thermal nuclear reactors, as described on the website http://en.wikipedia.org/wiki/Neutron. The thus generated primary neutrons with energy in the MeV range must be braked in the moderator area of the reactor to energies in the range of 1 eV (thermal neutrons) and then fed into beam tubes for the neutron scattering experiments.

In the thermal nuclear reactor, an extended moderator (usually light or heavy water) is an integral part of the nuclear reactor because the nuclear chain reaction is maintained only by thermal neutrons. In modern research reactors, e.g. At the FRM II in Garching, the neutrons for the scattering instruments are extracted from the moderator volume by tangential beam tubes. These lances are sac tubes made of a neutron transparent material that extend into the moderator and penetrate the biological shield of the reactor. Their orientation is such that the reactor core is not visible through the opening in the reactor shield, so that high-energy primary neutrons can not reach the experiments directly through the jet pipe.

The openings of the jet pipes and the angular range, under which neutrons can escape, are so large that several instruments are supplied from a jet pipe can be shielded by the neutron beam in the angular range between the instruments with a neutron absorber.

Alternatively, primary neutrons are generated by nuclear reactions triggered by charged particles (e.g., electrons, protons, or deuterons) that have been accelerated to high energies in an accelerator. In such accelerator-driven neutron sources there is a target, also called a converter, which is bombarded with high-energy charged particles from the accelerator. There arise high-energy neutrons, which are then moderated in a downstream moderator. Such a presenter is, according to the current state of the art, a massive block of water or polyethylene, which in many cases is surrounded by a graphite or beryllium reflector in order to reduce the neutron losses at the edge of the moderator. The jet pipes are usually connected to the surface of the moderator block. They penetrate the reflector through an opening provided and record at the moderator surface all the divergence experienced by the neutrons as they travel through the moderation volume. This results in a uniform neutron flux over the entire moderator surface.

Due to the fact that the neutron beams are removed in the case of accelerator-driven neutron sources at the periphery of the moderator, there is so much moderator volume on the neutron path that many already moderated neutrons, which actually move in the direction of the desired beam, are scattered out of this direction again. The neutrons that reach the point where a beam is to be extracted come randomly randomly from all directions, so that the tapped beam has a strong isotropy. In addition, the neutron flux is distributed over the entire moderator surface. Therefore, there are no excellent sites where more neutrons can be extracted than at others, and there are no excellent directions in which many neutrons are preferentially emitted from the moderator volume. The neutron beams generated in this way therefore have no preferred direction and thus a low intensity (beam density per unit area) and a low brilliance (intensity per solid angle unit).

In order to increase the thermal neutron flux provided to the experiment, the prior art has forced it to increase the performance of the accelerator. For commercial accelerators, various technologies are in use. Each of these technologies is limited to a certain power range, with the jump from a power range to a higher one with a disproportionately high technical outlay. With the increase in the accelerator performance is accompanied by an increase in the amount of heat deposited by the primary ion beam in the target. A therefore necessary increase in the cooling capacity at the target is associated with a further technical effort.

For most research methods, the direction of the neutron beam in the scattering instrument must be better defined than 2 °. Therefore, collimation systems remove all neutrons from the beam that are outside the accepted angular range.

The object of the invention is to extract as many suitable and as few as possible unsuitable neutrons from a moderator in beam tubes, also called neutron guides, to the neutron scattering instruments.

The object of the invention is achieved by a device having the features of claim 1. To achieve the object, a method comprises the features of the independent claim. Advantageous embodiments emerge from the dependent claims.

The object of the invention is achieved by a device comprising an accelerator- or laser-driven neutron source and a moderator for moderating neutrons of the neutron source, characterized in that one or more channels are present in the moderator, or from a central region in the interior of the Guide the moderator to the moderator's interface.

A channel according to the present invention is much more transparent, d. H. permeable, for thermal neutrons compared to the adjoining material of the moderator. A channel according to the present invention may be drilled in the moderator material if the material of the moderator is a solid. If the material of the moderator is a liquid, then a metallic tube such as an aluminum tube can serve as a channel in the sense of the present invention.

Fast neutrons lose their energy through elastic scattering with the atomic nuclei of the moderator and thus get into the thermal after a few shocks Energy sector. On average, thermal neutrons lose no further energy in the event of elastic shocks, but are deflected along their trajectory. A moderator suitable material therefore also acts as a reflector for thermal neutrons. Thermal neutrons, which move from an inner, central region of the moderator to the surface of the moderator, basically have a suitable direction, also referred to below as the forward direction, in order to be able to escape from the surface of the moderator. From such a central region, the neutron intensity in the forward direction decreases toward the outside since the thermal neutrons can be scattered or absorbed on their way to the extraction site. By using a channel from the central area of the moderator to the moderator surface, the neutron intensity in the forward direction is completely retained. It is therefore possible to increase the intensity of thermal neutrons in the forward direction, which are led out via one channel from the moderator. As a result, the neutron flux provided for the experiments is increased, which on the one hand benefits the time required for measurement and the measurement accuracy limited by the neutron flux.

