arXiv subjects

Chen-Yu Liu

Publications and source records attributed to Chen-Yu Liu.

64 records · Page 4Linked to original sources

arXiv2021

Incoherent approximation for neutron up-scattering cross sections and its corrections for slow neutrons and low crystal temperatures

The incoherent approximation (IA) is often used for calculating the one-phonon inelastic neutron scattering cross section for arbitrary solids. It is valid for thermal neutrons but for slow neutrons it requires a correction, which is significant for isotopes that are strong coherent scatterers. In this article, we present the extension of the Placzek--Van Hove corrections for slow neutrons in the limit of low temperatures using the example of solid \emph{ortho}-deuterium (sD$_2$). Our approach yields realistic one-phonon up-scattering cross sections for sD$_2$ and shows the IA to be a factor of 2 to 5 too high for ultracold neutron (UCN) up-scattering in sD$_2$. Our calculations are compared with previously published Monte Carlo calculations of the one-phonon cross section based on the dynamic structure function $S(q,ω)$ of polycrystalline \emph{ortho}-deuterium and are found to be consistent with them. Furthermore, we provide the means for easily replicable calculations of the one-phonon up-scattering cross sections of solid \emph{ortho}-deuterium for slow neutrons. These should from now on be used in calculations and simulations of UCN scattering in sD$_2$.

nucl-th↗

arXiv2021

Compact ultracold neutron source concept for low energy accelerator-driven neutron sources

The concept of a small-scale, pulsed-proton accelerator based compact ultracold neutron (UCN) source is presented. The essential idea of the compact UCN source is to enclose a volume of superfluid $^{4}\mathrm{He}$ converter with a supercold moderator in the vicinity of a low-radiation neutron production target from (p, n) reactions. The supercold moderator should possess an ability to produce cold neutron flux with a peak brightness near the single-phonon excitation band of the superfluid $^{4}\mathrm{He}$ converter, thereby augmenting the UCN production in the compact UCN source even with very low intensity of neutron brightness. The performance of the compact UCN source is studied in terms of the UCN production and thermal load in the UCN converter. With the proposed concept of the compact UCN source, a UCN production rate of $P_{\mathrm{UCN}}=80\mathrm{UCN}/\mathrm{cc}/\mathrm{sec}$ in the UCN converter could be obtained while maintaining thermal load of on the superfluid $^{4}\mathrm{He}$ and its container at a level of $22\mathrm{mW}$. This study shows that the compact UCN source can produce a high enough density of UCN at a small-scale, low-energy, pulsed-proton beam facility with reduced efforts on the cooling and radiation protection.

physics.ins-det↗

arXiv2020

Random Sampling Neural Network for Quantum Many-Body Problems

The eigenvalue problem of quantum many-body systems is a fundamental and challenging subject in condensed matter physics, since the dimension of the Hilbert space (and hence the required computational memory and time) grows exponentially as the system size increases. A few numerical methods have been developed for some specific systems, but may not be applicable in others. Here we propose a general numerical method, Random Sampling Neural Networks (RSNN), to utilize the pattern recognition technique for the random sampling matrix elements of an interacting many-body system via a self-supervised learning approach. Several exactly solvable 1D models, including Ising model with transverse field, Fermi-Hubbard model, and spin-$1/2$ $XXZ$ model, are used to test the applicability of RSNN. Pretty high accuracy of energy spectrum, magnetization and critical exponents etc. can be obtained within the strongly correlated regime or near the quantum phase transition point, even the corresponding RSNN models are trained in the weakly interacting regime. The required computation time scales linearly to the system size. Our results demonstrate that it is possible to combine the existing numerical methods for the training process and RSNN to explore quantum many-body problems in a much wider parameter regime, even for strongly correlated systems.

cond-mat.dis-nn↗

arXiv2018

Monte Carlo Simulations of Trapped Ultracold Neutrons in the UCNτ Experiment

In the UCNτ experiment, ultracold neutrons (UCN) are confined by magnetic fields and the Earth's gravitational field. Field-trapping mitigates the problem of UCN loss on material surfaces, which caused the largest correction in prior neutron experiments using material bottles. However, the neutron dynamics in field traps differ qualitatively from those in material bottles. In the latter case, neutrons bounce off material surfaces with significant diffusivity and the population quickly reaches a static spatial distribution with a density gradient induced by the gravitational potential. In contrast, the field-confined UCN -- whose dynamics can be described by Hamiltonian mechanics -- do not exhibit the stochastic behaviors typical of an ideal gas model as observed in material bottles. In this report, we will describe our efforts to simulate UCN trapping in the UCNτ magneto-gravitational trap. We compare the simulation output to the experimental results to determine the parameters of the neutron detector and the input neutron distribution. The tuned model is then used to understand the phase space evolution of neutrons observed in the UCNτ experiment. We will discuss the implications of chaotic dynamics on controlling the systematic effects, such as spectral cleaning and microphonic heating, for a successful UCN lifetime experiment to reach a 0.01% level of precision.

