CN1595125A - Neutron diffraction enhanced imaging device - Google Patents

Abstract

A neutron diffraction enhanced imaging device includes a collimated neutron beam and a detector, wherein a monochromator is arranged in the advancing direction of the collimated neutron beam, the monochromator forms a grazing incidence angle θ with the neutron beam, an analyzer is placed in parallel with the monochromator, the direction of the neutron beam diffracted by the analyzer is the detector, an aluminum reflector is placed at 45° on the optical axis of the visible light converted by the detector, a CCD camera is arranged on the reflected light path of the aluminum reflector, the output end of the CCD camera is connected to a display, and the detector, the aluminum reflector and the CCD camera are sealed in a dark box. The invention has the advantages of being able to eliminate the scattering noise generated when neutrons and nuclei interact and the interference of directly transmitted neutrons, and improving the contrast and resolution of neutron phase contrast imaging.

Description

Neutron Diffraction Enhanced Imaging Device

technical field

The invention relates to neutron imaging, in particular to a neutron diffraction enhanced imaging device, which has extremely wide applications in defense industry, biomedicine and the like.

Background technique

Neutrons belong to baryons, one of the four types of elementary particles. They have certain basic characteristics such as a certain rest mass, no charge, a spin of 1/2, a negative magnetic moment, and free neutrons are unstable and undergo beta decay.

Neutrons interact with matter. Unlike charged particles and electromagnetic waves (X and γ-rays), neutrons have no charge. When they are injected into matter, they have almost no interaction with extranuclear electrons and do not need to overcome the Coulomb force barrier of charges, so the energy is very small Low neutrons can also enter the nucleus (target nucleus) to cause various nuclear reactions, and the probability of the reaction is often very high.

The interaction of neutrons with nuclei can be viewed as the interaction of neutron waves with nuclei. According to the wave theory of matter and de Broglie formula, in non-relativity, the particle wavelength is:

λ=h/p=h/mv

λ is the De Broglie wavelength of the particle, h is Planck's constant, p is the momentum of the particle, m is the mass of the particle, and v is the velocity. From this, the formula λ(Å)=0.286T _n ⁻¹² can be deduced, where Tn is neutron kinetic energy (electron volts).

It can be known from quantum mechanics that the longer the wavelength of neutrons, the greater the probability of nuclear reactions caused by neutrons.

When X-rays pass through a substance, X-rays interact with the electron cloud of atoms in the substance. Therefore, its attenuation depends on the charge density of the electron cloud, and it increases with the increase of the atomic number of the sample; unlike X-rays, in Neutrons interact with the nucleus, and the total neutron cross-section is related to the properties of the nucleus. There are also great differences between different elements and even isotopes.

The neutrons that bombard the target nucleus do not interact with all the particles in the nucleus, but only with some of the closest particles. Therefore, the nucleon binding energy of all nuclei is about the same. The interaction potential of a neutron with a complex nucleus is of the same order of magnitude as that of a neutron with a single nucleus.

The interaction of neutrons with atomic nuclei takes many forms, including elastic scattering, inelastic scattering, reactions that emit charged particles, nuclear fission, and radiation capture.

Since neutrons cannot directly cause ionization of atoms in matter, it can only be detected based on the strong interaction between neutrons and atomic nuclei. Neutrons with different energies have different nuclear processes, so there are different detection methods, mainly There are recoil nuclear method (suitable for fast neutrons), nuclear reaction method (mainly used for slow neutrons), nuclear fission method and activation method.

The detection methods of neutron imaging mainly include film method and CCD, etc., no matter which one needs to be used together with the conversion screen. This is because the direct detection efficiency of these detectors such as film is very low for neutrons, and indirect detection is required. The role of the conversion screen is to make neutrons interact with it to produce secondary radiation such as α, β or γ, and visible light, and detectors such as film record their intensities. Therefore, the efficiency and quality of the conversion screen directly affects the results of neutron imaging.

