分类: 物理学 >> 核物理学 提交时间: 2024-03-11
摘要: Fast neutron flux measurements with high count rates and high time resolution have important applications in equipment such as tokamaks. In this study, real-time neutron and gamma discrimination was implemented on a self-developed 500-Msps, 12-bit digitizer, and the neutron and gamma spectra were calculated directly on an FPGA . A fast neutron flux measurement system with BC-501A and EJ-309 liquid scintillator detectors was developed and a fast neutron measurement experiment was successfully performed on the HL-2M tokamak at the Southwestern Institute of Physics, China. The experimental results demonstrated that the system obtained the neutron and gamma spectra with a time accuracy of 1ms. At count rates of up to 1 Mcps, the figure of merit was greater than 1.05 for energies between 50 keV and 2.8 MeV.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Coronal mass ejections (CMEs) result from eruptions of magnetic flux ropes (MFRs) and can possess a three-part structure in white-light coronagraphs, including a bright front, dark cavity and bright core. In the traditional opinion, the bright front forms due to the plasma pileup along the MFR border, the cavity represents the cross section of the MFR, and the bright core corresponds to the erupted prominence. However, this explanation on the nature of the three-part structure is being challenged. In this paper, we report an intriguing event occurred on 2014 June 14 that was recorded by multiple space- and ground-based instruments seamlessly, clearly showing that the CME front originates from the plasma pileup along the magnetic arcades overlying the MFR, and the core corresponds to a hot-channel MFR. Thus the dark cavity is not an MFR, instead it is a low-density zone between the CME front and a trailing MFR. These observations are consistent with a new explanation on the CME structure. If the new explanation is correct, most (if not all) CMEs should exhibit the three-part appearance in their early eruption stage. To examine this prediction, we make a survey study of all CMEs in 2011 and find that all limb events have the three-part feature in the low corona, regardless of their appearances in the high corona. Our studies suggest that the three-part structure is the intrinsic structure of CMEs, which has fundamental importance for understanding CMEs.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Long-term synoptic observations of the Sun are critical for advancing our understanding of Sun as an astrophysical object, understanding the solar irradiance and its role in solar-terrestrial climate, for developing predictive capabilities of solar eruptive phenomena and their impact on our home planet, and heliosphere in general, and as a data provider for the operational space weather forecast. We advocate for the development of a ground-based network of instruments provisionally called ngGONG to maintain critical observing capabilities for synoptic research in solar physics and for the operational space weather forecast.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Precise measurements of neutron star (NS) velocities provide critical clues to the supernova physics and evolution of binary systems. Based on Gaia Data Release 2 (DR2), we selected a sample of 24 young (<3 Myr) pulsars with precise parallax measurements and measured the velocity of their local standard of rest (LSR) and the velocity dispersion among their respective local stellar groups. The median velocity difference between thus calculated LSRs and the Galactic rotation model is ~7.6km/s, small compared to the typical velocity dispersion of ~27.5km/s. For pulsars off the Galactic plane, such differences grow significantly to as large as ~40 km/s. More importantly, the velocity dispersion of stars in the local group of low-velocity pulsars can be comparable to their transverse velocities, suggesting that the intrinsic velocities of NS progenitors should be taken into account when we consider their natal kicks and binary evolution. We also examined the double NS systems J0737- 3039A/B, and measured its transverse velocity to be 26(+18, -13) km/s assuming nearby Gaia sources being representative of its birth environment. This work demonstrated the feasibility and importance of using Gaia data to study the velocity of individual systems and velocity distribution of NSs.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: X-ray emission provides the most direct diagnostics of the energy-release process in solar flares. Occasionally, a superhot X-ray source is found to be above hot flare loops of ~10 MK temperature. While the origin of the superhot plasma is still elusive, it has conjured up an intriguing image of in-situ plasma heating near the reconnection site high above the flare loops, in contrast to the conventional picture of chromospheric evaporation. Here we investigate an extremely long-duration solar flare, in which EUV images show two distinct flare loop systems that appear successively along a Gamma-shaped polarity inversion line (PIL). When both flare loop systems are present, the HXR spectrum is found to be well fitted by combining a hot component (Te ~12 MK) and a superhot component (Te ~30 MK). Associated with a fast CME, the superhot X-ray source is located at top of the flare arcade that appears earlier, straddling and extending along the long "arm" of the Gamma-shaped PIL. Associated with a slow CME, the hot X-ray source is located at the top of the flare arcade that appears later and sits astride the short "arm" of the Gamma-shaped PIL. Aided by observations from a different viewing angle, we are able to verify that the superhot X-ray source is above the hot one in projection, but the two sources belong to different flare loop systems. Thus, this case study provides a stereoscopic observation explaining the co-existence of superhot and hot X-ray emitting plasmas in solar flares.