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This Week at Physics

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Events on Friday, April 20th, 2018

JIMFEST 2018
High energy density plasma simulations using ultracold neutral plasmas
Time: 1:00 pm
Place: 2241 Chamberlin Hall
Speaker: Scott Bergeson, Brigham Young University
Abstract: Strongly-coupled Coulomb systems are typically fluid-like plasmas characterized by the ratio of the nearest-neighbor electrical potential energy to the average kinetic energy. When this ratio is near unity, the plasmas are non-ideal and the foundational assumptions of plasma physics are no longer valid. Astronomical examples include the crusts of white dwarf stars, the interior of Jovian planets, and the dust belts of Saturn. Laboratory examples include laser-driven compression shocks, exploding wires, inertially confined plasmas, and, surprisingly, photo-ionized laser-cooled atoms. Transport properties in these widely disparate plasmas scale with the ratio of potential-to-kinetic energy (the Coulomb coupling parameter). Even systems that are widely disparate in temperature and density can be thermodynamically similar if this ratio is the same. I'll talk about our work in photo-ionized laser-cooled atoms, presenting progress in thermalization, transport, and equilibration.
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JIMFEST 2018
UW Laboratory Astrophysics
Time: 1:30 pm
Place: 2241 Chamberlin Hall
Speaker: Elizabeth DenHartog, University of Wisconsin-Madison
Abstract: In this talk I will describe the laboratory astrophysics program of the Jim Lawler group at the University of Wisconsin – Madison, focusing on the laboratory techniques and instruments used for the measurement of transition probabilities. Transition probabilities are determined from the combination of radiative lifetimes and branching fractions. Radiative lifetimes are measured using time-resolved laser-induced fluorescence on a slow atomic/ionic beam. This experiment has been extremely productive over the 3+ decades of its operation. The atomic beam source at the heart of this experiment was designed by Jim in the early 1980’s, and can produce a gas phase neutral and singly-ionized sample of any metal, and many non-metals. This source is a big reason for the success and productivity of this experiment. Branching fractions are measured in emission using Fourier transform or grating spectroscopy, or a combination thereof. Fourier Transform spectrometers have many advantages for branching fraction work, but have one major drawback. As with any interferometric device, the noise from all lines in the spectrum distribute evenly throughout the spectrum, making the measurement of very weak branches difficult to impossible. I will describe a more recent instrument of Jim’s design – the 3 meter echelle spectrograph – which is ideal for the measurement of the branching fractions of very weak spectral lines.
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JIMFEST 2018
Spectroscopic study of the 7p J=1/2 and 7p J=3/2 states in neutral cesium-133
Time: 2:00 pm
Place: 2241 Chamberlin Hall
Speaker: Maria-Teresa Herd, Mt. Holyoke College
Abstract: I will start with a few observations of connections between spectroscopy and other (seemingly unrelated) research topics, then focus on Doppler free spectroscopy on the 7p J=1/2 and 7p J=3/2 states in neutral cesium-133. A frequency doubled titanium sapphire laser stabilized to a temperature stabilized ultra-low expansion optical cavity was used to find the absolute frequencies for the centers of gravity of the two cesium states. These measurements are an improvement of a factor of 650 and 500 over previously reported measurements. The magnetic dipole and electric quadrupole constants were also measured and found to be consistent with previously published values.
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JIMFEST 2018
A Fresh Look at Abundances of Iron-group Elements in Very Metal-Poor Stars
Time: 2:45 pm
Place: 2241 Chamberlin Hall
Speaker: Chris Sneden, University of Texas, Austin
Abstract: Studies of very metal-poor stars have focused attention on the neutron-capture (Z > 30) elements, whose dramatic star-to-star variations give us insights to element production in extreme, explosive nucleosynthesis environments. Neglected in this fascinating exotic element concentration is that the Fe group (Z = 21-30) often have inadequate abundances to say much of anything useful to massive star evolution theorists. Major improvements are ongoing in treatment of radiative transfer and stellar atmosphere structure. But these modeling improvements will completely wasted if the basic atomic data cannot be trusted. Jim Lawler's Wisconsin atomic physics group is nearly completion of lab work on neutrals and first ions of nearly all of the Fe-group elements. This talk will describe them and show how they are being used to overturn prevailing notions of early synthesis of the most abundant heavy elements in our Galaxy.