physics.ins-det↗

arXiv2018

Chaotic Motion of Charged Particles around a Weakly Magnetized Kerr-Newman Black Hole

In this paper, we consider a charged particle moves around a Weakly Magnetized Kerr-Newman black hole. We first study its circular motion with a detailed analysis in the innermost stable circular orbits(ISCO). Then the dynamics of a particle, kicked out from the circular orbit, is also explored. The final states can be characterized into three different types: bound motion, captured by black hole or escape to spatial infinity. The respective trajectories are shown, and are mainly determined by the energy of a particle after a kick and the initial condition, in which we use Basins of attraction to represent the relation between different initial condition and final states.

gr-qc↗

arXiv2017

Geodesic Motion of Neutral Particles around a Kerr-Newman Black Hole

We examine the dynamics of a neutral particle around a Kerr-Newman black hole, and in particular focus on the effects of the charge of the spinning black hole on the motion of the particle. We first consider the innermost stable circular orbits (ISCO) on the equatorial plane. It is found that the presence of the charge of the black hole leads to the effective potential of the particle with stronger repulsive effects as compared with the Kerr black hole. As a result, the radius of ISCO decreases as charge $Q$ of the black hole increases for a fixed value of black hole's angular momentum $a$. We then consider a kick on the particle from its initial orbit out of the equatorial motion. The perturbed motion of the particle will eventually be bounded, or unbounded so that it escapes to spatial infinity. Even more, the particle will likely be captured by the black hole. Thus we analytically and numerically determine the parameter regions of the corresponding motions, in terms of the initial radius of the orbital motion and the strength of the kick. The comparison will be made with the motion of a neutral particle in the Kerr black hole.

gr-qc↗

arXiv2016

Project X: Physics Opportunities

Part 2 of "Project X: Accelerator Reference Design, Physics Opportunities, Broader Impacts". In this Part, we outline the particle-physics program that can be achieved with Project X, a staged superconducting linac for intensity-frontier particle physics. Topics include neutrino physics, kaon physics, muon physics, electric dipole moments, neutron-antineutron oscillations, new light particles, hadron structure, hadron spectroscopy, and lattice-QCD calculations. Part 1 is available as arXiv:1306.5022 [physics.acc-ph] and Part 3 is available as arXiv:1306.5024 [physics.acc-ph].

hep-ex↗

arXiv2013

The Upscattering of Ultracold Neutrons from the polymer $[C_6 H_{12}]_n$

It is generally accepted that the main cause of ultracold neutron (UCN) losses in storage traps is the upscattering to the thermal energy range by hydrogen adsorbed on the surface of the trap walls. However, the data on which this conclusion is based are poor and contradictory. Here, we report a measurement, performed at the Los Alamos National Laboratory UCN source, of the average energy of the flux of upscattered neutrons after the interaction of UCN with hydrogen bound in semicrystalline polymer PMP (tradename TPX), [C$_{6}$H$_{12}$]$_n$. Our analysis, performed with the MCNP code based on the application of the neutron scattering law to UCN upscattered by bound hydrogen in semicrystalline polyethylene, [C$_{2}$H$_{4}$]$_n$, leads us to a flux average energy value of 26$\pm3$ meV in contradiction with previously reported experimental values of 10 to 13 meV and in agreement with the theoretical models of neutron heating implemented in the MCNP code.

cond-mat.mtrl-sci↗

arXiv2013

Measurements of ultracold neutron upscattering and absorption in polyethylene and vanadium

The study of neutron cross sections for elements used as efficient ``absorbers'' of ultracold neutrons (UCN) is crucial for many precision experiments in nuclear and particle physics, cosmology and gravity. In this context, ``absorption'' includes both the capture and upscattering of neutrons to the energies above the UCN energy region. The available data, especially for hydrogen, do not agree between themselves or with the theory. In this report we describe measurements performed at the Los Alamos National Laboratory UCN facility of the UCN upscattering cross sections for vanadium and for hydrogen in CH$_2$ using simultaneous measurements of the radiative capture cross sections for these elements. We measured $σ_{up}=1972\pm130$ b for hydrogen in CH$_2$, which is below theoretical expectations, and $σ_{up} < 25\pm9$ b for vanadium, in agreement with the expectation for the neutron heating by thermal excitations in solids.

nucl-ex↗

arXiv2011

A high dynamic range data acquisition system for a solid-state electron Electric Dipole Moment experiment

We have built a high precision (24-bit) data acquisition (DAQ) system with eight simultaneously sampling input channels for the measurement of the electric dipole moment (EDM) of the electron. The DAQ system consists of two main components, a master board and eight individual analog-to-digital converter (ADC) boards. This custom DAQ system provides galvanic isolation, with fiber optic communication, between the master board and each ADC board to reduce the possibility of ground loop pickups. In addition, each ADC board is enclosed in its own heavy-duty radio frequency shielding enclosure and powered by DC batteries, to attain the ultimate low levels of channel cross-talk. In this paper, we describe the implementation of the DAQ system and scrutinize its performance.

physics.ins-det↗