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  • Dating the First Case of COVID-19 Epidemic from a Probabilistic Perspective

    分类: 数学 >> 应用数学 提交时间: 2021-09-22

    摘要: In the early days of the epidemic of coronavirus disease 2019 (COVID-19), due to insufficient knowledge of the pandemic, inadequate nucleic acid tests, lack of timely data reporting, etc., the origin time of the onset of COVID-19 is difficult to determine. Therefore, source tracing is crucial for infectious disease prevention and control. The purpose of this paper is to infer the origin time of pandemic of COVID-19 based on a data and model hybrid driven method. We model the testing positive rate to fit its actual trend, and use the least squares estimation to obtain the optimal model parameters. Further, the kernel density estimation is applied to infer the origin time of pandemic given the specific confidence probability. By selecting 12 representative regions in the United States for analysis, the dates of the first infected case with 50% confidence probability are mostly between August and October 2019, which are earlier than the officially announced date of the first confirmed case in the United States on January 20, 2020. The experimental results indicate that the COVID-19 pandemic in the United States starts to spread around September 2019 with a high confidence probability. In addition, the existing confirmed cases are also used in Wuhan City and Zhejiang Province in China to infer the origin time of COVID-19 and provide the confidence probability. The results show that the spread of COVID-19 pandemic in China is likely to begin in late December 2019.

  • BL02U1: The Relocated Macromolecular Crystallography Beamline at the Shanghai Synchrotron Radiation Facility

    分类: 物理学 >> 核物理学 提交时间: 2023-11-15

    摘要: Macromolecular crystallography beamline BL17U1 at the Shanghai Synchrotron Radiation Facility has been relocated, upgraded, and given a new ID (BL02U1). It now delivers X-rays in the energy range of 6–16 keV, with a focused beam of 11.6 µm × 4.8 µm and photon flux greater than 10^12 phs/s. The high credibility and stability of the beam and good timing synchronization of the equipment significantly improve the experimental efficiency. Since June 2021, when it officially opened to users, over 4200 h of beamtime have been provided to over 200 research groups to collect data at the beamline. Its good performance and stable operation have led to the resolution of several structures based on data collected at the beamline.
     

  • Development of Short Prototype of Dual Aperture Quadrupole Magnet for CEPC Ring

    分类: 物理学 >> 核物理学 提交时间: 2023-06-16

    摘要: Abstract Main quadrupole magnets are critical for the Circular Electron and Positron Collider (CEPC), and are specifically designed as dual aperture quadrupole (DAQ) magnets. However, the field crosstalk between the two apertures presents challenges. As the CEPC will work at four beam energies of Z, W, Higgs and ttbar mode, the DAQ magnets will operate at four field gradients spanning from 3.18 to 12.63 T/m. The first short quadrupole magnet prototype with the bore diameter of 76 mm and magnetic length of 1.0 m revealed the problems of large magnetic field harmonics and a magnetic center shift within the beam energy range. Accordingly, a compensation method was proposed in this work to solve the field crosstalk effect. By adjusting the gap height at the middle of the two apertures, the field harmonics and magnetic center shift are significantly reduced. After optimization, the short prototype was modified using a new scheme. The field simulations are validated from the magnetic measurement results. Further, the multipole field meets the requirements at the four beam energies. The detailed magnetic field optimization, field harmonics adjustment, and measurement results are presented herein.

  • GOAT: a simulation code for high intensity beams

    分类: 物理学 >> 核物理学 提交时间: 2023-05-31

    摘要: A simulation code, GOAT, is developed to simulate single-bunch intensity-dependent effects and their interplay in the proton ring (pRing) of the Electron-Ion Collider in China (EicC) project. GOAT is a scalable and portable macroparticle tracking code written in Python and coded by object-oriented programming technology. It allows for transverse and longitudinal tracking, including impedance, space charge effect, electron cloud effect, and beam-beam interaction. In this paper, physical models and numerical approaches for the four types of high-intensity effects, together with the benchmark results obtained through other simulation codes or theories, are presented and discussed. In addition, a numerical application of the cross-talk simulation between the beam-beam interaction and transverse impedance is shown, and a dipole instability is observed below the respective instability threshold. Different mitigation measures implemented in the code are used to suppress the instability. The flexibility, completeness, and advancement demonstrate that GOAT is a powerful tool for beam dynamics studies in the EicC project or other high-intensity accelerators.