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Physics Department Colloquium
The astrophysical r-process: what we are learning from gravitational waves, dwarf galaxies, and stellar archaeology
Time: 3:30 pm
Place: 2241 Chamberlin Hall
Speaker: Ian Roederer, University of Michigan, Ann Arbor
Abstract: Understanding the origin of the elements is one of the major challenges of modern astrophysics. The rapid neutron-capture process, or r-process, is one of the fundamental ways that stars produce the elements listed along the bottom two-thirds of the periodic table, but key aspects of the r-process are still poorly understood. I will describe three major advances in the last few years that have succeeded in confirming neutron star mergers as an important site of the r-process. These include the detection of freshly produced r-process material powering the kilonova associated with the merger of neutron stars detected via gravitational waves (GW170817), the detection of a dwarf galaxy where most of the stars are highly enhanced in r-process elements (Reticulum II), and advances in deriving abundances of previously-undetected r-process elements (Se, Te, Pt) in ultraviolet and optical spectra of metal-poor stars in the Milky Way halo field. I will describe future prospects that connect these three research directions and future rare isotope accelerators to associate specific physics with specific sites of the r-process. Finally, I will highlight the major impact of Jim Lawler's atomic spectroscopy group at Wisconsin in enabling these advances.
Host: Jim Lawler
Video: https://vod.physics.wisc.edu/media/2018_04_20.m4v
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JIMFEST 2018
Measurements of branching fractions using Fourier transform and grating spectroscopy
Time: 5:00 pm
Place: 2241 Chamberlin Hall
Speaker: Gillian Nave, National Institute of Standards and Technology
Abstract: The measurement of lifetimes using laser-induced fluorescence at the University of Wisconsin-Madison has been one of the most productive experiments in the measurement of atomic spectroscopic data. The NIST bibliographic database on Atomic Transition Probabilities lists over 60 papers with Jim Lawler as an author or coauthor on lifetimes of neutral and singly-ionized elements ranging from carbon to mercury. The applications of these measurements range from astrophysics to lighting research. However, it is necessary to combine these lifetimes with branching fractions in order to obtain the atomic transition probabilities, and these branching fractions are now frequently the limiting factor in the accuracy of the measurements. I shall describe how the combination of measurements of branching fractions using Fourier transform spectroscopy at NIST with those of weak lines using the UW echelle spectrograph can improve the reliability of these measurements and provide the atomic data for weak lines that are challenging to measure using Fourier transform spectroscopy alone.
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JIMFEST 2018
Downstream hydrogen plasma cleaning for mass metrology at NIST
Time: 5:30 pm
Place: 2241 Chamberlin Hall
Speaker: Eric Benck, National Institute of Standards and Technology
Abstract: On International Metrology Day, May 20, 2019, the redefinition of the International System of Units (SI) is scheduled to be implemented. When this occurs the kilogram, the last remaining unit defined by a physical artifact, will be redefined to be based on a fundamental physical constant, the Planck constant. As a consequence, the NIST mise en pratique, a series of steps for the realization and dissemination of the revised definition of the kilogram, must be significantly modified. In particular, the mise en pratique will need to address for the first time the measurement and handling of masses in a vacuum. At NIST this will includes 5 major components: NIST-4 Kibble Balance, Magnetic Suspension Mass Comparator (MSMC), Mass-in-Vacuum Comparator, storage chambers, and mass transport vehicles (MTV). In addition to these components, there also is a need to develop new procedures to maintain and clean masses in vacuum. Over long periods of time masses can become coated with a carbon containing contamination layer, even while stored in a vacuum. A downstream hydrogen plasma source is being investigated as a means of gently removing these carbonaceous layers. Atomic hydrogen can react with carbon on the mass surface and convert it to volatile hydrocarbons which can then be pumped out of the system. Initial results demonstrating the etching of carbon coatings off a quartz crystal microbalance (QCM) in our downstream hydrogen plasma cleaner will be presented.
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