High-resolution x-ray spectrograph for laser tail field acceleration and measuring method of high-resolution x-ray spectrograph

Abstract

A focus lens and a flat field grating are arranged in a front beryllium window and a rear beryllium window of a vacuum chamber, and an IP plate is arranged behind the beryllium windows. The wavelength range measured by the spectrometer is 1.05 nm to 5 nm, and the resolution is 1000 near 2 nm. According to the invention, an X-ray CCD is replaced by the IP board, so that the problem that other spectrometers cannot perform multiple irradiation spectrum formation in a laser wake field acceleration scene is solved, multiple irradiation spectrum formation can be realized, and an X-ray absorption spectrum changing along with time in a dynamic ultrafast process can be obtained by utilizing the characteristics of ultrastrong and ultrashort laser.

Description

Technical Field

[0001] The present invention belongs to the technical field of high-resolution spectrometers, and specifically relates to a spectrometer and a measurement method for measuring X-rays generated by the interaction of ultra-intense ultrashort lasers and plasmas. The spectrometer is mainly used in laser wakefield acceleration (LWFA) scenarios, and can realize multiple irradiation and spectrum formation of low-brightness, femtosecond pulse X-rays, and is suitable for X-ray absorption spectrum research of dynamic ultrafast processes. Background Art

[0002] X-rays are electromagnetic waves with wavelengths between 0.01nm and 10nm. Because their wavelengths are close to the size of atoms, they can be used to observe physical and chemical processes occurring at the atomic scale. Traditional methods use X-rays from synchrotron radiation sources. However, these sources are enormous, with diameters of approximately 200m, making them difficult to miniaturize and impossible to implement in a laboratory. Furthermore, they are expensive, costing billions of RMB. The pulse widths of X-rays produced by synchrotron radiation are on the order of nanoseconds, making it difficult to measure shorter atomic-scale processes.

[0003] In recent years, laser wakefield acceleration (LWFA), generated by the interaction of ultraintense, ultrashort lasers with gas targets, has provided a new approach to addressing these challenges. The X-rays generated by LWFA (called betatron radiation) have femtosecond pulse widths, consistent with the pulse width of the driving laser (e.g., 30 femtoseconds), making them ideal sources for studying ultrafast dynamic processes. However, the photon count of betatron radiation is several orders of magnitude lower than that of synchrotron radiation (typically 10^11 photons per shot). Furthermore, due to the influence of laser and plasma parameters, the energy spectrum and spatial distribution exhibit significant fluctuations, necessitating the accumulation of multiple irradiations (typically more than 100 shots) to improve signal-to-noise ratio and consistency.

[0004] Spectrometers can measure the intensity of X-rays at different energies. Specifically, when X-rays are absorbed by matter, studying the energy and degree of absorption can reveal information about the material's composition and structure. Existing spectrometers are designed for use with synchrotron radiation sources, which produce high-brightness X-rays that can be spectrally generated with a single exposure. However, X-rays generated by laser wakefield acceleration are lower in brightness and require multiple exposures to form a spectrum, making them impractical for existing spectrometers. Summary of the Invention

[0005] To overcome the above-mentioned deficiencies of the prior art, the present invention provides a high-resolution X-ray spectrometer suitable for laser wakefield acceleration, which avoids the limitation of the above-mentioned spectrometer on multiple irradiations and can be used in the context of laser wakefield acceleration.

[0006] The X-ray radiation produced in laser wakefield acceleration (LWFA) is called betatron radiation.

[0007] The number of photons emitted by a single electron in the oscillation process of Betatron radiation is

[0008]

[0009] Where L is the electron oscillation length and the betatron frequency is The lateral amplitude is

[0010] The typical betatron radiation plasma density is 10^(-18)cm^(-3), L=2mm, a0=2, and the number of photons emitted by a single electron is 4800. The charge of each electron is 100 picoules, that is, 6.24×10^8 electrons, and a total of 2.995×10^12 photons are radiated. In fact, due to the deviation of various conditions in the experiment from the ideal situation and the influence of divergence angle and occlusion, the actual number of photons is only 10^11 at best. The synchrotron radiation source used in traditional X-ray absorption spectroscopy can produce up to 10^20 photons per second.

