Design method of self-supporting thin film photocathode for improving dynamic range of transmission grating spectrometer

Abstract

The application discloses a self-supporting thin film photocathode design method for improving the dynamic range of a transmission grating spectrometer, and the photocathode is designed by using a self-supporting thin film and an equal-width photocathode slit, so that the difference in photoelectron density in the high-energy area and the low-energy area of a diffraction spectrum is reduced, the dynamic range of the time-resolved transmission grating spectrometer is improved, and the time resolution of the spectrometer along the length direction of the photocathode is kept unchanged. Compared with a conventional equal-width photocathode and a flat-response photocathode, the self-supporting thin film photocathode can reduce the difference in photoelectron density in the high-energy area and the low-energy area of the diffraction spectrum on the photocathode, and improve the dynamic range of the spectrometer. Compared with a width-second-gradual photocathode, the self-supporting thin film photocathode can keep the time resolution of the time-resolved transmission grating spectrometer along the length direction of the photocathode unchanged, and solve the problem that the quantitative spectral analysis is difficult due to the change of the time resolution of the width-second-gradual photocathode along the length direction of the photocathode.

Description

Technical Field

[0001] This invention relates to the field of X-ray spectroscopy diagnostics, and more specifically to a design method for a self-supporting thin-film photocathode to improve the dynamic range of a transmission grating spectrometer. Background Technology

[0002] In indirect-drive laser inertial confinement fusion (ICF), X-ray spectroscopy is an important diagnostic tool for diagnosing the plasma state. However, the X-ray emission spectrum of laser plasma within a black cavity has a wide energy range and its state changes rapidly. A high-time-resolution, wide-energy-range transmission grating spectrometer, composed of a transmission grating (TG) and an X-ray streak camera (XSC), is one of the important measurement devices for conducting X-ray spectroscopic measurements of laser plasma. Its geometry and working principle are as follows: Figure 1 As shown. However, the intensity of X-ray emission spectra from laser plasma varies greatly across different energy regions, and its absolute intensity also changes drastically with the evolution of plasma states. This poses a severe challenge to the dynamic range of time-resolved transmission grating spectrometers. Therefore, improving the dynamic range of time-resolved transmission grating spectrometers and enhancing their robustness in spectral measurements is of great significance for their widespread application.

[0003] The dynamic range of X-ray streak cameras is a bottleneck for the dynamic range of time-resolved transmission grating (TRC) spectrometers. Given the limited dynamic range of X-ray streak cameras, reducing the sensitivity of the photocathode to high-energy X-rays and increasing its sensitivity to low-energy X-rays, thereby reducing the spatial distribution difference of photoelectrons generated by the transmission grating diffraction spectrum on the XSC photocathode, is an important way to improve the dynamic range of TRC spectrometers. Commonly used Au or CsI photocathodes in X-ray streak cameras typically consist of a CH film of hundreds of nm and an Au film of tens of nm or a CsI film on the order of hundreds of nm. On the one hand, the CH film, which provides support, absorbs low-energy X-rays much more strongly than high-energy X-rays. On the other hand, the energy of low-energy X-rays is mainly deposited on the front surface of Au or CsI and generates photoelectrons. The absorption of photoelectrons by the Au or CsI material itself makes it difficult for photoelectrons to be transported to the rear surface of the photocathode to form effective photoelectrons. Therefore, conventional Au or CsI photocathodes are more sensitive to high-energy X-rays than to low-energy X-rays. The sensitivity of conventional photocathodes to X-rays contradicts the requirements for improving the dynamic range of TRC spectrometers. While flat-response photocathodes make the sensitivity of photocathodes to high and low energy X-rays nearly uniform, they still cannot meet the requirements of time-resolved transmission grating spectrometers. Although width-gradient photocathodes can reduce the difference in the number of photoelectrons generated by X-rays in different energy regions on the photocathode, the change in its width causes the time resolution of the spectrometer to change along the length of the photocathode, introducing new challenges for subsequent quantitative inversion of the X-ray source energy spectrum. Summary of the Invention

[0004] In view of this, the present invention provides a self-supporting thin-film photocathode design method for improving the dynamic range of a time-resolved transmission grating spectrometer. While reducing the difference in photoelectron density between the high and low energy regions of the diffraction spectrum and improving the dynamic range of the time-resolved transmission grating spectrometer, it ensures that the time resolution of the spectrometer remains unchanged along the length of the photocathode.

