WO2021109313A1 - Neutron ghost imaging method and apparatus - Google Patents

Abstract

A neutron ghost imaging method and apparatus, comprising: a neutron source (1) used for emitting a neutron beam; a neutron baffle (2) provided with a through hole used for limiting the shape and size of the neutron beam; a modulator (3) comprising a plurality of modulation patterns and used for adjusting the mode of the neutron beam, the neutron beam outputted from the neutron baffle (2) only covering one of the plurality of modulation patterns; a sample rack used for placing an object (5) to be measured; a probe (P) used for collecting a corresponding time-of-flight spectrum after each modulated neutron beam passing through the plurality of modulation patterns passes through the object (5) to be measured; and a data processing unit (6) for acquiring an image of the object (5) to be measured on the basis of the time-of-flight spectrum. The neutron ghost imaging apparatus is high resolution, and has low probe requirements, and low costs.

Description

Method and device for neutron intensity correlation imaging

cross reference

This application claims the priority of a Chinese patent application filed on December 2, 2019 with the application number 201911210815.1 and the invention titled "Method and Apparatus for Neutron Intensity Correlation Imaging", the entire content of which is included in the present invention by reference.

Technical field

The invention belongs to the field of neutron imaging, and in particular relates to a method and device for neutron intensity associated imaging (Neutron ghost imaging, NGI).

Background technique

As a kind of elementary particles, neutrons are very different from electrons, protons or x-rays in their interaction with matter: they can penetrate most common metal materials deeply, and at the same time, they are very sensitive to light elements (such as hydrogen, Hydrogen-containing substances or lithium) have high sensitivity. This makes neutrons very suitable for research on materials used for energy storage and conversion, such as batteries, hydrogen storage, fuel cells, etc. In addition, their wave characteristics can be used for diffraction, phase contrast, and dark-field imaging experiments. Their magnetic moments can be used to distinguish the magnetic properties of bulk samples. Relying on different types of interactions between neutrons and samples provides various imaging contrast mechanisms, and can obtain information and images of the three-dimensional shape, structure, and chemical composition of the object. The new contrast mechanism combined with the advanced imaging device has made the application of neutron imaging in many fields have been greatly developed, and it has become a common non-destructive analysis tool in many research fields. Many of these technologies rely on energy (wavelength) resolution measurement. achieve. At present, the commonly used neutron sources mainly include reactor neutron sources, radionuclide neutron sources, and the recently rapidly developed spallation neutron source. The time-of-flight (ToF) imaging mode can make full use of the pulse time characteristics of the spallation source for wavelength/energy resolution, which brings new and unprecedented possibilities, but also brings about the available detectors Technology (requiring a combination of spatial and temporal resolution) and effective data analysis are a great challenge. Therefore, for neutron imaging technology, how to improve the spatial resolution as much as possible while satisfying the energy resolution is a very important topic.

Traditional imaging is that light is transmitted or reflected by the target object and then is perceived and imaged by the detector. However, it is noteworthy that there is a kind of non-local intensity correlation imaging that subverts people’s perception of traditional imaging: if a beam of light with fluctuations in space is irradiated on an object and the light field on the object is known Distribution (reference light), the signal (object light) obtained by the barrel detector (single-pixel detector) without spatial resolution ability can be used to restore the image of the object through intensity correlation, so it is also called "ghost imaging". This phenomenon was discovered experimentally for the first time in 1995 using entangled photon pairs. This bizarre phenomenon has aroused people's strong interest. Through in-depth research on this phenomenon, people have discovered that the peculiar imaging method of "ghost imaging" is not only the characteristic of entangled light, as long as it is other sources with related characteristics (including Daily LED light, sunlight, and even various particle sources (such as electrons, atoms, etc.) can be realized, breaking the limit of sources in ghost imaging. In addition, ghost imaging finally obtains the image of the object through correlation calculation. Its imaging resolution is only related to the distribution of the light field, which can break through the limitation of the source size; according to its imaging process, only a very weak light beam is needed in the signal light path. It is sufficient to illuminate the object, with high sensitivity and good anti-noise performance; moreover, the detector required by ghost imaging technology does not require spatial resolution capability, which greatly reduces the imaging cost.

