Physics Colloquium Series - Fall 2017

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Coherent elastic neutrino-nucleus scattering (CEvNS) has eluded detection for over 40 years despite having the largest interaction cross-section for low-energy neutrinos. A first CEvNS measurement is difficult because it requires sensitivity to low-energy nuclear recoils in a potentially high-background environment. Despite this difficulty, CEvNS provides a valuable tool to study nuclear structure, supernovae, and neutrino oscillations. The COHERENT experiment recently made a first observation of the CEvNS process at a 6.7-sigma confidence level by deploying a 14.6-kg CsI[Na] scintillating crystal at the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory. Beyond a first measurement, COHERENT is deploying a suite of other low-energy-threshold detector technologies to study the CEvNS process in detail, e.g., the neutron-number-squared dependence of the nuclear target. In this talk, I will discuss the first observation of CEvNS as well as describe the ongoing and future work by the COHERENT collaboration to study the CEvNS at the SNS.

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The Air Force employs semiconductor detectors in space sensors for many applications, including launch detection/missile warning, space situational awareness, and intelligence, surveillance, and reconnaissance. A range of semiconductor materials, each with their own advantages and disadvantages, are used to detect visible through long-wavelength infrared regions of the electro-magnetic spectrum. These materials must be able to withstand the harsh environment of space, including alpha, proton, and heavy-ion radiation. Our team evaluates the performance of these detectors under irradiation, models new designs for increased performance and radiation tolerance, and explores new materials for lower size, weight, power, and cost to benefit Air Force applications. A review of our work using solid state semiconductor physics, material science, and radiation tolerance will be given. In addition, opportunities for collaboration and employment will be presented.

The neutron has proven to be an excellent tool for studying all four of the fundamental forces of nature. Studies of the free neutron's decay to a proton, electron, and antineutrino can give us our most precise understanding of the charged weak interaction of quarks and allows us to search for hints of new physics and interactions missing from the Standard Model of Particle Physics. The next generation of experiments to measure the lifetime and angular correlations in neutron beta decay faces significant challenges to push to even higher precision and stronger limits. I will give an overview of how some of the cutting edge experiments today are approaching these challenges, and discuss in detail the upcoming Nab experiment at the Spallation Neutron Source at Oak Ridge National Laboratory. Nab will measure the correlation between the emitted electron and antineutrino and the shape of the electron energy spectrum. The thick, large area silicon detectors that will be used in Nab have excellent energy and timing resolution and position sensitivity and represent a significant advance for neutron beta decay measurements.

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We present a simple, fast and affordable experiment for the exploration of optical properties of colored materials in the visible spectrum. The optical measurements provide access to reliable transmission and absorption coefficients for wavelengths of 425 nm to 850 nm. The obtained spectra, after background and light source correction, contain information about electronic states within the energy range 1.5 eV to 2.9 eV below accessible final states for light absorption, encompassing a large fraction of the visible light and near-infrared. Our tests of RGB color mixing rules with optical filters show that perceived color alone is insufficient for a unique characterization of spectral ranges of increased light transmission or absorption. Our wavelength dependent optical measurements overcome this limitation and provide insight into defect-induced coloration in ionic materials, such as fluorites (CaF2). The results suggest that the green color in certain samples is due to additive coloration with lanthanum and color center formation is due to a possible temperature-dependent change of its oxidation state. In contrast, the violet color seen in other samples is of different origin and may be explained through metallic calcium colloid formation. Each measurement takes only a few seconds and is therefore well suited as a demonstration experiment of defect-induced property changes of materials in modern physics courses, solid state physics courses and laboratories.

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Engaging more than 100 students (mostly undergraduates) from more than a dozen universities and assembling a database with nearly 1000 atmospheric profiles, the Tropospheric Ozone Pollution Project has used weather balloons with special instruments to measure ozone to help identify sources of pollution: natural or anthropogenic, local or remote. With more than 600 soundings in Houston, Texas, TOPP has assembled the world’s largest ozone profile database for an urban megacity. As part of a Fulbright study in Japan, TOPP augmented its suite of measurements developing an approach, using the same instrument, to measure profiles of sulfur dioxide from volcanic or anthropogenic sources as well. This talk provides an overview of TOPP and some of our favorite observations from more than a decade in the field.