Wang Xun, Zhang Feng-Qi, Chen Wei, Guo Xiao-Qiang, Ding Li-Li, and Luo Yin-Hong

The experiment of neutron single event effect was carried out at China Spallation Neutron Source (CSNS) back-n on 13 kinds of commercial SRAM. The single event upset (SEU) cross section of each device was obtained, and multiple cell upsets (MCU) were extracted from the SEUs using a statistical method without layout information. The influences of the test pattern, feature size and device layout on the SEU cross section and MCU were studied. The results show that the test pattern has little influence on the SEU cross section of the devices, but has a great influence on the MCU ratio of some devices. The feature size has influence both on the SEU cross section and the MCU ratio of the devices. The influence on SEU cross section is not definite. The influence on the MCU ratio is definite. Both the ratio and the maximum size of the MCUs increase with the decrease of the feature size. The difference of layout has great influence both on the SEU cross section and the MCU ratio of the device. In addition, compared with the results of plateau irradiation, the ratio of MCU in CSNS back-n is less than that of plateau irradiation. There are two reasons for this difference. One is that the energy spectrum of CSNS back-n is softer than that of the atmospheric neutron. The other is the neutron beam at CSNS back-n is perpendicular to the device under test. Therefore, evaluating the atmospheric neutron SEE using CSNS back-n line may underestimate the MCU ratio of the device under test. The experimental data, analytical methods and results obtained in this paper are valuable for the researchers to carry out the atmospheric neutron SEE test and the evaluation of devices on atmospheric neutron SEE.The experiment of neutron single event effect was carried out at China Spallation Neutron Source (CSNS) back-n on 13 kinds of commercial SRAM. The single event upset (SEU) cross section of each device was obtained, and multiple cell upsets (MCU) were extracted from the SEUs using a statistical method without layout information. The influences of the test pattern, feature size and device layout on the SEU cross section and MCU were studied. The results show that the test pattern has little influence on the SEU cross section of the devices, but has a great influence on the MCU ratio of some devices. The feature size has influence both on the SEU cross section and the MCU ratio of the devices. The influence on SEU cross section is not definite. The influence on the MCU ratio is definite. Both the ratio and the maximum size of the MCUs increase with the decrease of the feature size. The difference of layout has great influence both on the SEU cross section and the MCU ratio of the device. In addition, compared with the results of plateau irradiation, the ratio of MCU in CSNS back-n is less than that of plateau irradiation. There are two reasons for this difference. One is that the energy spectrum of CSNS back-n is softer than that of the atmospheric neutron. The other is the neutron beam at CSNS back-n is perpendicular to the device under test. Therefore, evaluating the atmospheric neutron SEE using CSNS back-n line may underestimate the MCU ratio of the device under test. The experimental data, analytical methods and results obtained in this paper are valuable for the researchers to carry out the atmospheric neutron SEE test and the evaluation of devices on atmospheric neutron SEE.

- Jan. 04, 2021
- Acta Physica Sinica
- Vol.69 Issue, 16 162901-1 (2020)
- DOI：10.7498/aps.69.20200265

Cao Ying-Yu, and Guo Jian-You

Based on the existing experimental data of nuclear radius, the previous formula of nuclear charge radius is verified and discussed. Comparing the formula of the single-parameter nuclear charge radius, it is proved that the formula of $Z^{1/3}$ law is better than the formula of $A^{1/3}$ law. We refitted the two-parameter formula and the three-parameter formula that have been proposed and confirmed that the two-parameter and three-parameter formula fit better than the single-parameter formula. It is shown that show that the deformation plays a key role in the nuclear charge radius. The electric quadrupole moment is an important physical quantity representing the properties of the nucleus. Its appearance indicates the deviation from spherical symmetry and also reflects the size of the nuclear deformation. The electric quadrupole moment is also one of the basic observations to understand the distribution of matter within the nucleus, to examine the nuclear model, and to observe nucleon-nuclear interactions. Taking into account the relationship between the nuclear quadrupole moment and the deformation, the electric quadrupole moment factor is added to the original three-parameter formula to obtain a new formula for the nuclear charge radius. Fitting the four-parameter formula, it is found that the theoretical value of the nuclear charge radius is in good agreement with the experimental value, the root-mean-square deviation is 0.0397 fm. Considering the relationship between the total spin and the electric quadrupole moment, the intrinsic electric quadrupole moment is obtained and brought into the formula for fitting, and the root-mean-square deviation further decreases,the root-mean-square deviation is 0.0372 fm. Finally, considering the universality of odd-even staggering, we add the $\delta$ term that can reflect the odd and even oscillation phenomenon, and the root-mean-square deviation obtained by the formula is 0.369 fm, which better reflects the relationship between the deformation and the nuclear charge radius. Compared with the formulas already proposed, the new formula can better reflect the variation trend of nuclear deformation, shell effect, odd-even staggering, etc., and the calculation accuracy is also improved, which can provide a useful reference for future experiments.Based on the existing experimental data of nuclear radius, the previous formula of nuclear charge radius is verified and discussed. Comparing the formula of the single-parameter nuclear charge radius, it is proved that the formula of $Z^{1/3}$ ![]()

