EP1468427B1 - Neutron-optical component array for the specific spectral shaping of neutron beams or pulses - Google Patents

Abstract

A neutron-optical component array in which the beam paths of the individual moderators are combined in a concerted manner so as to create a superimposed neutron beam with an effective mean beam direction. The superimposed neutron beam has a multi spectrum composed of the single spectrums of several moderators, whereby a larger spectral width is obtained, making various applications in different neutron energy fields possible. The multi spectrum can be further improved in terms of the intensity thereof and the beam quality by adding further neutron-optical components, particularly in the form of an energy-depending switching super reflector, and by switching between moderators.

Description

The invention relates to a neutron-optical component arrangement for the targeted spectral design of neutron beams or pulses between a fast neutron source with a plurality of closely arranged moderators of different embodiments for the production of slow neutrons different energy spectra and their radiation in predetermined Radiation directions and at least one experimental space.

Neutron beams serve a broad spectrum of scientific investigations ranging from pure basic research to application-oriented investigations in the field of material structure research. Here, neutrons act as probes that penetrate into matter. Neutrons striking atoms of structured matter are either scattered in a characteristic way for the atoms or absorbed by the atoms by emitting characteristic radiation. For most applications, such as neutron scattering, it is necessary to provide slow neutrons produced by slowing down fast neutrons obtained from nuclear reactions. Intensive neutron radiation of fast neutrons is generated primarily by either constant-time flow in research reactors by fission enriched uranium or in pulsed form in spallation sources by disintegration of heavy nuclei.

The targeted slowing down of the fast neutrons occurs primarily through so-called "moderators", which are brought into contact with the fast neutron radiation. This is simply stated Accumulations of matter in gaseous, liquid or solid form with special properties at a given temperature. Due to the interaction of the fast neutrons with the lightest possible atoms of the moderator matter, the high-energy neutrons are strongly decelerated until their energies and wavelengths have the values suitable for the experiments on condensed matter. It produces a neutron gas with a kinetic energy distribution that can be approximated by a Maxwellian velocity distribution at a given temperature. This is a theoretically derived function that assigns their relative abundances to the velocities of the atoms of a gas. However, the effective temperature of the Maxwellian spectrum of the neutron gas is slightly higher than the temperature of the moderator material. It should be mentioned in this context that even neutron reflectors, such as (heavy) water, lead, beryllium, graphite, etc., produce slow neutrons, but with a different spectrum than the range approximated by the moderators by the maxwave spectrum. Nevertheless, reflectors, which mainly serve to increase the neutron flux, contribute to the neutron deceleration, so that in a broader sense they can also be assigned to the group of moderators as neutron-optical components. Similarly, premoders such as water or all other structures of a neutron source are added to the group of moderators who can even emit slow-neutrons.

Depending on the temperature of the moderator material, the slow neutrons are differentiated into "hot", "thermal" and "cold" neutrons, whereby the moderators can also be distinguished into "hot", "thermal" and "cold" moderators. With slow neutrons in the present context, neutrons with a kinetic energy in the range of 1 eV and less are called. Higher speed, lower wavelength hot neutrons have energy in the range above 100 meV and are particularly useful for scattering experiments on liquids suitable. Thermal neutrons have a kinetic energy in the range between 10 meV and 100 meV and the cold neutrons have kinetic energies in a range between 0.1 meV and 10 meV. Cold neutrons with a relatively low velocity and a large wavelength are of particular importance for applications of neutron scattering for the investigation of biological substances. Moderators exist in different forms of training. Hot, thermal, and cold moderators are distinguished by the nature of their primarily produced slow neutrons. An overview of possible moderator structures in a spallation source can be found in the article I by D. Filges et al. The ESC Mercury Target - Moderator - Reflector System (available from the Internet under http://www.hmi.de/bereiche/SF/ess/ESS_moderators3.pdf, as of 18.01.2002) Examples are the liquid hydrogen moderator with an operating temperature in the range of 25 K for the production of cold neutrons and the water moderator with the ambient temperature as operating temperature for the production of thermal neutrons, whereby a cold moderator also produces thermal and hot and a thermal moderator also cold and hot neutrons, but always with at least an order of magnitude smaller flux than the moderator, which mainly serves for the generation of cold, thermal or hot neutrons.