[0005] To achieve the above objectives, the technical solution of the present invention is as follows:

[0006] A method for designing a self-supporting thin-film photocathode to improve the dynamic range of a transmission grating spectrometer, the key of which includes the following steps:

[0007] Step S1: Design a self-supporting thin-film photocathode. The self-supporting thin-film photocathode includes a grid structure layer and a photoelectric conversion thin film layer. The grid structure layer supports the photoelectric conversion thin film layer, which is used to realize the photoelectric conversion of X-rays to generate photoelectrons. The grid structure layer has circular through holes distributed on it.

[0008] Step S2: Establish a spatial distribution model of the photoelectron density emitted by the photocathode of the streak camera in a time-resolved transmission grating spectrometer. At a distance x from the zeroth order in the diffraction spectral recording region, the photoelectron flux density emitted by the photocathode is:

[0009] (1)

[0010] (2)

[0011] In equation (1), the distance x from the diffraction spectrum to the zero order and the X-ray energy are related. The relationship is derived from the grating equation. It is determined that d is the grating constant in mm, and in equation (1) These represent the widths of the photocathode and the grating slits, respectively, in mm. The distance from the light source to the grating. This represents the distance from the photocathode to the grating, in mm. The energy of the light source to be measured is X-ray emission intensity, expressed in J / s / eV. The nth-order diffraction efficiency of the grating Photocathode sensitivity, measured in A / W. The spectrometer line dispersion is calculated by equation (2) and its unit is eV / mm;

[0012] Step S3: Establish a sensitivity model for a self-supporting thin-film photocathode with a grid structure:

[0013] (3)

[0014] In equation (3), The material density of the cathode, The surface absorption coefficient of the cathode material. The secondary electron escape depth in the material, expressed in μm. It is energy The absorption length of X-rays in the photocathode material, measured in μm. The thickness of the grid structure layer, The thickness of the photoelectric conversion thin film layer is expressed in μm. It is the ratio of the photoelectric conversion thin film layer in the photocathode to the total area of ​​the self-supporting thin film photocathode;

[0015] Step S4: Establish the evaluation function for the homogenization of photoelectron flux density in the photocathode diffraction spectral recording region. :

[0016]

[0017] In the above formula, and These represent the shortest and longest distances, in mm, between the photocathode diffraction spectral recording region I and the zeroth order of the diffraction spectrum. The average photocurrent density in the photocathode diffraction spectral recording region I is given by... get;

[0018] Step S5, with Minimize as the objective , and Optimization calculations are performed on the parameters to be optimized to obtain... minimize , and value.

[0019] By employing the above structure, compared to conventional uniform-width photocathodes and flat-response photocathodes, the self-supporting thin-film photocathode can reduce the difference in photoelectron density generated on the cathode in the high and low energy regions of the diffraction spectrum, thereby improving the dynamic range of the spectrometer. Compared to photocathodes with a second-gradient width, the self-supporting thin-film photocathode allows the time resolution of the time-resolved transmission grating spectrometer to remain constant along the length of the photocathode, solving the problem of quantitative spectral interpretation difficulties caused by the variation of time resolution along the length of the photocathode in photocathodes with a second-gradient width.

[0020] As a preferred option: in step S3 The calculation formula is as follows:

[0021]

[0022] In the above formula, b is the diameter of the through hole, and a is the distance between the centers of adjacent through holes.

[0023] Preferably, the mesh structure layer and the photoelectric conversion thin film layer are prepared using the same photoelectric sensitive metal material.

[0024] Preferably, the photosensitive metal material is aluminum.

[0025] Preferably, the mesh structure layer is oriented towards the X-ray source.