![]()

law is better than the formula of $A^{1/3}$ ![]()

![]()

law. We refitted the two-parameter formula and the three-parameter formula that have been proposed and confirmed that the two-parameter and three-parameter formula fit better than the single-parameter formula. It is shown that show that the deformation plays a key role in the nuclear charge radius. The electric quadrupole moment is an important physical quantity representing the properties of the nucleus. Its appearance indicates the deviation from spherical symmetry and also reflects the size of the nuclear deformation. The electric quadrupole moment is also one of the basic observations to understand the distribution of matter within the nucleus, to examine the nuclear model, and to observe nucleon-nuclear interactions. Taking into account the relationship between the nuclear quadrupole moment and the deformation, the electric quadrupole moment factor is added to the original three-parameter formula to obtain a new formula for the nuclear charge radius. Fitting the four-parameter formula, it is found that the theoretical value of the nuclear charge radius is in good agreement with the experimental value, the root-mean-square deviation is 0.0397 fm. Considering the relationship between the total spin and the electric quadrupole moment, the intrinsic electric quadrupole moment is obtained and brought into the formula for fitting, and the root-mean-square deviation further decreases,the root-mean-square deviation is 0.0372 fm. Finally, considering the universality of odd-even staggering, we add the $\delta$ ![]()

![]()

term that can reflect the odd and even oscillation phenomenon, and the root-mean-square deviation obtained by the formula is 0.369 fm, which better reflects the relationship between the deformation and the nuclear charge radius. Compared with the formulas already proposed, the new formula can better reflect the variation trend of nuclear deformation, shell effect, odd-even staggering, etc., and the calculation accuracy is also improved, which can provide a useful reference for future experiments.

- Jan. 04, 2021
- Acta Physica Sinica
- Vol.69 Issue, 16 162101-1 (2020)
- DOI：10.7498/aps.69.20191643

Deng Li, Li Rui, Wang Xin, and Fu Yuan-Guang

Monte Carlo method is an ideal way to simulate criticality, shielding and nuclear detection. JMCT is a multipurpose 3D Mont Carlo (MC) neutron-photon-electron and coupled neutron /photon /electron transport code which is developed by IAPCM. The program is developed based on the combinatorial geometry parallel infrastructure JCOGIN and has the most functions of general Monte Carlo particle transport code, including the various variance reduction techniques. In addition, some new algorithms, such as Doppler broadening on-the-fly (OTF), uniform tally density (UTD), consistent adjoint driven importance sampling (CADIS), fast criticality search of boron concentration (FCSBC), the domain decomposition (DD), the two-level parallel computation of MPI and OpenMP, etc. have been developed, where the number of geometry zones, materials, tallies, depletion zones, memories and period of random number are big enough to simulate various extremely complicated problems. Also the JMCT is hybrid the discrete ordinate SN program JSNT to generate source biasing factors and weight window parameters for deep-penetration shielding problems. The input is based on the CAD modeling, and the result is a visualized output. The JMCT can provide technology support for radiation shielding design, reactor physics and criticality safe analysis. Especially, the JMCT is coupled depletion and thermal-hydraulic code for simulating the reactor feedback effect, including depletion, thermal feedback. In recent years, new function of γ-ray spectrum analysis has been developed.In this paper, the working principles of timing measure are introduced. The advanced calibration count is developed for distinguishing between inelastic γ-ray and capture γ-ray based on time bin tally. On the other hand, when neutron collides with nuclide, the secondary photon is labeled into the primary line photon and primary continuous photon, where energy of primary line photon does not change with the incident neutron energy, such as carbon spectral-line at 4.43 MeV and oxygen spectral-line at 6.13 MeV. The element components of detected object can be determined by the primary line photon. On the other hand, expect value estimator (EVE) is used to produce the secondary photons. The advantage of EVE does not leak any event even with a small probability which is important for detecting the hide exploder. However the shortage of the EVE results in producing a great number of photons with small weight. If all of these small weight photons are simulated one by one, a great amount of computation time and memory will be consumed. For avoiding this case, a new algorithm is design by coupling EVE and DE (direct estimator). The all of secondary photons from EVE only make the direct tally take a little computing time, then end the photon history and return to the DE production photon model (one photon production at most). Final, the total tally is a summation of EVE direct tally and DE scattering tally. The use of new algorithm to realize the analysis of γ-ray spectrum will increase only a little computing time. The numerical tests are done by using own Monte Carlo code JMCT. The correctness and validity of the algorithm are shown preliminarily. Monte Carlo method is an ideal way to simulate criticality, shielding and nuclear detection. JMCT is a multipurpose 3D Mont Carlo (MC) neutron-photon-electron and coupled neutron /photon /electron transport code which is developed by IAPCM. The program is developed based on the combinatorial geometry parallel infrastructure JCOGIN and has the most functions of general Monte Carlo particle transport code, including the various variance reduction techniques. In addition, some new algorithms, such as Doppler broadening on-the-fly (OTF), uniform tally density (UTD), consistent adjoint driven importance sampling (CADIS), fast criticality search of boron concentration (FCSBC), the domain decomposition (DD), the two-level parallel computation of MPI and OpenMP, etc. have been developed, where the number of geometry zones, materials, tallies, depletion zones, memories and period of random number are big enough to simulate various extremely complicated problems. Also the JMCT is hybrid the discrete ordinate SN program JSNT to generate source biasing factors and weight window parameters for deep-penetration shielding problems. The input is based on the CAD modeling, and the result is a visualized output. The JMCT can provide technology support for radiation shielding design, reactor physics and criticality safe analysis. Especially, the JMCT is coupled depletion and thermal-hydraulic code for simulating the reactor feedback effect, including depletion, thermal feedback. In recent years, new function of γ-ray spectrum analysis has been developed. In this paper, the working principles of timing measure are introduced. The advanced calibration count is developed for distinguishing between inelastic γ-ray and capture γ-ray based on time bin tally. On the other hand, when neutron collides with nuclide, the secondary photon is labeled into the primary line photon and primary continuous photon, where energy of primary line photon does not change with the incident neutron energy, such as carbon spectral-line at 4.43 MeV and oxygen spectral-line at 6.13 MeV. The element components of detected object can be determined by the primary line photon. On the other hand, expect value estimator (EVE) is used to produce the secondary photons. The advantage of EVE does not leak any event even with a small probability which is important for detecting the hide exploder. However the shortage of the EVE results in producing a great number of photons with small weight. If all of these small weight photons are simulated one by one, a great amount of computation time and memory will be consumed. For avoiding this case, a new algorithm is design by coupling EVE and DE (direct estimator). The all of secondary photons from EVE only make the direct tally take a little computing time, then end the photon history and return to the DE production photon model (one photon production at most). Final, the total tally is a summation of EVE direct tally and DE scattering tally. The use of new algorithm to realize the analysis of γ-ray spectrum will increase only a little computing time. The numerical tests are done by using own Monte Carlo code JMCT. The correctness and validity of the algorithm are shown preliminarily.

