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X-ray Optics|5 Article(s)
Research on Accuracy of Photon Arrival Time Labeling Based on APD Single Photon Detector
Baoquan LI, Fan LI, Yang CAO, and Peng SANG
X-ray pulsar navigation technology uses pulsars as navigation beacons, and multiple pulsars with orderly distribution in space are selected to form a function similar to the existing navigation satellite network. X-ray detectors receive highly stable pulse signals emitted by distant pulsars, record photon arrival time, and then determine the position, attitude, speed and time of spacecraft through algorithms to provide navigation information for spacecraft and realize autonomous navigation. Accurate measurement of photon arrival time is a very important part of X-ray pulsar navigation, and photon arrival time accuracy is an important factor affecting the navigation accuracy. Therefore, a measuring system for photon arrival time accuracy of X-ray detector is established. The system mainly consisted of pulse X-ray generator, arbitrary waveform generator, Avalanche Photodiode (APD) and time-marked photon counter. The pulse X-ray generator consists of X-ray modulation tube and high-voltage and control circuit. In the X-ray modulation tube, the cathode filament is energized and heated to release electrons, which are accelerated at high anode pressure, the modulating electrode adjusts the number of electrons passing through, the focusing pole focuses the electron beam, and finally the electron beam hits the anode target to produce X-rays. Among them, the electric field formed by the modulating pole voltage is equivalent to the external electric field of the hot cathode. When the modulator voltage is negative, the electron beam can be blocked from passing through the modulator. After the negative voltage gradually increases, the electron beam is completely blocked and cannot stimulate the X-ray. This voltage is the cut-off voltage, which is -3 V after test. The arbitrary waveform generator generates a pulse control signal to control the modulation pole of the X-ray modulation tube. When an arbitrary waveform generator controls the pulse X-ray generator to simulate the pulsar signal, the control pulse with corresponding pulse profile can be generated. In this experiment, in order to simplify the experiment, the function of arbitrary waveform generator is replaced by a signal generator, and the pulse control signal is set as a narrow pulse varying from -3 V to 1 V, with a pulse width of 25 ns. When the modulation pole voltage is 1 V, the pulse X-ray generator can normally emit X-ray photons. When the modulation pole voltage is -3 V, no X-ray photon is produced. The narrow pulse waveform is designed to ensure that only one photon or a certain probability of no photon is generated in an X-ray modulation tube in a pulse period, so as to avoid the occurrence of multiple photons and reduce the error interference. At the same time, the narrow pulse can also ensure that the X-ray photons can be concentrated at the same time during the very short time when the X-ray modulation tube is on, so as to avoid the scattering of photon emission time. The APD detector receives the X-ray photon signal, each photon received generates a negative pulse with a pulse width of about 20 ns and a signal amplitude of about 150 mV, and the output pulse is connected to the time-marked photon counter. The time-marked photon counter measures the time delay between the control pulse signal generated by the arbitrary waveform generator and the measured output signal of the APD detector. Then the distribution of delay is studied, and the results show that compared with the control signal, the time delay of the APD output signal is about 9.03 ns, and the standard deviation is 2.23 ns, that is, the photon arrival time precision of the APD is 2.23 ns. The results show that the APD can realize the fast time response and high precision labeling of the single X-ray photon. X-ray pulsar navigation technology uses pulsars as navigation beacons, and multiple pulsars with orderly distribution in space are selected to form a function similar to the existing navigation satellite network. X-ray detectors receive highly stable pulse signals emitted by distant pulsars, record photon arrival time, and then determine the position, attitude, speed and time of spacecraft through algorithms to provide navigation information for spacecraft and realize autonomous navigation. Accurate measurement of photon arrival time is a very important part of X-ray pulsar navigation, and photon arrival time accuracy is an important factor affecting the navigation accuracy. Therefore, a measuring system for photon arrival time accuracy of X-ray detector is established. The system mainly consisted of pulse X-ray generator, arbitrary waveform generator, Avalanche Photodiode (APD) and time-marked photon counter. The pulse X-ray generator consists of X-ray modulation tube and high-voltage and control circuit. In the X-ray modulation tube, the cathode filament is energized and heated to release electrons, which are accelerated at high anode pressure, the modulating electrode adjusts the number of electrons passing through, the focusing pole focuses the electron beam, and finally the electron beam hits the anode target to produce X-rays. Among them, the electric field formed by the modulating pole voltage is equivalent to the external electric field of