[0011] Although the number of photons emitted by betatron radiation is several orders of magnitude lower than that of synchrotron radiation. However, the light produced by synchrotron radiation is continuous in time and does not have time resolution, so it can only be used to perform X-ray absorption spectra of static samples. The X-ray pulses of betatron radiation produced by LWFA have the same pulse width as the ultra-intense ultra-short laser used, that is, 30 femtoseconds, so it can be regarded as the X-ray absorption spectrum of the sample at a certain moment. By adjusting the time difference between the arrival of the x-ray pulse and the sample reaction process, the x-ray absorption spectrum of the sample reaction at different stages can be obtained, which is very critical for studying the evolution of the sample reaction. In particular, due to its femtosecond pulse width, it can be used to study the ultrafast process of the sample. In fact, to study the ultrafast process of the sample, the x-ray absorption spectrum of the betatron light source must be used instead of the synchrotron radiation light source.

[0012] Using a betatron light source for X-ray absorption spectroscopy with a single shot can result in a poor signal-to-noise ratio due to insufficient photon counts. Furthermore, the energy spectrum of betatron radiation can vary significantly from shot to shot due to the influence of laser and plasma parameters. Furthermore, the spatial distribution of radiation can also vary due to variations in electron beam oscillation. Therefore, using multiple shots can average out fluctuations and improve consistency, while suppressing random noise and increasing the signal-to-noise ratio. Typically, a betatron absorption spectrum is generated by accumulating more than 100 shots.

[0013] Traditional X-ray absorption spectrum signal receivers use X-ray CCDs. CCDs convert X-ray photons into electric charges through the photoelectric effect and read them out pixel by pixel. CCDs are usually set to external trigger mode, which synchronizes the CCD with the laser pulse. Each X-ray produces a frame of image. When the CCD reads out the data, the charge is cleared, so the charge signal cannot be directly accumulated in the CCD readout circuit.

[0014] IP plate, also known as imaging plate, has a unique photostimulated luminescence (PSL) mechanism, the core of which is a photostimulated fluorescent material (such as BaFBr:Eu 2+ ). When x-ray photons hit electrons in the fluorescent material, they jump from the valence band to metastable trap energy levels (such as F-center defects). These bound electrons store information for a long time, with a half-life measured in days, forming a latent image on the IP board. When a 633nm helium-neon laser is used to scan the IP board point by point, its bound electrons are released and emit 390nm light. A digital image can be obtained by collecting the light signal using a photomultiplier tube. The remaining bound electrons can be completely released by irradiating the IP board with a white light LED for 10 minutes, and the IP board returns to its original state and can be reused.

[0015] The number of bound electrons is proportional to the number of photons, and the bound electrons generated by multiple irradiations can be linearly superimposed and stored. At the same time, the capacity of the trap energy level is large enough not to be saturated by a single irradiation, and its dynamic range is as high as 10 5 :1. At the same time, the bound electrons hardly relax spontaneously before readout, and the signal can be preserved for a long time.

[0016] Compared with CCD data acquisition, IP board has the following advantages in Betatron spectroscopy experiments

[0017] The readout noise is low. The readout noise of the IP board depends on the photomultiplier tube, and the photomultiplier tube has extremely high sensitivity to reduce the readout noise. However, the X-ray CCD is limited by the existence of the readout circuit and has a higher readout noise.

[0018] The dynamic range is large, and the IP board has a capacity of 10 due to the trap energy level 5 :1 dynamic range, while the dynamic range of CCD is only 10 due to the limitation of photoelectric effect process. 3 :1

[0019] It can be used for multiple irradiations. The photon signal of the IP board is stored in the bound electrons in the trap energy level, which has a very long relaxation time. The signal generated by multiple photons can be saved before one-time reading. The photon signal of the CCD is stored in the charge generated by the photoelectric effect. Each X-ray pulse will read the signal once and clear the charge. When the next photon is collected, the signal of the previous photon cannot be saved. Therefore, the signal generated by multiple photons cannot be saved, and it cannot be used for multiple irradiations.

