A polarized neutron radio frequency flipper

Abstract

The application discloses a polarized neutron radio frequency flipper, and relates to the technical field of polarized neutron scattering. The polarized neutron radio frequency flipper comprises a shell, a first opening and a second opening are oppositely arranged along the length direction of the shell, a magnetic field generating device is arranged in the shell, the magnetic field generating device comprises a radio frequency magnetic field generating device for generating a radio frequency alternating magnetic field and a gradient magnetic field generating device for generating a gradient magnetic field. The radio frequency magnetic field generating device comprises a solenoid, the axis of the solenoid is arranged in line with the connecting line of the first opening and the second opening; the gradient magnetic field generating device comprises a first metal plate and a second metal plate, the first metal plate and the second metal plate are arranged on the two sides of the solenoid and are symmetrically arranged along the axis of the solenoid; the distance between the first metal plate and the second metal plate and the axis of the solenoid gradually decreases in the direction from the first opening to the second opening.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of polarized neutron scattering technology, and particularly relates to a polarized neutron radio frequency flipper. BACKGROUND

[0002] As a basic scientific research method, polarized neutron scattering is widely used in the fields of materials, engineering, biology and basic physics research. Polarized neutron technology has achieved fruitful results in many fields dominated by condensed matter physics. Current applications include magnetic structure, spin density, magnetic fluctuation and their intrinsic state, measurement of large-scale structure, separation of coherent and incoherent scattering, inelastic scattering, neutron spin modulation, complete polarization analysis and polarized neutron imaging. In addition, polarized neutrons also have important applications in basic particle physics. The radio frequency flipper of polarized neutrons is a basic device of polarized neutron scattering technology, which can flip the vector of multi-wavelength polarized neutrons through the coupling of radio frequency field and constant magnetic field. Because the flipper is a device on the neutron path, its performance will directly affect all experimental results based on it.

[0003] Foreign polarized neutron flippers started very early, the earliest experiments began in the 1970s. They are mainly divided into Mezei flippers, superconductor polarized neutron flippers and thin film flippers. These flippers have important applications in various neutron sources due to their different characteristics. Polarized neutron flippers have mature commercial products internationally, but there is no independent research and development technology in China. Therefore, the localization research of polarized neutron radio frequency flippers is urgently needed and has important significance in China. SUMMARY

[0004] The technical problem solved by the present application is to provide a polarized neutron radio frequency flipper for flipping the spin vector of polarized neutrons.

[0005] According to a first aspect, in an embodiment, a polarized neutron radio frequency flipper is provided, comprising:

[0006] a housing, the housing is oppositely provided with a first opening and a second opening along the length direction; the housing is provided with a magnetic field generating device, the magnetic field generating device includes a radio frequency magnetic field generating device for generating a radio frequency alternating magnetic field and a gradient magnetic field generating device for generating a gradient magnetic field;

[0007] The radio frequency magnetic field generating device includes a solenoid, the axis of the solenoid is arranged in line with the connecting line of the first opening and the second opening;

[0008] The gradient magnetic field generating device comprises a first metal plate and a second metal plate, which are arranged on both sides of the solenoid and symmetrically along the axis of the solenoid; the distance between the first metal plate and the second metal plate from the axis of the solenoid gradually decreases along the direction from the first opening to the second opening.

[0009] The magnetic field frequency of the center point of the solenoid is equal to the magnetic field frequency of the center magnetic field of the effective magnetic field in the gradient magnetic field.

[0010] In one embodiment, the polarized neutron radio frequency flipper further comprises a test coil, which is sleeved on the outer periphery of the solenoid, and the axis of the test coil passes through the center point of the solenoid.

[0011] In one embodiment, the polarized neutron radio frequency flipper further comprises a guide magnetic field generating device, which comprises a third metal plate and a fourth metal plate, which are arranged in parallel on the side of the magnetic field generating device close to the second opening, the third metal plate and the fourth metal plate are symmetrically arranged along the axis of the solenoid, the distance between the third metal plate and the fourth metal plate is less than or equal to the shortest distance between the first metal plate and the second metal plate, and the distance between the third metal plate and the fourth metal plate is greater than the diameter of the solenoid.