the hot cathode. When the modulator voltage is negative, the electron beam can be blocked from passing through the modulator. After the negative voltage gradually increases, the electron beam is completely blocked and cannot stimulate the X-ray. This voltage is the cut-off voltage, which is -3 V after test. The arbitrary waveform generator generates a pulse control signal to control the modulation pole of the X-ray modulation tube. When an arbitrary waveform generator controls the pulse X-ray generator to simulate the pulsar signal, the control pulse with corresponding pulse profile can be generated. In this experiment, in order to simplify the experiment, the function of arbitrary waveform generator is replaced by a signal generator, and the pulse control signal is set as a narrow pulse varying from -3 V to 1 V, with a pulse width of 25 ns. When the modulation pole voltage is 1 V, the pulse X-ray generator can normally emit X-ray photons. When the modulation pole voltage is -3 V, no X-ray photon is produced. The narrow pulse waveform is designed to ensure that only one photon or a certain probability of no photon is generated in an X-ray modulation tube in a pulse period, so as to avoid the occurrence of multiple photons and reduce the error interference. At the same time, the narrow pulse can also ensure that the X-ray photons can be concentrated at the same time during the very short time when the X-ray modulation tube is on, so as to avoid the scattering of photon emission time. The APD detector receives the X-ray photon signal, each photon received generates a negative pulse with a pulse width of about 20 ns and a signal amplitude of about 150 mV, and the output pulse is connected to the time-marked photon counter. The time-marked photon counter measures the time delay between the control pulse signal generated by the arbitrary waveform generator and the measured output signal of the APD detector. Then the distribution of delay is studied, and the results show that compared with the control signal, the time delay of the APD output signal is about 9.03 ns, and the standard deviation is 2.23 ns, that is, the photon arrival time precision of the APD is 2.23 ns. The results show that the APD can realize the fast time response and high precision labeling of the single X-ray photon.
Acta Photonica Sinica
- Publication Date: Jul. 25, 2023
- Vol. 52, Issue 7, 0734003 (2023)
High-efficiency Reconstruction Algorithm of Source Translation Scan Computed Tomography for High-voltage Cable
Song NI, Jie CHEN, Haijun YU, Xiaojiao DUAN, Dabing CHEN, and Jianjun LIU
Cross-linked Polyethylene (XLPE) cable is widely used in urban power supply systems because of its excellent performance. Its inner structure-the water-blocking buffer layer-is prone to suffer from ablation defects after a long time of working, which will threaten the safety of the city's power supply. Therefore, the accurate detection of such cable defects is an urgent problem to be solved. In our previous work, we proposed a Local Source-translation Computed Tomography (L-STCT) method for cable detection. This method focuses on scanning part of the cable around the water-blocking buffer layer, and can intuitively observe the internal structure and identify defects with the reconstructed image. However, the used reconstruction method-Simultaneous Iterative Reconstruction Technique (SIRT)-requires massive iterations for final images, lowering reconstruction efficiency. To meet the high-efficiency reconstruction in engineering applications, we propose a Local-detecting Filtered Backprojection (LFBP) method in this paper, as the analytical method is much fast for image reconstruction. The typical analytic algorithm-Filtered Backprojection (FBP)-requires complete and detruncated projection for artifacts-free reconstruction. As L-STCT scans only part of the cable, the X-ray beam cannot cover the entire object, resulting in projection truncation. At the same time, limited source trajectory and detector width cannot achieve at least 180° angular coverage, resulting in projection missing. The L-STCT scan suffers from the problems of projection truncation and limited-angle computed tomography. Performing analytic reconstruction with raw incomplete and truncated projection will produce severe artifacts, affecting defect identification. To suppress the potential artifacts, the projection smoothing method is introduced according to projection characteristics. Projection smoothing mainly uses the cosine function to interpolate data along the detector direction and source sampling direction respectively. At both ends of the detector, the new part of virtual data gradually smoothing to 0 is splicing with the original data, avoiding data dropping to 0 suddenly and alleviating projection truncation. Similarly, starting from the projection sampled at the start and end source sampling points, smoothing is carried out on the part of the original projection collected near the two ends of the source sampling trajectory. The processed data gradually smooth to 0 along the two ends of the X-ray source trajectory to suppress limited-angle artifacts. In the experiment, we respectively used SIRT, FBP, and LFBP methods to reconstruct the image with the same set of projections, and calculated the SSIM and RMSE values to evaluate the image quality quantitatively. The results show that images reconstructed by the FBP method suffer from severe limited-angle and