Combined cold neutron and thermal neutron speed reducer

Abstract

The invention relates to a combined cold and thermal neutron retarder (10) having a longitudinal axis (t) and at least one neutron exit window (50) at a first end (11, 12), and a cold retarder (20), a thermal pre-retarder (30) having a pre-retarder cover (32) and a vacuum space (40) arranged along the longitudinal axis (t), the invention relates to a prereducer (30) for a cold reducer (20), comprising a prereducer jacket (32) surrounding the cold reducer (20) along a longitudinal axis (t) and separated from the cold reducer (20) by a vacuum space (40), characterized in that the prereducer (30) comprises a prereducer ring (34) extending from the prereducer jacket (32) towards the longitudinal axis (t) at least at a first end (11, 12), the pre-reducer ring substantially covers a portion (50) of the vacuum space (40) between the cold reducer (20) and the pre-reducer cover (32) in the direction of the central outlet window such that at least one end (21, 22) of the cold reducer (20) is substantially unblocked at the ends (11, 12).

Description

[0001] The present invention relates to a combined cold neutron and thermal neutron moderator, the moderator having a longitudinal axis and a neutron outlet window at at least one end, and comprising a cold moderator, a hot pre-moderator and a vacuum space arranged along the longitudinal axis, the hot pre-moderator having a pre-moderator cover, the pre-moderator cover surrounding the cold moderator along the longitudinal axis and separated from the cold moderator by the vacuum space. Background Art

[0002] One of the most important components of a modern neutron source is the so-called moderator, which reduces the energy of the fast neutrons produced in the nuclear reaction to the thermal or cold energy range. Both energy ranges can be used for different measurements, so the purpose of the end device determines the optimal slow neutron output.

[0003] Based on previous studies (see K. Batkov, A. Takibayev, L. Zanini, F. Mezei: Unperturbed moderator brightness in pulsed neutron sources, NIMA, Vol. 729, 2013, pp. 500-505 and Tetsuya Kai, Masahide Harada, Makoto Teshigawara, Noboru Watanabe, Yujiro Ikeda: Coupled hydrogen moderator optimization with ortho / parahydrogen ratio, NIMA, Vol. 523, No. 3, 2004, pp. 398-414), one of the most favorable moderator types for these sources in terms of neutron yield-“brightness” (n / Å / cm2 / s / sr) is a low-dimensional (2D, flat, 1D, “tubular”) neutron moderator 1, which consists of a cold moderator 2 and a hot pre-moderator 3 (see Figure 1a ). In a "tubular" neutron moderator 1, the medium of the central cold moderator 2 (e.g. 20K liquid para-H2) is surrounded by a vacuum space 4, followed by a pre-moderator 3 (e.g. room temperature water), which is typically formed as a pre-cooling shroud of a few centimeters (1-3 cm) thickness.

[0004] At each end of the neutron moderator 1 there is an exit window 5 through which neutrons can leave the neutron moderator 1 and enter a neutron outlet tube (not shown) parallel to the longitudinal axis of the neutron moderator 1 . Figure 1b The neutron moderator 1 is shown from the direction of one of the exit windows 5. The exit window 5 can be divided into three nested areas: an exit window section 5a in front of the cold moderator 2, an exit window section 5b in front of the pre-moderator 3, and an exit window section 5c in front of the vacuum space 4.

[0005] Such an arrangement allows for more efficient neutron beam emission compared to higher dimensional moderator types. In addition, pre-moderator 3 allows for multi-spectrum neutron emission, since cold neutrons can be emitted from cold moderator 2, while thermal neutrons can be emitted from an adjacent pre-moderator 3 in close spatial proximity.