[0012] In one embodiment, the guide magnetic field generating device further comprises a fifth metal plate and a sixth metal plate, which are arranged in parallel on the side of the magnetic field generating device close to the first opening, the fifth metal plate and the sixth metal plate are symmetrically arranged along the axis of the solenoid, and the distance between the fifth metal plate and the sixth metal plate is greater than or equal to the longest distance between the first metal plate and the second metal plate.

[0013] In one embodiment, the radio frequency magnetic field generating device further comprises a power amplification module and a radio frequency amplification module;

[0014] The power amplification module is used to obtain an alternating voltage, and amplify the alternating voltage according to a set amplification multiple to generate an output voltage;

[0015] The radio frequency amplification module comprises a resistor and a capacitor, the first end of the resistor is connected to the first output end of the power amplification module, the second end of the resistor is connected to the first end of the capacitor, the second end of the capacitor is connected to the first end of the solenoid, and the second end of the solenoid is connected to the second output end of the power amplification module; the radio frequency amplification module obtains the output voltage and inputs the output voltage into the solenoid to generate a radio frequency alternating magnetic field.

[0016] In one embodiment, the radio frequency magnetic field generating device includes: adjusting the amplification factor to adjust the radio frequency alternating magnetic field.

[0017] In one embodiment, the resistor is a ceramic resistor.

[0018] In one embodiment, the first metal plate, the second metal plate, the third metal plate, the fourth metal plate, the fifth metal plate, and the sixth metal plate are permanent magnets.

[0019] In one embodiment, the radio frequency amplification module is provided with a protective housing, and a cooling fan is provided on the protective housing.

[0020] In one embodiment, the protective outer shell and the housing are made of bakelite board.

[0021] According to the polarized neutron radio frequency (RF) reversing device of the above embodiment, the RF reversing device includes a housing, on which a first opening, a second opening, and a magnetic field generating device are provided. The magnetic field generating device includes an RF magnetic field generating device and a gradient magnetic field generating device. The RF magnetic field generating device includes a solenoid coil, and the axis of the solenoid coil is collinear with the connecting line of the first opening and the second opening. The gradient magnetic field generating device includes a first metal plate and a second metal plate, which are disposed on both sides of the solenoid coil and symmetrically arranged along the axis of the solenoid coil. The distance of the first metal plate and the second metal plate from the axis of the solenoid coil gradually decreases along the direction from the first opening to the second opening. Furthermore, the magnetic field frequency at the center point of the solenoid coil is equal to the magnetic field frequency of the center magnetic field of the effective magnetic field in the gradient magnetic field. This application can change the spin state of multi-wavelength polarized neutrons by coupling the gradient magnetic field generated by the first metal plate and the second metal plate with a fixed included angle with the RF alternating magnetic field generated by the solenoid coil. Attached Figure Description

[0022] Figure 1 A three-dimensional structural schematic diagram of a polarized neutron radio frequency flipper according to one embodiment;

[0023] Figure 2 This is a circuit diagram of an RLC oscillator circuit according to one embodiment;

[0024] Figure 3 This is a schematic diagram of neutron spin vector flipping in one embodiment;

[0025] Figure 4 This is a graph showing the experimental measurement results of one embodiment. Detailed Implementation

[0026] The application will be described in further detail below with specific reference to the drawings. Like elements in different embodiments are denoted by like reference numerals. In the following description, numerous specific details are described to provide a thorough understanding of the application. However, it will be apparent to one skilled in the art that the application can be practiced without these specific details. In other instances, well-known methods have not been described in detail in order not to unnecessarily obscure the application. In the following description, numerous specific details are described to provide a thorough understanding of the application. However, one skilled in the art will recognize that the application can be practiced without these specific details. In some instances, well-known structures have not been described in detail in order not to unnecessarily obscure the application.

[0027] In addition, the features, operations or characteristics described in the specification can be combined in any appropriate manner to form various embodiments. At the same time, the steps or actions in the method description can also be sequentially adjusted or adjusted in a manner that can be obviously seen by those skilled in the art. Therefore, the various sequences in the specification and drawings are only for the purpose of clearly describing a certain embodiment, and do not mean that the sequence is necessary, unless otherwise stated that a certain sequence must be followed.