If the ghost imaging technology is extended to neutrons and combined with the ToF imaging mode, energy-resolved imaging technology with high spatial resolution can be realized. In contrast, traditional neutron imaging modes and large, fixed imaging devices can achieve The same effect requires tens of millions of costs. Therefore, the neutron ghost imaging technology has very considerable application prospects. However, due to the strong penetrating ability of neutrons, it is difficult to use optical elements to split, focus or other optical transformations. Therefore, low-cost, high-spatial resolution neutron ghost imaging (neutron intensity correlation imaging) technology has always been It is difficult to achieve.

Summary of the invention

Therefore, the purpose of the present invention is to overcome the above-mentioned defects of the prior art and provide a neutron intensity correlation imaging device, including:

Neutron source, used to emit neutron beams;

The neutron baffle has a through hole for limiting the shape and size of the neutron beam;

The modulator includes a plurality of modulation patterns for adjusting the mode of the neutron beam, wherein the neutron beam output from the neutron baffle covers only one of the plurality of modulation patterns;

Sample rack, used to place the object to be tested;

A detector for collecting the corresponding time flight spectrum of the neutron beam modulated by each of the plurality of modulation patterns after passing through the object to be measured; and

The data processing unit obtains the image of the object to be measured based on the time-of-flight spectrum.

According to the neutron intensity correlation imaging device of the present invention, preferably, the neutron source is a pulsed spallation neutron source.

According to the neutron intensity correlation imaging device of the present invention, preferably, the neutron baffle is a cadmium neutron baffle or a gadolinium neutron baffle.

According to the neutron intensity correlation imaging device of the present invention, preferably, the through hole is a square hole.

According to the neutron intensity correlation imaging device of the present invention, preferably, each of the multiple modulation patterns is obtained by the following method: a 1024×1024 Hadamard matrix is generated, and each row of the Hadamard matrix is reshaped A 32×32 matrix is used as a modulation pattern.

According to the neutron intensity correlation imaging device of the present invention, preferably, the modulation pattern includes at least a first part and a second part, and the absorption of neutrons by the first part is stronger than the absorption of neutrons by the second part.

According to the neutron intensity correlation imaging device of the present invention, preferably, the data processing unit performs the following steps:

Obtaining the corresponding object light signal intensity based on the time flight spectrum corresponding to each of the plurality of modulation patterns;

Performing normalization processing based on the background signal intensity and the object light signal intensity to obtain the normalized signal intensity;

Perform correlation operations on the normalized signal intensity.

The present invention also provides a neutron intensity correlation imaging method, which includes the following steps:

The object light signal acquisition step: the neutron beam irradiates each of the multiple modulation patterns in turn, then passes through the object to be measured, and collects the corresponding time flight spectrum, and obtains the corresponding object light signal intensity based on the time flight spectrum;

Normalization processing step: subtracting the background signal intensity from the object light signal intensity to obtain the normalized signal intensity; and

Data processing step: performing correlation operations on the normalized signal intensity to obtain an image of the object to be measured.

According to the neutron intensity correlation imaging method of the present invention, preferably, the background signal intensity is obtained by the following method:

The neutron beam irradiates each of the multiple modulation patterns of the modulator in turn, collects the corresponding time flight spectrum and obtains the corresponding background signal intensity.

According to the neutron intensity correlation imaging method of the present invention, preferably, the correlation calculation uses the following formula:

Wherein said modulation pattern is a plurality of N modulation pattern, N being a positive integer, x, y represent the coordinates in the plane coordinate system, G (x, y) is the image of the object under test, S _i (x, y) represents The i-th modulation pattern, 1≤i≤N, I _i represents the light signal intensity value of the normalized compound for the i-th modulation pattern, and <S(x,y)I> represents the N modulation patterns and the corresponding normalized compound The average value of the product of optical signal intensity values, <S(x,y)> represents the average value of N modulation patterns, and <I> represents the average value of N normalized light signal intensity values.

Compared with the prior art, the present invention has the advantages of high resolution, low requirements on the detector, and low cost.

