Search for: All records

Halogen (F, Cl, Br, and I) concentrations for 129 loess samples from worldwide localities yield geometric means of 517 ± 53 μg/g F, 150 ± 20 μg/g Cl, 1.58 ± 0.16 μg/g Br, 1.16 ± 0.11 μg/g I (2 standard errors). These concentrations, notably for Br and I, are substantially higher than previous estimates for the average upper continental crystalline bedrocks, with enrichment factors of 1.3 +0.7/−0.4 (F), 1.8 +2.4/−0.8 (Cl), 3.8 +1.3/−1.0 (Br), and 39 +71/−16 (I) (95%confidence), documenting enrichment of halogens on the continental surface. These surface halogens are likely sourced from the oceans and may be influenced by climate fluctuations. Halogen ratios (Br/Cl, I/Cl, and Br/I) in loess are similar to those of organic-rich soils/sediments from both terrigenous and marine settings, suggesting that terrigenous and marine organic matter have indistinguishable halogen ratios. The Br/I ratios differ from those in the fine grained matrix of glacial diamictites, indicating that another process (beyond biological influence) is responsible for fractionating halogens in the upper continental crust. Using a mixing model, we calculate that over 80–90 % of loess originates from crystalline bedrocks, while the remainder (<10–20 %) derives from the halogen- and organic-rich sedimentary cover or other sources (e.g., marine aerosols). 

This research focused on documenting student computer science (CS) interest, value, and self-efficacy across K-8 grade levels. It also examined differences between various student subgroups. The research questions were: Are there differences in K-8 students’ ratings of CS interest, task value, and self-efficacy? How does interest, task value, and self-efficacy change across grades K-8? Are there differential effects depending on student gender, ethnicity, and locale (rural/urban)? Results showed a continuing decline in the three student outcomes across grades, mirroring the larger body of STEM research. Students’ perception of the value of computer science was significantly higher than their interest, which in turn was higher than their self-efficacy. This result suggests that student recognize the importance of CS and find it fun, but not easy. Moderation results showed no gender differences at the elementary level, but significantly higher middle school results for males on CS interest and self-efficacy. Ethnicity findings tended to favor majority (white) students, with the exception that grade 6 – 8 minority students had significantly higher interest ratings. Urban students had significantly higher results for all three outcomes at grades K-3 and 6 -8. The research extends research documenting the decrease of adolescent student STEM interest by focusing specifically on computer science and including results at the elementary level. 

ABSTRACT Interferometric experiments designed to detect the highly redshifted 21-cm signal from neutral hydrogen are producing increasingly stringent constraints on the 21-cm power spectrum, but some k-modes remain systematics-dominated. Mutual coupling is a major systematic that must be overcome in order to detect the 21-cm signal, and simulations that reproduce effects seen in the data can guide strategies for mitigating mutual coupling. In this paper, we analyse 12 nights of data from the Hydrogen Epoch of Reionization Array and compare the data against simulations that include a computationally efficient and physically motivated semi-analytic treatment of mutual coupling. We find that simulated coupling features qualitatively agree with coupling features in the data; however, coupling features in the data are brighter than the simulated features, indicating the presence of additional coupling mechanisms not captured by our model. We explore the use of fringe-rate filters as mutual coupling mitigation tools and use our simulations to investigate the effects of mutual coupling on a simulated cosmological 21-cm power spectrum in a ‘worst case’ scenario where the foregrounds are particularly bright. We find that mutual coupling contaminates a large portion of the ‘EoR Window’, and the contamination is several orders-of-magnitude larger than our simulated cosmic signal across a wide range of cosmological Fourier modes. While our fiducial fringe-rate filtering strategy reduces mutual coupling by roughly a factor of 100 in power, a non-negligible amount of coupling cannot be excised with fringe-rate filters, so more sophisticated mitigation strategies are required.