[0028] The serial numbers of the components in this paper, such as "first", "second", etc., are only used to distinguish the described objects, and do not have any order or technical meaning. The "connection" and "coupling" in this application include direct and indirect connection (coupling) unless otherwise specified.

[0029] The application provides a polarized neutron radio frequency flipper. The polarized neutron radio frequency flipper controls the spin vector of the polarized neutron through the coupling of the gradient magnetic field and the radio frequency alternating magnetic field, and rotates the polarized neutron vector by 180° under the adiabatic condition. The application has a polarized flip efficiency of more than 90% for multi-wavelength polarized neutrons, can realize the spin control of polarized neutrons, meets the needs of various neutron experiments of the spallation source, and will be described in detail below.

[0030] Please refer to Figure 1 An embodiment provides a polarized neutron radio frequency flipper. An o-xyz coordinate system for representing a direction is constructed in the polarized neutron radio frequency flipper. The polarized neutron radio frequency flipper includes a shell 100. The shell 100 is relatively provided with a first opening 101 and a second opening 102 along a length direction (i.e. an x-axis in the figure). Figure 1 The shell 100 is provided with a magnetic field generating device inside. The magnetic field generating device includes a radio frequency magnetic field generating device 110 for generating a radio frequency alternating magnetic field, a gradient magnetic field generating device 120 for generating a gradient magnetic field, a test coil 130, and a guide magnetic field generating device 140.

[0031] In an embodiment, the radio frequency magnetic field generating device 110 comprises a solenoid 111, an axis of the solenoid 111 is arranged in line with a connecting line of the first opening 101 and the second opening 102 of the housing 100, that is, the axis of the solenoid 111 is parallel to the x-axis.

[0032] In an embodiment, the radio frequency magnetic field generating device 110 further comprises a radio frequency amplification unit, the radio frequency amplification unit comprises a power amplification module 113 and a radio frequency amplification module 114.

[0033] In an embodiment, the power amplification module 113 is configured to obtain an alternating voltage, and amplify the alternating voltage according to a set amplification factor to generate an output voltage.

[0034] Please refer to Figure 2 In an embodiment, the radio frequency amplification module 114 comprises a resistor R and a capacitor C, the resistor R, the capacitor C and the solenoid 111 form an RLC oscillation circuit, wherein a first end of the resistor R is connected to a first output end of the power amplification module 113, a second end of the resistor R is connected to a first end of the capacitor C, a second end of the capacitor C is connected to a first end of the solenoid 111, and a second end of the solenoid 111 is connected to a second output end of the power amplification module 113. The radio frequency amplification module 114 obtains the output voltage output by the power amplification module 113 and inputs the output voltage to the solenoid 111, so as to generate a radio frequency alternating magnetic field. When the radio frequency alternating magnetic field needs to be adjusted, the amplification factor of the power amplification module 113 can be directly adjusted.

[0035] In an embodiment, a protective shell is arranged outside the radio frequency amplification module 114, and a cooling fan is arranged on the protective shell. The cooling fan can effectively reduce the surface temperature of the electronic components and improve the heat dissipation efficiency, so as to ensure that the polarized neutron radio frequency flip-flop still maintains good electrical parameters under long-time work, and the output power of the solenoid 111 is not reduced. The protective shell is made of bakelite.