In one embodiment of the invention, the interior region of the moderator is adjacent to the neutron source converter or generally adjacent to the primary neutron source. This source of primary neutrons is surrounded by moderator material. Adjacent to this source, there is basically a high flux of thermal neutrons, especially in comparison to near-surface areas within the moderator, so that such an arrangement further improves the yield of thermal neutrons. Adjacent means that the distance between the inner region and the neutron source is less than the distance from the inner region to a surface of the moderator. Preferably, the distance from the central region to the neutron source is at least half as small as the smallest distance between the central region and a surface of the moderator. According to the invention, the inner region is a determined region with a maximum flux of thermal neutrons. The center is determined in particular by computer simulation, whereby a further increase in the intensity of the thermal neutrons can be achieved. Computer simulation programs, with which the spatial distribution of thermal neutrons and thus a flux maximum inside a moderator can be determined, are commercially available, so that This is a simple and reliable way to determine such a region with a maximum flow, also called center.

With increasing moderator volume, the thermal flux in the moderator material increases to a certain saturation value, since already thermalized neutrons (ie thermal neutrons) can be reflected back from the outer layers of the moderator. However, at the surface of the moderator, where the neutrons are usually extracted for the experiment, the thermal flow decreases again beyond a certain moderator volume. In one embodiment of the invention, the volume of the moderator is therefore 50-200 liters, preferably 100-150 liters, in order to be able to moderately moderate primary neutrons on the one hand and to manage on the other hand with suitable channel lengths. The moderator is preferably in the form of a block, which may be spherical, cube-shaped, cuboidal, conical or ellipsoidal. Particularly preferably, the moderator is cylindrical, since with this form a particularly suitable spatial distribution of the thermal neutrons is achieved and thus a plurality of thermally brilliant neutron beams are obtained particularly well. One side length of the block is generally not many times longer than other side lengths, so that the shape is compact. In principle, a diameter does not exceed the height many times in the case of a cylindrical shape so as to obtain a compact shape. So the moderator does not have a particularly elongated shape.

In one embodiment of the invention, at least four, preferably at least six channels are present, which lead from an inner region of the moderator to the surface, and in particular in a star shape. Preferably, no more than 10, more preferably no more than 8, channels are present which lead from an inner region of the moderator to the surface. It can be further improved so the yield of thermal neutrons can be increased, in which z. B. several neutron scattering instruments are supplied by one channel each with thermal neutrons. First, an increasing number of channels leads to an improved yield of thermal neutrons. However, if too many channels are present, primary neutrons or primary neutrons can no longer be adequately moderated so that the yield can not be continuously improved by increasing the number of channels, but, on the contrary, the intensity in each individual channel decreases.

In an advantageous embodiment of the invention, the distance between channels on the surface of the moderator is at least twice the free path of thermal neutrons in the moderator material in order to increase the yield further improved.

The neutron source used is in particular a linear accelerator with a converter which can be produced with relatively little technical effort and which does not require any thermal neutrons for a chain reaction, so that a considerably smaller moderator volume than in a thermal nuclear reactor is possible.

Particularly suitable material for the moderator beryllium has been found in particular. The material of the moderator is then a solid into which the one or more channels are drilled.

Preferably, channels are polished internally to provide a further improved thermal neutron yield by being able to be reflected from the inner portion of the moderator to the surface on the walls of the channel as they pass through the channel. Mechanical polishing is sufficient, in particular in the case of beryllium, to further increase the yield of thermal neutrons. In the case of aluminum, the inside of the channel is preferably further coated so as to be able to conduct thermal neutrons out of the moderator in an appropriate manner. Polishing reduces surface roughness created by making the channel. Thus, in a first step, the channel is produced, for example by drilling. Subsequently, a second processing step takes place, through which the surface roughness is reduced. In a third processing step, in one embodiment, the polished surface is coated, particularly in the case of aluminum tubes, to obtain further improved results. The material suitable for the coating is, in particular, nickel, which has a high reflection coefficient for neutrons. Even multiple layers (so-called super-mirrors) are suitable as a coating of the inner walls of the channels. These coatings are z. B. applied by sputtering or galvanic on the already polished inner walls of the flow channels.

