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Life has been described as information flowing in molecular streams (Dawkins, 1996).Our growing understanding of the impact of horizontal gene transfer on evolutionary dynamics reinforces this fluid-like flow of molecular information (Joyce, 2002). The diversity of nucleic acid sequences, those known and yet to be characterized across Earth's varied environments, along with the vast repertoire of catalytic and structural proteins, presents as more of a dynamic molecular river than a tree of life. These informational biopolymers function as a mutualistic union so universal as to have been termed the Central Dogma (Crick, 1958). It is the distinct folding dynamics-the digital-like base pairing dominating nucleic acids, and the environmentally responsive and diverse range of analog-like interactions dictating protein folding (Goodwin et al., 2012)-that provides the basis for the mutualism. The intertwined functioning of these analog and digital forms of information (Goodwin et al., 2012) unified within diverse chemical networks is heralded as the Darwinian threshold of cellular life (Woese, 2002).
The discovery of prion diseases (Chien et al., 2004; Jablonka and Raz, 2009; Paravastu et al., 2008) introduced the paradigm of protein templates that propagate conformational information, suggesting a new context for Darwinian evolution. When taking both protein and nucleic acid moelcular evolution into consideration (Cairns- Smith, 1966; Joyce, 2002), the conceptual framework for chemical evolution can be generalized into three orthogonal dimensions as shown in Figure 5.1 (Goodwin et al., 2014). The 1st dimension manifests structural order through covalent polymerization reactions and includes chain length, sequence, and linkage chemistry inherent to a dynamic chemical network. The 2nd dimension extends the order in dynamic conformational networks through noncovalent interactions of the polymers. This dimension includes intramolecular and intermolecular forces, from macromolecular folding to supramolecular assembly to multicomponent quaternary structure. Folding in this 2nd dimension certainly depends on the primary polymer sequence, and the folding/assembly diversity yields an additional set of environmentally constrained supramolecular folding codes. For example, double-stranded DNA assemblies are dominated by the rules of complementary base pairing, while the self-propagating conformations of prions are based on additional noncovalent, environmentally-dependent interactions.
A non-destructive neutron scattering method was developed to precisely measure the uptake of total hydrogen in nuclear grade Zircaloy-4 cladding. The hydriding apparatus consists of a closed stainless steel vessel that contains Zircaloy-4 specimens and hydrogen gas. By controlling the initial hydrogen gas pressure in the vessel and the temperature profile, target hydrogen concentrations from tens of ppm to a few thousands of ppm have been successfully achieved. Following hydrogen charging, the hydrogen content of the hydrided specimens was measured using the vacuum hot extraction method (VHE), by which the samples with desired hydrogen concentration were selected for the neutron study. Small angle incoherent neutron scattering (SAINS) were performed in the High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory (ORNL). Our study indicates that a very small amount (≈ 20 ppm) hydrogen in commercial Zircaloy-4 cladding can be measured very accurately in minutes for a wide range of hydrogen concentration by a nondestructive method. The hydrogen distribution in a tube sample was obtained by scaling the neutron scattering rate with a factor, which is determined by calibration process with direct chemical analysis method on the specimen. This scale factor can be used for future test with unknown hydrogen concentration, thus provide a nondestructive method for absolute hydrogen concentration determination.
Significant new opportunities for astrophysics and cosmology have been identified at low radio frequencies. The Murchison Widefield Array is the first telescope in the southern hemisphere designed specifically to explore the low-frequency astronomical sky between 80 and 300 MHz with arcminute angular resolution and high survey efficiency. The telescope will enable new advances along four key science themes, including searching for redshifted 21-cm emission from the EoR in the early Universe; Galactic and extragalactic all-sky southern hemisphere surveys; time-domain astrophysics; and solar, heliospheric, and ionospheric science and space weather. The Murchison Widefield Array is located in Western Australia at the site of the planned Square Kilometre Array (SKA) low-band telescope and is the only low-frequency SKA precursor facility. In this paper, we review the performance properties of the Murchison Widefield Array and describe its primary scientific objectives.
A superior cavopulmonary connection is commonly performed before the Fontan procedure in patients with a functionally univentricular heart. Data are limited regarding associations between a prior superior cavopulmonary connection and functional and ventricular performance late after the Fontan procedure.