  • A Creativity Survey of Fully Dynamic Maximal Independent Set in Expected Poly-log Update Time

    分类: 计算机科学 >> 计算机软件 提交时间: 2022-02-24

    摘要: This paper focus on the researches of Maximal Independent Set (MIS). Based on reading and analysis of several recent papers, we divide the MIS problems into several classifications. The first is the classification based on the research objects, including the solution and maintenance of MIS; the second is the classification based on research methods, including serial, parallel, deterministic and randomized algorithms; the third is experimental analysis, including worst time complexity and expected time complexity.

  • Preparation and electrochemical properties of high purity mixed-acid electrolytes for high energy density vanadium redox flow battery

    分类: 材料科学 >> 材料化学 分类: 材料科学 >> 金属与冶金 提交时间: 2017-08-15

    摘要: All-vanadium redox flow battery (VRFB) is a large-scale electrochemical energy storage technology with numerous potential applications because of its inherent safety and long service life. In previous years, a novel mixed-acid electrolyte system, vanadium electrolytes with mixture of sulfuric acid and hydrochloric acid, has been developed by the Pacific Northwest National Laboratory (PNNL) to increase vanadium solubility, which can effectively raise the electrolyte energy density from 25 Wh/L to 40 Wh/L. To further improve mixed-acid VRFBs, in present work, high purity mixed-acid electrolytes were prepared using a novel direct dissolution – electrochemical reduction process from high purity vanadium oxytrichloride (VOCl3). And the purity and electrochemical properties were investigated, comparing with common mixed-acid electrolytes and regular sulfate electrolytes prepared from metallurgical grade vanadium pentoxide. It was found that the novel process demonstrated a great potential for the low cost and high efficiency production of high purity electrolytes with excellent electrochemical properties for mixed-acid VRFBs. Comparing the traditional high purity electrolytes preparation process, the present novel method will dramatically cut the cost by more than 90 percent, which will considerably facilitate the commercial application of high performance and high density VRFBs.

  • Pulse-shaping method for real-time neutron/gamma discrimination at low sampling rates

    分类: 物理学 >> 核物理学 提交时间: 2023-09-11

    摘要: The NaI:Tl scintillator is an innovative material for dual-gamma-ray and neutron detection with a low 6Li concentration. To achieve real-time n/γ discrimination, a zero-crossing time comparison algorithm based on trapezoidal pulse shaping was developed. The algorithm can operate efficiently at low sampling rates and was implemented on a single-probe portable digital n/γ discriminator based on a feld-programmable gate array (FPGA). The discriminator and NaI:Tl,6Li detector were tested in a neutron-gamma mixed field produced by an 241Am-Be neutron source to evaluate the performance of the algorithm. The Figure of Merits (FoM) was measured as 2.88 at a sampling rate of 50 MHz, indicating that the discriminator with its embedded algorithm has a promising n/γ discrimination capability. Efficient discrimination at sampling rates of 40 and 25 MHz demonstrates that the capability of this method is not limited by low sampling rates.

  • Optical hiding with visual cryptography

    分类: 物理学 >> 电磁学、光学、声学、传热、经典力学和流体动力学 分类: 计算机科学 >> 信息安全 提交时间: 2017-03-28

    摘要: Classical optical hiding methods are symmetric, being apt to realize but not secure. The security is improved in existing non-symmetric hiding techniques, yet all of them fails in convenient extractions, still not optically realized so far. Here, we propose an asymmetric optical hiding method based on visual cryptography, achieving the high security and the easy extraction at the same time. In the hiding process, we convert the secret information into a set of fabricated phase-keys, which are completely independent of each other, intensity-detected-proof, and image-covered, this complex hiding procedure leading to the high security. Correspondingly, during the extraction process, the covered phase-keys are illuminated with laser beams and then incoherent superposed to extract the hidden information directly by human visual system, without complicated optical implementations and any additional computation, resulting in the convenience of extracting. Optical experiments verify that both the high security and the easy extraction are obtainable in the visual-cryptography-based optical hiding.