truncated artifacts, which interfere with the basic structure of the cable and potential defect identification. The images reconstructed by SIRT and LFBP are similar in quality, as both methods suppressed artifacts and clarified the basic structure of the cable. A lower RMSE and higher SSIM mean better image quality. Obviously, for ?100 mm cable imaging, the values of RMSE and SSIM are 0.117 0 and 0.923 8 for SIRT, which is close to that of LFBP (0.130 3 and 0.917 2). It proves that the image quality is comparable for both methods. By contrast, a higher RMSE (0.191 6) and lower SSIM (0.657 7) further indicates that artifacts greatly degrade reconstructed image for FBP. A similar result also can be found for ?160 mm cable imaging. For computational consumption, LFBP costs 1.38 s, FBP costs 1.42 s, and SIRT costs 62.46 s. Due to the added additional projection in the projection smoothing process, LFBP reconstruction is slightly longer than that of FBP but can be completely negligible. As the SIRT method gets the final image after 500 iterations, its reconstruction speed is much lower than the other two methods. Therefore, the LFBP method takes into account the reconstruction image quality and reconstruction efficiency, which is more suitable for practical engineering applications. Cross-linked Polyethylene (XLPE) cable is widely used in urban power supply systems because of its excellent performance. Its inner structure-the water-blocking buffer layer-is prone to suffer from ablation defects after a long time of working, which will threaten the safety of the city's power supply. Therefore, the accurate detection of such cable defects is an urgent problem to be solved. In our previous work, we proposed a Local Source-translation Computed Tomography (L-STCT) method for cable detection. This method focuses on scanning part of the cable around the water-blocking buffer layer, and can intuitively observe the internal structure and identify defects with the reconstructed image. However, the used reconstruction method-Simultaneous Iterative Reconstruction Technique (SIRT)-requires massive iterations for final images, lowering reconstruction efficiency. To meet the high-efficiency reconstruction in engineering applications, we propose a Local-detecting Filtered Backprojection (LFBP) method in this paper, as the analytical method is much fast for image reconstruction. The typical analytic algorithm-Filtered Backprojection (FBP)-requires complete and detruncated projection for artifacts-free reconstruction. As L-STCT scans only part of the cable, the X-ray beam cannot cover the entire object, resulting in projection truncation. At the same time, limited source trajectory and detector width cannot achieve at least 180° angular coverage, resulting in projection missing. The L-STCT scan suffers from the problems of projection truncation and limited-angle computed tomography. Performing analytic reconstruction with raw incomplete and truncated projection will produce severe artifacts, affecting defect identification. To suppress the potential artifacts, the projection smoothing method is introduced according to projection characteristics. Projection smoothing mainly uses the cosine function to interpolate data along the detector direction and source sampling direction respectively. At both ends of the detector, the new part of virtual data gradually smoothing to 0 is splicing with the original data, avoiding data dropping to 0 suddenly and alleviating projection truncation. Similarly, starting from the projection sampled at the start and end source sampling points, smoothing is carried out on the part of the original projection collected near the two ends of the source sampling trajectory. The processed data gradually smooth to 0 along the two ends of the X-ray source trajectory to suppress limited-angle artifacts. In the experiment, we respectively used SIRT, FBP, and LFBP methods to reconstruct the image with the same set of projections, and calculated the SSIM and RMSE values to evaluate the image quality quantitatively. The results show that images reconstructed by the FBP method suffer from severe limited-angle and truncated artifacts, which interfere with the basic structure of the cable and potential defect identification. The images reconstructed by SIRT and LFBP are similar in quality, as both methods suppressed artifacts and clarified the basic structure of the cable. A lower RMSE and higher SSIM mean better image quality. Obviously, for ?100 mm cable imaging, the values of RMSE and SSIM are 0.117 0 and 0.923 8 for SIRT, which is close to that of LFBP (0.130 3 and 0.917 2). It proves that the image quality is comparable for both methods. By contrast, a higher RMSE (0.191 6) and lower SSIM (0.657 7) further indicates that artifacts greatly degrade reconstructed image for FBP. A similar result also can be found for ?160 mm cable imaging. For computational consumption, LFBP costs 1.38 s, FBP costs 1.42 s, and SIRT costs 62.46 s. Due to the added additional projection in the projection smoothing process, LFBP reconstruction is slightly longer than that of FBP but can be completely negligible. As the SIRT method gets the final image after 500 iterations, its reconstruction speed is much lower than the other two methods. Therefore, the LFBP method takes into account the reconstruction image quality and reconstruction efficiency, which is more suitable for practical engineering applications.