- Dec. 02, 2020
- Acta Physica Sinica
- Vol.69 Issue, 11 112801-1 (2020)
- DOI：10.7498/aps.69.20200279

Li Qian-Li, Hu Ya-Hua, Ma Yi-Fan, Sun Zhi-Xiang, Wang Min, Liu Xiao-Lin, Zhao Jing-Tai, and Zhang Zhi-Jun

X-ray scintillation screens as the core component of X-ray imaging detectors have widespread applications in the medical imaging, security inspection, high energy physics, radiochemistry, and so on. For a long time, the development of X-ray scintillation screen mainly focuses on improving the light yield in order to enhance its detection efficiency. However, a novel tendency has recently emerged for ultrafast time performance of the X-ray imaging detector. The indium doping zinc oxide (ZnO:In) with high radiation hardness, higher light yield(>10000 photons/MeV) and subnanosecond decay time is a promising scintillation material for ultrafast detections. In order to satisfy the requirements of X-ray scintillation screens with ultrafast and high-spatial-resolution in the existing and upcoming high energy physics experiments, the ZnO:In nanorod arrays have been prepared on a 100-nm-thick ZnO-seeded substrate by hydrothermal reaction method and then treated by hydrogen plasma in present work. The results of SEM demonstrate the average diameter and length of the ZnO:In nanorods are about 0.5 and 12 μm, respectively. The XRD shows the ZnO:In nanorods are highly aligned perpendicular to the substrate alongc-axis direction. The X-ray excited luminescence spectra show that two luminescence bands are observed, i.e. an ultraviolet emission peak located at about 395 nm and a visible emission band at 450–750 nm. It is particularly important to point out that hydrogen plasma treatment can enhance the ultraviolet emission of ZnO:In nanorod arrays and suppress its visible emission. The reason is attributed to the formation of shallow donors through hydrogen entering the ZnO and the combination of VO and Oi. In addition, the fluorescence decay times of the ultraviolet and visible emissions for the ZnO:In nanorod arrays are subnanosecond and nanosecond, respectively, satisfying the demand of the fast X-ray imaging. The spatial resolution of ZnO:In nanorod arrays has been characterized in X-ray imaging beamline at the Shanghai Synchrotron Radiation Facility. Under excitation of the X-ray beam with the energy of 20 keV, a system spatial resolution of 1.5 μm could be achieved by using an 12 μm thickness ZnO:In nanorod arrays as the scintillation screen, which is exceeded the highest level had ever been reported on ZnO:In nanorod arrays scintillation screen. In conclusion, this present work shows that it is a feasible solution for X-ray detection and imaging with high temporal and spatial resolution by using ZnO:In nanorod arrays as the X-ray scintillation screen.X-ray scintillation screens as the core component of X-ray imaging detectors have widespread applications in the medical imaging, security inspection, high energy physics, radiochemistry, and so on. For a long time, the development of X-ray scintillation screen mainly focuses on improving the light yield in order to enhance its detection efficiency. However, a novel tendency has recently emerged for ultrafast time performance of the X-ray imaging detector. The indium doping zinc oxide (ZnO:In) with high radiation hardness, higher light yield(>10000 photons/MeV) and subnanosecond decay time is a promising scintillation material for ultrafast detections. In order to satisfy the requirements of X-ray scintillation screens with ultrafast and high-spatial-resolution in the existing and upcoming high energy physics experiments, the ZnO:In nanorod arrays have been prepared on a 100-nm-thick ZnO-seeded substrate by hydrothermal reaction method and then treated by hydrogen plasma in present work. The results of SEM demonstrate the average diameter and length of the ZnO:In nanorods are about 0.5 and 12 μm, respectively. The XRD shows the ZnO:In nanorods are highly aligned perpendicular to the substrate along*c*-axis direction. The X-ray excited luminescence spectra show that two luminescence bands are observed, i.e. an ultraviolet emission peak located at about 395 nm and a visible emission band at 450–750 nm. It is particularly important to point out that hydrogen plasma treatment can enhance the ultraviolet emission of ZnO:In nanorod arrays and suppress its visible emission. The reason is attributed to the formation of shallow donors through hydrogen entering the ZnO and the combination of V_{O} and O_{i}. In addition, the fluorescence decay times of the ultraviolet and visible emissions for the ZnO:In nanorod arrays are subnanosecond and nanosecond, respectively, satisfying the demand of the fast X-ray imaging. The spatial resolution of ZnO:In nanorod arrays has been characterized in X-ray imaging beamline at the Shanghai Synchrotron Radiation Facility. Under excitation of the X-ray beam with the energy of 20 keV, a system spatial resolution of 1.5 μm could be achieved by using an 12 μm thickness ZnO:In nanorod arrays as the scintillation screen, which is exceeded the highest level had ever been reported on ZnO:In nanorod arrays scintillation screen. In conclusion, this present work shows that it is a feasible solution for X-ray detection and imaging with high temporal and spatial resolution by using ZnO:In nanorod arrays as the X-ray scintillation screen.