[0036] It should be noted that the size, frequency and working stability of the radio frequency alternating magnetic field generated by the radio frequency amplification unit will affect the flipping efficiency of the polarized neutron radio frequency flipper. Therefore, in view of the characteristics of the radio frequency alternating magnetic field, the radio frequency amplification unit is arranged in the polarized neutron radio frequency flipper, and the alternating current amplification source is input into the series RLC oscillation circuit in the radio frequency amplification unit. The inductance of the solenoid 111 generates a controlled radio frequency alternating magnetic field, and the series RLC oscillation circuit is used as a circuit for directly generating an alternating magnetic field. In order to improve the output power of the radio frequency alternating magnetic field, the resistance R in the circuit is selected as a high-temperature resistant ceramic resistance. By connecting it as a load in the circuit, the effective output frequency range of the RLC oscillation circuit can be expanded while adjusting the working current. In this way, the influence of the thermal effect on the electronic components is reduced, and the thermal stability of the overall circuit is improved. After determining the resistance value, a plurality of resistors are connected in parallel to generate the load resistance, the purpose of which is to increase the heat dissipation area and reduce the damage of the thermal effect to the resistance. The solenoid 111 in the circuit is made by winding a certain length of copper wire outside the anti-magnetic tube. In order to improve the inductance, a copper wire with small radius, large load current and good toughness is selected for winding. When the resonance condition is met, the alternating signal input into the solenoid 111 will generate an alternating magnetic field with a certain intensity. In one embodiment, in order to provide stable and sufficient alternating current signal for the RLC oscillation circuit, an alternating current source is made by combining the alternating current amplification source and the power amplification module 113. A stable alternating current signal meeting the design index is generated by a signal generator, and then the power amplification module 113 is used to amplify the alternating current signal by a set amplification multiple without loss and stably input into the radio frequency amplification module 114. When it is needed to change the size of the radio frequency alternating magnetic field, only the amplification multiple of the power amplification module 113 needs to be adjusted to directly complete the change.

[0037] In one embodiment, the gradient magnetic field generating device 120 includes a first metal plate 121 and a second metal plate 122, the first metal plate 121 and the second metal plate 122 are arranged on both sides of the solenoid and are symmetrically arranged along the axis of the solenoid, and the distance between the first metal plate 121 and the second metal plate 122 and the axis of the solenoid gradually decreases in the direction from the first opening 101 to the second opening 102.

[0038] It should be noted that the gradient magnetic field generating device 120 is composed of a plurality of symmetrically distributed permanent magnets and metal plates. After the symmetrically distributed permanent magnets magnetize the metal plates, the metal plates also become permanent magnets. The metal plates form a gradient magnetic field with a relatively uniform distribution perpendicular to the neutron path on the path. The first metal plate 121 includes a first face and a second face in the thickness direction, and the second metal plate 122 includes a third face and a fourth face in the thickness direction. The first metal plate 121 and the second metal plate 122 are arranged on both sides of the solenoid, that is, the second face of the first metal plate 121 faces the solenoid, and the fourth face of the second metal plate 122 faces the solenoid. The main purpose of the gradient magnetic field generating device 120 generating the gradient magnetic field is to give a gradient magnetic field distribution perpendicular to the neutron path, so that the field strength at a specific position of the gradient magnetic field has a certain value, and the maximum value of the gradient magnetic field before and after the specific position is far away from the specific position. The included angle between the first metal plate 121 and the second metal plate 122 along the x-axis in the present application is 13° and -13°.

[0039] In an embodiment, in the polarized neutron radio frequency flip-flop, the method of coupling the gradient magnetic field and the radio frequency alternating magnetic field is used to change the spin direction of the polarized neutron by 180°. The magnetic field frequency of the center point of the solenoid 111 needs to be equal to the magnetic field frequency of the center magnetic field of the effective magnetic field in the gradient magnetic field.

[0040] It should be noted that the axis of the solenoid 111 is perpendicular to the cross section of the solenoid 111 along the zoy plane at the midpoint of the length of the solenoid 111 along the x-axis direction at this time. The center of the cross section is the center point of the solenoid 111. In the gradient magnetic field, the gradient magnetic field overlapping with the radio frequency alternating magnetic field is defined as the effective magnetic field of the gradient magnetic field, and the magnetic field at the midpoint of the effective magnetic field along the x-axis direction is defined as the center magnetic field.

[0041] In an embodiment, the spin polarization of the neutron produces precession in the external magnetic field, which is called Larmor precession. This motion can be described as the dynamics equation of the polarization vector using the Bloch equation:

[0042]

[0043] wherein, represents the spin polarization vector of the neutron, γ n represents the gyromagnetic ratio of the neutron, γ n = -1.83247171 × 10 -8 rad s-1T-1, represents the vector magnetic field strength of the external magnetic field.