We compared characteristics of those with and without prior superior cavopulmonary connection among 546 subjects enrolled in the Pediatric Heart Network Fontan Cross-Sectional Study. We further compared different superior cavopulmonary connection techniques: bidirectional cavopulmonary anastomosis (n equals 229), bilateral bidirectional cavopulmonary anastomosis (n equals 39), and hemi-Fontan (n equals 114).
A prior superior cavopulmonary connection was performed in 408 subjects (75%); the proportion differed by year of Fontan surgery and centre (p-value less than 0.0001 for each). The average age at Fontan was similar, 3.5 years in those with superior cavopulmonary connection versus 3.2 years in those without (p-value equals 0.4). The type of superior cavopulmonary connection varied by site (p-value less than 0.001) and was related to the type of Fontan procedure. Exercise performance, echocardiographic variables, and predominant rhythm did not differ by superior cavopulmonary connection status or among superior cavopulmonary connection types. Using a test of interaction, findings did not vary according to an underlying diagnosis of hypoplastic left heart syndrome.
After controlling for subject and era factors, most long-term outcomes in subjects with a prior superior cavopulmonary connection did not differ substantially from those without this procedure. The type of superior cavopulmonary connection varied significantly by centre, but late outcomes were similar.
Using 0.5 ps pulses of 5.9 eV light to excite electron-hole concentrations varied up to 2x1020 e-h/cm3 corresponding to energy deposition within electron tracks, we measure dipole-dipole quenching rate constants K2 in SrI2 and CsI. We previously reported determination of K2 directly from the time dependence of quenched STE luminescence in CsI. The nonlinear quenching rate decreases rapidly within a few tens of picoseconds as the host excitation density drops below the Förster threshold. In the present work, we measure the dependence of integrated light yield on excitation density in the activated scintillators SrI2:Eu2+ and CsI:Tl+. The “z-scan” method of yield vs. irradiance is applicable to a wider range of materials, e.g. when the quenching population is not the main light-emitting population. Furthermore, because of using an integrating sphere and photomultiplier for light detection, the signal-to-noise is substantially better than the time-resolved method using a streak camera. As a result, both 2nd and 3rd orders of quenching (dipole-dipole and Auger) can be distinguished. Detailed comparison of SrI2 and CsI is of fundamental importance to help understand why SrI2 achieves substantially better proportionality than CsI in scintillator applications. The laser measurements, in contrast to scintillation, allow evaluating the rate constants of nonlinear quenching in a population which has small enough spatial gradient to suppress the effect of carrier diffusion.
We report on the optical and charge transport properties of novel alkali metal chalcogenides, Cs2Hg6S7 and Cs2Cd3Te4, pertaining to their use in radiation detection. Optical absorption, photoconductivity, and gamma ray response measurements for undoped crystals were measured. The band gap energies of the Cs2Hg6S7 and Cs2Cd3Te4 compounds are 1.63 eV and 2.45 eV, respectively. The mobility-lifetime products for charge carriers are of the order of ~10-3 cm2/V for electrons and ~10-4 cm2/V for holes. Detectors fabricated from the ternary compound Cs2Hg6S7 shows well-resolved spectroscopic features at room temperature in response to ϒ -rays at 122 keV from a 57Co source, indicating its potential as a radiation detector.
We have previously described a numerical model for carrier diffusion and nonlinear quenching in the track of an electron in a scintillator. Significant inequality of electron and hole mobilities predicts a characteristic “hump” in the light yield vs gamma energy, whereas low mobility of either or both carriers accentuates the universal roll-off due to nonlinear quenching at low gamma energy (high dE/dx). The material parameter basis of the two major trends in nonproportionality of scintillators can be related to the effective diffusion coefficient of excitations and the difference of electron and hole mobilities, respectively. Activator concentration, type of activator, and effect of transport anisotropy are associated with minor trends. The predicted trends are qualitatively consistent with empirical measures of nonproportionality including electron yield curves.
