分类： 核科学技术 >> 粒子加速器 提交时间： 2023-06-02
Mou, Hongjin
Mao, Guangbo
Zhang, Jinquan
Zhao, Can
Guo, Jinlong
Liu, Wenjintg
Wu, Ruqun
Shen, Cheng
Zhang, Lei
Li, Junshuai
Du, Guanghua
摘要： High-energy proton microbeam facilities are powerful tools in space science, biology, and cancer therapystudies. The primary limitationsof the 50 MeV proton microbeam system are the poor beam quality provided by the cyclotron and the problem of intense scattering in the slit position. Here, we present an optical design for a cyclotron-based 50 MeV high-energy proton microbeam system with a micron-sized resolution. The microbeam system,which hasan Oxford triplet lens configuration,has relatively small spherical aberrations and is insensitive to changes in the beam divergence angle and momentum spread. In addition, the energy filtrationincluded in the system can reducethe beam momentum spread from 1% to 0.02%. The effects of lens parasitic aberrations and the lens fringe fieldon the beam spot resolution are also discussed. In addition, owing to the severe scattering of 50 MeV protons in slit materials, a slit system model based on the Geant4 toolkit enables the quantitative analysis of scattered protons and secondary particles. For the slit system settings under a 10-micron final beam spot, very few scattered protons can enter the quadrupole lens system and affect the focusing performance of the microbeam system, but thesecondary radiation of neutrons and gamma rays generatedat the collimationsystem should be considered for the 50 MeV protonmicrobeam. These data demonstrate thata 50 MeV proton microbeam system with a micron-sized beam spot based on a cyclotron is feasible.
分类： 物理学 >> 普通物理:统计和量子力学,量子信息等 提交时间： 2017-05-02
Rong, Yun
Dandapat, Anirban
Huang, Youju
Sasson, Yoel
Zhang, Lei
Dai, Liwei
Zhang, Jiawei
Guo, Zhiyong
Chen, Tao
摘要： We describe the synthesis of bimetallic dendritic platinum decorated gold nanorods (AuNRs) by the spatial control of Pt growth over gold nanorods using a heterogeneous seed-mediated growth method. The amounts of the Au seed and Pt-precursor were changed to achieve a tunable volume fraction of Pt coverage on the Au NRs surface. Pt nanostructures were spatially separated from each other, which was highly favorable for promising optical and catalytic properties. The dendritic-Pt decorated AuNRs with variable Au/Pt ratios were exploited to study their surface plasmonic properties and catalytic activities. Interestingly, the Pt decorated AuNRs showed strong surface plasmon resonance (SPR) peak due to noncompact dendritic Pt shell in contrast to the conventional core–shell Au@Pt nanoparticles (NPs). Moreover, the longitudinal peak of the AuNRs was finely tuned from 820 to 950 nm (NIR region) by controlling the volume fraction of the Pt decoration over the AuNRs. The catalytic activity of the dendritic-Pt decorated AuNRs on the reduction of 4-nitrophenol (4-NP) by sodium borohydride (NaBH4) as reducing agent was studied and found to be superior to the activities compared to the monometallic Au NRs. Considering practical applications, dendritic-Pt decorated AuNRs nanostructures were immobilized successfully on the hydrophilic polyvinylidene difluoride (PVDF) film as an efficient reusable catalyst.
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-13
摘要： Superhalo electrons appear to be continuously present in the interplanetary medium, even during very quiet times, with a power-law spectrum at energies above similar to 2 keV. Here we numerically investigate the generation of superhalo electrons by magnetic reconnection in the solar wind source region, using magnetohydrodynamics and test particle simulations for both single X-line reconnection and multiple X-line reconnection. We find that the direct current electric field, produced in the magnetic reconnection region, can accelerate electrons from an initial thermal energy of T similar to 10(5) K up to hundreds of keV. After acceleration, some of the accelerated electrons, together with the nascent solar wind flow driven by the reconnection, propagate upwards along the newly-opened magnetic field lines into interplanetary space, while the rest move downwards into the lower atmosphere. Similar to the observed superhalo electrons at 1 AU, the flux of upward-traveling accelerated electrons versus energy displays a power-law distribution at similar to 2 - 100 keV, f (E) similar to E-delta, with a delta of similar to 1.5 - 2.4. For single (multiple) X-line reconnection, the spectrum becomes harder (softer) as the anomalous resistivity parameter alpha (uniform resistivity eta) increases. These modeling results suggest that the acceleration in the solar wind source region may contribute to superhalo electrons.