In order to be able to provide the right, required neutron spectrum for various experiments with slow neutrons, the known neutron sources work with different moderators in combinations. From the essay II of Jose R. Alonso "The Spallation Neutron Source Project" from the Proceedings of the 1999 Particle Accelerator Conference, New York, 1999, pp 574-578 (available from the Internet at http://accelconf.web.cern.ch/accelconf/p99/PAPERS/FRAL1.pdf, as of 18.01.2002) it is known that two temperature moderated water moderators below the level with the target material to be crushed and position two supercritical hydrogen moderators at 20K operating temperature above the target level. Each of the moderators now exclusively supplies one or more of eighteen different experimental sites via neutron guides with the slow neutron spectrum that he has generated (see Figure 9 and Chapter 6 of Appendix II). A similar structure is also known from the article III of N. Watanabe "5.3 - Material Issues for Spallation Target by GeV Proton Irradiation" (available from the Internet at http://wwwndc.tokai.jaeri.go.jp/nds/proceedings /1998/watanabe_n.pdf, as of 18.01.2002). Here, a target moderator configuration for performing high-intensity and high-resolution experiments with cold neutrons is described, in which a coupled cold moderator with premodulator and two thermal moderators are closely adjacent in the region of highest and fastest neutron radiation at the target. (see Supplement III, Chapters 4 (2) to (4) and FIG. 2 ). As an important point in this paper, it is pointed out that in spite of the close discernment, a crosstalk between the individual moderators, which has an effect on the neutron intensity, should be avoided (see article III, chapter 4 (ii)). The moderators are therefore arranged at such angles to one another that their respective forward and rearward-oriented emission directions or emitted neutron beams are oriented in different spatial directions and do not overlap. Each moderator supplies in this way about four to eight experimental stations with a neutron beam with a characteristic spectrum. Reflectors for separating the spectra are also arranged between the two levels.

LA Charlton et al .: "Spallation neutron source moderator design", NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH, SECTION - A: ACCELERATORS, SPECTROMETERS, DETECTORS AND ASSOCIATED EQUIPMENT, ELSEVIER, AMSTERDAM, NL, Bd. 411, No. 2-3 , July 11, 1998, pages 494-502, XP004133296, ISSN: 0168-9002 discloses the modification of the spectrum of a single radiating moderator surface with a single emission direction through the moderator composition.

GJ Russell et al .: "Target station design for a 1 MW pulsed spallation neutron source", TWELFTH MEETING OF THE IN TERNATIONAL COLLABORATION ON ADVACED NEUTRON SOURCES, 24-28 May 1993, LU-UR-94-404, 7 February 1994 , Pages 1-13, The Consener's House, Abingdon, Oxfordshire, UK discloses two independent target stations with central fixed targets, central fixed moderators, and each with more than ten radial solid experimental sites, with a neutron beam directed at each experimental site, emitted from the surface of exactly one moderator.

Starting from the known prior art for the well-known use of moderators, as described, for example, in the cited article III, it can be seen that both the provision of a neutron spectrum from a slow neutron required for a particular experiment and its generation pose major problems , Especially with regard to the very complex and expensive and high In the prior art, there is no flexibility in providing a neutron spectrum for a single experimentation site requiring protection for neutron optical devices. Each space is supplied with a neutron spectrum whose maximum indicates the mainly generated slow neutrons from a directly assigned moderator type. Changes in the spectrum of the neutron beam at a place of experimentation can only be realized through major structural changes in the moderator setup during long periods of operation of the neutron source. Experiments in broader energy ranges than those of a single slow neutron shape are not possible or very inefficient.

Object of the present invention is therefore to provide such an arrangement of neutron optical components for the targeted design of the spectrum of a neutron beam of the aforementioned type, which has great flexibility with respect to the provision of a neutron beam at a training area, so that here no costly conversion measures changed Requirements are required. In particular, experiments with neutrons from a larger energy range should be possible. Furthermore, the neutron beam which can be provided by the invention should have a high quality. The means for realizing the invention should be simple in construction and manageable and therefore relatively inconvenient and inexpensive. Existing safety aspects should be considered, additional risks should be avoided.