[0020] Traditional X-ray absorption spectrometers are used at synchrotron radiation sources, where the photon count is high enough to generate a single-shot spectrum. X-ray CCDs are often used as signal receivers. However, betatron radiation emits a lower number of photons, requiring multiple-shot spectrum generation. Traditional spectrometers are not suitable for multiple-shot spectrum generation and are therefore unsuitable for betatron sources. The spectrometer described in the present invention overcomes this limitation by using an IP (Internet Protocol) board for signal acquisition, making it suitable for X-ray absorption spectroscopy using betatron sources.

[0021] The technical solutions of the present invention are as follows:

[0022] A high-resolution X-ray spectrometer for laser tailfield acceleration is characterized by comprising:

[0023] A vacuum chamber, wherein a front beryllium window and a rear beryllium window are provided on the front and rear surfaces thereof respectively;

[0024] A focusing mirror and a concave flat field grating disposed in the vacuum chamber;

[0025] A black box is arranged outside the rear beryllium window, and an IP board is arranged in the black box;

[0026] The centers of the front beryllium window, focusing mirror, concave flat field grating, rear beryllium window and IP plate are all located on the same optical axis and have the same height;

[0027] X-rays are incident from the front beryllium window, focused by the focusing mirror, diffracted and split by the concave flat-field grating, emitted from the rear beryllium window, and focused onto the IP plate.

[0028] The vacuum chamber has a size of 250 mm × 100 mm × 100 mm, and is provided with beryllium windows on both the front and rear surfaces. The beryllium windows are made of metallic beryllium, have a thickness of 10 mm, and a diameter of 20 mm, and the center of the circle is the geometric center of the front and rear surfaces.

[0029] The breadboard is fixed to the bottom of the vacuum chamber, with holes arranged in a square pattern, a hole spacing of 25.4 mm, and a size of 320 mm × 120 mm. Its geometric center is the center of the bottom of the vacuum chamber.

[0030] The front surface of the focusing mirror is perpendicular to the optical axis. The focusing mirror has a diameter of 60 mm and a focal length of 100 mm. The optical axis is in the direction of the incident light and is 100 mm away from the front beryllium window.

[0031] The size of the concave flat-field grating is 120mm×160mm, the average grating period is 60nm, the radius of curvature is 200mm, the distance from the focusing lens is 100mm, and the angle between the plane where it is located and the optical axis is 1 degree.

[0032] The black box is fixed behind the rear beryllium window and has a size of 40mm×10mm×30mm. Its rear surface is detachable and an IP plate is attached to the front of the rear surface. The size is 35mm×22mm and its geometric center is the center of the rear beryllium window.

[0033] The center of the front beryllium window, the center of the focusing mirror, the center of the concave flat field grating, the center of the rear beryllium window and the center of the IP plate are all at the same height and located on the optical axis;

[0034] The X-ray source generated by the laser tail wave field before the front beryllium window can pass through the front beryllium window, the focusing mirror, be deflected by the concave flat field grating, and then reach the IP plate through the rear beryllium window.

[0035] An X-ray absorption spectrum measurement method, using the above-mentioned high-resolution X-ray spectrometer, is characterized in that it includes the following steps:

[0036] Aim the spectrometer at the X-ray source generated by laser tailfield acceleration;

[0037] Measure the original Betatron radiation spectrum without sample;

[0038] Place a sample in front of the front beryllium window and measure the absorption spectrum of X-rays after they pass through the sample;

[0039] By comparing the two measurement results, the X-ray absorption characteristics of the sample are obtained and its elemental composition and structural information are analyzed.

[0040] Compared with the prior art, the present invention has the following beneficial effects:

[0041] A concave flat-field grating replaces traditional concave mirrors to eliminate coma caused by off-axis incidence and improve resolution. The grating design is optimized for flat-field focusing, and the IP plate can receive the full energy dispersion spectrum (covering 5–20 keV) without the need for additional aberration correction elements.