- Nov. 30, 2020
- Acta Physica Sinica
- Vol.69 Issue, 10 102902-1 (2020)
- DOI：10.7498/aps.69.20200282

Zhang Mei, Li Kui-Nian, Li Yang, Sheng Liang, and Zhang Yan-Hong

Scintillating array image plates are allowed high resolution through a thicker detector which increases quantum efficiency without degrading the imaging resolution substantially. Due to limitations imposed by process capability, scintillator fiber array with pixel diameter less than 0.2 mm is hardly manufactured to improve performance. Therefore, a liquid scintillator capillary array with 0.1 mm pixel is developed to improve the detection efficiency and spatial resolution of image plate for low intensity radiation imaging. Its performances are studied and tested by simulation and experiment, and are compared with those of scintillating fiber array. Especially in order to gain high fidelity representation of modulation transfer function of the array image plate, a method of simulating and measuring the slanted knife edge response and an iterative algorithm are introduced. For 14 MeV neutron and 1.25 MeV gamma, the slanted knife edge responses of these array image plates with pixel dimensions in a range from 0.1 mm to 0.5 mm are respectively simulated by MCNPx program and the modulation transfer function (MTF) are obtained. The simulation results show that compared with scintillating fiber array, the liquid scintillator capillaries array has an obvious merit in spatial resolution because of greater stopping power for secondary charged particle in the capillary quartz glass wall with 0.02 mm in thickness. Its ultimate resolution can reach to 1.8 lp/mm for 14 MeV neutron by simulation. At the 4000 Ci 60Co facility, a 5-cm-thick tungsten bar, one side of which has a curvature of 0.1 radian to minimize the misalignment effect, is made a knife edge. The MTF of the scintillating fiber array with 0.3 mm and 0.5 mm pixel and newly developed liquid scintillator capillary array is measured through this tungsten knife edge. Experimental measurement results have also verified that the liquid scintillator capillary array performs well in spatial resolution and luminescent uniformity for 1.25 MeV gamma. The ultimate spatial resolution, 0.9 lp/mm is gained, and those of other scintillating fiber arrays are all less than 0.5 lp/mm. Moreover, experimental test validates the simulating method and simulated results, although the measured value is slight less than the simulated value because of the effect of dimension of 60Co source.Scintillating array image plates are allowed high resolution through a thicker detector which increases quantum efficiency without degrading the imaging resolution substantially. Due to limitations imposed by process capability, scintillator fiber array with pixel diameter less than 0.2 mm is hardly manufactured to improve performance. Therefore, a liquid scintillator capillary array with 0.1 mm pixel is developed to improve the detection efficiency and spatial resolution of image plate for low intensity radiation imaging. Its performances are studied and tested by simulation and experiment, and are compared with those of scintillating fiber array. Especially in order to gain high fidelity representation of modulation transfer function of the array image plate, a method of simulating and measuring the slanted knife edge response and an iterative algorithm are introduced. For 14 MeV neutron and 1.25 MeV gamma, the slanted knife edge responses of these array image plates with pixel dimensions in a range from 0.1 mm to 0.5 mm are respectively simulated by MCNPx program and the modulation transfer function (MTF) are obtained. The simulation results show that compared with scintillating fiber array, the liquid scintillator capillaries array has an obvious merit in spatial resolution because of greater stopping power for secondary charged particle in the capillary quartz glass wall with 0.02 mm in thickness. Its ultimate resolution can reach to 1.8 lp/mm for 14 MeV neutron by simulation. At the 4000 Ci ^{60}Co facility, a 5-cm-thick tungsten bar, one side of which has a curvature of 0.1 radian to minimize the misalignment effect, is made a knife edge. The MTF of the scintillating fiber array with 0.3 mm and 0.5 mm pixel and newly developed liquid scintillator capillary array is measured through this tungsten knife edge. Experimental measurement results have also verified that the liquid scintillator capillary array performs well in spatial resolution and luminescent uniformity for 1.25 MeV gamma. The ultimate spatial resolution, 0.9 lp/mm is gained, and those of other scintillating fiber arrays are all less than 0.5 lp/mm. Moreover, experimental test validates the simulating method and simulated results, although the measured value is slight less than the simulated value because of the effect of dimension of ^{60}Co source.

