arXiv subjects

Y. K. Sun

Publications and source records attributed to Y. K. Sun.

3 recordsLinked to original sources

arXiv2024

Constraints on Triton atmospheric evolution from occultations: 1989-2022

Context - Around the year 2000, Triton's south pole experienced an extreme summer solstice that occurs every about 650 years, when the subsolar latitude reached about 50°. Bracketing this epoch, a few occultations probed Triton's atmosphere in 1989, 1995, 1997, 2008 and 2017. A recent ground-based stellar occultation observed on 6 October 2022 provides a new measurement of Triton's atmospheric pressure which is presented here. Aims- The goal is to constrain the Volatile Transport Models (VTMs) of Triton's atmosphere that is basically in vapor pressure equilibrium with the nitrogen ice at its surface. Methods - Fits to the occultation light curves yield Triton's atmospheric pressure at the reference radius 1400 km, from which the surface pressure is induced. Results - The fits provide a pressure p_1400= 1.211 +/- 0.039 microbar at radius 1400 km (47 km altitude), from which a surface pressure of p_surf= 14.54 +/- 0.47 microbar is induced (1-sigma error bars). To within error bars, this is identical to the pressure derived from the previous occultation of 5 October 2017, p_1400 = 1.18 +/- 0.03 microbar and p_surf= 14.1 +/- 0.4 microbar, respectively. Based on recent models of Triton's volatile cycles, the overall evolution over the last 30 years of the surface pressure is consistent with N2 condensation taking place in the northern hemisphere. However, models typically predict a steady decrease in surface pressure for the period 2005-2060, which is not confirmed by this observation. Complex surface-atmosphere interactions, such as ice albedo runaway and formation of local N2 frosts in the equatorial regions of Triton could explain the relatively constant pressure between 2017 and 2022.

astro-ph.EP↗

arXiv2021

Research on proton beam spot imaging based on pixelated gamma detector

The primary secondary particles from the spallation target of the China Spallation Neutron Source are mainly gammas and neutrons, which are related to the distribution of the incident proton. The reconstruction of proton beam spot could be implemented based on the distribution of secondary particles. The methods of pinhole imaging and Compton imaging are developed by measuring the gamma distribution based on the pixelated detector. The secondary gammas could be detected by the pixelated gamma detector directly. The neutron can be identified by detecting the characteristic (478 keV) $γ$-rays from the $^{10}B$(n, $α$) reactions. In order to detect secondary neutrons, a layer of $^{10}B$ converter is added before the pixelated gamma detector. The pixelated gamma detector is sensitive to the characteristic (478 keV) $γ$-rays and then the neutron imaging could be achieved based on measuring the distribution of the characteristic gamma.

physics.ins-det↗

arXiv2004

Magic Angle Electron Energy Loss Spectroscopy (MAEELS) of core electron excitation in anisotropic systems

A general theory for the core-level electron excitation of anisotropic systems using angular integrated electron energy-loss spectroscopy has been derived. We show that it is possible to define a magic angle condition at which the specimen orientation has no effect on the electron energy-loss spectra. We have not only resolved the existing discrepancy between different studies of the magic angle condition, but also extended its applicability to all anisotropic systems. We have demonstrated that magic angle electron energy loss spectroscopy is equivalent to the orientation averaged EELS, although the specimen remains stationary. Our analysis provides the theoretical framework for the comparison between theoretical calculation and experimental measurement of core-level electron excitation spectra in anisotropic systems. In addition to MAEELS, we have also discovered a magic orientation condition which will also give rise to orientationally averaged spectra. It's relation with the magic angle X-ray absorption spectroscopy and magic angle spinning nuclear magnetic resonance is demonstrated.

cond-mat.mtrl-sci↗