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1. Neutron phase contrast imaging of PbWO ₄ crystals for G experiment test masses using a Talbot-Lau neutron interferometer

   https://doi.org/10.1088/1361-6382/aca1a4  

   Assumin-Gyimah, K T; Dutta, D; Hussey, D S; Snow, W M; Langlois, C; Lee, V (November 2022, Classical and Quantum Gravity)

   Abstract The use of transparent test/source masses can benefit future measurements of Newton’s gravitational constant G . Such transparent test mass materials can enable nondestructive, quantitative internal density gradient measurements using optical interferometry and allow in-situ optical metrology methods to be realized for the critical distance measurements often needed in a G apparatus. To confirm the sensitivity of such optical interferometry measurements to internal density gradients it is desirable to conduct a check with a totally independent technique. We present an upper bound on possible internal density gradients in lead tungstate (PbWO 4 ) crystals using a Talbot-Lau neutron interferometer on the Cold Neutron Imaging Facility at NIST. We placed an upper bound on a fractional atomic density gradient in two PbWO 4 test crystals of 1 N d N d x < 0.5 × 10 − 6  cm −1 . This value is about two orders of magnitude smaller than required for G measurements. We discuss the implications of this result and of other nondestructive methods for characterization of internal density inhomogeneties which can be applied to test masses in G experiments. 

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2. Maximal Distortion of Geodesic Diameters in Polygonal Domains

   Adrian Dumitrescu; Csaba D. Tóth (June 2023, Proc. 34th International Workshop on Combinatorial Algorithms)

   For a polygon P with holes in the plane, we denote by ρ(P ) the ratio between the geodesic and the Euclidean diameters of P . It is shown that over all convex polygons with h convex holes, the supremum of ρ(P ) is between Ω(h1/3) and O(h1/2). The upper bound improves to O(1 + min{h3/4∆, h1/2∆1/2}) if every hole has diameter at most ∆ ·diam2(P ); and to O(1) if every hole is a fat convex polygon. Furthermore, we show that the function g(h) = supP ρ(P ) over convex polygons with h convex holes has the same growth rate as an analogous quantity over geometric triangulations with h vertices when h → ∞ 

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3. An Approximate Criterion for Morphological Transformations in Small Vapor Grown Ice Crystals

   https://doi.org/10.1175/JAS-D-23-0131.1  

   Harrington, Jerry Y.; Pokrifka, Gwenore F. (February 2024, Journal of the Atmospheric Sciences)

   Abstract Observations and measurements show that crystals remain relatively compact at low ice supersaturations, but become increasingly hollowed and complex as the ice supersaturation rises. Prior measurements at temperatures >−25°C indicate that the transition from compact, solid ice to morphologically complex crystals occurs when the excess vapor density exceeds a threshold value of about 0.05 g m⁻³. A comparable threshold is not available at low temperatures. A temperature-dependent criterion for the excess vapor density threshold (Δρ_thr) that defines morphological transformations to complex ice is derived from laboratory measurements of vapor grown ice at temperatures below −40°C. This criterion depends on the difference between the equilibrium vapor density of liquid () and ice (ρ_ei) multiplied by a measurement-determined constant,. The new criterion is consistent with prior laboratory measurements, theoretical estimates, and it reproduces the classical result of about 0.05 g m⁻³above −25°C. Since Δρ_thrdefines the excess vapor density above which crystals transition to a morphologically complex (lower density) growth mode, we can estimate the critical supersaturation (s_crit) for step nucleation during vapor growth. The derived values ofs_critare consistent with previous measurements at temperatures above −20°C. No direct measurements ofs_critare available for temperatures below −40°C; however, our derived values suggest some measurement-based estimates may be too high while estimates from molecular dynamics simulations may be too low. 

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4. Fabrication of waveguides in flexible glass via femtosecond laser micromachining and visualization of ultrafast dynamics of the laser-glass interaction

   Agnes, J; Nagar, G; Dempsey, D; Batista, N; Sutherland, J; Shim, B (November 2021, 63rd Annual Meeting of the APS Division of Plasma Physics)

   We fabricate waveguides in Corning® flexible glass using Femtosecond Laser Micromachining (FLM) and visualize the ultrafast plasma dynamics which lead to waveguide formation via time-resolved interferometry. Due to minimal thermal effects and highly-nonlinear optical processes [1], FLM is an ideal tool to fabricate waveguides in glass with high precision and without post processing. We optimize laser fabrication of waveguides by varying scanning speed and pulse energy and, in particular, achieve waveguides with circular cross-sections using slit beam shaping [2]. Further optimization requires investigation of the underlying dynamics of how structural changes in glass are made during and after laser-glass interactions. Thus, we visualize the creation and recombination of plasma in glass which leads to the formation of waveguides using time- resolved interferometry [3]. [1] Rafael R. Gattass and Eric Mazur, Nature Photonics 2, 219–225 (2008)); [2] M. Ams et al. Opt. Express 13, 5676-5681 (2005); [3] G. C. Nagar, D. Dempsey, and B. Shim, Communications Physics 4, 96 (2021). 

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5. From the stellar properties of HD 219134 to the internal compositions of its transiting exoplanets

   https://doi.org/201936259  

   R. Ligi, C. Dorn (January 2019, Astronomy and astrophysics)

   Exoplanets’ properties are directly linked to that of their host star. This is even more true in the case of transiting exoplanets, where the planetary radius cannot be derived if the stellar radius is unknown. Interferometry seems the best technique in this context, as it provides in a quasi-direct way and with exquisite precision the stellar radii. Moreover, the transit light curve can be used to directly obtain the stellar density, and thus the stellar mass. We apply this technique to the system of HD 219134, which hosts two transiting super-Earths. Using these observa- tional techniques and the correlations between the measured parameters, we directly derive new stellar radius, density and mass : R⋆ = 0.726 ± 0.014 R⊙ , ρ⋆ = 1.82 ± 0.19 ρ⊙ , M⋆ = 0.696±0.078 M⊙ . This yields new planetary parameters, and in particular, we find that the two transiting exoplanets show different densities despite similar masses. This can be explained by three hypothesis, among which one suggests that tides heat the internal part of the innermost planet, leading to a molten mantle with lower density. 

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