- Nov. 19, 2020
- Acta Physica Sinica
- Vol.69 Issue, 6 062801-1 (2020)
- DOI：10.7498/aps.69.20191545

Luo Duan, Hui Dan-Dan, Wen Wen-Long, Li Li-Li, Xin Li-Wei, Zhong Zi-Yuan, Ji Chao, Chen Ping, He Kai, Wang Xing, and Tian Jin-Shou

One of the grand challenges in ultrafast science is real-time visualization of the microscopic structural evolution on atomic time and length scales. A promising pump-probe technique using a femtosecond laser pulse to initiate the ultrafast dynamics and another ultrashort electron pulse to probe the resulting changes has been developed and widely used to study ultrafast structural dynamics in chemical reactions, phase transitions, charge density waves, and even biological functions. In the past three decades, a number of different ultrafast electron guns have been developed to generate ultashort electron sources, mainly including hybrid electron gun with radio-frequency (RF) cavities for compressing the pulse broadening, relativistic electron gun for suppressing the coulomb interaction, single-electron pulses without space charge effect and compact direct current (DC) electron gun for minimizing the electron propagation distance. At present, these developments with different final electron energy and available total charge have improved the time response of ultrafast electron diffraction (UED) setups to a new frontier approaching to 100 fs regime. Although enormous efforts have been made, the superior capabilities and potentials of ultrafast electron diffraction (UED) are still hindered by space-charge induced pulse broadening. Besides, the penetration depth of electrons increases with the electron energy, while the scattering probability of electrons has the opposite consequence. Thus, in addition to the temporal resolution enhancement, it is also important that the electron energy should be tunable in a wide range to meet the requirements for samples with different thickness. Here in this work, we design a novel ultra-compact electron gun which combines a well-designed cathode profile, thereby providing a uniform field and a movable anode configuration to achieve a temporal resolution on the order of 100 fs over an accelerating voltage range from 10 kV to 125 kV. By optimizing the design of the high-voltage electrode profile, the field enhancement factor on the axis and along the cathode surface are both less than ~4% at different cathode-anode spacings, and thus the maximum on-axis field strength of ~10 MV/m is achieved under various accelerating voltages. This effectively suppresses the space charge broadening effect of the electron pulse. Furthermore, the anode aperture is designed as a stepped hole in which the dense sample grid can be placed, and the sample under study is directly supported by the grid and located at the anode, which reduces the cathode-to-sample distance, thus minimizing the electron pulse broadening from the cathode to sample. Moreover, the defocusing effect caused by the anode hole on the electron beam can be effectively reduced, therefore improving the lateral focusing performance of the electron beam.One of the grand challenges in ultrafast science is real-time visualization of the microscopic structural evolution on atomic time and length scales. A promising pump-probe technique using a femtosecond laser pulse to initiate the ultrafast dynamics and another ultrashort electron pulse to probe the resulting changes has been developed and widely used to study ultrafast structural dynamics in chemical reactions, phase transitions, charge density waves, and even biological functions. In the past three decades, a number of different ultrafast electron guns have been developed to generate ultashort electron sources, mainly including hybrid electron gun with radio-frequency (RF) cavities for compressing the pulse broadening, relativistic electron gun for suppressing the coulomb interaction, single-electron pulses without space charge effect and compact direct current (DC) electron gun for minimizing the electron propagation distance. At present, these developments with different final electron energy and available total charge have improved the time response of ultrafast electron diffraction (UED) setups to a new frontier approaching to 100 fs regime. Although enormous efforts have been made, the superior capabilities and potentials of ultrafast electron diffraction (UED) are still hindered by space-charge induced pulse broadening. Besides, the penetration depth of electrons increases with the electron energy, while the scattering probability of electrons has the opposite consequence. Thus, in addition to the temporal resolution enhancement, it is also important that the electron energy should be tunable in a wide range to meet the requirements for samples with different thickness. Here in this work, we design a novel ultra-compact electron gun which combines a well-designed cathode profile, thereby providing a uniform field and a movable anode configuration to achieve a temporal resolution on the order of 100 fs over an accelerating voltage range from 10 kV to 125 kV. By optimizing the design of the high-voltage electrode profile, the field enhancement factor on the axis and along the cathode surface are both less than ~4% at different cathode-anode spacings, and thus the maximum on-axis field strength of ~10 MV/m is achieved under various accelerating voltages. This effectively suppresses the space charge broadening effect of the electron pulse. Furthermore, the anode aperture is designed as a stepped hole in which the dense sample grid can be placed, and the sample under study is directly supported by the grid and located at the anode, which reduces the cathode-to-sample distance, thus minimizing the electron pulse broadening from the cathode to sample. Moreover, the defocusing effect caused by the anode hole on the electron beam can be effectively reduced, therefore improving the lateral focusing performance of the electron beam.