Acta Photonica Sinica
- Publication Date: Jul. 25, 2023
- Vol. 52, Issue 7, 0734002 (2023)
Design and Experimental Study of a Desktop Monochromatic X-ray Source Based on Spherical Bent-crystal Focusing Structure
Yue YU, Haoxuan SI, Zuhua YANG, Shengzhen YI, and Zhanshan WANG
X-ray optics and systems have been widely used for applications such as plasma diagnostics, X-ray spectroscopy, astronomical observation and material analysis. The spectral characteristics of the optics and systems need to be accurately measured, for which a high-brightness monochromatic X-ray source is indispensable. Synchrotron radiation facilities provide the most reliable metrology, but their measurement timeliness is limited because of the allocated user beam time and long distances to reach the facilities. Commercial X-ray measurement equipment, such as X-ray fluorescence spectrometers, X-ray reflectometers and diffractometers, are only capable of measurements of small-size optics at a few fixed energy points. Desktop monochromatic X-ray sources that combine X-ray tubes with dispersive optics have provided a laboratory-based approach to calibrating X-ray optics, where the dispersive optics usually refers to multilayers, gratings and crystals. Multilayers are distinguished by their high luminous flux and wide working energy band, but their spectral resolution is relatively low; X-ray gratings allow for much higher spectral resolution, but the fabrication processes are quite difficult for extremely dense grooves for X-ray diffraction. Crystals have a naturally unique dispersion capability in the X-ray band that can substantially improve the spectral resolution. Common-used planar and cylindrical crystals have limited effects to enhance the system throughput because of relatively weak focusing capability in one or both dimensions; complex aspheric bent-crystals, such as toroidal, parabolic and hyperbolic crystals, can reduce aberrations and focus in both meridional and sagittal directions so that system throughput and spectral resolution can be significantly improved. However, aspheric bent-crystals are difficult to fabricate and have many limitations for application; and the aberration would deteriorate abruptly if the incident angle deviates from the designed value. Spherically bent-crystals enable two-dimensional focusing and are relatively easy to be fabricated; therefore, they have been widely used in X-ray spectrometers. Conventional spherical bend-crystal spectrometers adopt the Johann scheme, where the light source and the image plane are located on a Rowland circle with a diameter equaling to the radius of the concave bend-crystal. By applying Johann scheme, better focusing and higher spectral resolution are obtained in the meridional direction, but the brightness of the spectral lines acquired in the image plane is largely decreased due to light divergence in the sagittal direction. Therefore, it is necessary to optimize the optical structure to improve the intensity of dispersed monochromatic X-ray. In this paper, a desktop method for obtaining high-brightness monochromatic X-ray is proposed based on a spherical bent-crystal focusing structure. It achieves monochromaticity in the meridional direction using crystal dispersion, and obtains high brightness in the sagittal direction simultaneously based on spherical-mirror focusing. A geometric model is developed to theoretically compare the efficiency of collecting light of the spherical and cylindrical bent-crystals; also, the dispersion and focusing performance of different optical layouts with spherical and cylindrical bent-crystals are simulated and evaluated. Both theoretical and simulation results show that the spherical bent-crystal focusing structure has an excellent performance in terms of focusing characteristics, with the brightness of the spectral line obviously increased, compared with the cylindrical bent-crystal. In view of the monochromatizing requirements of the Al Kα1 line, a device based on low-power Al-target X-ray tube is designed and adjusted, and its performance was tested by spectroscopic experiments. Measurement results show that with the X-ray tube working at 7 W power and CCD exposure for 10 minutes, the count of the full-field spectrum of the Al Kα1 line is greater than 2×105 with a spectral broadening of about 0.592 eV; when introducing a 200 μm beam-limiting slit, the spectral broadening is further reduced to 0.493 eV and the CCD count is about 2×104. The results have verified that the device can effectively obtain the high-brightness