  • On Two Factors Affecting the Efficiency of MILP Models in Automated Cryptanalyses

    分类: 数学 >> 应用数学 提交时间: 2023-02-15

    摘要: In recent years, mixed integer linear programming (MILP, in short) gradually becomes a popular tool of automated cryptanalyses in symmetric ciphers, which can be used to search differential characteristics and linear approximations with high probability/correlation. A key problem in the MILP method is how to build a proper model that can be solved efficiently in the MILP solvers like Gurobi or Cplex. It is known that a MILP problem is NP-hard, and the numbers of variables and inequalities are two important measures of its scale and time complexity. Whilst the solution space and the variables in many MILP models built for symmetric cryptanalyses are fixed without introducing dummy variables, the cardinality, i.e., the number of inequalities, is a main factor that might affect the runtime of MILP models. We notice that the norm of a MILP model, i.e., the maximal absolute value of all coefficients in its inequalities, is also an important factor affecting its runtime. In this work we will illustrate the effects of two parameters cardinality and norm of inequalities on the runtime of Gurobi by a large number of cryptanalysis experiments. Here we choose the popular MILP solver Gurobi and view it a black box, construct a large number of MILP models with different cardinalities or norms by means of differential analyses and impossible differential analyses for some classic block ciphers with SPN structure, and observe their runtimes in Gurobi. As a result, our experiments show that although minimizing the number of inequalities and the norm of coefficients might not always minimize the runtime, it is still a better choice in most situations.

  • Experimental investigation of cylindrical rotating detonation engine with film cooling

    分类: 动力与电气工程 >> 工程热物理学 提交时间: 2024-01-30

    摘要: We conduct an experimental investigation on the integration of film cooling for thermal protection in a 72-mm cylindrical rotating detonation engine (RDE). The cooling scheme employs the injection of cooling air through a series of cat-ear-shaped film cooling holes densely distributed along the outer wall of the cylindrical combustor. Our findings reveal successful initiation of the RDE and sustained propagation of the rotating detonation wave (RDW) when film cooling is activated.

  • Image recognition and empirical application of desert plant species based on convolutional neural network

    分类: 生物学 >> 植物学 提交时间: 2022-12-12 合作期刊: 《干旱区科学》

    摘要:In recent years, deep convolution neural network has exhibited excellent performance in computer vision and has a far-reaching impact. Traditional plant taxonomic identification requires high expertise, which is time-consuming. Most nature reserves have problems such as incomplete species surveys, inaccurate taxonomic identification, and untimely updating of status data. Simple and accurate recognition of plant images can be achieved by applying convolutional neural network technology to explore the best network model. Taking 24 typical desert plant species that are widely distributed in the nature reserves in Xinjiang Uygur Autonomous Region of China as the research objects, this study established an image database and select the optimal network model for the image recognition of desert plant species to provide decision support for fine management in the nature reserves in Xinjiang, such as species investigation and monitoring, by using deep learning. Since desert plant species were not included in the public dataset, the images used in this study were mainly obtained through field shooting and downloaded from the Plant Photo Bank of China (PPBC). After the sorting process and statistical analysis, a total of 2331 plant images were finally collected (2071 images from field collection and 260 images from the PPBC), including 24 plant species belonging to 14 families and 22 genera. A large number of numerical experiments were also carried out to compare a series of 37 convolutional neural network models with good performance, from different perspectives, to find the optimal network model that is most suitable for the image recognition of desert plant species in Xinjiang. The results revealed 24 models with a recognition Accuracy, of greater than 70.000%. Among which, Residual Network X_8GF (RegNetX_8GF) performs the best, with Accuracy, Precision, Recall, and F1 (which refers to the harmonic mean of the Precision and Recall values) values of 78.33%, 77.65%, 69.55%, and 71.26%, respectively. Considering the demand factors of hardware equipment and inference time, Mobile NetworkV2 achieves the best balance among the Accuracy, the number of parameters and the number of floating-point operations. The number of parameters for Mobile Network V2 (MobileNetV2) is 1/16 of RegNetX_8GF, and the number of floating-point operations is 1/24. Our findings can facilitate efficient decision-making for the management of species survey, cataloging, inspection, and monitoring in the nature reserves in Xinjiang, providing a scientific basis for the protection and utilization of natural plant resources.