- Nov. 18, 2020
- Acta Physica Sinica
- Vol.69 Issue, 5 052901-1 (2020)
- DOI：10.7498/aps.69.20191157

Chen Feng, Xu Hai-Bo, Zheng Na, Jia Qing-Gang, She Ruo-Gu, and Li Xing-E

The angle-cut collimator plays an important role in high-energy proton radiography. By using the collimator, the image contrast can be improved, and the material diagnosis and density reconstruction can be realized through secondary imaging. As all these techniques depend on the flux value, it is of great significance to reduce the error of the detected flux value. The ideal collimator is a much thin surface, but thick enough to block protons outside the collimation region. It is designed by stretching the aperture of the collimation plane. The shape is cylindrical, and it will increase the error of the flux value with the angle truncation. The initial bunch is defined and the phase diagram of the bunch within the angle-cut is ideal in the theoretical model. The equation of designing the collimator is given by theoretical analysis. It is given by the transfer matrix, the radius of the object and the angle-cut. The pore structure is oval-shaped by calculating and simulating. The proton imaging system of 1.6 GeV is established by Geant4 program, and the detector is ideal. The round copper plate and the concentric spheres are chosen as objects respectively. The parameters of the designed collimator is given by this method. The ideal collimator, tensile collimator and designed collimator are used in simulation, the radius of object is 5 cm and the angle-cut is 2 mrad and 3.5 mrad. The results show that when using the ideal and the designed angle-cut collimator, the flux distributions are in good agreement, while when using the tensile collimator, the result is quite different from that obtained by using the ideal collimator. Therefore, the collimator designed by this method can effectively reduce the error of the detected flux value.The angle-cut collimator plays an important role in high-energy proton radiography. By using the collimator, the image contrast can be improved, and the material diagnosis and density reconstruction can be realized through secondary imaging. As all these techniques depend on the flux value, it is of great significance to reduce the error of the detected flux value. The ideal collimator is a much thin surface, but thick enough to block protons outside the collimation region. It is designed by stretching the aperture of the collimation plane. The shape is cylindrical, and it will increase the error of the flux value with the angle truncation. The initial bunch is defined and the phase diagram of the bunch within the angle-cut is ideal in the theoretical model. The equation of designing the collimator is given by theoretical analysis. It is given by the transfer matrix, the radius of the object and the angle-cut. The pore structure is oval-shaped by calculating and simulating. The proton imaging system of 1.6 GeV is established by Geant4 program, and the detector is ideal. The round copper plate and the concentric spheres are chosen as objects respectively. The parameters of the designed collimator is given by this method. The ideal collimator, tensile collimator and designed collimator are used in simulation, the radius of object is 5 cm and the angle-cut is 2 mrad and 3.5 mrad. The results show that when using the ideal and the designed angle-cut collimator, the flux distributions are in good agreement, while when using the tensile collimator, the result is quite different from that obtained by using the ideal collimator. Therefore, the collimator designed by this method can effectively reduce the error of the detected flux value.

- Nov. 10, 2020
- Acta Physica Sinica
- Vol.69 Issue, 3 032901-1 (2020)
- DOI：10.7498/aps.69.20191691