[0006] The "primary" neutrons from the neutron source (produced in nuclear reactions) are scattered by a sufficiently large reflector and arrive at the pre-moderator 3 with a nearly uniform directional distribution, and the neutrons thermalized there leave the pre-moderator 3 with a typical cosine directional distribution close to a normal distribution, such as Figure 1a As shown at the point marked with P in the figure. Neutrons colliding in the pre-moderator 3 and leaving the pre-moderator 3 mainly fall into the thermal energy range, which is between about 0.025-0.3 eV energy. The actual energy range of the thermal neutrons leaving the pre-moderator 3 is determined by the material and temperature of the pre-moderator. Neutrons leaving from the surface of the pre-moderator 3 toward the cold moderator 2 usually reach the cold moderator 2, where, for example in the case of liquid secondary H2 at 20K, they typically dissipate a significant part of their energy in a single collision and continue as cold neutrons. Neutrons colliding in the cold moderator 2 and leaving it belong to the cold energy range, which is between about 0.00001 and 0.025 eV. The actual energy range of the obtained cold neutrons is determined by the material and temperature of the cold moderator 2. The cold neutrons leaving the cold moderator 2 through one of the two exit windows 5 are in Figure 1a In the figure, only the middle arrow (marked A) is shown on the left side of the neutron moderator 1. Thermal neutrons leaving the pre-moderator 3 in the direction of the exit window 5 can also be guided and utilized, which is also shown in FIG. Figure 1a The left side of is shown by the lower and upper arrows B. The useful neutron yield of the neutron moderator 1 is determined, among other factors, by the size of the exit surface (Liouville's theorem).

[0007] The inventors of the present invention have recognized that the vacuum space 4 separating the pre-moderator 3 from the cold moderator 2 in the neutron moderator 1 represents a significant loss in the yield of cold and thermal neutrons. One of the reasons is that only a very small portion of the neutrons that leave the pre-moderator 3 at the surface facing the cold moderator 2 leave at a sufficiently small angle to reach the exit window 5 through the vacuum space 4, and in addition, such thermal neutrons travel a longer distance in the tank wall of the pre-moderator 3, which is lossy due to the absorption of neutrons in the wall. The same is true for cold neutrons that leave the cold moderator 2 at a small angle. Another reason is that the vacuum space 4 in the volume between the cold moderator 2 and the pre-moderator 3 is a complete "blind zone" as a neutron source, and no neutrons come from there. Therefore, the neutron beam that comes out completely lacks neutron paths that can only come from the vacuum space 4 between the cold moderator 2 and the pre-moderator 3. Therefore, it can be expected that the neutron emission intensity from the vacuum space 4 around the cold moderator 2 is significantly lower than the neutron emission intensity from the cold moderator 2 and the pre-moderator 3. Therefore, the outlet window portion 5c adjacent to the vacuum space 4 is substantially a "dead space".

[0008] The present invention aims to provide a device which does not have the disadvantages of prior art solutions. In particular, the present invention aims to increase the combined cold and thermal neutron yield of a "tubular" neutron moderator.

[0009] The inventors have realized that by forming a ring around the cold neutron exit window at both ends of the neutron moderator 1 on the pre-moderator 3, which covers the exit opening of the vacuum space 4, for the same total beam cross section, this solution can significantly increase the total neutron yield of the moderator while keeping the cold neutron yield unchanged and increasing the thermal neutron yield.

[0010] Based on the above recognition, the problem is solved by a combined cold neutron and thermal neutron moderator according to the present invention, wherein the pre-moderator comprises a pre-moderator ring, which extends from at least one end of the pre-moderator cover to the longitudinal axis, and substantially covers a portion of the vacuum space between the cold moderator and the pre-moderator cover from the direction of the neutron outlet window, so that at least one end of the cold moderator at the pre-moderator ring is substantially unobstructed. Preferably, the neutron moderator has a neutron outlet window at each end, and a pre-moderator ring is formed at each outlet window to cover the vacuum space.

[0011] Compared to the low-dimensional moderators with pre-moderators studied so far, the innovation lies in the shape of the pre-moderator, which includes a pre-moderator ring around the cold neutron exit window. The role of the pre-moderator ring is to bridge the dead space caused by the vacuum space separating the cold moderator and the hot pre-moderator, increase the surface area of ​​the exit window portion for thermal neutrons, and thereby increase the thermal neutron yield of the total neutron moderator. The inventors have realized that the exit neutron intensity from the pre-moderator ring can be expected to be similar to the exit neutron intensity expected from any surface of the pre-moderator cover, and for this reason, the thickness of both the pre-moderator cover and the pre-moderator ring should be greater than the mean free path of neutrons in the pre-moderator. In the prior art arrangement, almost no thermal or cold neutrons escape from the vacuum space 4, because a very small part of the thermal or cold neutrons leaving the pre-moderator or cold moderator leave at an angle small enough to pass through the vacuum space 4 and the exit window 5, and the vacuum space as a neutron source is completely "blind" and there is no neutron path starting from there. On the other hand, further collisions in the pre-moderator ring of the present invention increase the probability of neutrons exiting in the longitudinal direction, because the collisions deflect the trajectories of some neutrons entering the pre-moderator ring towards the exit window.