Description of the drawings

The following further describes the embodiments of the present invention with reference to the accompanying drawings, in which:

Fig. 1 is a neutron intensity correlated imaging device and a light path diagram according to an embodiment of the present invention;

Figure 2 is a modulation pattern according to an embodiment of the present invention;

Figure 3 shows the time-of-flight-wavelength/energy relationship according to an embodiment of the present invention;

Fig. 4 shows an initial image on an object to be measured according to an embodiment of the present invention; and

Figure 5 shows images acquired for different bands and average counts.

Detailed ways

In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below through specific embodiments with reference to the accompanying drawings. It should be understood that the specific embodiments described here are only used to explain the present invention, but not used to limit the present invention.

The first embodiment

The first embodiment of the present invention provides a neutron intensity correlation imaging device and a corresponding method. Referring to the optical path diagram of the neutron intensity correlation imaging of this embodiment shown in FIG. 1, the neutron intensity correlation imaging device includes:

Pulse spallation neutron source 1. This embodiment uses the No. 20 beam line of the Dongguan Spallation Neutron Source (CSNS) in China. The outlet of the source is a circular hole with a diameter of 2 cm, and the divergence angle is 1 to 2°. Thermal neutrons The flux is 10 ⁶ /cm ² /s, and the neutron pulse frequency is 25 Hz;

The cadmium neutron baffle 2 has a thickness of 4 mm and a square shape with a side length of 10 cm. The center of the baffle plate 2 is a square hole with a side length of 1.6 mm. The shape of the hole is the same as the outline of the subsequent modulation pattern. The neutron baffle is used to control the size of the neutron beam diameter emitted by the pulse spallation neutron source, considering factors such as the divergence angle and propagation distance of the neutron beam, so that the size of the spot after the neutron beam passes through the baffle can illuminate the back For a modulation pattern on the mentioned modulator, those skilled in the art can understand that the shape and size of the neutron beam spot emitted by the baffle does not have to be consistent with the contour of the modulation pattern, as long as it can cover only one modulation pattern;

The modulation module includes an electric translation stage 4 and a modulator 3 fixed on the electric translation stage. The modulator 3 has a plurality of modulation patterns for adjusting the mode of the neutron beam. The modulator 3 includes a substrate and a modulation pattern set on the substrate. The substrate can be, for example, a rectangular plate, which uses a material that absorbs little neutrons, such as silicon wafers, sapphire, etc., using ion beam etching or laser drilling, etc. The basic pattern is obtained on the substrate, and then the basic pattern is filled with fillers. The fillers can include metal particles, gadolinium oxide powder and other materials that have strong absorption of thermal neutrons (attenuation of more than 1 order), and then use a small amount of Encapsulated with glue or thin quartz glass (SiO ₂ ) to produce multiple modulation patterns S ₁ , S ₂ ,..., S _N. In this embodiment, a Hadamard matrix with orthogonal normalization properties is selected. The inventors have generated a 1024×1024 Hadamard matrix, and each row of the matrix (ie 1×1024) is reshaped into 32×32 As a modulation pattern, 1024 different modulation patterns are obtained. Refer to the modulation pattern according to the embodiment of the present invention shown in FIG. 2. The black part of the pattern is etched and filled with those that have strong absorption of thermal neutrons. material. Considering factors such as the divergence angle and propagation distance of the neutron beam, the size of the spot after the neutron beam passes through the baffle can illuminate a modulation pattern on the modulator. When the neutron beam is irradiated on the modulator, because different parts of the modulator absorb neutrons differently, a series of neutron beams precisely modulated according to the pre-designed modulation pattern can be obtained. The number of modulation patterns on the modulator determines the total number of neutron beam patterns. Preferably, the 1024 modulation patterns are sorted according to the absolute value of Haar wavelet transform coefficients from small to large. Under this optimal sorting, the image can be reconstructed with a sampling rate of 25%, and the sampling speed can be increased by 4 times;

The sample holder (not shown in the figure) is used to place the object 5 to be imaged;

The barrel detector P, which has time resolution capability, is used to collect the time flight spectrum of the neutron beam after passing through the imaging object. The time flight spectrum represents the number of neutrons detected by the detector at each time t;

The data processing unit is implemented by the computer 6. In this embodiment, matlab software is used to perform subsequent data processing based on the time flight spectrum obtained by the barrel detector P, and finally an image of the object to be measured is obtained.

