Single Photon Counting Detectors for Low Light Level Imaging Applications

This dissertation presents the current state-of-the-art of semiconductor-based photon counting detector technologies. HgCdTe linear-mode avalanche photodiodes (LM-APDs), silicon Geiger-mode avalanche photodiodes (GM-APDs), and electron-multiplying CCDs (EMCCDs) are compared via their present and future performance in various astronomy applications. LM-APDs are studied in theory, based on work done at the University of Hawaii. EMCCDs are studied in theory and experimentally, with a device at NASA\u27s Jet Propulsion Lab. The emphasis of the research is on GM-APD imaging arrays, developed at MIT Lincoln Laboratory and tested at the RIT Center for Detectors. The GM-APD research includes a theoretical analysis of SNR and various performance metrics, including dark count rate, afterpulsing, photon detection efficiency, and intrapixel sensitivity. The effects of radiation damage on the GM-APD were also characterized by introducing a cumulative dose of 50 krad(Si) via 60 MeV protons. Extensive development of Monte Carlo simulations and practical observation simulations was completed, including simulated astronomical imaging and adaptive optics wavefront sensing. Based on theoretical models and experimental testing, both the current state-of-the-art performance and projected future performance of each detector are compared for various applications. LM-APD performance is currently not competitive with other photon counting technologies, and are left out of the application-based comparisons. In the current state-of-the-art, EMCCDs in photon counting mode out-perform GM-APDs for long exposure scenarios, though GM-APDs are better for short exposure scenarios (fast readout) due to clock-induced-charge (CIC) in EMCCDs. In the long term, small improvements in GM-APD dark current will make them superior in both long and short exposure scenarios for extremely low flux. The efficiency of GM-APDs will likely always be less than EMCCDs, however, which is particularly disadvantageous for moderate to high flux rates where dark noise and CIC are insignificant noise sources. Research into decreasing the dark count rate of GM-APDs will lead to development of imaging arrays that are competitive for low light level imaging and spectroscopy applications in the near future

Examining the Current Perceptions of K-3 Elementary School Teachers, and their Students, in Regard to Handwriting Instruction in the Modern Day Classroom

The intent of this thesis is to explore the current views and opinions that elementary school teachers, particularly Kindergarten through third grade teachers, have on the topic of handwriting instruction in their classrooms. Apart from the teachers, the views and opinions of selected Kindergarten through third grade students are also taken into consideration. Due to the advancement of technology and its prevalence in modern day classrooms, many consider handwriting, particularly cursive, a dying art, with keyboarding taking its place. However, research has proven that handwriting provides students with developmental benefits and helps to refine certain motor skills that keyboarding does not. To better understand the views of both teachers and students alike, the research design consists of teacher surveys and student focus groups to provide a better scope of understanding as to how teachers and students view this change in writing. The data is presented in the form of graphs and charts, which are then examined in detail

Characterization of silicon Geiger-mode avalanche photodiodes with novel device architecture

Geiger-mode avalanche photodiode (GM APD) detectors are capable of counting single photons, measuring arrival times with high resolution, and generating zero read noise (when operated with a CMOS digital readout circuit) due to their unique internal gain characteristics. These capabilities make them exceptionally suited to tasks that require precise arrival time measurements or characterization of faint signals (low photon flux). Laser ranging systems use their arrival time measurement capabilities to build three-dimensional images, while adaptive optics applications have recently begun to capitalize on their low noise and high-speed operation for correcting wavefront imperfections due to atmospheric interference. There is now growing interest in using GM APDs for imaging applications where accurate measurements of faint signals are necessary, such as in astronomy. MIT Lincoln Laboratory and the RIT Center for Detectors have developed silicon GM APDs with unique architecture, utilizing scupper regions to minimize detector noise. This thesis investigates the performance of these detectors in terms of dark count rate (DCR). There are a number of mechanisms that produce dark counts, the most prominent being thermal excitation of carriers. Thermal carrier generation rates are generally only dependent on the temperature of the diode and may be constant under certain controlled conditions. Afterpulsing results from the release of carriers trapped in intermediate energy states (states with energy in the band gap of the material). Unlike thermal carrier generation, afterpulsing is dependent on the quenching time of the device (during which the device is unable to detect a carrier). Another mechanism, called self re-triggering, occurs when relaxing carriers emit photons during an avalanche. These photons can be absorbed in the substrate and generate dark carriers. Self-retriggering is also dependent on the quenching time of the device. Theories for afterpulsing and self-retriggering are discussed. Specialized test circuitry is used with a customized data acquisition technique, and the author develops a method for parameter extraction from the raw data. Device characteristics derived from experimental results are examined. The author also develops a simulation program to approximate the dark count rate (among other parameters) of a device based on semiconductor characteristics and testing conditions. This thesis makes conclusions about the dependence of DCR on device architecture and how individual carrier generation mechanisms affect device performance

Disentangling the role of photosynthesis and stomatal conductance on rising forest water-use efficiency