[0042] Traditional CCD detectors are limited by the frame readout time (milliseconds) and cannot accumulate multi-pulse X-ray signals. The IP board is based on the photoluminescence effect, and the signal accumulation time can reach minutes. It improves the signal-to-noise ratio through multiple laser pulse irradiation (≥100 times), and adapts to the low brightness characteristics of the laser wakefield X-ray source. The linear response range of the IP board is 10 5 Photons / mm 2 , covering single pulse to cumulative pulse signal intensity to avoid detector saturation.

[0043] Through IP board signal accumulation, concave flat-field grating optimization and integrated optical path design, the present invention provides an efficient and high-resolution measurement tool for laser tailfield accelerated X-ray research, promoting the development of ultrafast science and plasma physics. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 This is a top view of the high-resolution X-ray spectrometer used for laser tailfield acceleration of the present invention.

[0045] In the figure: 1-front beryllium window; 2-rear beryllium window; 3-focusing mirror; 4-concave flat-field grating; 5-black box; 6-IP plate DETAILED DESCRIPTION

[0046] The present invention will be further described below with reference to the embodiments and drawings, but the scope of protection of the present invention shall not be limited thereby.

[0047] Please see first Figure 1 , Figure 1 This is a top view of the high-resolution X-ray spectrometer for laser tailfield acceleration according to the present invention. As shown in the figure, it includes a vacuum chamber consisting of a chamber wall and front and rear beryllium windows. The dimensions are 250mm×100mm×100mm. A front beryllium window 1 and a rear beryllium window 2 are provided on the front and rear surfaces. The beryllium windows are made of metallic beryllium, have a thickness of 10mm and a diameter of 20mm, and their center is the geometric center of the front and rear surfaces.

[0048] The breadboard base measures 320 mm × 120 mm and is fixed to the center of the bottom surface of the vacuum chamber. The surface is arrayed with M6 threaded holes (25.4 mm spacing, arranged in a square pattern) for mechanical positioning of optical components.

[0049] The front surface of the focusing mirror 3 is perpendicular to the optical axis. The focusing mirror has a diameter of 60 mm and a focal length of 100 mm. Its optical axis is in the direction of the incident light and is 100 mm away from the front beryllium window. It is used to focus the incident X-rays onto the flat field grating.

[0050] The size of the concave flat-field grating 4 is 120 mm × 160 mm, its average grating period is 60 nm, the radius of curvature is 200 mm, it is 100 mm away from the focusing lens, and the angle between the grating plane and the optical axis is 1 degree, achieving diffraction focusing.

[0051] The black box 5 is fixed behind the rear beryllium window, with dimensions of 40mm×10mm×30mm. Its rear surface is detachable, and an IP plate 6 with dimensions of 35mm×22mm is attached to the front of the rear surface. Its geometric center is the center of the rear beryllium window 2.

[0052] The center of the front beryllium window 1, the center of the focusing mirror 3, the center of the concave flat field grating 4, the center of the rear beryllium window 2 and the center of the IP plate 6 are all at the same height and located on the optical axis;

[0053] The X-ray source generated by laser wake field acceleration passes through the front beryllium window 1, focusing mirror 3, deflected by the concave flat field grating 4, and reaches the IP plate 6 through the rear beryllium window 2, realizing the integration of energy dispersion and spatial focusing.

[0054] The working process of this embodiment:

[0055] Align the optical axis of the spectrometer with the X-rays generated by laser wakefield acceleration, without placing the sample in front of the front beryllium window 1. The X-rays enter the spectrometer through the front beryllium window 1, are first focused by the focusing mirror 3, eliminate other stray light and background light, and are collimated. They are then diffracted and dispersed by the concave flat-field grating 4, and converge on the IP plate 6 after passing through the rear beryllium window 2. After multiple irradiations, the IP plate 6 is stimulated by an external laser and processed by a computer to obtain the spectrum distribution of the X-rays generated by the laser wakefield acceleration.