[0026] Compared with the prior art, the beneficial effects of the present invention are:

[0027] 1. The self-supporting thin-film photocathode design method for improving the dynamic range of a transmission grating spectrometer provided by this invention can reduce the difference in photoelectron intensity between the high-energy and low-energy regions on the photocathode in a time-resolved transmission grating spectrometer, thereby improving the dynamic range of the time-resolved transmission grating spectrometer.

[0028] 2. The self-supporting thin-film photocathode design method for improving the dynamic range of a transmission grating spectrometer provided by this invention features a self-supporting thin-film photocathode with equal width along its length and a photocathode with a secondary gradient in relative width, which reduces the difficulty of spectral interpretation.

[0029] 3. The self-supporting thin-film photocathode design method for improving the dynamic range of transmission grating spectrometers provided by this invention uses the same material for the grid structure layer and the photoelectric conversion thin film layer, which eliminates the problem of X-ray absorption complexity caused by the C absorption edge of the CH substrate material in traditional Au or CsI photocathodes. Attached Figure Description

[0030] Figure 1 A schematic diagram illustrating the structure and principle of a time-resolved transmission grating spectrometer based on an X-ray fringe camera;

[0031] Figure 2 The diagram shows a self-supporting thin-film photocathode A, where (a) is a schematic diagram of the overall layout of the photocathode, (b) is a partial schematic diagram of the photocathode, and (c) is a schematic diagram of the cross-section of the photocathode thin film.

[0032] Figure 3 This is a radiation spectrum distribution diagram at the peak moment of a 1ns high-temperature radiation source;

[0033] Figure 4 This is a diagram showing the diffraction efficiency of a transmission grating.

[0034] Figure 5 A comparison of the normalized sensitivity of a traditional Au photocathode, a flat-response Au photocathode, and a self-supporting Al thin-film photocathode;

[0035] Figure 6 A comparison diagram of the spatial distribution of normalized photocurrent density on three photocathodes. Detailed Implementation

[0036] The present invention will be further described below with reference to the embodiments and accompanying drawings.

[0037] like Figure 2 As shown, a method for designing a self-supporting thin-film photocathode to improve the dynamic range of a transmission grating spectrometer includes the following steps:

[0038] Step S1: Design a self-supporting thin-film photocathode A. The self-supporting thin-film photocathode A consists of a grid structure layer A1 and a photoelectric conversion thin film layer A2. The grid structure layer A1 supports the photoelectric conversion thin film layer A2, which is used to achieve photoelectric conversion of X-rays, generating photoelectrons. Both the grid structure layer A1 and the photoelectric conversion thin film layer A2 are made of the same photosensitive metal material. In use, the grid structure layer A1 faces the X-ray source. The grid structure layer A1 has uniformly distributed circular through-holes A11.

[0039] Step S2: Establish a spatial distribution model of the photoelectron density emitted by the photocathode of the streak camera in a time-resolved transmission grating spectrometer. At a distance x (in mm) from the zero order in the diffraction spectral recording region, the photoelectron flux density emitted by the photocathode is:

[0040] (1)

[0041] (2)

[0042] In equation (1), the distance x from the diffraction spectrum to the zero order and the X-ray energy are related. The relationship is derived from the grating equation. It is determined that d is the grating constant in mm, and in equation (1) These represent the widths of the photocathode and the grating slits, respectively, in mm. The distance from the light source to the grating. This represents the distance from the photocathode to the grating, in mm. The energy of the light source to be measured is X-ray emission intensity, expressed in J / s / eV. The nth-order diffraction efficiency of the grating Photocathode sensitivity, measured in A / W. is the spectrometer line dispersion, which is calculated by equation (2) and has the unit eV / mm.

[0043] Step S3: Establish a sensitivity model for a self-supporting thin-film photocathode with a grid structure:

[0044] (3)

[0045] In equation (3), The material density of the cathode, The surface absorption coefficient of the cathode material. The secondary electron escape depth in the material, expressed in μm. It is energy The absorption length of X-rays in the photocathode material, measured in μm. The thickness of the mesh structure layer A1, The thickness of the photoelectric conversion thin film layer A2 is expressed in μm. This represents the ratio of the photoelectric conversion thin film layer A2 in the photocathode to the total area of ​​the self-supporting thin film photocathode A. According to... Figure 2 The structure shown, The calculation formula is as follows:

[0046]

[0047] Where b is the diameter of the through hole A11, and a is the center-to-center distance between adjacent through holes A11.

