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Time-Dependent Behavior of Radiation Induced Conductivity of Polymers
Get PDFThe conductivity of insulating materials can be enhanced above the baseline dark conductivity by incident radiation via inelastic scattering that imparts energy to electrons in trapped states and excites them into the conduction band, without depositing charge. Such radiation-induced conductivity (RIC), caused by ionizing radiation present in harsh space plasma environments, can play a critical role in space charge dissipation within highly insulating materials used in spacecraft. An equilibrium value for RIC, RIC, is attained after prolonged exposure to an incident dose rate; this follows a standard theoretical power law model proposed by Rose/Fowler/Vissenberg, RIC(T, Ḋ) = kRIC ∙ Ḋ∆, where kRIC(T, Ḋ) and Δ(T, Ḋ) are material parameters dependent on both temperature T and dose rate, Ḋ. RIC behavior of real materials requires a finite amount of time for the measured current to come to equilibrium after radiation is turned on and to return to dark conductivity after radiation is turned off. The time-dependent onset of RIC (characterized by an exponential increase in RIC) and of delayed RIC after the dose is terminated (characterized by a hyperbolic inverse time-dependent decay), are controlled by defect energy-dependent time constants for carrier trapping and recombination. The model employed here to fit time-dependent RIC curves for various polymeric materials considers two different time constants for shallow and deep traps, which can themselves be dependent on T and Ḋ. Time-dependent RIC data sets are not common, particularly over ranges of temperature. Data from a large RIC database from Utah State University are analyzed for six polymeric materials—polyether ether ketone (PEEK), polyimide (PI, Kapton HNTM and Kapton ETM), polytetrafluoroethylene (PTFE, TeflonTM), ethylene-tetrafluoroethylene (ETFE, TefzelTM), and low density polyethylene (LDPE)—for ranges of temperatures from ~110 K to ~350 K and dose rates from ~50 μGy/s to ~0.1 Gy/s; the values of kRIC(T, Ḋ) and Δ(T, Ḋ) at equilibrium for these data have been previously investigated. The temperature and dose rate dependence of the parameters of this time-dependent RIC model are determined and compared for the different materials
The Primacy of Density-Mediated Indirect Effects in a Community of Wolves, Elk, And Aspen
Get PDFThe removal or addition of a predator in an ecosystem can trigger a trophic cascade, whereby the predator indirectly influences plants and/or abiotic processes via direct effects on its herbivore prey. A trophic cascade can operate through a density-mediated indirect effect (DMIE), where the predator reduces herbivore density via predation, and/or through a trait-mediated indirect effect (TMIE), where the predator induces an herbivore trait response that modifies the herbivore\u27s effect on plants. Manipulative experiments suggest that TMIEs are an equivalent or more important driver of trophic cascades than are DMIEs. Whether this applies generally in nature is uncertain because few studies have directly compared the magnitudes of TMIEs and DMIEs on natural unmanipulated field patterns. A TMIE is often invoked to explain the textbook trophic cascade involving wolves (Canis lupus), elk (Cervus canadensis), and aspen (Populus tremuloides) in northern Yellowstone National Park. This hypothesis posits that wolves indirectly increase recruitment of young aspen into the overstory primarily through reduced elk browsing in response to spatial variation in wolf predation risk rather than through reduced elk population density. To test this hypothesis, we compared the effects of spatiotemporal variation in wolf predation risk and temporal variation in elk population density on unmanipulated patterns of browsing and recruitment of young aspen across 113 aspen stands over a 21-year period (1999–2019) in northern Yellowstone National Park. Only 2 of 10 indices of wolf predation risk had statistically meaningful effects on browsing and recruitment of young aspen, and these effects were 8–28 times weaker than the effect of elk density. To the extent that temporal variation in elk density was attributable to wolf predation, our results suggest that the wolf–elk–aspen trophic cascade was primarily density-mediated rather than trait-mediated. This aligns with the alternative hypothesis that wolves and other actively hunting predators with broad habitat domains cause DMIEs to dominate whenever prey, such as elk, also have a broad habitat domain. For at least this type of predator–prey community, our study suggests that risk-induced trait responses can be abstracted or ignored while still achieving an accurate understanding of trophic cascades
Academic Standards Subcommittee Minutes November 21, 2024
Get PDFCOMMITTEE BUSINESS Follow-up Topic ACTION ITEMS New Topic ACTION ITEM
General Education Subcommittee Agenda December 05, 2025
Get PDFCall to Order Approval of Minutes Course Approvals/Removals/Syllabi Approvals New Business Additional Items Adjourn: 9:30a
Electro-Optic Time Transfer With Femtosecond Stability
Get PDFOptical two-way time and frequency transfer is an enabling technology that has applications ranging from fundamental investigations of relativity to the operation of global navigation satellite systems. While fiber frequency combs have demonstrated the most stable optical links, they are not ideal for applications that require very low SWaP-C. Here, we demonstrate two-way time and frequency transfer using electro-optic combs that have a direct path to full chip-scale integration. This two-way electro-optic time and frequency transfer system demonstrated instabilities as low as 15 fs at 1 s of averaging time. These results show a pathway to highly stable, agile and low SWaP-C time transfer networks
Facilitating Mathematics and Computer Science Connections: A Cross-Curricular Approach
Get PDFIn the United States, school curricula are often created and taught with distinct boundaries between disciplines. This division between curricular areas may serve as a hindrance to students\u27 long-term learning and their ability to generalize. In contrast, cross-curricular pedagogy provides a way for students to think beyond the classroom walls and make important connections across disciplines. The purpose of this paper is a theoretical reflection on our use of Expansive Framing in our design of lessons across learning environments within the school. We provide a narrative account of our early work in using this theoretical framework to co-plan and enact interdisciplinary mathematics and computer science (CS) tasks with a team of elementary school educators and school district personnel. The unit focuses on the concepts of exponents in mathematics and repeat loops as a control structure in computer science. Using a narrative approach, we describe what occurred during the collaborative planning of lessons and subsequent enactments in two fifth-grade classrooms and one computer lab and provide a practitioner-oriented account of our experience