[0012] The cold moderator contains a material having a neutron scattering probability that is preferably energy-dependent, wherein the mean free path of the cold neutrons is greater than the mean free path of the thermal neutrons in the cold moderator. One is preferably at least 2 times, more preferably at least 5 times, and most preferably at least 10 times greater than the other.

[0013] The length of the cold moderator is preferably at least 1 time and at most 3 times the mean free path of cold neutrons in the cold moderator, more preferably at least 1 time and at most 2.5 times the mean free path of cold neutrons, more preferably at least 1.5 times and at most 2 times the mean free path of cold neutrons. Preferably, the thickness of the cold moderator is at least 1 time, more preferably at least 1.5 times the mean free path of thermal neutrons in the cold moderator.

[0014] In a particularly preferred embodiment, the cold moderator comprises secondary H 2 as the material having an energy-dependent neutron scattering probability. It preferably has a temperature of about 20K.

[0015] Preferably, the pre-moderator material has no energy-dependent neutron scattering probability, so that thermal neutrons from the neutron source and moderated thermal and cold neutrons have approximately the same mean free path therein. In the case of room temperature water, this is about 1 cm.

[0016] The thickness of the prereducer cage and the prereducer ring is preferably selected to be at least 1 times, preferably at least 1.5 times, the mean free path in the prereducer material.

[0017] According to a further preferred embodiment, the pre-reducer extends beyond the cold-reducer in both directions along the longitudinal axis, and the longitudinal position of the two pre-reducer rings along the longitudinal axis is between the end of the cold-reducer and the outlet window.

[0018] Preferably, the longitudinal distance between the two pre-reducer rings and the ends of the cold reducers adjacent thereto is 0.5-2 mm, preferably about 1 mm. Such a vacuum gap already provides sufficient thermal insulation between the cold reducers and the hot pre-reducers.

[0019] The following will explain further details of the present invention using examples in conjunction with the accompanying drawings. In the accompanying drawings:

[0020] Figure 1a is a schematic cross-sectional view of a neutron moderator of the prior art along the longitudinal axis;

[0021] Figure 1b is based on the direction from one of the neutron exit windows Figure 1a Schematic diagram of a neutron moderator;

[0022] Figure 2a is a schematic cross-sectional view along the longitudinal axis of an exemplary neutron moderator according to the present invention;

[0023] Figure 2b is based on the direction from one of the neutron exit windows Figure 2a Schematic diagram of a neutron moderator;

[0024] Figure 3 A possible arrangement of a neutron moderator according to 2a and 2b within an exemplary neutron source is shown.

[0025] Figure 2a and 2b A combined cold and thermal neutron moderator 10 according to the invention is shown. The neutron moderator 10 comprises a cold moderator 20, a thermal pre-moderator 30 and a vacuum space 40 arranged along a longitudinal axis t. In the present embodiment, both ends 11 and 12 of the neutron moderator 10 are used as neutron exit windows 50. In contrast, embodiments are also possible in which only one of the ends 11 or 12 of the neutron moderator 10 is used as an exit window 50, i.e., from which the decelerated neutron beam is coupled. In this case, the other ends 11, 12 of the neutron moderator 10 do not have an opening on the pre-moderator 30 for the cold neutrons to leave, but rather the pre-moderator 30 covers the cold moderator 20.

[0026] exist Figure 2a and 2bIn the figure, the pre-reducer 30 has a pre-reducer cover 32, which surrounds the cold reducer 20 along the longitudinal axis t and is separated from the cold reducer 20 by a vacuum space 40. In the pre-reducer 30, the cold reducer 20 can be fixed in the usual way by an insulating spacer, which is not shown in the figure.

