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  1. Abstract The gravitational three-body problem is a fundamental problem in physics and has significant applications to astronomy. Three-body configurations are often considered stable as long the system is hierarchical; that is, the two orbital distances are well-separated. However, instability, which is often associated with significant energy exchange between orbits, takes time to develop. Assuming two massive objects in a circular orbit and a test particle in an eccentric orbit, we develop an analytical formula estimating the time it takes for the test particle’s orbital energy to change by an order of itself. We show its consistency with results from N -body simulations. For eccentric orbits in particular, the instability is primarily driven not by close encounters of the test particle with one of the other bodies, but by the fundamental susceptibility of eccentric orbits to exchange energy at their periapsis. Motivated by recent suggestions that the galactic center may host an intermediate-mass black hole (IMBH) as a companion to the massive black hole Sgr A*, we use our timescale to explore the parameter space that could harbor an IMBH for the lifetime of the S-cluster of stars surrounding Sgr A*. Furthermore, we show that the orbit of an S-star can be stable for long timescales in the presence of other orbital crossing stars, thus suggesting that the S-cluster may be stable for the lifetimes of its member stars. 
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    Free, publicly-accessible full text available July 20, 2024
  2. A series of multi-doped yttrium pyrosilicate (YPS) nanoparticles were synthesized using a high temperature multi-composite reactor, and used to explore the radioluminescent properties that have potential for biological applications. The luminescent activators explored in this work were cerium, terbium, and europium. A series of mono-doped YPS nanoparticles were synthesized that have optical and X-ray luminescent properties that span the entire visible spectrum. Energy transfer experiments were investiagted to increase the photo- and X-ray luminescence of terbium and europium. Cerium was used as a sensitizer for terbium where X-ray luminescence was enhanced. Similar results were also obtained using cerium as a sensitizer and terbium as an energy bridge for europium. By leveraging different energy transfer mechanisms X-ray luminescence can be enhanced for YPS nanoparticles. 
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  3. Hacking exercises are a common tool for security education, but there is limited investigation of how they teach security concepts and whether they follow pedagogical best practices. This paper enumerates the pedagogical practices of 31 popular online hacking exercises. Specifically, we derive a set of pedagogical dimensions from the general learning sciences and educational literature, tailored to hacking exercises, and review whether and how each exercise implements each pedagogical dimension. In addition, we interview the organizers of 15 exercises to understand challenges and tradeoffs that may occur when choosing whether and how to implement each dimension.We found hacking exercises generally were tailored to students’ prior security experience and support learning by limiting extraneous load and establishing helpful online communities. Conversely, few exercises explicitly provide overarching conceptual structure or direct support for metacognition to help students transfer learned knowledge to new contexts. Immediate and tailored feedback and secure development practice were also uncommon. Additionally, we observed a tradeoff between providing realistic challenges and burdening students with extraneous cognitive load, with benefits and drawbacks at any point on this axis. Based on our results, we make suggestions for exercise improvement and future work to support organizers. 
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  4. A high temperature reactor was developed to synthesize new scintillating nanoparticles that traditionally would sinter. Yttrium pyrosilicate nanoparticles were synthesized with optical properties suitable for x-ray biomedical applications. 
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  5. Core/shell nanoparticles composed of a silica core over which a propargyl methacrylate (PMA) shell was polymerized around were synthesized. To employ the shell coating, the surface of the silica nanoparticles (SiNPs) was modified with an alkene-terminated organometallic silane linker that allowed for the covalent attachment of a poly(propargyl methacrylate) (pPMA) shell. The alkyne groups resulting from the pPMA shell were utilized in copper(I)-catalyzed azide/alkyne cycloaddition (CuAAC) reactions to attach azide-modified Förster resonance energy transfer (FRET) pairs of naphthalimide (azNap), rhodamine B (azRhod), and silicon phthalocyanine (azSiPc) derivatives to the shell surface. The luminescence of the system was manipulated by the covalent attachment of one, two, or three of the fluorophores resulting in no energy transfer, one energy transfer, or two energy transfers, respectively. When all three fluorophores were attached to the core/shell particles, an excitation of azNap with a wavelength of 400 nm resulted in the sequential energy transfer between two FRET pairs and the sole emission of azSiPc at 670 nm. These particles may have applications as bioimaging probes as their luminescence is easily detected using fluorescence microscopy.

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