[0044] In an embodiment, under the condition of the static magnetic field, if the magnetic field direction of the static magnetic field is along the z-axis, the component Pz The static magnetic field remains unchanged, while the vertical component P of the polarized neutron is perpendicular to the direction of the magnetic field x and P y then precesses in a left-handed spiral.

[0045] In one embodiment, the angle of the precession of the vertical component of the neutron is directly proportional to the strength of the external magnetic field and the time of the precession of the neutron in the external magnetic field, wherein the angle of the precession of the vertical component of the neutron is the Larmor precession angle:

[0046]

[0047] ω L = γ n B

[0048] wherein, represents the Larmor precession angle, ω L represents the Larmor precession frequency, γ n represents the gyromagnetic ratio of the neutron, γ n = -1.83247171 x 10 -8 rad s-1T-1, B represents the scalar magnetic field strength of the external magnetic field, and t represents the time of the precession of the neutron in the external magnetic field.

[0049] In one embodiment, the evolution of the neutron in the changing magnetic field is more complex than in the static magnetic field. When the magnetic field changes with time at a certain frequency ω B , if the Larmor precession frequency ω L of the neutron is much larger than the changing frequency ω B of the magnetic field, the phenomenon is called adiabatic transformation. Under adiabatic transformation, the conical angle of the neutron spin polarization vector precessing along the external magnetic field changes with the change of the direction of the external magnetic field.

[0050] In this way, the polarization vector flipping process of the polarized neutron radio frequency flipper is achieved by coupling a gradient magnetic field B0 with a direction unchanged but gradually decaying or enhancing intensity and a radio frequency alternating magnetic field B1 with a direction orthogonal to the gradient magnetic field B0 (or the direction is parallel to the neutron beam direction). The polarized neutron radio frequency flipper uses the magnetic field generated by the coupling to flip the polarized neutron that satisfies the quasi-adiabatic flipping relationship, and the specific process is as follows.

[0051] In one embodiment, the magnetic induction intensity of the gradient magnetic field B0 changes with the position of the neutron path (along the x-axis), and the frequency of the radio frequency alternating magnetic field B1 is f1, then f1 needs to satisfy the following formula, that is, the frequency of the center point of the radio frequency alternating magnetic field B1 needs to satisfy the magnetic field frequency of the center magnetic field of the effective magnetic field in the gradient magnetic field B0:

[0052] 2πf1 = γ n B static

[0053] B0= B0+ B1 ststic represents the static magnetic field size of the center point of the radio frequency alternating magnetic field.

[0054] In one embodiment, under the constraint of the formula, the magnetic field in the polarized neutron radio frequency flipper can be equivalent to a rotating coordinate system rotating at the angular frequency of B ststic n B'0in the direction of γ ststic B'0in the direction of B ststic The static magnetic field generated by the radio frequency alternating magnetic field B1 is the magnetic induction strength in the direction perpendicular to B

[0055] In one embodiment, when the gradient magnetic field in the rotating coordinate system and the radio frequency alternating magnetic field satisfy the adiabatic transformation condition, the spin polarization vector of the neutron can be adiabatically flipped in the rotating coordinate system, please refer to Figure 3 , the vector direction is first transformed along the z-axis direction to the xoy plane, and then changed to the -z-axis direction, which is equivalent to completing the 180° transformation of the polarized neutron vector and the guide magnetic field B ststic in the laboratory coordinate system. The polarized neutron radio frequency flipper provided by the present application is suitable for all neutrons that satisfy the adiabatic condition, so it has good versatility for multi-wavelength neutrons, especially for neutron scattering experiments based on pulsed neutron sources.

[0056] In one embodiment, in order to detect the output of the radio frequency alternating magnetic field in the working state of the RLC oscillation circuit in real time, the polarized neutron radio frequency flipper provided by the present application further comprises a test coil 130, the test coil 130 is sleeved on the outer periphery of the solenoid 111, and the axis of the test coil 130 passes through the center point of the solenoid 111, the test coil 130 and the solenoid 111 adopt wires of the same material, and an oscilloscope is connected at both ends of the test coil 130. When the radio frequency alternating magnetic field is generated in the solenoid 111, the induced electromotive force generated by the test coil 130 can directly calculate the magnetic field strength of the controlled radio frequency alternating magnetic field according to Faraday's law of electromagnetic induction. When the induced electromotive force of the test coil 130 changes obviously, it indicates that the RLC oscillation circuit is damaged, the output power is unstable, and the circuit needs to be stopped immediately. The present application uses the test coil 130 to supervise and protect the polarized neutron radio frequency flipper.