Multiple lines of evidence suggest that plant water-use efficiency (WUE) -the ratio of carbon assimilation to water loss- has increased in recent decades. Although rising atmospheric CO2 has been proposed as the principal cause, the underlying physiological mechanisms are still being debated, and implications for the global water cycle remain uncertain. Here, we addressed this gap using 30-y tree ring records of carbon and oxygen isotope measurements and basal area increment from 12 species in 8 North American mature temperate forests. Our goal was to separate the contributions of enhanced photosynthesis and reduced stomatal conductance to WUE trends and to assess consistency between multiple commonly used methods for estimating WUE. Our results show that tree ring-derived estimates of increases in WUE are consistent with estimates from atmospheric measurements and predictions based on an optimal balancing of carbon gains and water costs, but are lower than those based on ecosystemscale flux observations. Although both physiological mechanisms contributed to rising WUE, enhanced photosynthesis was widespread, while reductions in stomatal conductance were modest and restricted to species that experienced moisture limitations. This finding challenges the hypothesis that rising WUE in forests is primarily the result of widespread, CO2-induced reductions in stomatal conductance

Height, selected genetic markers and prostate cancer risk:Results from the PRACTICAL consortium

Background: Evidence on height and prostate cancer risk is mixed, however, recent studies with large data sets support a possible role for its association with the risk of aggressive prostate cancer. Methods: We analysed data from the PRACTICAL consortium consisting of 6207 prostate cancer cases and 6016 controls and a subset of high grade cases (2480 cases). We explored height, polymorphisms in genes related to growth processes as main effects and their possible interactions. Results: The results suggest that height is associated with high-grade prostate cancer risk. Men with height 4180cm are at a 22% increased risk as compared to men with height o173cm (OR 1.22, 95% CI 1.01–1.48). Genetic variants in the growth pathway gene showed an association with prostate cancer risk. The aggregate scores of the selected variants identified a significantly increased risk of overall prostate cancer and high-grade prostate cancer by 13% and 15%, respectively, in the highest score group as compared to lowest score group. Conclusions: There was no evidence of gene-environment interaction between height and the selected candidate SNPs. Our findings suggest a role of height in high-grade prostate cancer. The effect of genetic variants in the genes related to growth is seen in all cases and high-grade prostate cancer. There is no interaction between these two exposures.</p

Many Labs 5:Testing pre-data collection peer review as an intervention to increase replicability

Replication studies in psychological science sometimes fail to reproduce prior findings. If these studies use methods that are unfaithful to the original study or ineffective in eliciting the phenomenon of interest, then a failure to replicate may be a failure of the protocol rather than a challenge to the original finding. Formal pre-data-collection peer review by experts may address shortcomings and increase replicability rates. We selected 10 replication studies from the Reproducibility Project: Psychology (RP:P; Open Science Collaboration, 2015) for which the original authors had expressed concerns about the replication designs before data collection; only one of these studies had yielded a statistically significant effect (p < .05). Commenters suggested that lack of adherence to expert review and low-powered tests were the reasons that most of these RP:P studies failed to replicate the original effects. We revised the replication protocols and received formal peer review prior to conducting new replication studies. We administered the RP:P and revised protocols in multiple laboratories (median number of laboratories per original study = 6.5, range = 3?9; median total sample = 1,279.5, range = 276?3,512) for high-powered tests of each original finding with both protocols. Overall, following the preregistered analysis plan, we found that the revised protocols produced effect sizes similar to those of the RP:P protocols (?r = .002 or .014, depending on analytic approach). The median effect size for the revised protocols (r = .05) was similar to that of the RP:P protocols (r = .04) and the original RP:P replications (r = .11), and smaller than that of the original studies (r = .37). Analysis of the cumulative evidence across the original studies and the corresponding three replication attempts provided very precise estimates of the 10 tested effects and indicated that their effect sizes (median r = .07, range = .00?.15) were 78% smaller, on average, than the original effect sizes (median r = .37, range = .19?.50)

Effects of Crown Scorch on Ponderosa Pine Resistance to Bark Beetles in Northern Arizona

We conducted a study linking mechanistic relationships among pre injury, tree physiological condition, and bark beetle (Coleoptera: Scolytidae) susceptibility. Crown scorch of 40 Pinus ponderosa trees was estimated in a naturally regenerated stand that was thinned and prescribed burned in winter and early spring 2000. Net photosynthetic rate (Pn) of undamaged foliage was higher in heavily and severely than moderately and lightly scorched trees. This trend was more pronounced in the dry season (June) than the wet season (August). This suggests the mechanism underlying the increase in Pn was an improvement in water relations. This was also supported by differences observed in predawn water potential and stomatal conductance (gs). Constitutive resin volume did not differ among crown scorch classes in June, but was negatively related to crown scorch intensity in September. This suggests that constitutive resin was formed and stored before pre injury, and therefore not strongly affected by crown scorch levels in June. Induced resin production (resin produced after depletion of constitutive resin) generally decreased with increasing crown scorch in both June and September. Induced resin appeared to be related to carbon available for resin synthesis. The proportion of successful colonization attempts by Ips spp. and Dendroctonus spp. was low throughout the season. Colonization of Ips sp. and Dendroctonus spp. was not spatially separated in host trees. Colonization attempts were generally positively related to intensity of crown scorch in both pheromone and no-pheromone treatments. This finding suggests that pheromones attracted insects to the tree, but host physiological condition or other factors ultimately determined host colonization. Our results also suggest that intensity of crown scorch may affect colonization attempts of Ips and Dendroctonus spp. on P. ponderosa trees