[0048] Step S4: Establish the evaluation function for the homogenization of photoelectron flux density in the photocathode diffraction spectral recording region. :

[0049]

[0050] In the above formula, and These represent the shortest and longest distances, in mm, between the photocathode diffraction spectral recording region I and the zeroth order of the diffraction spectrum. The average photocurrent density in the photocathode diffraction spectral recording region I is given by... get.

[0051] Step S5, with Minimize as the objective , and Optimization calculations are performed on the parameters to be optimized to obtain... minimize , and value.

[0052] Based on the above design, the advantages of this invention are as follows: The photoelectric conversion layer of the photocathode adopts a thin-film design and uses a hollowed-out material of the same type as the thin film as the thin-film substrate, thus realizing a self-supporting thin-film photocathode. The self-supporting thin-film photocathode adopts a uniform width design along its length, which not only minimizes the spatial distribution difference of photocurrent density generated by the transmission grating diffraction spectrum on the X-ray streak camera photocathode, improving the dynamic range of the time-resolved transmission grating spectrometer, but also ensures that the time resolution of the spectrometer remains unchanged along the length of the photocathode, reducing the difficulty of subsequent quantitative spectral interpretation.

[0053] The following is a simulation verification conducted using the self-supporting thin-film photocathode design method proposed in this invention. The simulation verification results demonstrate the feasibility of the self-supporting thin-film photocathode prepared by this design method in reducing the spatial distribution difference of photocathode photocurrent density and improving the dynamic range of the time-resolved transmission grating spectrometer. In this embodiment, the grid structure layer A1 and the photoelectric conversion thin film layer A2 are made of Al (aluminum).

[0054] This simulation verification example is based on obtaining a 1ns high-temperature radiation source from a large laser. The radiation spectrum of this source at its peak moment is as follows: Figure 3 As shown. This spectrum was diffracted by a transmission grating with a grating constant of 0.3 μm, and its diffraction efficiency is as follows. Figure 4 As shown, the diffraction spectrum was recorded by an Au photocathode fringe camera with a width of 200 μm and a length of 28 mm. The sensitivity and flat response of a conventional uniform-width Au photocathode (200 nm CH + 30 nm Au) are as follows: [The text abruptly ends here, so the translation stops as well.] Figure 5 As shown, the spatial distribution of the photocurrent density generated on the photocathode by the transmission grating diffraction spectrum can be calculated using equation (1). ,like Figure 6 As shown.

[0055] The spatial distribution of photocurrent density generated by conventional Au photocathodes, flat-response Au photocathodes, and self-supporting Al thin-film photocathodes was simulated and compared. Within the energy range of 0.13 keV to 3.0 keV, the ratio of maximum to minimum current density for both conventional and flat-response Au photocathodes was 248. However, the ratio for the self-supporting Al thin-film photocathode was 19. Compared to conventional and flat-response Au photocathodes, time-resolved transmission grating spectrometers using self-supporting Al thin-film photocathodes can reduce the dynamic range requirement of the X-ray streak camera by 13 times. For a given time-resolved transmission grating spectrometer using an X-ray streak camera, the self-supporting Al thin-film photocathode can improve the spectrometer's dynamic range by 13 times.

[0056] Therefore, the photocathode of this invention employs a self-supporting thin film and a photocathode slit design of equal width. This reduces the difference in photoelectron density between high and low energy regions of the diffraction spectrum, improving the dynamic range of the time-resolved transmission grating spectrometer, while ensuring that the spectrometer's time resolution remains constant along the length of the photocathode. Compared to conventional photocathodes of equal width and planar response, the self-supporting thin film photocathode can reduce the difference in photoelectron density generated on the cathode in the high and low energy regions of the diffraction spectrum, thus improving the dynamic range of the spectrometer. Compared to photocathodes with a second-gradient width, the self-supporting thin film photocathode can keep the time resolution of the time-resolved transmission grating spectrometer constant along the length of the photocathode, solving the problem of quantitative spectral interpretation difficulties caused by the variation of time resolution along the length of the photocathode in photocathodes with a second-gradient width.