According to the product characteristics of the interaction between the conversion material and neutrons in the conversion screen, the conversion screen can be divided into two types: transient screen and activation screen. The transient screen refers to the neutron imaging conversion screen used in the direct exposure method, such as ⁶ LiF-ZnS(Ag), organic plastic + Gd ₂ O ₂ S, Gd, etc., and the secondary radiation it emits during the neutron imaging process And the formation of the object is done in an instant. The activation screen is the conversion screen used in the indirect exposure method, such as In indium, Dy dysprosium, etc., which form radioactive nuclei with a certain lifespan under neutron irradiation. After the latent image of the daughter nucleus is established on the conversion screen, the The conversion screen is closely attached to the film, so that the decay radiation of the sub-nucleus of the conversion screen forms an image on the film.

Neutron imaging is developed on the basis of X-ray imaging, but neutron imaging has some advantages over X-ray imaging in some aspects, and can solve some problems that X-ray imaging is difficult to solve. Therefore, it is generally considered to be a good auxiliary means of X-ray imaging. For example, a distinct difference in the interaction of neutrons and X-rays with matter is reflected in the mass absorption coefficients of the various elements. The thermal neutron mass absorption coefficient of hydrogen is very large, while the neutron mass absorption coefficient of some heavy elements is very small, so it is particularly effective for neutron imaging of objects composed of hydrogen-containing substances and heavy metals. For example, the neutron imaging inspection of bullets can not only show the explosives loaded inside through the metal casing, but also observe whether the explosives have a uniform density and whether there are gaps. For objects with complex structures that contain hydrogen and a combination of plastic and metal, using neutron and X-ray imaging and comparing them can obtain a more accurate understanding of the internal structure of the object. Some elements with adjacent atomic numbers or different isotopes of the same element often have very different neutron mass absorption coefficients, so they can be easily distinguished by neutron imaging. In biology and medicine, neutron imaging can be used to check periosteum colloid and cancer cells, check dental pulp, and conduct pathological research.

Perhaps inspired by X-ray phase-contrast imaging technology, a joint Australian and European team developed a coaxial phase-contrast imaging method. They used cold neutrons, and the corresponding de Brogile wavelength was 0.433nm, and successfully observed Some details of the wasp's leg joints and wings.

We know that whether it is a light wave or a matter wave, when it passes through an object, it will produce scattering and absorption, and a clear sample absorption contrast image will be obtained at an appropriate distance from the sample, which is the imaging basis of conventional microscopy and tomography.

We already know from X-rays: the refractive index of X-rays

n _x =1-δ, δ=r ₀ λ ² N _at F/2π, where λ is the X-ray wavelength, r ₀ is the classical electron radius, N _at is the atomic number density per unit volume, and f is the atomic scattering factor . It is known from neutronics that the refractive index of neutrons has the same form as that of X-rays,

n _n =1-λ _n ² N(b±p)/2π, where N is also the number of protons per unit volume, λ is the neutron wavelength, b is the nuclear scattering coefficient, and p is the magnetic flux caused by electron spin. scattering coefficient. It can be seen from the above that the two forms of the refractive index n are almost the same, and for the same wavelength, neutron δ(λ _n ² N(b±p)/2π) is an order of magnitude smaller than that of X-rays. Although the difference between 1-δ and 1 is only 10 ^-6 , even modest changes in thickness or density can produce considerable phase distortion when very small values of λ are used. If coherent light or partially coherent light passes through an object, in addition to absorption, a phase change will also occur, that is, distortion of the wavefront will occur. This wavefront distortion causes the propagation direction of part of the wavefront to change, causing the wavefronts to overlap and form interference. In this way, the phase change is converted into an intensity change. This is the physical basis of phase contrast imaging and also the physical basis of phase contrast tomography, which is more important. The most striking thing is that this kind of image can directly obtain the phase change image without any reconstruction, which is the fundamental difference between phase contrast and holography.

In 2000, B.E.Allman et al. completed a neutron phase contrast experiment, and the experimental device is shown in Figure 1. After the neutron beam 1 emitted from the neutron source passes through the pinhole, it becomes a spherical wave incident on the sample 3, and a detector 5 is placed at a distance of 1.8 meters from the sample 3 to obtain phase contrast imaging of the sample 3 [See prior art: B.E. Allman et al: Nature 2000, 408, 158].