Cd0.9Zn0.1Te (CZT) detector grade crystals were grown from zone refined Cd, Zn, and Te (7N) precursor materials, using the tellurium solvent method. These crystals were grown using a high temperature vertical furnace designed and installed in our laboratory. The furnace is capable of growing up to 8” diameter crystals, and custom pulling and ampoule rotation functions using custom electronics were furnished for this setup. CZT crystals were grown using excess Te as a solvent with growth temperatures lower than the melting temperatures of CZT (1092°C). Tellurium inclusions were characterized through IR transmittance maps for the grown CZT ingots. The crystals from the grown ingots were processed and characterized using I-V measurements for electrical resistivity, thermally stimulated current (TSC), and electron beam induced current (EBIC). Pulse height spectra (PHS) measurements were carried out using a 241Am (59.6 keV) radiation source, and an energy resolution of ~4.2% FWHM was obtained. Our investigation demonstrates high quality detector grade CZT crystals growth using this low temperature solvent method.
In this present work we report the growth of Cd0.9Zn0.1Te doped with In by a modified THM technique. It has been demonstrated that by controlling the microscopically flat growth interface, the size distribution and concentration of Te inclusions can be drastically reduced in the as-grown ingots. This results in as-grown detector-grade CZT by the THM technique. The three-dimensional size distribution and concentrations of Te inclusions/precipitations were studied. The size distributions of the Te precipitations/inclusions were observed to be below the 10-μm range with the total concentration less than 105 cm-3. The relatively low value of Te inclusions/precipitations results in excellent charge transport properties of our as-grown samples. The (μτ)e values for different as-grown samples varied between 6-20 x10-3 cm2/V. The as-grown samples also showed fairly good detector response with resolution of ∼1.5%, 2.7% and about 3.8% at 662 keV for quasi-hemispherical geometry for detector volumes of 0.18 cm3, 1 cm3 and 4.2 cm3, respectively.
The imperfect quality of CdZnTe (CZT) crystals for radiation detectors seriously diminishes their suitability for different applications. Dislocations and other dislocation-related defects, such as sub-grain boundaries and dislocation fields around Te inclusions, engender significant charge losses and, consequently, cause fluctuations in the detector’s output signals, thereby hindering their spectroscopic responses. In this paper, we discuss our results from characterizing CZT material by using a high-spatial-resolution X-ray response mapping system at BNL’s National Synchrotron Light Source. In this paper, we emphasize the roles of these dislocation-related defects and their contributions in degrading the detector’s performance. Specifically, we compare the effects of the sub-grain- and coherent twin-boundaries on the X-ray response maps.
We address the issue of decreasing band-gap with increasing atomic number, inherent in semiconducting materials, by introducing a concept we call dimensional reduction. The concept leads to semiconductor compounds containing high atomic number elements and simultaneously exhibiting a large band gap and high mass density suggesting that dimensional reduction can be successfully employed in developing new γ-ray detecting materials. As an example we discuss the compound Cs2Hg6S7 that exhibits a band-gap of 1.65eV and mobility-lifetime products comparable to those of optimized Cd0.9Zn0.1Te.
Residual impurities in manganese (Mn) are a big obstacle to obtaining high- performance CdMnTe (CMT) X-ray and gamma-ray detectors. Generally, the zone-refining method is an effective way to improve the material’s purity. In this work, we purified the MnTe compounds combining the zone-refining method with molten Te that has a very high solubility. We confirmed the improved purity of the material by glow-discharge mass spectrometry (GDMS). We also found that CMT crystals from a multiple refined MnTe source, grown by the vertical Bridgman method, yielded better performing detectors.
We investigated defects in CdZnTe crystals produced from various conditions and their impact on fabricated devices. In this study, we employed transmission and scanning transmission electron microscope (TEM and STEM), because defects at the nano-scale are not observed readily under an optical or infrared microscope, or by most other techniques. Our approach revealed several types of defects in the crystals, such as low-angle boundaries, dislocations and precipitates, which likely are major causes in degrading the electrical properties of CdZnTe devices, and eventually limiting their performance.
The Czochralski pulling process is the most valuable and cost efficient method for producing large oriented single crystals of the group IV and III-V semiconductors. However, there have been only a small number of reported attempts to use the Czochralski process for growing the wide bandgap compound semiconductors, needed for the room temperature operated gamma-ray detectors. The main difficulty is in the low chemical stability and high vapor pressure of the group II, V and VI elements, leading to off-stoichiometric composition, and various related defects. Among the heavy metal halides, indium iodide and indium bromide present an interesting exception. InI has a high molecular disassociation energy and a low vapor pressure, allowing for Czochralski pulling. We will describe the procedures used and the results obtained by Czochralski growth and characterization of indium iodide and the related ternary compounds that appear to be quite encouraging.