According to an embodiment of the present invention, a method for performing neutron correlation imaging using the imaging device of FIG. 1 includes the following steps:

Step 1: Move the object to be imaged out of the light path for background calibration.

As shown in Figure 1(b), the neutron from the neutron source passes through the neutron baffle and irradiates _{the position of the first modulation pattern S 1} of the modulator, and the barrel detector P is used to collect the time flight corresponding to this position. The data processing unit obtains the corresponding intensity value B ₁ by integrating the count of the thermal neutron energy band. The barrel detector P uses, for example, a ³ He tube.

In this embodiment, the neutron pulse generating place is 8.95 m from the neutron exit port, the distance from the exit hole to the barrel detector ( ³ He tube) is 75 cm, and the ^{time resolution of the 3} He tube is 10 μs. The flight time is ^{synchronously measured by the 3} He tube relative to the spallation time. The spallation time is the time for protons to bombard the tungsten target to spall the tungsten target to produce neutrons.

The distance L from the place where the neutron pulse is generated to the detector is: L=8.95m+0.75m=9.7m;

The mass of neutron m is: m=1.67×10 ^-27 kg;

The wavelength λ and energy E corresponding to the neutrons detected at time t are:

Among them, h is Planck's constant and v is the velocity of neutrons.

From this, the relationship between the flight time and the corresponding wavelength/energy can be obtained, as shown in Figure 3. In this embodiment, the modulation pattern contains gadolinium oxide powder, because the gadolinium oxide powder only has strong absorption of neutrons at and below the thermal neutron energy range (that is, the neutron modulator can only modulate the thermal neutron energy Band and below), so in the subsequent data processing process, only select the wavelength range in the

The neutrons inside are imaged.

Based on the time-of-flight-wavelength/energy relationship shown in Fig. 3, the time-neutron number relationship can be obtained by conversion, and the intensity value B ₁ can be obtained by integrating the selected segments according to the energy/wavelength.

Next, the electronic translation stage is used to adjust the position of the modulator in turn, so that the neutron beams irradiate the modulation patterns S ₂ , ..., S _N in sequence. Using the same method as the above, the time flight spectra corresponding to each modulation pattern are sequentially collected and obtained The corresponding intensity values B ₂ ,..., B _N , N intensity values B ₁ , B ₂ ,..., B _{N are} the background signals.

In the present invention, the time for the barrel detector P to measure neutrons is called the acquisition time (exposure time), and the acquisition time for each modulation pattern is called the single exposure time. The single exposure time of the above background signal acquisition can be set arbitrarily, as long as a clear signal is acquired. The longer the exposure time, the higher the contrast of the restored image. In a specific embodiment, the single exposure time for each modulation pattern is 40s. The longer the exposure time, the better the recovery effect of the image, because the number of collected neutrons increases, which can eliminate statistical errors. However, the increase of the exposure time means the increase of the imaging time cost, therefore, the exposure time needs to be adjusted appropriately.

Using the method of calibration background can largely eliminate the errors caused by the processing technology and enhance the practicability and stability of the imaging system. For a modulation module including a given modulator, it only needs to be calibrated once in advance, and then it is no longer necessary to collect the background signal again for imaging any object.

Step 2: Move the object to be imaged into the optical path to collect the object light signal, see Figure 1(a).

The method is similar to that of background signal acquisition. Move the electric translation stage so that the neutron beam irradiates _{the position of the modulation pattern S 1} and adjusts the front and back positions of the object to be imaged to ensure that the neutrons cover the entire area to be imaged. The barrel detector P obtains the time corresponding to the _{modulation pattern S 1} In the flight spectrum, the corresponding object light intensity value I′ _{1 is} obtained by integrating the counting of the thermal neutron energy range. In a similar way, by controlling the electric translation stage to change the position of the neutron irradiated on the modulator, N object light intensity values corresponding to the _{modulation patterns S 1} , S ₂ ,..., S _{N can be sequentially collected. I '1, I' 2,} ..., I 'N optical signals as a set of objects. The object light intensity value mentioned here refers to the total thermal neutron intensity value of the neutron beam passing through each pre-designed modulation pattern before passing through the sample.