[0057] ​In an embodiment, the polarized neutron radio frequency flipper requires stable polarization transmission efficiency. In order to reduce the influence of external stray magnetic field on the internal magnetic field distribution of the polarized neutron radio frequency flipper, a guide magnetic field generating device 140 is added at the front and back of the gradient magnetic field of the polarized neutron radio frequency flipper, and the guide magnetic field generating device 140 is used to ensure that the polarization state of the polarized neutron will not be lost when entering and leaving the polarized neutron radio frequency flipper. At the same time, the guide magnetic field generating device 140 with a longer length makes the external magnetic field have little influence on the magnetic field at the center point of the solenoid 111, and will not reduce the normal flipping efficiency of the polarized neutron radio frequency flipper.

[0058] In an embodiment, the guide magnetic field generating device 140 includes a third metal plate 141 and a fourth metal plate 142, the third metal plate 141 and the fourth metal plate 142 are arranged in parallel on the side of the magnetic field generating device close to the second opening 102, the third metal plate 141 and the fourth metal plate 142 are symmetrically arranged along the axis of the solenoid 111, the distance between the third metal plate 141 and the fourth metal plate 142 is less than or equal to the shortest distance between the first metal plate 121 and the second metal plate 122, and the distance between the third metal plate 141 and the fourth metal plate 142 is greater than the diameter of the solenoid 111.

[0059] It should be noted that the shortest distance between the first metal plate 121 and the second metal plate 122 is the straight line distance of the first metal plate 121 and the second metal plate 122 closest to the second opening 102 along the z-axis. The third metal plate 141 and the fourth metal plate 142 are also permanent magnets.

[0060] In an embodiment, the guide magnetic field generating device 140 further includes a fifth metal plate 143 and a sixth metal plate 144, the fifth metal plate 143 and the sixth metal plate 144 are arranged in parallel on the side of the magnetic field generating device close to the first opening, the fifth metal plate 143 and the sixth metal plate 144 are symmetrically arranged along the axis of the solenoid 111, and the distance between the fifth metal plate 143 and the sixth metal plate 144 is greater than or equal to the longest distance between the first metal plate 121 and the second metal plate 122.

[0061] It should be noted that the longest distance between the first metal plate 121 and the second metal plate 122 is the straight line distance of the first metal plate 121 and the second metal plate 122 closest to the first opening 101 along the z-axis. The fifth metal plate 143 and the sixth metal plate 144 are also permanent magnets.

[0062] In one embodiment, in order to protect the long-term stable operation of the polarized neutron radio frequency flipper, the application designs a non-magnetized shell 100 which is formed by splicing multiple bakelite boards. In addition to being non-magnetized, the shell is also very sturdy, effectively ensuring the stability of the main body of the polarized neutron radio frequency flipper. The gradient magnetic field generated by the permanent magnets installed on both sides of the first metal plate 121 and the second metal plate 122 which are fixed at a horizontal angle can be coupled with the radio frequency alternating magnetic field generated by the radio frequency magnetic field generating device 110 to change the spin state of the multi-wavelength polarized neutron.

[0063] In one specific embodiment, in order to verify the flipping ability of the application on polarized neutrons, a polarized neutron measurement experiment was conducted on the neutron technology research beam line (BL-20) on the polarized neutron radio frequency flipper. The polarized super mirror was used as a polarizer, and the online polarized helium three neutron spin filter device was used as a polarization analyzer. The experimental measurement results are shown in FIG. 6. The preliminary verification showed that the device has a 90% polarization flipping efficiency on neutrons with a wavelength of 2.5 angstroms or more. The flipping efficiency of the polarized neutron increases with the increase of the wavelength. When the neutron wavelength is greater than 4 angstroms, the polarization flipping efficiency is higher than 97%. Figure 4