As a solution to this, according to the invention, the features according to claim 1, are provided in a neutron-optical component arrangement for the targeted influencing of neutron beams or pulses of the type described above

With the neutron-optical component arrangement according to the invention, the energy spectra of different moderators are combined with one another to form a "multi-spectrum". A neutron beam (or neutron pulse - this alternative should always be included in the use of the term "neutron beam") with such a multi-spectrum is particularly versatile. Since it has a larger energy spectrum than the neutron beams generated by only one moderator at a time, neutron experiments in a wide energy range of the incident neutrons, for example between 0.1 meV and 100 meV, can be carried out with high efficiency with the superimposed neutron beam according to the invention. The composition of the multi-spectrum of the superimposed neutron beam depends on the type and number of moderators used. For example, a cold and a thermal or a cold, a thermal and a hot moderator can be combined in their propagation direction. In the same way, different embodiments of a moderator type for achieving a particularly broad or specially designed multispectrum can be merged in their emission. The combination of different modifier gates here are only constructive limits set, since a combination of the radiation directions must still be technically feasible with reasonable effort. In this connection, it should be mentioned that other neutron-optical components present in the neutron system as well as parts of the neutron source itself with other main functions which have a decelerating effect on the neutrons, such as reflectors, neutron guides and primary moderators, are integrated into the composition of the multispectrum by unification of the emitted ones Radiation can be included in the common neutron beam concretely. This results in a single or multiple superimposed, versatile usable neutron beam. The focus of the invention is the union of the individual neutron beams in a common neutron beam with a correspondingly expanded energy spectrum. So far, the state of the art has always been based on an explicit and consistent separation of the moderator action ranges, since this appeared to be the only possibility, without too much technical effort, to provide suitable slow neutron beams for achieving usable measurement results. The disadvantage of the low flexibility and the limitation of the feasible experiments was accepted and corresponding numbers of different experimental places conceived.

A superimposition of the individual neutron beams of the moderators used to form a common neutron beam can take place both in the neutron guide and at the experimental station. In the first case, a superimposed neutron beam is generated, which, like a single neutron beam, is also conducted in a neutron guide to the experimental station and to the sample. In the second case, the different neutron beams are as it were focused on the sample to be examined, so that the superimposed neutron beam occurs directly in the sample. The advantage of this superimposed irradiation at the experimentation site itself can be seen in the relatively low technical complexity for combining the emission directions of the individual moderators. In the simplest case, the neighboring moderators are to be aligned at such angles to one another that an intersection of the emission directions in the sample or shortly before results. In this case, after a continuation of the neutron-optical component arrangement according to the invention, it can be advantageously provided that in the case of a direct superimposition of the emission directions they can be determined in the experimental space by a predetermined coding scheme. For the evaluation of the measurement results, it may be important to know the different emission directions from which the different types of neutrons impinge on the sample. This can be done in particular by monitoring the neutron flight time with a pulsed neutron source. With a continuous neutron source, the neutron beam must be chopped accordingly. That I Within the slow neutrons, the cold, thermal and hot neutrons differ by their energy spectrum and thus by their velocity distribution, by the knowledge of the individual neutron flight times from the pulses predominantly an assignment to the individual moderators and thus to their radiation directions in relation to the sample can be made ,

However, for most applications in experiments, it is important that the neutrons all hit from a common spatial direction on the sample to be examined. This common spatial direction is referred to below by the term "effective, mean beam direction". To achieve a common beam direction, a superimposition of the individual neutron beams by further neutron-optical components is required. For targeted guidance of neutron beams, various components are known, which are in principle all suitable for bringing about a combination of the moderator emissions in the arrangement according to the invention. This also includes the neutron guide itself, which may be coated on its inner surface with nickel according to an embodiment of the invention (see. DE 44 23 781 A1 ) and reflected at certain, particularly shallow angles incident neutrons flat in the tube interior. If, for example, two neutron beams coming from different directions now enter the entrance area of the neutron guide, they are directed in the course of the neutron guide through its internal reflections into the desired effective, mean beam direction.