Step 3: Normalization processing, that is, the background signal obtained in Step 1 is subtracted from the object-light signal obtained in Step 2. The specific formula is:

Wherein, I _i represents for the i-th modulation pattern normalized object beam intensity values, I _'i denotes the object beam intensity value for the i-th modulation pattern, <I'> represents the mean of the N object light signal intensity , <B> is the average value of N background signal strengths.

Step 4: Correlate the N modulation patterns S ₁ , S ₂ , ..., S _N with the N normalized object light intensity values I ₁ , I ₂ , ..., I _N to obtain the image G of sample A (x,y), specifically using the formula:

Where x, y represent the coordinates of the plane coordinate system, S _i (x, y) represents the i-th modulation pattern, and <S(x,y)I> represents the product of the N modulation patterns and the corresponding normalized light intensity value Average value, <S(x,y)> represents the average value of N modulation patterns.

In order to embody the effect of the present invention, the inventor selected N=256, 512, and 1024 respectively in the experiment, and obtained the image obtained by the corresponding neutron correlation imaging method. The initial image on the object to be measured is a hollowed out "N" on a 4mm thick cadmium sheet, as shown in Figure 4.

The corresponding collected image is shown in Figure 5.

In Figure 5, for different N, imaging is performed for different wavebands and average counts respectively.

For example, the data obtained in the experiment is that the neutron wavelength is at

Within the range, each wavelength has a corresponding neutron count, for example, the wavelength is

The neutron count is 100, and the wavelength is

The neutron count is 150,......, the wavelength is

The neutron count is 200, the inventor chooses

The band is the sum of the neutron counts of these bands.

Mean (i.e., the modulation pattern corresponding to each of the average neutron count) associated with the modulation pattern, after one band is selected, according to each different modulation patterns S _i we obtain the corresponding I _'i, is the average number of

In Figure 5, the pixel size of each image is 100μm. The pixel size of the generated image depends on the pixel size of the modulation pattern. The pixel size of the modulation pattern selected in the experiment is 100μm, so the final pattern size is also 100μm. Neutron high spatial resolution ghost imaging. The method of the present invention has successfully achieved neutron spectral ghost imaging, but spectral analysis will bring about a decrease in the count, because the full band is

In this example, the spectral analysis selected

with

For the two bands, their counts will obviously be lower than the neutron counts of the full band, sacrificing imaging quality to some extent. The higher the count, the lower the statistical error and the better the imaging quality. As the number of samples increases, the imaging effect gets better and better.

The modulation pattern of Fig. 2 corresponds to N=1024, the modulation pattern of N=256 is the first 256 pictures, and the modulation pattern of N=512 is the first 512 pictures.

Second embodiment

In this embodiment, a barrel detector that does not have time resolution capability is used, and only the neutron count (the count of neutrons in the all-energy section) is obtained, instead of the relationship between time t and neutron number, so there is no way to perform energy spectroscopy. Analysis, the final imaging result is neutron imaging of all energy bands, which is neutron ghost imaging. The neutron ghost imaging corresponds to the neutron spectral ghost imaging of all bands selected, that is, the band in Figure 5:

the result of.

According to other embodiments of the present invention, a detector with both spatial resolution capability and time resolution capability can be selected, but this will greatly increase the cost.

Neutron ghost imaging can be achieved by selecting a detector with no spatial resolution capability and time resolution capability. Furthermore, when a barrel detector with time resolution capability is used, neutron spectral ghost imaging can be achieved. According to one aspect of the present invention, neutron ghost imaging is realized, which has the advantage of being able to realize low-cost, simple and portable equipment, and high spatial resolution neutron imaging; according to another aspect of the present invention, neutron spectral ghost imaging is realized In addition to the advantages of neutron ghost imaging, it has a very important significance compared to other neutron imaging methods in that it can achieve energy spectrum analysis and high spatial resolution at the same time.

It is well known to those skilled in the art that neutron sources are divided into continuous neutron sources (for example, radioisotope neutron sources, nuclear reactor neutron sources) and pulsed neutron sources (spacing neutron sources). According to other embodiments of the present invention, when a continuous neutron source is used, a chopper must be added after the neutron source to achieve neutron spectral ghost imaging, otherwise only neutron ghost imaging without energy resolution can be achieved.