[0064] The above application of specific examples is used to illustrate the application, which is only used to help understand the application and does not limit the application. For those skilled in the art to which the application belongs, according to the idea of the application, several simple deductions, deformations or substitutions can be made.​

Claims

1. A polarized neutron radio frequency inverter, characterized in that, include: The housing has a first opening and a second opening arranged opposite each other along its length; a magnetic field generating device is provided inside the housing, the magnetic field generating device including a radio frequency magnetic field generating device for generating a radio frequency alternating magnetic field and a gradient magnetic field generating device for generating a gradient magnetic field. The radio frequency magnetic field generating device includes a solenoid coil, the axis of which is collinear with the connecting line between the first opening and the second opening; The gradient magnetic field generating device includes a first metal plate and a second metal plate, which are disposed on both sides of the solenoid coil and symmetrically arranged along the axis of the solenoid coil. The first metal plate includes a first surface and a second surface in the thickness direction, and the second metal plate includes a third surface and a fourth surface in the thickness direction. The second surface of the first metal plate faces the solenoid coil, and the fourth surface of the second metal plate faces the solenoid coil. The distance between the first metal plate and the axis of the solenoid coil gradually decreases along the direction from the first opening to the second opening. The first metal plate and the second metal plate are permanent magnets. The magnetic field frequency at the center point of the solenoid is equal to the magnetic field frequency of the center magnetic field of the effective magnetic field in the gradient magnetic field.

2. The polarized neutron radio frequency inverter as described in claim 1, characterized in that, The polarized neutron radio frequency inverter also includes a test coil, which is sleeved on the outer periphery of the solenoid, and the axis of the test coil passes through the center point of the solenoid.

3. The polarized neutron radio frequency inverter as described in claim 2, characterized in that, The polarized neutron radio frequency flipper also includes a guiding magnetic field generating device, which includes a third metal plate and a fourth metal plate. The third metal plate and the fourth metal plate are arranged parallel to each other on the side of the magnetic field generating device near the second opening. The third metal plate and the fourth metal plate are symmetrically arranged along the axis of the solenoid. The distance between the third metal plate and the fourth metal plate is less than or equal to the shortest distance between the first metal plate and the second metal plate, and the distance between the third metal plate and the fourth metal plate is greater than the diameter of the solenoid.

4. The polarized neutron radio frequency inverter as described in claim 3, characterized in that, The guiding magnetic field generating device further includes a fifth metal plate and a sixth metal plate, which are arranged in parallel on the side of the magnetic field generating device near the first opening. The fifth metal plate and the sixth metal plate are symmetrically arranged along the axis of the solenoid, and the distance between the fifth metal plate and the sixth metal plate is greater than or equal to the longest distance between the first metal plate and the second metal plate.

5. The polarized neutron radio frequency inverter as described in claim 1, characterized in that, The radio frequency magnetic field generating device also includes a power amplification module and a radio frequency amplification module; The power amplifier module is used to acquire AC voltage and amplify the AC voltage according to a set amplification factor to generate an output voltage; The radio frequency amplification module includes a resistor and a capacitor. The first end of the resistor is connected to the first output terminal of the power amplification module, the second end of the resistor is connected to the first end of the capacitor, the second end of the capacitor is connected to the first end of the solenoid, and the second end of the solenoid is connected to the second output terminal of the power amplification module. The radio frequency amplification module acquires the output voltage and inputs the output voltage to the solenoid to generate a radio frequency alternating magnetic field.

6. The polarized neutron radio frequency inverter as described in claim 5, characterized in that, The radio frequency magnetic field generating device includes: adjusting the amplification factor to adjust the radio frequency alternating magnetic field.

7. The polarized neutron radio frequency inverter as described in claim 5, characterized in that, The resistor is a ceramic resistor.

8. The polarized neutron radio frequency inverter as described in claim 4, characterized in that, The third, fourth, fifth, and sixth metal plates are permanent magnets.

9. The polarized neutron radio frequency inverter as described in claim 5, characterized in that, The radio frequency amplification module is provided with a protective shell, and a cooling fan is provided on the protective shell.

10. The polarized neutron radio frequency inverter as described in claim 9, characterized in that, The protective outer shell and the housing are made of bakelite board.

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