Furthermore, in a superimposition of the emission directions by further neutron optical components to achieve an effective, central beam direction of the superimposed neutron beam according to another embodiment of the invention may be provided that another neutron optical device is designed as an oscillating mirror synchronously with a pulsed neutron source or with the chopped neutron beam a continuous neutron source oscillates. Through the oscillating mirror The neutron beams of different moderators are alternately superimposed in the superimposed neutron beam with the effective, central beam direction. For example, if the mirror oscillates back and forth between a cold and a thermal moderator to the beat of a neutron pulse source and if it has the correct angle for the cold neutrons, it first reflects the cold neutron pulse in the central beam direction. Then the mirror angle is adjusted in the pulse clock, so that the thermal neutrons impinge and the thermal neutron pulse is coupled. The other neutron pulse is deflected outside the central beam direction. In a continuous neutron beam from a nuclear reactor, mechanical or otherwise operating chopper arrangements can be used to chop the continuous neutron beam into individual pulses. The measurements on the sample are to be made in this embodiment in time with the neutron pulses or in the oscillator cycle.

It has already been explained above that in the energy spectra of the individual moderators there are two border areas each with neutron energies, which are to be generated in the main by the other moderators. If, for example, only cold neutrons have been supplied to the sample during an experiment, there are still hot and thermal neutrons in the neutron beam, albeit in a significantly smaller number. According to another continuation of the neutron optical component arrangement according to the invention, it is particularly advantageous if a further neutron optical component is designed with an energy-dependent switching function. In this embodiment, switching is not switched back and forth between the individual neutron beams with an active, moving mirror, but a neutron-optical system is provided which simultaneously accesses all incident neutron beams. In this case, a neutron-optical component is used which has an energy-selective switching function. Such components can be designed and aligned so that they pass, for example, the central energy of each moderator with the largest number of neutrons targeted to be generated and in the coupling effective, middle beam direction, whereas they lock the edge regions with the energetic deviating neutrons. Through the switching function, the multi-spectrum of the superimposed neutron beam can be assembled by passing only the corresponding neutrons from the moderators producing them in maximum number for the individual neutron species. Thus, for both cold and thermal and hot neutrons, maximum neutron flux can be achieved for the experiments.

Neutron optical components with an energy-selective switching function can be realized primarily by special neutron mirrors. Therefore, according to a further embodiment of the invention, it is provided that the further neutron-optical component with an energy-dependent switching function is designed as a neutron mirror, which transmits or reflects incident neutrons continuously or graduated by a corresponding angular orientation as a function of their energy. To further explain the functional interaction of the neutron mirrors to achieve the switching effect described above, reference is hereby made to the specific part of the description in order to avoid repetition. According to another embodiment of the invention, it can be advantageously provided that the neutron mirrors are formed in self-supporting or on a neutron-transparent substrate form as a single-layered or multi-layered neutron mirror, wherein the coating is applied to one or both sides of the substrate. The multi-layered, neutron mirrors are so-called "super-mirrors" with interfering properties (cf. DE 198 44 300 A1 ). As substrates, for example, silicon or sapphire are suitable. All of these neutron optical components are relatively simple and thus cost compared to other neutron optical components. A particularly favorable and compact embodiment of the invention results when, according to another invention continuation, the further neutron-optical components with an energy-dependent switching function in the Neutron conductor are integrated. Reference is also made to this embodiment to avoid repetition in the specific description part.

Figur 1

eine erfindungsgemäße neutronenoptische Bauelementanordnung zur Erzeugung eines Multispektrums und

Figur 2

die mit der Anordnung gemäß Figur 1 erzeugte Schaltfunktion zur Erzeugung des Multispektrums.

A specific embodiment of the invention is explained in more detail below by way of example embodiment with reference to the schematic figure for a further understanding of the invention. Showing:

FIG. 1

a neutron-optical component arrangement according to the invention for generating a multi-spectrum and

FIG. 2

with the arrangement according to FIG. 1 generated switching function for generating the multispectrum.

