Measurement and Analysis of III-V & II-VI Infrared Detectors: Radiometric, Noise Spectrum, and Radiation Tolerance Performance

Infrared (IR) hybrid detector arrays and discrete detectors operated in the space environment may be subjected to a variety of sources of natural radiation while in orbit. This means IR detectors intended for applications such as space-based intelligence, surveillance, and reconnaissance (ISR) or space-situational awareness (SSA) must not only have high performance (high quantum efficiency, h and low dark-current density, JD, and preferably minimal 1/f noise content), but also their radiation tolerance or ability to withstand the effects of the radiation they would expect to encounter in space must be characterized and well understood. As the effects of proton interactions with hybrid detector arrays can dominate in space, a specific detector’s radiation tolerance is typically characterized by measuring its performance degradation as a function of proton fluence, FP, up to a total ionizing dose (TID) of typically 100 krad(Si), which is 3-5 times the maximum expected on-orbit TID value for typical space-based E/O applications. Now for other applications such as astronomy, planetary science, and imaging associated with nuclear medicine applications, the TID requirement can be much higher. When comparing the performance of novel IR detector technologies, it has also proven valuable to determine the rate of performance degradation induced by radiation, referred to as a damage factor. It has also proven valuable to perform temperature-dependent measurements of JD, which are used to determine the dark current limiting mechanism via an Arrhenius-analysis, and the degree to which any thermal annealing of the irradiation induced defects may occur have provided unique insights. Finally, given the potential sensor/system impact it is of the upmost importance to understand the frequency dependent contributions to the overall noise in IR detectors. This body of work contains in-depth measurements and analysis of these performance metrics for both III-V- and II-VI-based IR detectors of various detector architectures. In this dissertation, the results of IR III-V-based InAs/GaSb and InAs/InAsSb type-II strained layer superlattice (T2SLS) and bulk detectors that employ unipolar barriers in their detector architecture and II-VI-based HgCdTe IR detectors are characterized in both clear and radiation environments. III-V-based IR detectors that employ unipolar barriers are now being considered for space applications due to their relative advantage in manufacturability as compared with conventional HgCdTe IR detectors that dominant space-based IR E/O imaging. T2SLS detectors are theoretically predicted to have lower Auger-limited dark currents compared with HgCdTe. However, this advantage is yet to be realized due to the lack of reliable passivation schemesand higher bulk defect densities in these materials, which lead to surface- and Shockley-Read-Hall (SRH)-limited dark currents, respectively. Unipolar-barrier architecture detectors, including the nBn, pBp, pBiBn, etc. detectors reported on here, have been introduced in an effort to mitigate these dark current limiting mechanisms. By deliberate choices of the absorber materials and device structure, the potential barriers in these detectors appear only in either the conduction or valence band to block the majority-carrier bulk and surface currents (e.g. in a nBn detector the potential barrier appears only in the conduction band). This results in an elegant detector architecture in which the ideal barrier layer limits the depletion by an external bias to itself so that the absorbing layer remains in the flatband condition, which eliminates Generation Recombination currents due to SRH defects that may be present in the absorbing layer that ultimately limit the diffusion length. Subjecting IR detectors to proton irradiation may lead to both TID and displacement damage effects, both of which occur on orbit. TID effects occur as incoming protons lose their kinetic energy to ionization of the detector material’s constituent atoms and the additional charges become trapped in oxide layers or surface traps. This additional charging may result in flat-band voltage shifts and increased surface leakage currents. TID effects generally are more visible at lower device temperatures, where charges generated in oxide layers are less mobile, and tend to anneal out at higher temperatures. Displacement damage effects result from the occasional non-ionizing energy loss of an incoming proton due to elastic or inelastic scattering with an atomic nucleus that is sufficient to knock the atom from its lattice site and generate vacancy-interstitial pairs, anti-sites, and defect complexes. In this work these defects were shown to manifest in lower h, due to the consequent reduction in minority carrier lifetime t, and higher JD, due to the SRH mechanism. The proton fluence required to alter the background doping levels, such that the fundamental Auger mechanism is enhanced, when using protons with an energy of 63 MeV is expected to be order’s of magnitude higher than the fluence levels used in this work. Thus, a vital step to characterizing a detector’s radiation tolerance is measuring h and JD as a function of FP, with all irradiation and measurements conducted in-situ stepwise at the detector’s expected operating temperature and bias. In this research, it was found that rate of degradation in quantum efficiency when irradiated with 63 MeV protons for a family of Sb-based MWIR detectors that employed unipolar barrier architectures was greater than 3 times that of conventional p-on-n HgCdTe photodiodes with similar cut-off wavelengths. Likewise, it was found that the rate of degradation in the lateral optical collection length for these same devices was greater than 20 times that of the equivalent MWIR HgCdTe photodiodes. This has been attributed to a degradation in minority carrier lifetime leading to a reduction in the diffusion length. This body of research provides unique insights into the radiation susceptibility and fundamental mechanisms taking place that directly contribute to performance degradation of III-V- and II-V-based IR detectors of various detector architectures