[0027] At each end of the pre-reducer cover 32, there is a pre-reducer ring 34 extending in the direction of the longitudinal axis t, which also forms part of the pre-reducer 30. The pre-reducer ring 34 is sized to substantially cover the portion of the vacuum space 40 between the cold moderator 20 and the pre-reducer cover 32 from the direction of the outlet window 50, and to substantially leave the ends 21, 22 of the cold moderator 20 unobstructed. Preferably, this is achieved by extending the pre-reducer 30 beyond the cold moderator 20 in both directions along the longitudinal axis t, and the two pre-reducer rings 34 are positioned along the longitudinal axis t so that they are between the ends 21, 22 of the cold moderator 20 closest thereto and the outlet window 50 closest thereto. Therefore, the two pre-reducer rings 34 are positioned outwardly relative to the cold moderator 20, allowing the pre-reducer rings 34 to extend all the way to the outlet window portion 50a close to the ends 21, 22 of the cold moderator 20, as shown in FIG. Figure 2b As shown, the boundary between the pre-reducer ring 34 and the pre-reducer cover 32 is represented by a dotted line. In this case, the two outlet windows 50 can therefore be divided into two parts: an outlet window part 50a close to the corresponding end 21 or 22 of the cold reducer 20 and an outlet window part 50b close to the hot pre-reducer 30 and outside it. The two pre-reducer rings 34 are at a distance d from the end 21 or 22 of the cold reducer 20 closer to it along the longitudinal axis t. The distance d is preferably 0.5 to 2 mm, more preferably about 0.8 to 1.2 mm, for example about 1 mm. Therefore, there is a vacuum-filled gap between the inner side of the cold reducer 20 and the pre-reducer ring 34, so that no heat conduction occurs between the lower temperature cold reducer 20 and the higher temperature pre-reducer 30.

[0028] In the present embodiment, the cross-section of the "tubular" neutron moderator 10 is rectangular, but of course other cross-sections are possible (eg, square, circular, etc.).

[0029] Preferably, pre-moderator 30 contains water at room temperature (about 20-24 degrees Celsius), but during use, the temperature may rise to 30-35 degrees Celsius even when the water is circulating. In this medium, high energy thermal neutrons collide within a few millimeters, dissipating most of their energy and leaving pre-moderator 30 primarily in the thermal energy range. Preferably, pre-moderator 30 is configured as a single space container, with pre-moderator cover 32 and pre-moderator ring 34 open to each other. The wall of the tank may be made of any commonly used material such as 2-3 mm thick aluminum alloy or zirconium alloy (e.g., material marketed as Zircaloy).

[0030] The material filling the cold moderator 20 has a neutron scattering probability that is preferentially energy-dependent, which means that the mean free path of neutrons in the material of the cold moderator 20 depends on the energy of the neutrons. The mean free path is the average distance that a moving particle (in this case a neutron) will travel before its direction of travel and energy are significantly changed due to collisions with other particles (in this case with particles in the material of the cold moderator 20 and in the pre-moderator 30 with particles of the material forming the pre-moderator 30).

[0031] The energy-dependent neutron scattering probability of the material of the cold moderator 20 is preferably such that the mean free path of the cold neutrons in the material of the cold moderator 20 is greater than the mean free path of the thermal neutrons from the pre-moderator 30. Preferably, the mean free path of the cold neutrons in the material of the cold moderator 20 is at least 2 times, more preferably at least 5 times, and most preferably at least 10 times the mean free path of the thermal neutrons.

[0032] In a particularly preferred embodiment, the cold moderator 20 comprises secondary H2 at about 15-40K, preferably about 20K, which is maintained in the desired temperature range by techniques known per se. In this liquid secondary H2 medium, the mean free path of thermal neutrons from the room temperature pre-moderator 30 is about 1 cm, while the mean free path of cold neutrons is about 10 cm. The wall of the cold moderator 20 is typically 2-3 mm thick and can be, for example, an aluminum alloy or a zirconium alloy (e.g., a material marketed as Zircaloy).

[0033] The energy-dependent neutron scattering probability of the material of the cold moderator 20 can be utilized so that the length of the cold moderator 20 is at least 1 and at most 3 times, preferably at least 1 and at most 2 times, more preferably about 1.5-2 times the mean free path of the cold neutrons along the longitudinal axis t of the neutron moderator 10. Cold neutrons are defined here as neutrons having an energy that has been cooled by collision with the material of the cold moderator 20, i.e., neutrons having a kinetic energy below about 50K.