The FIG. 1 shows a neutron-optical component arrangement according to the invention NOA for the targeted spectral design of neutron beams or pulses. In the chosen embodiment, a cold moderator CNM for neutrons is arranged closely adjacent to a thermal moderator TNM for neutrons. Both moderators CNM, TNM have a cross section of 12 cm x 12 cm and are adjacent to each other with a gap of 0.5 cm. Instead of showing an angled arrangement between the two moderators CNM, TNM , their emission directions CBL, TBL are indicated at an angle to one another. The cold moderator CNM emits a neutron spectrum with a maximum at the cold neutrons CCN and a smaller contribution at the thermal neutrons TCN. Conversely, the thermal moderator TNM generates a maximum at the thermal neutrons TTN and a smaller number of cold neutrons CTN. The thermal moderator TNM is arranged directly opposite a neutron guide NGT , which makes the coupled neutrons one in the FIG. 1 not further illustrated experimental station passes. The neutron guide NGT has a cross section of 6 cm x 10 cm and extends from the also in of the FIG. 1 not shown neutron source at a distance of 32 m. It is nickel plated to improve its reflective properties on the inner surface INS . By multiple flat reflections of the flat incident neutron beams CCN, TTN he concentrates them in an effective, central beam direction EBL to a superimposed neutron beam SBL with a multi-spectrum. By achieving the effective mean beam direction EBL , the neutrons strike the sample to be analyzed, as it were, from all directions.

The superimposed neutron beam SBL generated in the neutron guide NGT by beam superimposition has a qualitatively very high-quality multispectrum, which is composed only of the maximum ranges of the spectra of the two moderators CNM, TNM . In order to achieve such a cleaned-up multispectrum, which can be used particularly well for experiments in a broad energy range, the neutron guide NGT at its end facing the two moderators CNM, TNM at a distance of 1.5 m from these further neutron optical devices NOC with integrated an energy-dependent switching function. In the selected exemplary embodiment, this is a simple neutron-conducting super mirror RSM and a further super mirror SSM lying opposite it . These are arranged at an angle of 0.72 ° with respect to the direction of the neutron guide NGT , so that the super mirror SSM reflects or transmits incident neutrons depending on their kinetic energy. If another angle is selected, the other dimensions of the components involved must be changed accordingly. Both supermirrors RSM, SSM are 6.5 m long and have a commercial quality m = 3, ie the section angle is three times the section angle of natural nickel. The supermirror SSM is deposited on a 0.75 mm thick neutron transparent Si substrate. While the supermirror RSM serves the pure reflection of emanating neutron beams, the opposite super-mirror has SSM an energy and angle dependent switching function. In the example chosen, the supermirror SSM is constructed and angularly adjusted (here, for example, 0.72 °) so as to reflect the cold neutrons CCN of the cold moderator CNM in the neutron guides NGT , whereas the cold neutrons CTN of the thermal moderator TNM be reflected away from the other mirror side of the area of the neutron guide NGT . In the opposite case, the thermal neutrons TCN of the cold moderator CNM are led out of the neutron guide NGT along the supermirror SSM , whereas the thermal neutrons TTN of the thermal moderator TNM can pass unhindered through the super-mirror SSM . In this way, it is ensured that the superimposed neutron beam SBL consists of preferentially emitted neutrons of the two moderators CNM, TNM . This ensures, on the one hand, that with each neutron energy the moderator with the respective higher neutron flux is switched and on the other hand the other moderator with possibly less favorable beam quality - eg pulse shape with pulsed sources - is faded out.