  • Association between hindrance stress and state anxiety: the moderating role of HPA-axis function to acute stress

    分类: 心理学 >> 应用心理学 提交时间: 2023-09-14

    摘要: Objective: Nowadays, young adults are facing stressors from several aspects. They have already become the most anxious groups in Chinese society and in risk of developing a series of anxiety disorders. The theory of challenge-hindrance stress was proposed to explain the positive and negative outcomes of different stressors. It has been widely tested mostly in the field of organization and management. In the current study, we used the challenge-hindrance stress theory to clarify the association between stress in daily life and anxiety. We also examined the HPA-axis function buffering the influence of daily stress on anxiety.
    Methods: we used the edited Chinese version of challenge-hindrance stress scales to measure challenge and hindrance stress over 6 months. The level of anxiety was measured by state-trait anxiety inventory. We also carried out a Trier Social Stress Test (TSST) in laboratory and recorded the change of cortisol level during the 60 minutes right after the acute stress.
    Results: Results show that the recent level of hindrance stress positively predicts trait anxiety, but the level of challenge stress does not predict trait anxiety. It is also found that, the cortisol decline rate during the recovery of acute stress moderates the association between stress and anxiety. To be exact, individuals with low cortisol decline rate could not recover to baseline level even after rather long rest, and hindrance stress in their lives would lead to higher level of anxiety. But for individuals who has high cortisol decline rate after acute stress, they recover fast to baseline after the stressor disappear, and they become less anxious although facing the same level of hindrance stress
    Limitations: Firstly, we only examined anxiety but left other distal outcomes of stress such as wellbeing to be further studied. Secondly, we choose the decline rate of cortisol to represent the HPA-axis function instead of taking different systems into consideration. Thirdly, stress appraisal could be further examined in the challenge-hindrance stress researches in addition to different stressors.
    Conclusions: The current study checked the association between stress and anxiety under the framework of challenge-hindrance stress. We examined the moderating mechanism of HPA-axis function, and discussed the effect of physiological toughness from the respective of resources and demands.

  • International UTC TAI comparison based on BDS PPP

    分类: 地球科学 >> 地球科学其他学科 提交时间: 2018-01-07

    摘要: The BeiDou navigation satellite system (BDS) developed by China, and provided official service for Asia-pacific region freely since 2012. With development of BDS, BDS-based time transfer has become an important research direction in BDS’s application fields. At present, the main method of BDS-based time transfer is BDS Common View (BDS CV), which can reach nanoseconds magnitude. Hence, we investigate the performance time transfer based on BDS precise point positioning (PPP) toward UTC/TAI compute. In this contribution, we focus on UTC/TAI comparison based on BDS PPP by using developed quad-constellation GNSS software, which called National Time System Center’s (NTSC) Bernese. And a long-term data analysis is presented. The experiments include two parts: (1) The reliability of the software and multi-GNSS products; (2) The performance of the BDS PPP toward International UTC/TAI comparison. The experimental results reveal that the accuracy of NTSC’s Bernese GPS PPP can reach about 0.1 ns relative to BIPM TAI PPP solutions. Compared with BIPM TAI PPP solutions, the accuracy of GPS PPP solutions can achieve about 0.2 ns by using multi-GNSS precise products, such as GBM, COM. For BDS PPP solutions, the solutions of GPS PPP are regarded as reference values. It is demonstrated that the accuracy of time transfer based on BDS PPP can reach better 1 ns toward UTC/TAI comparison for the statistics of 30 days-arc solutions, while 0.1 ns magnitude can be achieved for the statistics of daily solutions due to the influence of the day boundary discontinuity. Moreover, four different processing strategies of BDS PPP which include tropospheric delay fixed, tropospheric delay and coordinates fixed, coordinates fixed, and BDS-only are tested. Results show that the comparable accuracy can be achieved for four processing strategies. Hence, one can conclude that the time transfer based on BDS-only PPP show a good performance toward UTC/TAI computation.