[0034] In contrast, the thickness of the cold moderator 20 perpendicular to the longitudinal axis t is preferably at least 1 times, more preferably about 1.5 times or more, the mean free path of thermal neutrons from the pre-moderator 30 in the cold moderator 20. Therefore, using secondary H2 with a temperature of about 20K as the cold moderator 20 and room temperature water as the pre-moderator 30, the length of the cold moderator 20 is preferably about 10-20 cm, and the thickness is preferably about 1-2 cm. The thickness of the cold moderator 20 refers to the width perpendicular to the longitudinal axis t. This may differ depending on the direction in which it is measured perpendicular to the longitudinal axis t. For example, in Figure 2b In the illustrated embodiment, the rectangular cross-section cold moderator 20 has two thicknesses (in the vertical and horizontal directions as shown). Preferably, both thicknesses fall within the given ranges.

[0035] In contrast, the material of the pre-moderator 30 does not have an energy-dependent neutron scattering probability, i.e. both thermal and cold neutrons, and even hot neutrons, have the same mean free path. The thickness of the pre-moderator cover 32 and the pre-moderator ring 34 is adapted to the mean free path, preferably at least 1 times, more preferably at least 1.5 times the mean free path.

[0036] Figure 3 An exemplary arrangement of a neutron moderator 10 according to the present invention in an exemplary neutron source 100 is shown.

[0037] The neutron source 100 contains a target 130, which is surrounded by a reflector 110 and a beam shield 120 (e.g., a concrete wall), which is externally connected to a proton beam channel (not shown). A proton beam in the energy range of 2-100 MeV can be guided to the target 130, such as lithium or beryllium, through the proton beam channel, so that when protons entering the proton beam channel collide with atomic nuclei of the target 130, fast neutrons in the neutron energy range of about 100 keV-100 MeV are mainly produced.

[0038] The neutron moderator 10 according to the present invention is located inside the reflector 110, and the two exit windows 50 are each connected to an exit channel 150 through which the slowed cold neutrons and thermal neutrons can exit. Thanks to the reflector 110, the fast neutrons generated in the target 130 enter the neutron moderator 10 through the pre-moderator cover 32 from almost all directions with approximately the same intensity.

[0039] The outlet window portion 50a of the neutron moderator 10 in front of the cold moderator 20 is preferably not sealed, and the outlet passage 150 is also vacuumed, and the outlet passage 150 is connected to the vacuum space 40 in the neutron moderator 10 through the outlet window portion 50a. However, other embodiments are also conceivable, in which the neutron moderator 10 is sealed at the outlet window portion 50a by a 2-3 mm thick wall made of, for example, an aluminum alloy or a zirconium alloy (for example, a material sold under the trade name "Zircaloy").

[0040] In the neutron source 100, the pre-moderator 30 can play a dual role: on the one hand, it thermalizes the incoming fast neutrons (or partially thermalized neutrons), thereby improving the cold neutron yield of the cold moderator 20. On the other hand, it also acts as a thermal insulator between the target 130 and the neutron moderator 10 together with the vacuum space, or shields the cold moderator 20 from the influence of energetic particles and thermal radiation leaving the target 130.

[0041] Most of the thermal neutrons leaving the room temperature water filling the pre-moderator 30 are moderated into cold neutrons in the liquid hydrogen (eg, 20K liquid para-H 2 ) filling the cold moderator 20 near the outer wall of the cold moderator 20 .

[0042] In the cold moderator 20, the thermal neutrons from the pre-moderator 30 have a mean free path of about 1 cm, and lose most of their kinetic energy in one collision. The obtained cold neutrons have a mean free path of about 10 cm in parahydrogen, that is, the cold neutrons can leave the cold moderator 20, which is preferably 10-20 cm long, towards the neutron beam 150 exit channel through the exit window portion 50a close to the exit window 50 of the cold moderator 20 without any change of direction, thereby ensuring the formation of a balanced cold neutron flux. Therefore, in order to achieve the best cold neutron intensity along the longitudinal axis t, a depth of 10-20 cm of cold parahydrogen is preferably required. Therefore, this determines the size of the cold moderator 20 in the direction of the exiting beam, that is, the length of the cold moderator 20 along the longitudinal axis t. In contrast, the mean free path of the thermal neutrons generated in the pre-moderator 30 filled with room temperature water is only a few millimeters, so that even a 1.5 cm thick water shield is sufficient to couple out the balanced thermal neutron flux. Therefore, a uniform thermal neutron flux can be expected from the entire exit window portion 50b enlarged by the pre-moderator ring 34. With this approach, the overall neutron yield of the neutron moderator 10 can be improved, with cold neutrons unchanged and thermal neutron yield increased.