Wu Yong-Cun, Yang Xing-Lin, Shi Jin-Shui, Zhao Liang-Chao, and He Xiao-Zhong

The high-frequency resonant cavity is affected by factors such as beam load, gravity and heat loss caused by cavity deformation during the actual operation of the medical cyclotron. The resonant frequency will shift to a certain extent, resulting in the high-frequency operation frequency varying with the resonant frequency of the resonator cavity. In order to meet the requirements for isochronous acceleration, the magnetic field strength should also be changed correspondingly when the high-frequency operation frequency changes, that is, the magnitude of the magnet current needs changing accordingly, so that the particle cyclotron frequency matches the high-frequency resonant frequency to overcome the sliding phase. Firstly, the static magnetic field model is established by finite element simulation software to simulate the average magnetic field of cyclotron under different magnet currents. Then the relationship between the magnetic field and the resonant frequency is theoretically analyzed. Finally, the relationship between the magnet current and the resonant frequency is obtained when the magnet current varies in a small interval. According to the optimal magnet current corresponding to different resonance frequencies, the automatic frequency tracking of magnet current is completed. In the case of ensuring the maximum carbon film beam, the optimal magnet current corresponding to different resonance frequencies is obtained, which makes the theory validated. According to the relationship, the magnet current is automatically adjusted, which overcomes the slip phase and ensures the stable output of the Faraday beam. The method enables the magnet current to be quickly and accurately find and track the cavity frequency, overcomes the slip phase caused by the frequency offset, and completes the stable output of the beam.The high-frequency resonant cavity is affected by factors such as beam load, gravity and heat loss caused by cavity deformation during the actual operation of the medical cyclotron. The resonant frequency will shift to a certain extent, resulting in the high-frequency operation frequency varying with the resonant frequency of the resonator cavity. In order to meet the requirements for isochronous acceleration, the magnetic field strength should also be changed correspondingly when the high-frequency operation frequency changes, that is, the magnitude of the magnet current needs changing accordingly, so that the particle cyclotron frequency matches the high-frequency resonant frequency to overcome the sliding phase. Firstly, the static magnetic field model is established by finite element simulation software to simulate the average magnetic field of cyclotron under different magnet currents. Then the relationship between the magnetic field and the resonant frequency is theoretically analyzed. Finally, the relationship between the magnet current and the resonant frequency is obtained when the magnet current varies in a small interval. According to the optimal magnet current corresponding to different resonance frequencies, the automatic frequency tracking of magnet current is completed. In the case of ensuring the maximum carbon film beam, the optimal magnet current corresponding to different resonance frequencies is obtained, which makes the theory validated. According to the relationship, the magnet current is automatically adjusted, which overcomes the slip phase and ensures the stable output of the Faraday beam. The method enables the magnet current to be quickly and accurately find and track the cavity frequency, overcomes the slip phase caused by the frequency offset, and completes the stable output of the beam.

- Oct. 30, 2019
- Acta Physica Sinica
- Vol.68 Issue, 12 122901-1 (2019)
- DOI：10.7498/aps.68.20190116

Shangguan Dan-Hua, Ji Zhi-Cheng, Deng Li, Li Rui, Li Gang, and Fu Yuan-Guang

Traditionally, the Monte Carlo criticality calculation must set a maximum inactive step by experience to ensure that a fission source distribution has converged. The tallying process can only be invoked after this maximum inactive step to avoid the system error caused by the non-converged fission source distribution. In the same way, the uniform fission site algorithm for increasing the whole efficiency of global tallying should also be invoked after the fission source distribution has converged fully. The calculation must reach a maximum iteration step, then, this process can be stopped and the tallies can be printed. This old strategy has two defects. Firstly, the appointed maximum inactive step can only be set by experience, which will be insufficient in some cases; secondly, some iteration steps can be wasted because the precision of tallies has been enough and no one knows it. So, a new strategy is proposed in this article to overcome these defects. Based on an on-the-fly diagnostic method for the convergence of Shannon entropy sequence corresponding to the fission source distribution of each iteration step, the uniform fission site algorithm will be invoked after the iteration step whose serial number is the maximum of the first active step and the first converged step diagnosed by the above-mentioned rule. This rule will be helpful in ensuring that the uniform fission site algorithm can use enough accurate data to bias the secondary fission neutron number, thus avoiding the system error to some degree. Then, a global precision index will be calculated at each fixed step to judge whether the precision standard is reached. If so, the whole calculation is stopped. This process will be repeated until the pre-set maximum step number is reached. In this way, superfluous calculations can be skipped but the calculation precision can be guaranteed. In a word, this new strategy is beneficial to increasing the efficiency of global tallying in the Monte Carlo criticality calculation when appropriate parameters are adopted. This conclusion can be proved by the numerical result from the C5G7 benchmark model.Traditionally, the Monte Carlo criticality calculation must set a maximum inactive step by experience to ensure that a fission source distribution has converged. The tallying process can only be invoked after this maximum inactive step to avoid the system error caused by the non-converged fission source distribution. In the same way, the uniform fission site algorithm for increasing the whole efficiency of global tallying should also be invoked after the fission source distribution has converged fully. The calculation must reach a maximum iteration step, then, this process can be stopped and the tallies can be printed. This old strategy has two defects. Firstly, the appointed maximum inactive step can only be set by experience, which will be insufficient in some cases; secondly, some iteration steps can be wasted because the precision of tallies has been enough and no one knows it. So, a new strategy is proposed in this article to overcome these defects. Based on an on-the-fly diagnostic method for the convergence of Shannon entropy sequence corresponding to the fission source distribution of each iteration step, the uniform fission site algorithm will be invoked after the iteration step whose serial number is the maximum of the first active step and the first converged step diagnosed by the above-mentioned rule. This rule will be helpful in ensuring that the uniform fission site algorithm can use enough accurate data to bias the secondary fission neutron number, thus avoiding the system error to some degree. Then, a global precision index will be calculated at each fixed step to judge whether the precision standard is reached. If so, the whole calculation is stopped. This process will be repeated until the pre-set maximum step number is reached. In this way, superfluous calculations can be skipped but the calculation precision can be guaranteed. In a word, this new strategy is beneficial to increasing the efficiency of global tallying in the Monte Carlo criticality calculation when appropriate parameters are adopted. This conclusion can be proved by the numerical result from the C5G7 benchmark model.