According to other embodiments of the present invention, the neutron baffle is a gadolinium neutron baffle, and the through hole on the neutron baffle is not limited to a square shape, as long as its shape matches the shape of the contour of the subsequent modulation pattern.

According to other embodiments of the present invention, the pattern on the neutron modulator is not necessarily a Hadamard matrix, but can also be another special matrix, such as a Gold matrix, a random matrix, or even a random speckle. In addition, the Hadamard matrix is not limited to the manner given in the embodiment. For example, a 4096X 4096 Hadamard matrix can be selected, and each row is reshaped into a 64X 64 matrix, and then the total number of modulation patterns is 4096.

Those skilled in the art can understand that in order to make the neutron beam irradiate different modulation patterns, it can also be achieved by adjusting the position of the neutron beam.

Although the present invention has been described through preferred embodiments, the present invention is not limited to the embodiments described here, and also includes various changes and changes made without departing from the scope of the present invention.

Claims (10)

1. A neutron intensity correlation imaging device includes:

   Neutron source, used to emit neutron beams;

   The neutron baffle has a through hole for limiting the shape and size of the neutron beam;

   The modulator includes a plurality of modulation patterns for adjusting the mode of the neutron beam, wherein the neutron beam output from the neutron baffle covers only one of the plurality of modulation patterns;

   Sample rack, used to place the object to be tested;

   A detector for collecting the corresponding time flight spectrum of the neutron beam modulated by each of the plurality of modulation patterns after passing through the object to be measured; and

   The data processing unit obtains the image of the object to be measured based on the time-of-flight spectrum.
2. The neutron intensity correlation imaging device according to claim 1, wherein the neutron source is a pulsed spallation neutron source.
3. The neutron intensity correlation imaging device according to claim 1, wherein the neutron baffle is a cadmium neutron baffle or a gadolinium neutron baffle.
4. The neutron intensity correlation imaging device according to claim 3, wherein the through hole is a square hole.
5. The neutron intensity correlation imaging device according to claim 1, wherein each of the plurality of modulation patterns is obtained by the following method: a 1024×1024 Hadamard matrix is generated, and each row of the Hadamard matrix is Reshape it into a 32×32 matrix as a modulation pattern.
6. The neutron intensity correlation imaging device according to claim 1, wherein the modulation pattern includes at least a first part and a second part, and the absorption of neutrons by the first part is stronger than the absorption of neutrons by the second part .
7. The neutron intensity correlation imaging device according to claim 1, wherein the data processing unit performs the following steps:

   Obtaining the corresponding object light signal intensity based on the time flight spectrum corresponding to each of the plurality of modulation patterns;

   Performing normalization processing based on the background signal intensity and the object light signal intensity to obtain the normalized signal intensity;

   Perform correlation operations on the normalized signal intensity.
8. A neutron intensity correlation imaging method includes the following steps:

   The object light signal acquisition step: the neutron beam irradiates each of the multiple modulation patterns in turn, then passes through the object to be measured, and collects the corresponding time flight spectrum, and obtains the corresponding object light signal intensity based on the time flight spectrum;

   Normalization processing step: subtracting the background signal intensity from the object light signal intensity to obtain the normalized signal intensity; and

   Data processing step: performing correlation operations on the normalized signal intensity to obtain an image of the object to be measured.
9. The neutron intensity correlation imaging method according to claim 7, wherein the background signal intensity is obtained by the following method:

   The neutron beam irradiates each of the multiple modulation patterns of the modulator in turn, collects the corresponding time flight spectrum and obtains the corresponding background signal intensity.
10. The neutron intensity correlation imaging method according to claim 9, wherein the correlation operation uses the following formula:

    

    Wherein said modulation pattern is a plurality of N modulation pattern, N being a positive integer, x, y represent the coordinates in the plane coordinate system, G (x, y) is the image of the object under test, S _i (x, y) represents The i-th modulation pattern, 1≤i≤N, I _i represents the light signal intensity value of the normalized compound for the i-th modulation pattern, <S(x,y)I> represents the N modulation patterns and the corresponding normalized compound The average value of the product of optical signal intensity values, <S(x,y)> represents the average value of N modulation patterns, and <I> represents the average value of N normalized optical signal intensity values.

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