[0043] It should be noted here that since the cold neutrons and thermal neutrons do not leave through the exit window 50 strictly parallel to the longitudinal axis t, the cold neutrons and thermal neutrons are mixed in the exit channel 150, such as Figure 3 shown.

[0044] It will be obvious to those skilled in the art that various modifications may be made to the embodiments disclosed above without departing from the scope of protection defined in the accompanying claims.

Claims

1. A combined cold and thermal neutron moderator (10) having a longitudinal axis (t) and a neutron outlet window (50) at at least a first end (11, 12), and comprising a cold moderator (20), a hot pre-moderator (30) and a vacuum space (40) arranged along the longitudinal axis (t), the hot pre-moderator (30) having a pre-moderator cover (32), the pre-moderator cover (32) surrounding the cold moderator (20) along the longitudinal axis (t) and separated from the cold moderator (20) by the vacuum space (40), characterized in that The pre-moderator (30) comprises at least a pre-moderator ring (34) at a first end (11, 12) extending from a pre-moderator cover (32) toward a longitudinal axis (t), wherein the pre-moderator ring (34) substantially covers a portion of a vacuum space (40) between a cold moderator (20) and the pre-moderator cover (32) from a direction of a neutron outlet window (50), so that at least one end (21, 22) of the cold moderator (20) at the first end (11, 12) is substantially unobstructed.

2. The neutron moderator according to claim 1, characterized in that: The cold moderator (20) comprises a material having an energy-dependent neutron scattering probability, wherein a first mean free path of cold neutrons decelerated in the cold moderator (20) is larger, preferably at least 2 times larger, more preferably at least 5 times larger, and most preferably at least 10 times larger, than a second mean free path of thermal neutrons decelerated in the pre-moderator (30).

3. The neutron moderator according to claim 2, characterized in that: The length of the cold moderator (20) is at least 1 time and at most 3 times, preferably at least 1 time and at most 2.5 times, more preferably at least 1.5 times and at most 2 times, of the first mean free path of cold neutrons, and the thickness of the cold moderator (20) is at least 1 time and preferably at least 1.5 times, of the second mean free path of thermal neutrons.

4. The neutron moderator according to claim 2 or 3, characterized in that: The cold moderator (20) includes secondary H2 as a material having an energy-dependent neutron scattering probability.

5. The neutron moderator according to any one of claims 1 to 4, characterized in that: The thickness of the pre-moderator cover (32) and the pre-moderator ring (34) is at least 1 times, preferably at least 1.5 times, the third mean free path of neutrons in the material of the pre-moderator (30).

6. The neutron moderator according to any one of claims 1 to 5, characterized in that: The pre-moderator (30) extends beyond the cold moderator (20) along the longitudinal axis (t) at least at a first end (11, 12), and the position of the pre-moderator ring (34) along the longitudinal axis (t) is between the end (21, 22) of the cold moderator (20) and the neutron exit window (50).

7. The neutron moderator according to claim 6, characterized in that: The distance (d) of the pre-reducer ring (34) from the adjacent ends (21, 22) of the cold reducer (20) along the longitudinal axis (t) is 0.5-2 mm, preferably about 1 mm.

8. A neutron moderator according to any one of claims 1 to 7, characterized in that It includes a neutron outlet window (50) at each end (11, 12) of the neutron moderator, and the pre-moderator (30) has a pre-moderator ring (34) at each end (11, 12) extending from the pre-moderator cover (32) toward the longitudinal axis (t), each pre-moderator ring (34) basically covers a part of the vacuum space (40) between the cold moderator (20) and the pre-moderator cover (32) from the direction of the adjacent neutron outlet window (50), so that the two ends (21, 22) of the cold moderator (20) are basically unobstructed.

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