The biggest disadvantage of this device is: due to the interference of various scattered waves and transmitted waves, the contrast and resolution of neutron phase contrast imaging are seriously affected.

Contents of the invention

The technical problem to be solved by the present invention is to provide a neutron diffraction enhanced imaging device to eliminate the scattering noise generated by the interaction between neutrons and nuclei and the interference of directly transmitted neutrons in view of the shortcomings of the above-mentioned prior art , to improve the contrast and resolution of neutron phase contrast imaging.

Technical solution of the present invention is as follows:

A neutron diffraction enhanced imaging device, comprising a collimated neutron beam and a detector, is characterized in that a monochromator is arranged in the advancing direction of the collimated neutron beam, and the monochromator is formed with the neutron beam Grazing incidence angle θ, place an analyzer parallel to the monochromator, the direction of the neutron beam diffracted by the analyzer is the detector, placed at 45° on the optical axis of the visible light converted by the detector An aluminum reflector, a CCD camera is arranged on the reflective light path of the aluminum reflector, the output terminal of the CCD camera is connected to a display, and the detector, the aluminum reflector and the CCD camera are hermetically housed in a dark box.

Working process of the present invention is as follows:

The sample to be tested is placed between the monochromator and the analyzer, the neutron beam collimated by the catheter is incident on the monochromator, and after monochromatization, it is incident on the sample to be tested placed in the direction of the neutron beam , the neutron beam emitted from the sample to be tested enters the analyzer placed parallel to the monochromator, and the neutron beam diffracted by the analyzer enters the detector and is converted into visible light, at an angle of 45° to the optical axis direction, set an aluminum reflector. The function of the aluminum reflector is to convert the visible light by 90° and send it to the CCD receiver to read the signal from the display.

The said neutron source is the neutrons radiated from the fission reactor neutron source and emitted by the collimator. The fission reactor neutron source is a device that uses fissile materials such as uranium and plutonium as fuel, and uses neutrons as a medium to maintain a controllable chain fission reaction. It is called a fission reactor. This device can obtain high-flux neutrons Radiation, up to 10 ¹³ ~ 10 ²⁰ neutrons per second, can run for a long time, and is collimated by a steel box or steel cylinder with a rectangular or circular cross-section, the neutrons emitted from the collimator, its divergence It is equal to the ratio of the aperture to the length. Obviously, as long as the aperture is reduced and the length is increased, the divergence can be greatly improved and quasi-parallel neutron beams can be obtained.

The monochromator is a piece of single crystal aluminum or copper. When the quasi-parallel incident neutron beam forms a grazing incidence angle θ with the monochromator, Bragg reflection occurs to produce a monochromatic neutron beam.

The sample is a neutron-transmissive material to be tested.

The analyzer is a single crystal aluminum or copper single crystal made of the same material as the monochromator, and its function is equivalent to a broadband filter, which can filter out various scattering generated when neutrons interact with the sample.

Said detector is a neutron scintillator, which is ZnS(Ag)-LiF. Since neutrons cannot directly cause ionization of atoms in the material, and there is no current output, ZnS(Ag)-LiF is used in the present invention. The diffracted neutron beam generated in the sample is perpendicularly incident on the screen of the scintillator, and each neutron produces a cascade of visible photons.

The aluminum reflector is used to reflect the visible light produced by the scintillator to the receiver CCD camera, and the CCD camera is a commercial CCD.

Said display is used to display the signal received by the CCD.

The so-called obscura is used to shield outside stray light.

Technical effect of the present invention is as follows:

The neutron diffraction enhanced imaging device of the present invention adopts a monochromator to disperse the neutrons. When the quasi-parallel neutron beam is irradiated on the sample to be tested, the neutron beam carries the sample information and also produces scattering. , this kind of scattered wave and diffracted wave are mixed together. If the coaxial holographic phase-contrast imaging method is adopted, the signal-to-noise ratio is bound to be low. Therefore, the present invention places an analyzer behind the sample, and this analyzer is parallel to the monochromator. . The role of the analyzer is to filter out the scattered waves, so only the phase information of the sample is received on the receiver, thus ensuring high signal-to-noise ratio, high contrast and high resolution. Compared with the prior art: the neutron diffraction enhanced imaging device and method of the present invention can filter neutron scattered waves and transmitted waves due to the use of an analyzer, which can improve the signal-to-noise ratio, contrast and resolution.