Al Kα1 line, and also provides a new technical approach for accurately measuring the spectral characteristics of X-ray optics and systems. X-ray optics and systems have been widely used for applications such as plasma diagnostics, X-ray spectroscopy, astronomical observation and material analysis. The spectral characteristics of the optics and systems need to be accurately measured, for which a high-brightness monochromatic X-ray source is indispensable. Synchrotron radiation facilities provide the most reliable metrology, but their measurement timeliness is limited because of the allocated user beam time and long distances to reach the facilities. Commercial X-ray measurement equipment, such as X-ray fluorescence spectrometers, X-ray reflectometers and diffractometers, are only capable of measurements of small-size optics at a few fixed energy points. Desktop monochromatic X-ray sources that combine X-ray tubes with dispersive optics have provided a laboratory-based approach to calibrating X-ray optics, where the dispersive optics usually refers to multilayers, gratings and crystals. Multilayers are distinguished by their high luminous flux and wide working energy band, but their spectral resolution is relatively low; X-ray gratings allow for much higher spectral resolution, but the fabrication processes are quite difficult for extremely dense grooves for X-ray diffraction. Crystals have a naturally unique dispersion capability in the X-ray band that can substantially improve the spectral resolution. Common-used planar and cylindrical crystals have limited effects to enhance the system throughput because of relatively weak focusing capability in one or both dimensions; complex aspheric bent-crystals, such as toroidal, parabolic and hyperbolic crystals, can reduce aberrations and focus in both meridional and sagittal directions so that system throughput and spectral resolution can be significantly improved. However, aspheric bent-crystals are difficult to fabricate and have many limitations for application; and the aberration would deteriorate abruptly if the incident angle deviates from the designed value. Spherically bent-crystals enable two-dimensional focusing and are relatively easy to be fabricated; therefore, they have been widely used in X-ray spectrometers. Conventional spherical bend-crystal spectrometers adopt the Johann scheme, where the light source and the image plane are located on a Rowland circle with a diameter equaling to the radius of the concave bend-crystal. By applying Johann scheme, better focusing and higher spectral resolution are obtained in the meridional direction, but the brightness of the spectral lines acquired in the image plane is largely decreased due to light divergence in the sagittal direction. Therefore, it is necessary to optimize the optical structure to improve the intensity of dispersed monochromatic X-ray. In this paper, a desktop method for obtaining high-brightness monochromatic X-ray is proposed based on a spherical bent-crystal focusing structure. It achieves monochromaticity in the meridional direction using crystal dispersion, and obtains high brightness in the sagittal direction simultaneously based on spherical-mirror focusing. A geometric model is developed to theoretically compare the efficiency of collecting light of the spherical and cylindrical bent-crystals; also, the dispersion and focusing performance of different optical layouts with spherical and cylindrical bent-crystals are simulated and evaluated. Both theoretical and simulation results show that the spherical bent-crystal focusing structure has an excellent performance in terms of focusing characteristics, with the brightness of the spectral line obviously increased, compared with the cylindrical bent-crystal. In view of the monochromatizing requirements of the Al Kα1 line, a device based on low-power Al-target X-ray tube is designed and adjusted, and its performance was tested by spectroscopic experiments. Measurement results show that with the X-ray tube working at 7 W power and CCD exposure for 10 minutes, the count of the full-field spectrum of the Al Kα1 line is greater than 2×105 with a spectral broadening of about 0.592 eV; when introducing a 200 μm beam-limiting slit, the spectral broadening is further reduced to 0.493 eV and the CCD count is about 2×104. The results have verified that the device can effectively obtain the high-brightness Al Kα1 line, and also provides a new technical approach for accurately measuring the spectral characteristics of X-ray optics and systems.