[0056] The optical axis of the spectrometer is aligned with the X-rays generated by the laser wakefield acceleration, and a sample is placed in front of the front beryllium window 1; the X-rays are selectively absorbed by the sample and then enter the spectrometer through the front beryllium window 1, first being focused by the focusing mirror 3 to eliminate other stray light and background light and be collimated, then being diffracted and dispersed and focused by the concave flat-field grating 4, and then converging on the IP plate 6 after passing through the rear beryllium window. After waiting for multiple irradiations, an external laser is applied to excite the IP plate 6 and the spectrum distribution of the X-rays generated by the laser wakefield acceleration selectively absorbed by the sample is obtained through computer processing. This is then compared with the above-mentioned sample without a spectrum placed to obtain the corresponding X-ray absorption spectrum of the sample, thereby obtaining information such as the elemental composition, atomic structure and spatial distribution of the sample.

[0057] Experiments show that the present invention solves the shortcoming that other current flat-field grating spectrometers cannot be applied to X-ray scenarios generated by laser wakefield acceleration by using a concave flat-field grating and an IP plate. The spectrometer operates in the soft X-ray band with a wavelength range of 1.05nm to 5nm and a resolution of 1000 near 2nm.

Claims

1. A high-resolution X-ray spectrometer for laser wakefield acceleration, characterized in that: include: A vacuum chamber, wherein a front beryllium window (1) and a rear beryllium window (2) are provided on the front and rear surfaces thereof respectively; A focusing mirror (3) and a concave flat field grating (4) are arranged in the vacuum chamber; a black box (5) arranged outside the rear beryllium window (2), wherein an IP board (6) is arranged inside the black box (5); The centers of the front beryllium window (1), the focusing mirror (3), the concave flat field grating (4), the rear beryllium window (2) and the IP plate (6) are all located on the same optical axis and have the same height. X-rays are incident from the front beryllium window (1), focused by the focusing mirror (3), diffracted and split by the concave flat field grating (4), emitted from the rear beryllium window (2), and focused onto the IP plate (6).

2. The high-resolution X-ray spectrometer for laser wakefield acceleration according to claim 1, characterized in that: The IP plate (6) is a light-stimulated luminescent imaging plate, and its fluorescent material is BaFBr:Eu 2 +, can excite 390nm fluorescence signals through 633nm laser scanning, and realize the cumulative storage and readout of X-ray photon signals.

3. The high-resolution X-ray spectrometer for laser wakefield acceleration according to claim 1, characterized in that: The black box (5) is used to shield ambient light interference, and its rear cover is detachable, making it easy to replace the IP board (6).

4. The high-resolution X-ray spectrometer for laser wakefield acceleration according to claim 1, characterized in that: The concave flat-field grating (4) is used for diffracting and focusing X-rays, and the resolution reaches 1000 at a wavelength of about 2 nm.

5. The high-resolution X-ray spectrometer for laser wakefield acceleration according to any one of claims 1 to 4, characterized in that: It also includes a breadboard fixed to the bottom of the vacuum chamber, which is used to fix the focusing mirror (3) and the concave flat field grating (4).

6. The high-resolution X-ray spectrometer for laser wakefield acceleration according to claim 5, characterized in that: The concave flat-field grating (4) has a size of 120 mm×160 mm, an average grating period of 60 nm, a curvature radius of 200 mm, is 100 mm away from the focusing mirror (3), and the angle between the plane where it is located and the optical axis is 1°.

7. A method for measuring X-ray absorption spectrum, using the high-resolution X-ray spectrometer according to any one of claims 1 to 6, characterized in that: The following steps are involved: Aim the spectrometer at the X-ray source generated by laser tailfield acceleration; Measure the original Betatron radiation spectrum without sample; A sample is placed in front of the front beryllium window (1) and the absorption spectrum of X-rays after passing through the sample is measured; By comparing the two measurement results, the X-ray absorption characteristics of the sample are obtained and its elemental composition and structural information are analyzed.

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