The Rise of the s-Process in the Galaxy

From newly-obtained high-resolution, high signal-to-noise ratio spectra the abundances of the elements La and Eu have been determined over the stellar metallicity range -3<[Fe/H]<+0.3 in 159 giant and dwarf stars. Lanthanum is predominantly made by the s-process in the solar system, while Eu owes most of its solar system abundance to the r-process. The changing ratio of these elements in stars over a wide metallicity range traces the changing contributions of these two processes to the Galactic abundance mix. Large s-process abundances can be the result of mass transfer from very evolved stars, so to identify these cases, we also report carbon abundances in our metal-poor stars. Results indicate that the s-process may be active as early as [Fe/H]=-2.6, alalthough we also find that some stars as metal-rich as [Fe/H]=-1 show no strong indication of s-process enrichment. There is a significant spread in the level of s-process enrichment even at solar metallicity.Comment: 64 pages, 15 figures; ApJ 2004 in pres

Anemone bleaching impacts the larval recruitment success of an anemone-associated fish

In marine environments, mutualisms such as those between corals or sea anemones and their algal symbionts (Symbiodiniaceae) play a key role for supporting surrounding biodiversity. However, as the breakdown of the mutualism between corals and/or anemones and Symbiodiniaceae (i.e. bleaching) become increasingly frequent and severe, the risk of losing the additional species that rely on them may also increase. While the effects of anemone bleaching on the biology and ecology of anemone-associated fishes have been the subject of recent research, relatively little is known about the impacts that anemone bleaching might have on the recruitment of larval fish. Here, we report that climate change-induced anemone bleaching impairs a secondary mutualism between anemones and an anemone-associated fish species, the threespot dascyllus (Dascyllus trimaculatus). Field-based monitoring over a 1-year period showed anemones that bleached experienced decreased recruitment of larval D. trimaculatus compared to those that did not bleach, with abundances of newly settled D. trimaculatus three times lower in bleached versus unbleached anemones. A visual choice experiment showed that this pattern is associated with fish being less attracted to bleached anemones, and a predation experiment demonstrated that fish associated with bleached anemones experienced higher mortality compared to those associated with unbleached anemones. These results suggests that the decreased recruitment of D. trimaculatus observed in bleached anemones may be driven by hampered pre-settlement (habitat selection) and post-settlement (survival to predation) processes for larval D. trimaculatus in bleached hosts. This study highlights the risk of cascading mutualism breakdowns in coral reefs as conditions deteriorate and stresses the importance of protecting these mutualisms for the maintenance of coral reef biodiversity

One Health approach to controlling a Q fever outbreak on an Australian goat farm

A recent outbreak of Q fever was linked to an intensive goat and sheep dairy farm in Victoria, Australia, 2012-2014. Seventeen employees and one family member were confirmed with Q fever over a 28-month period, including two culture-positive cases. The outbreak investigation and management involved a One Health approach with representation from human, animal, environmental and public health. Seroprevalence in non-pregnant milking goats was 15% [95% confidence interval (CI) 7&ndash;27]; active infection was confirmed by positive quantitative PCR on several animal specimens. Genotyping of Coxiella burnetii DNA obtained from goat and human specimens was identical by two typing methods. A number of farming practices probably contributed to the outbreak, with similar precipitating factors to the Netherlands outbreak, 2007-2012. Compared to workers in a high-efficiency particulate arrestance (HEPA) filtered factory, administrative staff in an unfiltered adjoining office and those regularly handling goats and kids had 5&middot;49 (95% CI 1&middot;29&ndash;23&middot;4) and 5&middot;65 (95% CI 1&middot;09&ndash;29&middot;3) times the risk of infection, respectively; suggesting factory workers were protected from windborne spread of organisms. Reduction in the incidence of human cases was achieved through an intensive human vaccination programme plus environmental and biosecurity interventions. Subsequent non-occupational acquisition of Q fever in the spouse of an employee, indicates that infection remains endemic in the goat herd, and remains a challenge to manage without source control