To further improve yield, the diameter of each channel is less than the mean free path of thermal neutrons in the moderator material. Such a channel is capable of thermal neutrons suitable from the moderator lead out. It also avoids unnecessarily dimensioning a channel, which would be detrimental to the efficiency of the moderator material.

The diameter of each channel depends on the requirements of the connected experiment and can be reduced to the technically feasible. The diameter to length ratio of the channel determines exit angles of thermal neutrons from the channel. The longer the channel is compared to the diameter, the smaller is the exit angle of thermal neutrons.

In one embodiment of the invention, filters may be provided inside each channel to filter out or prevent primary neutrons from exiting the respective channel. Suitable filter materials are, for example, lead and sapphire single crystals. As a result, it is better avoided that primary neutrons, ie neutrons with high energy in the MeV range, reach instruments or samples, but without weakening the proportion of thermal neutrons in the beam.

Preferably, the moderator comprises an outer reflective sheath of, for example, graphite or polyethylene. As a result, the yield can be improved further improved.

Especially for scattering experiments, the intensity of the extracted thermal neutrons in a certain preferred direction, the direction of the experiment, and thus the brilliance of particular importance. It is the forward component of the thermal neutron flux on the axis of the neutron instrumentation in the direction of the experiment.

In the center of the moderator is not only the total flux of thermal neutrons maximum but also its forward component (or radial component to the outside). Viewed from the center, both the total flux and the forward component of the flux decrease outwardly as the thermal neutrons can be scattered or absorbed on their way to the extraction site on the moderator surface. By using a channel from the center of the moderator, the point of maximum thermal flux, to the moderator surface, the forward component of the flux is fully conserved and can be passed without attenuation, for example to the beginning of the neutron guide of the connected neutron scattering instrument. Since the forward component at this point is independent of the size of the moderator, the volume of the Moderators or the reflector are chosen so that the thermal flux in the center and thus also the forward component can be increased.

Investigations have shown that the thermal intensity in the forward direction (θ≤2 ° with respect to the axis of the flow channel) and thus the brilliance can be increased by a factor of 6, depending on the position of the flow channel in the moderator.

A cylindrical moderator made of beryllium with a radius of r = 31cm and a length of l = 41cm was used, which corresponds to a volume of 125 liters. Parameter studies have shown that this volume represents a good compromise between maximum thermal flow and technical effort. The moderator is surrounded by a 20 cm thick layer of graphite as a reflector, whereby the thermal flow at the surface of the moderator can be increased by an order of magnitude. The axis of the cylinder lies on the beam axis of the primary ions (eg deuterium ions in the energy range of about 20 MeV energy), with the reflector at the entry point of the ions is interrupted, so that the accelerated by a linear accelerator ions strike the surface of the beryllium moderator and in the first layer of about 0.4 cm are converted into neutrons. This area, where the ions are converted into neutrons, is called a converter. In principle, the converter can also be moved further into the moderator. In this case, the converter is also surrounded on its back by moderator and reflector material, so that backscattered neutrons in favor of the overall thermal flux are more likely to remain in the moderator. However, this variant is associated with a higher design effort, since the converter must be laboriously cooled in most cases.

Due to its compact design, the moderator can be operated with many different sources of fast neutrons, resulting in a high flexibility for the use and the use. The low-price segment would include commercial fusion-based neutron generators, which essentially have an isotropic neutron field. To achieve higher neutron intensities, linear accelerators or cyclotrons come into question. Even high-intensity, short-pulse lasers are capable of generating intense primary ion beams, so that a more compact design of the entire system can be achieved with such a laser.

The increase of the forward thermal flux at the opening of the flow channel depends on the underlying geometric shape and arrangement of the flow channel River channel in the moderator. Different angles of attack between the flow channel and the axis of the primary ion beam are possible, depending on the intended use.

With an inclination of 40 ° with respect to the direction of the primary ion beam, the thermal intensity at the opening of the flow channel in the forward direction can be increased by a factor of 6, compared to an arrangement without a channel, as studies have shown. If a smaller proportion of fast neutrons is desired, in one embodiment, the channel is positioned orthogonal to the direction of irradiation of the primary ions. The intensity of thermal neutrons is stronger by a factor of 3 in this case.

The neutron flux can be strongly influenced by the choice of moderator material. In question are water, graphite, heavy water or beryllium, these materials increase in both moderation capacity and in terms of effort in turn. Taking into account the economic and radiological aspect, life, radiation exposure and technical complexity, for example, a combination of beryllium as a moderator surrounded by a layer of graphite as a reflector is a preferred solution.