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-12
摘要： MHD discontinuities are ubiquitous in the solar wind and are often found at the origin of turbulence intermittency. They may also play a key role in the turbulence dissipation and heating of the solar wind. The tangential discontinuities (TDs) and rotational discontinuities (RDs) are the two most important types of discontinuities. Recently, the connection between turbulence intermittency and proton thermodynamics has been observationally investigated. Here, we present numerical results from a three-dimensional MHD simulation with pressure anisotropy and we define new methods for identifying and distinguishing TDs and RDs. Three statistical results obtained for the relative occurrence rates and heating effects are highlighted: (1) RDs tend to take up the majority of the discontinuities along with time;(2) the thermal states embedding TDs tend to be associated with extreme plasma parameters or instabilities while RDs do not;(3) TDs have a higher average T as well as perpendicular temperature T.. The simulation shows that TDs and RDs evolve and contribute to solar wind heating differently. These results will improve our understanding of the mechanisms that generate discontinuities and cause plasma heating.
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-12
Yang, Liping
Zhang, Lei
He, Jiansen
Tu, Chuanyi
Wang, Linghua
Marsch, Eckart
Wang, Xin
Zhang, Shaohua
Feng, Xueshang
摘要： Measurements of solar wind turbulence reveal the ubiquity of discontinuities. In this study we investigate how the discontinuities, especially rotational discontinuities (RDs), are formed in MHD turbulence. In a simulation of the decaying compressive three-dimensional (3D) MHD turbulence with an imposed uniform background magnetic field, we detect RDs with sharp field rotations and little variations of magnetic field intensity, as well as mass density. At the same time, in the de Hoffman-Teller frame, the plasma velocity is nearly in agreement with the Alfven speed, and is field-aligned on both sides of the discontinuity. We take one of the identified RDs to analyze its 3D structure and temporal evolution in detail. By checking the magnetic field and plasma parameters, we find that the identified RD evolves from the steepening of the Alfven wave with moderate amplitude, and that steepening is caused by the nonuniformity of the Alfven speed in the ambient turbulence.
分类： 地球科学 >> 空间物理学 提交时间： 2016-05-12
Yang, Liping
Zhang, Lei
He, Jiansen
Peter, Hardi
Tu, Chuanyi
Wang, Linghua
Zhang, Shaohua
Feng, Xueshang
摘要： The Atmospheric Imaging Assembly instrument on board the Solar Dynamics Observatory has directly imaged the fast-propagating magnetosonic waves (FMWs) successively propagating outward along coronal magnetic funnels. In this study we perform a numerical investigation of the excitation of FMWs in the interchange reconnection scenario, with footpoint shearing flow being used to energize the system and drive the reconnection. The modeling results show that as a result of magnetic reconnection, the plasma in the current sheet is heated up by Joule dissipation to similar to 10 MK and is ejected rapidly, developing the hot outflows. Meanwhile, the current sheet is torn into plasmoids, which are shot quickly both upward and downward. When the plasmoids reach the outflow regions, they impact and collide with the ambient magnetic field there, which consecutively launches FMWs. The FMWs propagate outward divergently away from the impact regions, with a phase speed of the Alfven speed of similar to 1000 km s(-1). In the k-omega. diagram of the Fourier wave power, the FMWs display a broad frequency distribution with a straight ridge that represents the dispersion relation. With the WKB approximation, at the distance of 15Mm from the wave source region, we estimate the energy flux of FMWs to be E similar to 7.0 x 10(6) erg cm(-2) s(-1), which is similar to 50 times smaller than the energy flux related to the tube-channeled reconnection outflow. These simulation results indicate that energetically and dynamically the outflow is far more important than the waves.
分类： 生物学 >> 生物物理学 >> 细胞生物学 提交时间： 2016-05-12
摘要： The Hippo signaling pathway restricts organ size by inactivating the Yorkie (Yki)/Yes-associated protein (YAP) family proteins. The oncogenic Yki/YAP transcriptional coactivator family promotes tissue growth by activating target gene transcription, but the regulation of Yki/YAP activation remains elusive. In mammalian cells, we identified Brg1, a major subunit of chromatin-remodeling SWI/SNF family proteins, which interacts with YAP. This finding led us to investigate the in vivo functional interaction of Yki and Brahma (Brm), the Drosophila homolog of Brg1. We found that Brm functions at the downstream of Hippo pathway and interacts with Yki and Scalloped (Sd) to promotes Yki-dependent transcription and tissue growth. Furthermore, we demonstrated that Brm is required for the Crumbs (Crb) dysregulation-induced Yki activation. Interestingly, we also found that crb is a downstream target of Yki-Brm complex. Brm physically binds to the promoter of crb and regulates its transcription through Yki. Together, we showed that Brm functions as a critical regulator of Hippo signaling during tissue growth and plays an important role in the feedback loop between Crb and Yki. (C) 2014 The Authors. Published by Elsevier Inc. This is an open access article under the CC BY-NC-ND license.
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