Namib Desert Soil Microbial Community Diversity, Assembly, and Function Along a Natural Xeric Gradient

The hyperarid Namib desert is a coastal desert in southwestern Africa and one of the oldest and driest deserts on the planet. It is characterized by a west/east increasing precipitation gradient and by regular coastal fog events (extending up to 75 km inland) that can also provide soil moisture. In this study, we evaluated the role of this natural aridity and xeric gradient on edaphic microbial community structure and function in the Namib desert. A total of 80 individual soil samples were collected at 10-km intervals along a 190-km transect from the fog-dominated western coastal region to the eastern desert boundary. Seventeen physicochemical parameters were measured for each soil sample. Soil parameters reflected the three a priori defined climatic/xeric zones along the transect (“fog,” “low rain,” and “high rain”). Microbial community structures were characterized by terminal restriction fragment length polymorphism fingerprinting and shotgun metaviromics, and their functional capacities were determined by extracellular enzyme activity assays. Both microbial community structures and activities differed significantly between the three xeric zones. The deep sequencing of surface soil metavirome libraries also showed shifts in viral composition along the xeric transect. While bacterial community assembly was influenced by soil chemistry and stochasticity along the transect, variations in community “function” were apparently tuned by xeric stress.The South African National Research Foundation (NRF; grant N00113-95565), the University of Pretoria and the Genomics Research Institute.http://link.springer.com/journal/2482019-01-01hj2017GeneticsMicrobiology and Plant Patholog

Standalone vertex finding in the ATLAS muon spectrometer

A dedicated reconstruction algorithm to find decay vertices in the ATLAS muon spectrometer is presented. The algorithm searches the region just upstream of or inside the muon spectrometer volume for multi-particle vertices that originate from the decay of particles with long decay paths. The performance of the algorithm is evaluated using both a sample of simulated Higgs boson events, in which the Higgs boson decays to long-lived neutral particles that in turn decay to bbar b final states, and pp collision data at √s = 7 TeV collected with the ATLAS detector at the LHC during 2011

Measurements of Higgs boson production and couplings in diboson final states with the ATLAS detector at the LHC

Measurements are presented of production properties and couplings of the recently discovered Higgs boson using the decays into boson pairs, H →γ γ, H → Z Z∗ →4l and H →W W∗ →lνlν. The results are based on the complete pp collision data sample recorded by the ATLAS experiment at the CERN Large Hadron Collider at centre-of-mass energies of √s = 7 TeV and √s = 8 TeV, corresponding to an integrated luminosity of about 25 fb−1. Evidence for Higgs boson production through vector-boson fusion is reported. Results of combined fits probing Higgs boson couplings to fermions and bosons, as well as anomalous contributions to loop-induced production and decay modes, are presented. All measurements are consistent with expectations for the Standard Model Higgs boson

Measurement of the top quark pair cross section with ATLAS in pp collisions at √s=7 TeV using final states with an electron or a muon and a hadronically decaying τ lepton

A measurement of the cross section of top quark pair production in proton-proton collisions recorded with the ATLAS detector at the Large Hadron Collider at a centre-of-mass energy of 7 TeV is reported. The data sample used corresponds to an integrated luminosity of 2.05 fb -1. Events with an isolated electron or muon and a τ lepton decaying hadronically are used. In addition, a large missing transverse momentum and two or more energetic jets are required. At least one of the jets must be identified as originating from a b quark. The measured cross section, σtt-=186±13(stat.)±20(syst.)±7(lumi.) pb, is in good agreement with the Standard Model prediction