Acta Photonica Sinica
- Publication Date: Jul. 25, 2023
- Vol. 52, Issue 7, 0734001 (2023)
Theoretical and Experimental Research of X-ray Communication in Shielded Environment
Xuehan ZHANG, Tong SU, Lizhi SHENG, Yongan LIU, and Baosheng ZHAO
Based on the theoretical model of the interaction between X-rays and matter, the transmission characteristics of X-rays in an electromagnetic shielding environment are studied, and the feasibility of X-rays communication for information transmission in an electromagnetic shielding environment is theoretically demonstrated. After that, a numerical simulation model of X-ray communication in a shielded environment is established, and the communication parameters of X-rays in an electromagnetic shielded environment are analyzed to achieve the index constraint of the core components. Finally, based on the key technologies of X-ray modulated emission and single-photon X-ray detection, an equivalent verification experiment of X-ray passing through the shielding material is conducted to realize the experimental verification of X-ray communication with a communication rate better than 23 kbps. The results are expected to provide some theoretical basis and experimental foundation for solving the radiation data transmission in a shielded environment. Based on the theoretical model of the interaction between X-rays and matter, the transmission characteristics of X-rays in an electromagnetic shielding environment are studied, and the feasibility of X-rays communication for information transmission in an electromagnetic shielding environment is theoretically demonstrated. After that, a numerical simulation model of X-ray communication in a shielded environment is established, and the communication parameters of X-rays in an electromagnetic shielded environment are analyzed to achieve the index constraint of the core components. Finally, based on the key technologies of X-ray modulated emission and single-photon X-ray detection, an equivalent verification experiment of X-ray passing through the shielding material is conducted to realize the experimental verification of X-ray communication with a communication rate better than 23 kbps. The results are expected to provide some theoretical basis and experimental foundation for solving the radiation data transmission in a shielded environment.
Acta Photonica Sinica
- Publication Date: Nov. 25, 2021
- Vol. 50, Issue 11, 1134002 (2021)
Theoretical and Technical Research of X-ray Communication in Plasma Sheath
Tong SU, Lizhi SHENG, Yongan LIU, Xuehan ZHANG, Yifan LIU, and Baosheng ZHAO
Because of the imperfect theoretical model and insufficient experimental verification technologies, the problem of information transmission in the plasma sheath has not yet been resolved. In this paper, the interaction mechanism between X-ray photons and plasma is studied firstly, and a modified theoretical model is provided through numerical calculation and theoretical modeling. Different from the conclusion that X-rays can penetrate plasma without attenuation in the traditional wave model, the modified theoretical model established in this paper points out that the transmittance of X-rays in plasma is closely related to plasma electron density and incident X-ray flux. Secondly, an experimental system was built using a grid-controlled X-ray modulation emission source, a single-photon X-ray detector, and a dynamic plasma generator. Using this system, the non-uniform plasma which electron density ranges from 109/cm3 to 1014/cm3 is generated, and the X-ray communication with 1 Mbps communication rante and 10-5 bit error rate is also verified. The experimental results indicate that the modified theoretical model can explain and predict the experimental phenomena, and the experimental system can provide the solution for solving the communication problems in the plasma sheath. Because of the imperfect theoretical model and insufficient experimental verification technologies, the problem of information transmission in the plasma sheath has not yet been resolved. In this paper, the interaction mechanism between X-ray photons and plasma is studied firstly, and a modified theoretical model is provided through numerical calculation and theoretical modeling. Different from the conclusion that X-rays can penetrate plasma without attenuation in the traditional wave model, the modified theoretical model established in this paper points out that the transmittance of X-rays in plasma is closely related to plasma electron density and incident X-ray flux. Secondly, an experimental system was built using a grid-controlled X-ray modulation emission source, a single-photon X-ray detector, and a dynamic plasma generator. Using this system, the non-uniform plasma which electron density ranges from 109/cm3 to 1014/cm3 is generated, and the X-ray communication with 1 Mbps communication rante and 10-5 bit error rate is also verified. The experimental results indicate that the modified theoretical model can explain and predict the experimental phenomena, and the experimental system can provide the solution for solving the communication problems in the plasma sheath.
Acta Photonica Sinica
- Publication Date: Nov. 25, 2021
- Vol. 50, Issue 11, 1134001 (2021)
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