In the FIG. 2 is the switching function for generating the multi-spectrum of the inventive arrangement in the exemplary embodiment selected according to FIG. 1 shown. Here, the relative transmission coefficient RTC of the entire neutron optical system is as a function of the neutron wavelength NWL in nm for both moderators CNM, TNM according to FIG FIG. 1 , which can be defined in comparison to the simple spectra in an identical neutron guide, which is located at 1.5 m distance either before the cold or before the thermal moderator CNM, TNM . If neutron energies of more than 20 meV (equivalent to a neutron velocity above 2000 m / s or equivalent to a neutron wavelength below 0.2 nm) are required in an experiment, only thermal neutrons TTN of the thermal moderator TNM are available in the combined multi- spectrum . When needed Neutron energies of less than 5 meV (corresponding to a neutron velocity of less than 1000 m / s or a neutron wavelength of more than 0.4 nm), the neutron supply is almost exclusively by the cold moderator CNM with cold neutrons CCN. In a transition region between 5 meV and 20 meV, the neutrons TTN, CCN from both moderators TNM, CNM are fed into the experiment in a mixed form with different proportions in the superimposed neutron beam SBL .

LIST OF REFERENCE NUMBERS

CBL

Radiation direction cold moderator

CCN

cold neutrons cold moderator

CNM

cold moderator for neutrons

CTN

cold neutrons thermal moderator

EBL

mean beam direction

INS

inner surface

NGT

neutron Guide

NOA

neutron-optical component arrangement

NOC

another neutron optical component

NWL

Neutron wavelength

RSM

reflective super mirror

RTC

relative transmission coefficient

SBL

superimposed neutron beam

SSM

switching super mirror

TBL

Radiation direction thermal moderator

TCN

thermal neutrons cold moderator

TNM

thermal moderator for neutrons

TTN

thermal neutrons thermal moderator

Claims (8)

1. A neutron-optical component array for the specific spectral shaping of neutron beams or pulses in a neutron guide or a steel tube between a fast neutron source with a plurality of moderators of different structures arrayed closely together for generating slow neutrons of different energy spectra as well as for their radiation in predetermined radiation directions and at least one place of experiment,
   characterised in that
   the radiation directions (CBL, TBL) of the moderators (CNM, TNM), as a result of the angular alignment thereof, are focused directly on the place of experiment or, by the provision of further neutron-optical components (RSM, SSM), which are used for the specific guidance of neutron beams, are superimposed in the neutron guide (NGT) or at the place of experiment, wherein in each case a superimposed neutron beam (SBL) with a multi-spectrum is formed, in which the slow neutrons (CCN, TTN) generated by the moderators (CNM, TNM) of different energy spectra are integrated in common.
2. The neutron-optical component array according to claim 1,
   characterised in that
   the radiation directions of the moderators (CNN, TNM), in the case of focusing in the place of experiment, can be ascertained by a predetermined coding scheme with a time-of-flight determination of the neutrons in the presence of a pulsed neutron source or with a chopped superimposed neutron beam (SBL).
3. The neutron-optical component array according to claim 1,
   characterised in that
   the neutron guide (NGT) is coated with nickel on its internal surface (INS).
4. The neutron-optical component array according to claim 1 or 3,
   characterised in that,
   in the case of superimposition of the radiation directions by further neutron-optical components for obtaining an effective mean radiation direction of superimposed neutron beam, a further neutron-optical component is constituted as an oscillating reflector which oscillates in synchronism with a pulsed neutron source or with the chopped neutron beam of a continuous neutron source.
5. The neutron-optical component array according to claim 1 or 3,
   characterised in that,
   in the case of superimposition of the radiation directions (CBL, TBL) by further neutron-optical components (NOC) for obtaining an effective mean radiation direction (EBL) of the superimposed neutron beam (SBL), a further neutron-optical component (SSM) is provided with an energy-dependent switching function.
6. The neutron-optical component array according to claim 5,
   characterised in that
   the further neutron-optical component (NOC) is constituted with an energy-dependent switching function as a neutron reflector (SSM), which by a suitable angular alignment continuously or intermittently passes or reflects impinging neutrons.
7. The neutron-optical component array according to claim 5 or 6,
   characterised in that
   the neutron reflectors (RSM, SSM) are constituted in a self-supporting form or applied on a neutron-transparent substrate or as a single- or multilayered neutron reflector, the coating being applied to one or both sides of the substrate.
8. The neutron-optical component array according to any one of claims 4 to 7,
   characterised in that
   the further neutron-optical components (NOC, RSM, SSM) are integrated into the neutron guide (NGT).

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