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: The Helium abundance, defined as $A_{He}=n_{He}/n_{H}\times 100$, is $\sim$8.5 in the photosphere and seldom exceeds 5 in fast solar wind. Previous statistics have demonstrated that $A_{He}$ in slow solar wind correlates tightly with sunspot number. However, less attention is paid to the solar cycle dependence of $A_{He}$ within interplanetary coronal mass ejections (ICMEs) and comparing the $A_{He}$ characteristics of ICMEs and solar wind. In this paper we conduct a statistical comparison of Helium abundance between ICMEs and solar wind near 1 AU with observations of \textit{Advanced Composition Explorer} from 1998 to 2019, and find that the ICME $A_{He}$ also exhibits the obvious solar cycle dependence. Meanwhile, we find that the $A_{He}$ is obviously higher within ICMEs compared to solar wind, and the means within 37\% and 12\% of ICMEs exceed 5 and 8.5, respectively. It is interesting to answer where and how the high Helium abundance originates. Our statistics demonstrate that 21\% (3\%) of ICME (slow wind) $A_{He}$ data points exceed 8.5 around solar maximum, which decreases dramatically near minimum, while no such high $A_{He}$ values appear in the fast wind throughout the whole solar cycle. This indicates that the high $A_{He}$ (e.g., $>$8.5) emanates from active regions as more ICMEs and slow wind originates from active regions around maximum, and supports that both active regions and quiet-Sun regions are the sources of slow wind. We suggest that the high $A_{He}$ from active regions could be explained by means of the magnetic loop confinement model and/or photoionization effect.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: The physics potential of detecting $^8$B solar neutrinos is exploited at the Jiangmen Underground Neutrino Observatory (JUNO), in a model independent manner by using three distinct channels of the charged-current (CC), neutral-current (NC) and elastic scattering (ES) interactions. Due to the largest-ever mass of $^{13}$C nuclei in the liquid-scintillator detectors and the potential low background level, $^8$B solar neutrinos would be observable in the CC and NC interactions on $^{13}$C for the first time. By virtue of optimized event selections and muon veto strategies, backgrounds from the accidental coincidence, muon-induced isotopes, and external backgrounds can be greatly suppressed. Excellent signal-to-background ratios can be achieved in the CC, NC and ES channels to guarantee the $^8$B solar neutrino observation. From the sensitivity studies performed in this work, we show that one can reach the precision levels of 5%, 8% and 20% for the $^8$B neutrino flux, $\sin^2\theta_{12}$, and $\Delta m^2_{21}$, respectively, using ten years of JUNO data. It would be unique and helpful to probe the details of both solar physics and neutrino physics. In addition, when combined with SNO, the world-best precision of 3% is expected for the $^8$B neutrino flux measurement.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: Coronal mass ejections (CMEs) result from eruptions of magnetic flux ropes (MFRs) and can possess a three-part structure in white-light coronagraphs, including a bright front, dark cavity and bright core. In the traditional opinion, the bright front forms due to the plasma pileup along the MFR border, the cavity represents the cross section of the MFR, and the bright core corresponds to the erupted prominence. However, this explanation on the nature of the three-part structure is being challenged. In this paper, we report an intriguing event occurred on 2014 June 14 that was recorded by multiple space- and ground-based instruments seamlessly, clearly showing that the CME front originates from the plasma pileup along the magnetic arcades overlying the MFR, and the core corresponds to a hot-channel MFR. Thus the dark cavity is not an MFR, instead it is a low-density zone between the CME front and a trailing MFR. These observations are consistent with a new explanation on the CME structure. If the new explanation is correct, most (if not all) CMEs should exhibit the three-part appearance in their early eruption stage. To examine this prediction, we make a survey study of all CMEs in 2011 and find that all limb events have the three-part feature in the low corona, regardless of their appearances in the high corona. Our studies suggest that the three-part structure is the intrinsic structure of CMEs, which has fundamental importance for understanding CMEs.