  • Design Study of Charge-Stripping Scheme of Heavy Ion Beams for HIAF-BRing

    分类: 物理学 >> 核物理学 提交时间: 2024-01-04

    摘要: Charge strippers play an essential role in heavy-ion accelerators by stripping the projectile ions to higher charge states to enhance the acceleration efficiency downstream of the stripper. In the high-energy mode of the booster ring (BRing) of the high-intensity heavy-ion accelerator facility, the pre-accelerated ions from the iLinac will be stripped by a carbon foil to higher charge states and then injected into the BRing. The key parameters of the stripper and stripped ions were calculated, and the influence of stripping on the beam quality was discussed. To get high stripping efficiencies, the foil thicknesses and resultant charge state distributions for the typical ions were determined by the code ETACHA. The equilibrium thickness was obtained for the U beam, while the stripper thicknesses for the Xe and Kr beams were determined based on a compromise between the stripped charge states and the stripping efficiency. The energy loss, energy straggling, and emittance growth due to stripping have a non-negligible impact on the transport of the stripped beams and the injection of the ring. Therefore, these parameters were simulated by GEANT4. In addition, the foil’s temperature evolution, which greatly affects the foil lifetime, was simulated by ANSYS. The maximum temperature of the foil bombarded by the U and Xe beams with the nominal parameters will exceed the safe value in terms of the impact of evaporation on the foil’s lifetime. Given the foil temperature constraint, the highest tolerable beam intensity and the injected ion number into the ring were derived for different beam sizes. The results of this paper will present important reference data for the optimization design and commissioning of the beamline and injection to the BRing for the stripped ions.

  • Applying Ricci flow to Manifold Learning

    分类: 计算机科学 >> 计算机软件 提交时间: 2017-04-10

    摘要: Traditional manifold learning algorithms often bear an assumption that the local neighborhood of any point on embedded manifold is roughly equal to the tangent space at that point without considering the curvature. The curvature indifferent way of manifold processing often makes traditional dimension reduction poorly neighborhood preserving. To overcome this drawback we propose a new algorithm called RF-ML to perform an operation on the manifold with help of Ricci flow before reducing the dimension of manifold.

  • A broadband KU-band microstrip reflectarray antenna using single-layer fractal elements

    分类: 地球科学 >> 空间物理学 提交时间: 2017-03-10

    摘要: A novel single-layer microstrip reflectarray element with fractal structure is proposed. Ansoft HFSS is used to analyze the reflect phase for the fractal element in honeycomb lattice. A 469-element prime focus microstrip reflectarray antenna composed of the proposed fractal elements is designed, manufactured, and measured. The measured gain level of 29.8 dB is obtained at the center frequency of 13.58 GHz with 1-dB gain bandwidth of 15.3%.

