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    Thermal Quenching of Luminescence in YAG:Ce.
    (Wheaton College. (Norton, Mass.), 2022-05-16) Osborn, Benjamin M.
    Yttrium aluminum garnet doped with cerium (YAG:Ce) is currently the phosphor of choice for LED lighting applications, due to its broad spectrum emission and relatively high efficiency at common LED operating temperatures. For applications where higher temperatures are reached, such as high-powered LED lighting, a decline in the luminescent efficiency of this phosphor is observed. This phenomenon is referred to as thermal quenching and can be explained by several nonradiative decay processes within the phosphor. This research investigated the luminescent properties of a sample of YAG:Ce with a 2.1% cerium concentration at various temperatures in order to better understand the relationship between temperature and luminescence in this material. Through measurements of continuous luminescence, excitation, and response to pulsed excitation, the properties of this material were determined and compared to previous findings as well as computer models. After fitting this data to various models, and considering previously published research, it was determined that the primary mechanism of thermal quenching in this sample is concentration quenching. By comparing the model for concentration quenching to the experimental data, it was found that, on average, energy transferred between 2.2 cerium ions before ending up in a killer center.
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    The 2821 Star Southern Hemisphere Optical SETI Survey: An Archival Search for Candidate Laser Signals in ESO’s HARPS Spectroscope Database
    (Wheaton College. (Norton, Mass.), 2022-05-16) Fields, Benjamin
    I conducted a survey of archived observations taken from 2821 stellar targets by the HARPS spectroscope for potential signatures of lasers from extraterrestrial civilizations. The HARPS instrument examines light across a spectral range of 378-691 nanometers down to a resolution of .01 angstroms–allowing for high wavelength range and sensitivity across the optical spectrum which makes it an ideal instrument for optical SETI surveys. This search operates under the premise that such lasers would be distinct from their stellar spectral background and appear as anomalously narrow emission lines. We develop an algorithm designed to search for statistically significant spikes at least 3.5 standard deviations above the median flux in each wavelength window. Using an initial sample set of 100 stars, we established a classification system as well as methodologies for ruling out a number of types of false positives, including cosmic rays, bleedthrough from the calibration lamp, night sky airglow lines from the Earth’s own atmosphere, and large clusters of emission lines which are likely the product of natural stellar activity and flaring. Our project split into two separate approaches–The Broad Search of one observation of each star in the 2821 star target list designed to detect constant lighthouse-like beacons, and a Deep Search of all observations of each star for more intermittent sources. The Deep Search is ongoing. The Broad Search is complete, and we investigated all 294 candidates our algorithm detected in all 2821 spectra, covering the entire range of stars. Of all 294 all candidates, 29.3% appeared to be in clusters indicative of natural emission lines, possibly from flaring activity. Cosmic rays were the most common type of false positives, accounting for 47.6% of all candidates. Bleedthrough from the calibration lamp accounted for 2.4% of all candidates and 3.1% of candidates were night sky atmospheric airglow lines which evaded the algorithm’s filters. Finally, 17.7% of all candidates remain unexplained and warrant further investigation as potential signatures of extraterrestrial lasers.
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    Using eclipses to probe physical conditions along the jet in SS433.
    (Wheaton College (MA), 2019) Liu, Xinyi
    The Galactic X-ray binary SS 433 is the only known astrophysical object to exhibit strong relativistically red- and blue-shifted lines from elements such as S, Si, Fe, Ni. The X-ray emission lines originate in a jet outflow that is launched somewhere very close to the compact accretion (a black hole or a neutron star). During 2018 August 10-14, SS433 was observed using the High Energy Transmission Grating Spectrometer system on the Chandra X-ray observatory. A total of 116 ksec of Chandra HETGS observation was made during 2018 August 10-14, which was split in one 20 ksec and one 96 ksec observation. The 20 ksec observation started at an orbital phase of 0.802, three days before an eclipse and the 96 ksec observation started at an orbital phase of 0.109 in the middle of the eclipse. The observations were designed to take advantage of the eclipse and carry out time-resolved spectroscopy and timing studies to infer spatial variation of physical properties such as composition, temperature, and density at different distances among the jet. In addition to phenomenological fits to determine properties of the observed emission lines, we will present results from fitting collisionally ionized plasma models. The observation indicates that the redshift of the western jet was 0.034 while the predicted redshift value is -0.0097. This suggests the independent motion of the two jets. The disappearance of the FeXXV line from the Eastern jet and the high photon index of the power law at the end of the long observation suggest the accretion might not has come out of the eclipse.
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    Analog Modeling of Contractional Strain on Europa
    (Wheaton College (MA), 2020-05-10) Hammond, Claire
    Jupiter's moon Europa shows evidence that it is creating new surface material. Since the moon is not increasing in surface area this material must be accommodated somehow. It is possible this may be due to contractional bands with in Europa sinking as plates collide. This was simulated with an analogue sand model. The model found that sinking faults result in more tightly packed faults that contract more horizontally than their non sinking counter parts. This plus other factors such as erosion of any crustal root the sinking fault creates by convection with Europa's ocean may be the way crust is recycled on Europa.
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    Characterizing the Dust and Cold-Gas Content of Nearby Star-Forming Galaxies
    (Wheaton College. (Norton, Mass.), 2020-05-10) MacBride, Sean
    Composed of baryonic and non-baryonic matter, star-forming galaxies are one of the many unique structures within the universe. Due to minimal emission contribution from their galactic core, star-forming galaxies emit similar signatures, particularly in Balmer emission. Balmer emission can be used with physical extinction processes to trace the overall composition of certain baryonic matter components within star- forming galaxies. The baryonic matter components of any galaxy can be separated into several elements: stars, gas, and dust. While accurate methods to measure the stellar component of galaxies are convenient and simple, methods to determine the dust and gas content are scant and rely on longer observations from ground and space-based telescopes. Surveys of the dust and cold-gas components exist, yet a law to describe the dust and cold-gas content as a function of Balmer emission does not currently exist. I present methods to constrain the dust and cold-gas content of star-forming galaxies using Bayesian fitting of the Balmer decrement (log(Hα/Hβ)). The constraints are applied to an independent galaxy sample (SDSS) with similar physical characteristics to test the validity of the dust and cold-gas calibrations. The constraints show low intrinsic scatter, and show satisfactory results when applied to a wider star-forming galaxy sample with independent Balmer decrement measurements. The effects of inclination dependent reddening on the calibration are significant when applied to the SDSS sample, indicating the calibration requires further modification.