分类: 天文学 >> 天文学 提交时间: 2023-02-19
摘要: It is generally accepted that CMEs result from eruptions of magnetic flux ropes, which are dubbed as magnetic clouds in interplanetary space. The composition (including the ionic charge states and elemental abundances) is determined prior to and/or during CME eruptions in the solar atmosphere, and does not alter during magnetic cloud propagation to 1 AU and beyond. It has been known that the composition is not uniform within a cross section perpendicular to magnetic cloud axis, and the distribution of ionic charge states within a cross section provides us an important clue to investigate the formation and eruption processes of flux ropes due to the freeze-in effect. The flux rope is a three dimensional magnetic structure intrinsically, and it remains unclear whether the composition is uniform along the flux rope axis as most magnetic clouds are only detected by one spacecraft. In this paper we report a magnetic cloud that was observed by ACE near 1 AU on 1998 March 4--6 and Ulysses near 5.4 AU on March 24--28 sequentially. At these times, both spacecraft were located around the ecliptic plane, and the latitudinal and longitudinal separations between them were $\sim$2.2$^{\circ}$ and $\sim$5.5$^{\circ}$, respectively. It provides us an excellent opportunity to explore the axial inhomogeneity of flux rope composition, as both spacecraft almost intersected the cloud center at different sites along its axis. Our study shows that the average values of ionic charge states exhibit significant difference along the axis for carbon, and the differences are relatively slight but still obvious for charge states of oxygen and iron, as well as the elemental abundances of iron and helium. Besides the means, the composition profiles within the cloud measured by both spacecraft also exhibit some discrepancies. We conclude that the inhomogeneity of composition exists along the cloud axis.
分类: 光学 >> 量子光学 提交时间: 2023-02-19
摘要: We demonstrate a non-invasive time-sorting method for ultrafast electron diffraction (UED) experiments with radio-frequency (rf) compressed electron beams. We show that electron beam energy and arrival time at the sample after rf compression are strongly correlated such that the arrival time jitter may be corrected through measurement of the beam energy. The method requires minimal change to the infrastructure of most of the UED machines and is applicable to both keV and MeV UED. In our experiment with ~3 MeV beam, the timing jitter after rf compression is corrected with 35 fs root-mean-square (rms) accuracy, limited by the 3x10^-4 energy stability. For keV UED with high energy stability, sub-10 fs accuracy in time-sorting should be readily achievable. This time-sorting technique allows us to retrieve the 2.5 THz oscillation related to coherent A1g phonon in laser excited Bismuth film and extends the temporal resolution of UED to a regime far beyond the 100-200 fs rms jitter limitation.
分类: 光学 >> 量子光学 提交时间: 2023-02-19
摘要: Particle accelerators that use electromagnetic fields to increase a charged particle's energy have greatly advanced the development of science and industry since invention. However, the enormous cost and size of conventional radio-frequency accelerators have limited their accessibility. Here we demonstrate a mini-accelerator powered by terahertz pulses with wavelengths 100 times shorter than radio-frequency pulses. By injecting a short relativistic electron bunch to a 30-mm-long dielectric-lined waveguide and tuning the frequency of a 20-period terahertz pulse to the phase-velocity-matched value, precise and sustained acceleration for nearly 100% of the electrons is achieved with the beam energy spread essentially unchanged. Furthermore, by accurately controlling the phase of two terahertz pulses, the beam is stably accelerated successively in two dielectric waveguides with close to 100% charge coupling efficiency. Our results demonstrate stable and scalable beam acceleration in a multistage mini-accelerator and pave the way for functioning terahertz-driven high-energy accelerators.
分类: 物理学 >> 核物理学 提交时间: 2016-09-05
摘要: The upgrade of the current BESIII Endcap TOF (ETOF) is carried out with the Multi-gap Resistive Plate Chamber (MRPC) technology. The installation of the new ETOF has been finished in October 2015. The first results of the MRPCs commissioning at BESIII are reported in this paper.
分类: 生物学 >> 植物学 >> 植物生物化学、植物生物物理学 提交时间: 2016-05-03
摘要: Although it has been reported previously that ethylene plays a critical role in sex determination in cucurbit species, how the andromonoecy that carries both the male and hermaphroditic flowers is determined in watermelon is still unknown. Here we showed that the watermelon gene 1-aminocyclopropane-1-carboxylate synthase 4 (CitACS4), expressed specifically in carpel primordia, determines the andromonoecy in watermelon. Among four single nucleotide polymorphism (SNPs) and one InDel identified in the coding region of CitACS4, the C364W mutation located in the conserved box 6 was co-segregated with andromonoecy. Enzymatic analyses showed that the C364W mutation caused a reduced activity in CitACS4. We believe that the reduced CitACS4 activity may hamper the programmed cell death in stamen primordia, leading to the formation of hermaphroditic flowers.