Description of drawings

Fig. 1 is a schematic diagram of a neutron phase contrast imaging device in the prior art.

Fig. 2 is a schematic diagram of the neutron diffraction enhanced imaging device of the present invention.

Detailed ways

Please refer to Fig. 2, Fig. 2 is a schematic diagram of the neutron diffraction enhanced imaging device of the present invention, as shown in Fig. 2, the neutron diffraction enhanced imaging device of the present invention is made up of 9 parts: in the advancing direction of the collimated neutron beam 1 A monochromator 2 is provided, the monochromator 2 forms a grazing incidence angle θ with the neutron beam 1, and an analyzer 4 is placed parallel to the monochromator 2, and the direction of the neutron beam diffracted by the analyzer 4 is the Detector 5, place an aluminum reflector 6 at 45° on the optical axis of visible light converted by the detector 5, a CCD camera 7 is arranged on the reflected light path of the aluminum reflector 6, and the output end of the CCD camera 7 Connected to a display 8, the detector 5, the aluminum reflector 6 and the CCD camera 7 are hermetically housed in a dark box 9.

Said collimated neutron beam 1 is a fission reactor and a neutron collimator, the wavelength is 0.4nm.

Said monochromator 2 is a single crystal aluminum, also can be single crystal copper.

Said test sample 3 is a biological sample, which is transparent to neutron beams.

Said analyzer 4 is a piece of single crystal aluminum, also can be single crystal copper, and it is exactly the same as the material of monochromator.

Said detector 5 is a scintillator whose material is ZnS-LiF, which is available in the market.

Said aluminum reflector 6 is an aluminized mirror.

Said CCD camera 7 is a commercially available charge coupled device sensitive to visible light.

Said display 8 is used to read out the signal received by the CCD camera.

Said dark box 9 is used for shielding stray light and avoiding interference to CCD.

The working process of the neutron diffraction enhanced imaging device of the present invention is: after the neutron from the collimated neutron beam 1 is dispersed by the monochromator 2, it is irradiated in the sample 3 to be tested, and the neutron and the sample 3 to be tested interact with each other After the action, part of the neutrons is refracted, scattered, absorbed and transmitted, and irradiates the analyzer 4, and only the part of neutrons refracted by the sample 3 to be measured contains the information of the sample, and is diffracted by the analyzer 4 to the detector 5 up, while the rest will be filtered out, thereby improving the signal-to-noise ratio, contrast, and resolution.

This neutron diffraction enhanced imaging device has a wide range of applications in biomedicine, material structure, aerospace, cosmochemistry, weapon industry, archaeology, etc.

Claims (3)

1. A neutron diffraction enhanced imaging device comprising a neutron beam source (1) and a detector (5), characterized in that the collimated neutron beam of the neutron beam source (1) is provided with a monochromatic device (2), the monochromator (2) forms a grazing incidence angle θ with the neutron beam, and the analyzer (4) is placed parallel to the monochromator (2), and the neutrons diffracted by the analyzer (4) The beam direction is the detector (5), and an aluminum reflector (6) is placed at 45° on the visible light axis converted by the detector (5), and the reflected light of the aluminum reflector (6) CCD camera (7) is arranged on the road, and the output terminal of this CCD camera (7) connects a display (8), and described detector (5), aluminum mirror (6) and CCD camera (7) are contained in airtightly In a camera obscura (9). 2. The neutron diffraction enhanced imaging device according to claim 1, characterized in that the monochromator (2) and the analyzer (4) are both single crystal aluminum or single crystal copper. 3. The neutron diffraction enhanced imaging device according to claim 1, characterized in that the detector (5) is a neutron scintillator.

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