Reburns of High Severity Patches: Effects on Ponderosa Pine Regeneration and Plant Community Composition
Get PDFThe area burned in high severity fire as well as the proportion of fires burning at high severity have been increasing in southwestern ponderosa pine (Pinus ponderosa) forests since the 1980s. These fires often contain large patches (\u3e100 ha) of 100% tree mortality, which are a concern as they can lead to the establishment of nonnative species and conversion to non-forest vegetation. And, with an increasing amount of fire on the landscape, high severity patches are reburning. However, information on the ecological trajectory of reburns of high-severity patches as well as the utility of remotely sensed burn severity metrics (dNBR) in these reburns is largely unknown. In this thesis I investigate how reburns of high severity patches affect ponderosa pine regeneration and plant community composition, as well as the ecological significance of dNBR in these reburns. Chapter 1 examines the effects of reburns of high severity patches on ponderosa pine presence and regeneration. Across nine fires in northern Arizona, I found no significant differences in total ponderosa pine regeneration between areas burned once at high severity and those that reburned. This suggests that after high severity fire, at least where seed sources exist nearby, ponderosa pine may be resilient to reburns. However, regeneration was highly variable among fires. Within reburned areas, I found that reburn burn severity (dNBR) had a negative association with post-reburn ponderosa pine regeneration density, suggesting that dNBR is capturing a measure of ecosystem change that affects post-reburn regeneration. Chapter 2 explores the effects of reburns of high severity patches on plant community composition. Across nine fires and 892 plots, I found that the initial high severity fire defines the ecological trajectory of reburned areas, and reburns generally reinforce changes initiated by the first fire. Additionally, reburns can bolster nonnative species cover, with a small but positive association between nonnative species and higher reburn burn severity. Collectively, these chapters expand our understanding of how reburns of high severity patches impact ponderosa pine regeneration and plant community composition and the utility of remotely sensed burn severity in these patches
Invasion Potential of Nonnative Fishes Through a Large Western Dam Into a Prized and Vulnerable Ecosystem
Get PDFNative river fish face threats from habitat fragmentation, water overallocation, invasive species establishment, and powerful synergies amongst these factors. In the Colorado River Basin, USA, these threats coalesce at Glen Canyon Dam (GCD), which impounds the Colorado River to create Lake Powell, a reservoir inhabited by nonnative fish species. There is widespread concern low water level in Lake Powell may allow nonnative fish to escape through the dam, adversely affecting native fish species downstream in the Grand Canyon. In this study we characterized the seasonal distribution of nonnative fishes in the vicinity of GCD and identified environmental conditions that may lead to increased entrainment and survival of such entrainment, to inform invasion risk into the lower Colorado River (entrainment refers to water and potentially fish drawn into and passed through the GCD hydroelectric infrastructure). When the reservoir was cold, most fishes selected gently sloping habitat such as that found in Wahweap Bay (5 km from GCD), and thus winter entrainment risk is likely minimal for most species. During the summer and fall, fish were found in the forebay surface waters within meters of the dam, increasing entrainment when the dam penstock (fixed withdrawal structure) is positioned near the surface, as it was during our study, rather than lower as it is at higher water levels.
To evaluate potential injuries to fish during entrainment, we measured conditions of passage through GCD using a pressure sensor and compared our results to previous studies. We estimated members of Centrarchidae (e.g., Smallmouth Bass) may survive entrainment at rates above 50% at low water levels, with survival expected to decrease as the water level rises, but remaining problematically high across all water levels where entrainment is expected. Fish of other families may survive entrainment at higher rates, depending on their physiology. Maintaining water levels higher than those in 2022 and 2023 would decrease both entrainment and survival of entrainment. Under a changing climate the current overallocation of Colorado River water will likely continue to result in conditions suitable for nonnative escapement through CGD, unless and until operational and engineering solutions are implemented
Applying Implementation Supports to Professional Development to Improve Implementation of the Caught Being Good Game
Get PDFDisruptive behavior in the classroom can cause challenges for both teachers and students, creating high levels of teacher stress and preventing students from accessing academic instruction. Training on evidence-based classroom management is often resource-intensive and has inconsistent impact on teacher behavior in the classroom. The purpose of this study was to investigate the impact of professional development on paraprofessionals\u27 treatment fidelity of the Caught Being Good Game. We used a multiple baseline design to provide a tiered model of professional development to provide the least intensive level necessary to support implementation. Results indicate that all four teachers successfully implemented most components of the Caught Being Good Game following direct training, with the exception of verbal feedback. One teacher required implementation planning followed by post-session feedback to reach mastery, and the final teacher did not reach mastery. Student disruption reduced slightly when the Caught Being Good Game was in place. Social validity results suggest teachers and students found the game to be highly acceptable and effective