- Oct. 30, 2019
- Acta Physica Sinica
- Vol.68 Issue, 12 122801-1 (2019)
- DOI：10.7498/aps.68.20182276

Wang Yi, Zhang Qiu-Nan, Han Dong, and Li Yuan-Jing

Particle identification is very important in nuclear and particle physics experiments. Time of flight system (TOF) plays an important role in particle identification such as the separation of pion, kaon and proton. Multi-gap resistive plate chamber (MRPC) is a new kind of avalanche gas detector and it has excellent time resolution power. The intrinsic time resolution of narrow gap MRPC is less than 10 ps. So the MRPC technology TOF system is widely used in modern physics experiments for particle identification. With the increase of accelerator energy and luminosity, the TOF system is required to indentify definite particles precisely under high rate environment. The MRPC technology TOF system can be defined as three generations according to the timing and rate requirement. The first-generation TOF is based on the float glass MRPC and its time resolution is around 80 ps, but the rate is relatively low (typically lower than 100 Hz/cm2). The typical systems are TOF of RHIC-STAR, LHC-ALICE and BES III endcap. For the second-generation TOF, its time resolution has the same order as that for the first generation, but the rate capability is much higher. Its rate capability can reach 30 kHz/cm2. The typical experiment with this high rate TOF is FAIR-CBM. The biggest challenge is in the third-generation TOF. For example, the momentum upper limit of $ {\rm{K}}/{\text{π}}$ separation is around 7 GeV/c for JLab-SoLID TOF system under high particle rate as high as 20 kHz/cm2, and the time requirement is around 20 ps. The readout electronics of first two generations is based on time over threshold method, and pulse shape sampling technology will be used in the third-generation TOF. In the same time, the machine learning technology LSTM network is also used to analyze the time performance. As a very successful sample, MRPC barrel TOF has been used in RHIC-STAR for more than ten years and many important physics results have been obtained. A prominent result is the observation of antimatter helium-4 nucleus. This discovery proves the existence of antimatter in the early universe. In this paper, we will describe the evolution of MRPC TOF technology and key technology of each generation of TOFs including MRPC detector and related electronics. The industrial and medical usage of MRPC are also introduced in the work finally.Particle identification is very important in nuclear and particle physics experiments. Time of flight system (TOF) plays an important role in particle identification such as the separation of pion, kaon and proton. Multi-gap resistive plate chamber (MRPC) is a new kind of avalanche gas detector and it has excellent time resolution power. The intrinsic time resolution of narrow gap MRPC is less than 10 ps. So the MRPC technology TOF system is widely used in modern physics experiments for particle identification. With the increase of accelerator energy and luminosity, the TOF system is required to indentify definite particles precisely under high rate environment. The MRPC technology TOF system can be defined as three generations according to the timing and rate requirement. The first-generation TOF is based on the float glass MRPC and its time resolution is around 80 ps, but the rate is relatively low (typically lower than 100 Hz/cm^{2}). The typical systems are TOF of RHIC-STAR, LHC-ALICE and BES III endcap. For the second-generation TOF, its time resolution has the same order as that for the first generation, but the rate capability is much higher. Its rate capability can reach 30 kHz/cm^{2}. The typical experiment with this high rate TOF is FAIR-CBM. The biggest challenge is in the third-generation TOF. For example, the momentum upper limit of $ {\rm{K}}/{\text{π}}$ ![]()

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separation is around 7 GeV/c for JLab-SoLID TOF system under high particle rate as high as 20 kHz/cm^{2}, and the time requirement is around 20 ps. The readout electronics of first two generations is based on time over threshold method, and pulse shape sampling technology will be used in the third-generation TOF. In the same time, the machine learning technology LSTM network is also used to analyze the time performance. As a very successful sample, MRPC barrel TOF has been used in RHIC-STAR for more than ten years and many important physics results have been obtained. A prominent result is the observation of antimatter helium-4 nucleus. This discovery proves the existence of antimatter in the early universe. In this paper, we will describe the evolution of MRPC TOF technology and key technology of each generation of TOFs including MRPC detector and related electronics. The industrial and medical usage of MRPC are also introduced in the work finally.

- Oct. 29, 2019
- Acta Physica Sinica
- Vol.68 Issue, 10 102901-1 (2019)
- DOI：10.7498/aps.68.20182192