[0057] Finally, it should be noted that the above description is merely a preferred embodiment of the present invention. Those skilled in the art, under the guidance of the present invention, can make various similar representations without departing from the spirit and claims of the present invention, and such modifications all fall within the protection scope of the present invention.

Claims

1. A method for designing a self-supporting thin-film photocathode to improve the dynamic range of a transmission grating spectrometer, characterized in that, Includes the following steps: Step S1: Design a self-supporting thin-film photocathode (A). The self-supporting thin-film photocathode (A) includes a grid structure layer (A1) and a photoelectric conversion thin film layer (A2). The grid structure layer (A1) is used to support the photoelectric conversion thin film layer (A2). The photoelectric conversion thin film layer (A2) is used to realize the photoelectric conversion of X-rays and generate photoelectrons. Circular through holes (A11) are distributed on the grid structure layer (A1). Step S2: Establish a spatial distribution model of the photoelectron density emitted by the photocathode of the streak camera in a time-resolved transmission grating spectrometer. At a distance x from the zeroth order in the diffraction spectral recording region, the photoelectron flux density emitted by the photocathode is: (1) (2) In equation (1), the distance x from the diffraction spectrum to the zero order and the X-ray energy are related. The relationship is derived from the grating equation. It is determined that d is the grating constant in mm, and in equation (1) These represent the widths of the photocathode and the grating slits, respectively, in mm. The distance from the light source to the grating. This represents the distance from the photocathode to the grating, in mm. The energy of the light source to be measured is X-ray emission intensity, expressed in J / s / eV. The nth-order diffraction efficiency of the grating Photocathode sensitivity, measured in A / W. The spectrometer line dispersion is calculated by equation (2) and its unit is eV / mm; Step S3: Establish a sensitivity model for a self-supporting thin-film photocathode with a grid structure: (3) In equation (3), The material density of the cathode, The surface absorption coefficient of the cathode material. The secondary electron escape depth in the material, expressed in μm. It is energy The absorption length of X-rays in the photocathode material, measured in μm. The thickness of the mesh structure layer (A1) The thickness of the photoelectric conversion thin film layer (A2) is expressed in μm. The ratio of the photoelectric conversion thin film layer (A2) in the photocathode to the total area of ​​the self-supporting thin film photocathode (A); Step S4: Establish the evaluation function for the homogenization of photoelectron flux density in the photocathode diffraction spectral recording region. :

2. In the above formula, and These represent the shortest and longest distances, in mm, between the photocathode diffraction spectral recording region I and the zeroth order of the diffraction spectrum. The average photocurrent density in the photocathode diffraction spectral recording region I is given by... get; Step S5, with Minimize as the objective , and Optimization calculations are performed on the parameters to be optimized to obtain... minimize , and value.

3. The self-supporting thin-film photocathode design method for improving the dynamic range of a transmission grating spectrometer according to claim 1, characterized in that: In step S3, The calculation formula is as follows:

4. In the above formula, b is the diameter of the through hole (A11), and a is the distance between the centers of adjacent through holes (A11).

5. The self-supporting thin-film photocathode design method for improving the dynamic range of a transmission grating spectrometer according to claim 1, characterized in that: The grid structure layer (A1) and the photoelectric conversion thin film layer (A2) are made of the same photoelectric sensitive metal material.

6. The self-supporting thin-film photocathode design method for improving the dynamic range of a transmission grating spectrometer according to claim 3, characterized in that: The photosensitive metal material is aluminum.

7. The self-supporting thin-film photocathode design method for improving the dynamic range of a transmission grating spectrometer according to claim 1, characterized in that: The mesh structure layer (A1) is oriented towards the X-ray source.

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