The channel can be equipped with a lead or sapphire filter to suppress the flow of fast neutrons without significantly affecting the intensity of forward thermal neutrons.

Investigations have shown that several flow channels in a thermal moderator do not have to interfere with each other up to a certain number. For this reason, it is possible to operate multiple thermal neutron flow channels on a single moderator system. The channels may be arranged in a star shape, for example, so that the channels can lead away from the central area in the moderator in two or three dimensions in different directions and the various experiments do not have to restrict their space requirements.

The FIG. 1 shows a moderator 1 with a linear accelerator 2. At the end 3 of the beam pipe for the primary ions from the linear accelerator 2 arise in a converter primary neutrons. The end 3 of the linear accelerator 2 formed by the converter extends into the moderator 1. In the direction of acceleration of the linear accelerator, adjacent to the end 3, there is a flux maximum 4 thermal neutrons. The flux maximum was determined by computer simulations. Starting from this maximum flux 4, four channels 5 designed as neutron conductors extend approximately in a star shape to the surface 6 of the moderator 1.

The FIG. 2 shows a further embodiment with six channels 5 and with an outer shell 6, for example made of graphite or polyethylene.

According to this embodiment FIG. 2 the device consists of a linear accelerator which accelerates deuterons to 25 MeV and leads through the tube 2 to the converter 3, which here consists of a 0.7 mm thick layer of beryllium. Moderator 1 is a cylinder of beryllium 62 cm in diameter and 41 cm in length. The end of tube 2 and the converter 3 located there is embedded 5 cm deep along the cylinder axis in the moderator. The moderator is surrounded by a 10 cm thick reflector layer 6 made of graphite. The thermal neutron flux has in this construction its maximum 4 in 15 cm distance from the converter along the cylinder axis. At this point begin the six flow channels 5, each of which is cylindrical holes with a diameter of 2 cm.

In this embodiment, the neutron beams from the two backward (backward in the direction of the ion beam) channels have a factor of 1.5 higher brilliance than the neutrons on the surface of the moderator, the neutron beams from the two channels orthogonal to the cylinder axis one around the Factor 3.5 higher brilliance than the neutrons on the surface of the moderator and the neutron beams from the two forward channels a brilliance higher by a factor of 6 than the neutrons on the surface of a moderator of the same dimensions without flow channels.

Claims (14)

1. A device for generating thermal neutrons for neutron scattering experiments, comprising an accelerator or laser driven neutron source (3) and a moderator (1) for the moderation of neutrons of the neutron source (2, 3), characterized in that there are one or more channels (5) in the moderator which extend(s) from an inner area (4) in the moderator (1) to the surface (6) of the moderator (1), characterized in that the inner area (4) is at or in the maximum flux of thermal neutrons.
2. A device according to claim 1, characterized in that the moderator (1) is made of a solid matter and the channels (5) are drilled into the solid matter.
3. A device according to the preceding claim, characterized in that the solid matter consists of beryllium.
4. A device according to one of the preceding claims, characterized in that the inner area (4) is arranged adjacent to the neutron source (3).
5. A device according to one of the preceding claims, characterized in that the neutron source is a converter (3) at the end of a linear accelerator (2).
6. A device according to the preceding claim, characterized in that the inner area (4) is placed, seen in the direction of acceleration of the linear accelerator (2), behind the linear accelerator.
7. A device according to one of the preceding claims, characterized in that the neutron source (3) is placed in the moderator (1).
8. A device according to one of the preceding claims, characterized in that the moderator comprises a volume comprised between 100 and 150 liters.
9. A device according to one of the preceding claims, characterized in that there are 4-10 channels (5) in the moderator (1), which channels (5) extend from an inner area (4) to the surface (6) of the moderator (1).
10. A device according to one of the preceding claims, characterized in that the diameter of each channel (5) is smaller than the double of the mean free path of the thermal neutrons in the moderator.
11. A device according to one of the preceding claims, characterized in that the moderator (1) is enclosed by an outer sheath made of graphite or polyethylene,
12. A method for manufacturing a device according to one of the preceding claims, characterized in that a maximum flux of thermal neutrons in the moderator will be determined and ends of the channels (5) will be arranged in the area of the determined maximum flux.
13. A method according to the preceding claim, characterized in that the surfaces of the channels will be polished on the inside or lined with a polished coating.
14. A method according to the preceding claims, characterized in that inside one or more channels there is a filter made of lead or sapphire single crystal, in order to filter out primary fast neutrons.

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