  • Model-based decomposition with adaptive selection of unitary transformations

    分类: 地球科学 >> 空间物理学 提交时间: 2017-03-10

    摘要: In this paper a three component model-based decomposition with adaptive selection of unitary transformations for polarimetric synthetic aperture radar (POLSAR) data processing is proposed. Singh et al implemented two unitary transformations on the coherency matrix to minimize the power of cross-polarization, and as a result the T23element of the coherency matrix becomes zero. Another two unitary transformations are proposed by us to carry out on the coherency matrix also to minimize the power of crosspolarization, and the T13element of the coherency matrix becomes zero. Here, we first implement Singh's two unitary transformations and the proposed two unitary transformations on the coherency matrix separately. Then we select the one which leads to the smaller T33. At last, we carry out the three component model-based decomposition proposed by Freeman and Durden based on the obtained coherency matrix. The smaller T33is obtained, the better the over-estimation of volume scattering in model-based decomposition can be suppressed. The RADARSAT-2 POLSAR data of San Francisco area is used to validate the improvement of the proposed method over the three component decomposition only with Singh's two unitary transformations.

  • High-Precision High-Voltage Detuning System for HIAF-SRing Electron Target

    分类: 核科学技术 >> 粒子加速器 提交时间: 2023-06-01

    摘要: 研制用于精确控制电子能量的失谐系统是一个电子靶用于离子储存环的重要挑战。为此,本文为HIAF-SRing的电子靶研制了一种高精度、高电压、失谐系统,使其能在实验中产生精确的电子离子相对能量调制。该系统由辅助电源和高压失谐电源组成。辅助电源级采用工作在软开关状态的LCC谐振变换器和LC滤波器输出正弦波形为高压失谐电源供电。失谐电源由高压脉冲放大器(HVPA)与高压直流(HVDC)模块串联组成。本文介绍了失谐系统的设计与开发,并详细介绍了实验装置,测试结果表明,失谐系统符合双电子复合(DR)实验的技术要求。最后,利用失谐量测量了Fe15+ DR谱,实验数据显示了良好的实验分辨率,验证了该方法的可靠性和可行性。

  • Development and application of high-precision multifunction astronomical plate digitizers in China

    分类: 天文学 >> 天文仪器与技术 提交时间: 2024-02-07 合作期刊: 《天文技术与仪器(英文)》

    摘要:Before charge-coupled device detectors became widely employed in observational astronomy, for more than a hundred years, the main detection method was photography on astronomical glass plates. Recently, in order to preserve these historical data and maintain their usability, the International Astronomical Union has appealed to all countries for global digitization of astronomical plates by developing or adopting advanced digitization technology. Specialized digitizers with high precision and high measuring speed represent key equipment for this task. The Shanghai Astronomical Observatory and the Nishimura Co., Ltd in Japan cooperated between 2013 and 2016 to develop the first Chinese high-precision astronomical plate digitizer, which was then used for complete digitization of all nighttime-observation astronomical plates in China. Then, in 2019–2021, the Shanghai Astronomical Observatory independently developed new models of plate digitizers that enabled countries such as Uzbekistan and Italy to digitize their astronomical plates. Additionally, a new high-precision and multifunction digitizer was also used to digitize valuable microscope slides from the Shanghai Natural History Museum, providing a successful example of crossdomain application of high-precision digitization technology.

  • The impact of COVID-19 on negative body image: Evidence based on social media data

    分类: 心理学 >> 应用心理学 提交时间: 2023-05-12

    摘要: 身体经验和条件关乎社会发展和人类的幸福体验,然而以往研究在COVID-19对消极身体意象的影响上尚未达成一致。为了探究COVID-19与消极身体意象的真实关系,本研究构建了消极身体意象词典,从海量微博数据中获取2010年1月至2022年5月中国31个省/市/自治区的身体意象面板数据,比较了疫情前后不同年份间消极身体意象水平的差异,并探究了疫情严重程度对消极身体意象的影响关系。结果发现,疫情爆发后,消极身体形象水平显著下降,并在两年内保持稳定。且在控制区域效应和时间效应后,新冠疫情的严重程度能够负向预测消极身体意象水平。本研究表明,在封锁期间,群众可能出于对死亡的恐惧和生活习惯的改变更容易接纳身体外观。研究结果对从宏观层面理解重大公共卫生事件影响下的大众身体意象的变化有重要意义。