A study was initiated to investigate the chronological deposition of uranium in certain species of trees growing on the site
of a former uranium metal processing facility. The Feed Materials Production Center (FMPC) is located in Fernald, Ohio, and
for roughly 40 years operated as a large scale uranium processing center. Core samples from several species of trees growing
in different locations throughout the site were extracted using a 12.5 mm incremental wood boring drill bit. After extraction,
each core sample was cut and packaged into individual sections representing 4 annual growth rings and submitted for instrumental
neutron activation analysis (INAA). The reaction 235U(n,f)140Ba→140La+γ was evaluated using high resolution germanium gamma-spectroscopy to detect the 1.596 MeV photon emission from the fission
product 140La following a minimum of a 3 week decay. A total of 106 samples representing 7 individual trees of 3 unique species were
irradiated. In addition to the tree-core samples, 18 quality control (QC) samples and 18 standard reference material (SRM)
Fly Ash samples were irradiated with the core samples for determining neutron flux. The activity in any one sample in a batch
was determined by comparison with the amount of natural uranium in the QC standards. No significantly measurable amount of
uranium was detected in any of the tree core samples, although 3 tree core samples were in excess of the minimum detectable
amount (30 ng). 相似文献
A general synthesis for the preparation of alkyl N,N'-beta-diketimines has been developed. The method reported here demonstrates the use of dimethyl sulfate for conversion of enaminoketones to beta-diketimines. The reaction can be performed without solvent, providing good yields. 相似文献
The capacitated single assignment hub location problem with modular link capacities is a variant of the classical hub location problem in which the cost of using edges is not linear but stepwise, and the hubs are restricted in terms of transit capacity rather than in the incoming traffic. We propose a metaheuristic algorithm based on strategic oscillation, a methodology originally introduced in the context of tabu search. Our method incorporates several designs for constructive and destructive algorithms, together with associated local search procedures, to balance diversification and intensification for an efficient search. Computational results on a large set of instances show that, in contrast to exact methods that can only solve small instances optimally, our metaheuristic is able to find high-quality solutions on larger instances in short computing times. In addition, the new method, which joins tabu search strategies with strategic oscillation, outperforms the previous tabu search implementation. 相似文献
Quinol esters 2b, 2c, and 3b and sulfonamide 4c were investigated as possible precursors to 4-alkylaryloxenium ions, reactive intermediates that have not been previously detected. These compounds exhibit a variety of interesting reactions, but with one possible exception, they do not generate oxenium ions. The 4-isopropyl ester 2b predominantly undergoes ordinary acid- and base-catalyzed ester hydrolysis. The 4-tert-butyl ester 2c decomposes under both acidic and neutral conditions to generate tert-butanol and 1-acetyl-1,4-hydroquinone, 8, apparently by an SN1 mechanism. This is also a minor decomposition pathway for 2b, but the mechanism in that case is not likely to be SN1. Decomposition of 2c in the presence of N3- leads to formation of the explosive 2,3,5,6-tetraazido-1,4-benzoquinone, 14, produced by N3--induced hydrolysis of 8, followed by a series of oxidations and nucleophilic additions by N3-. No products suggestive of N3--trapping of an oxenium ion were detected. The 4-isopropyl dichloroacetic acid ester 3b reacts with N3- to generate the two adducts 2-azido-4-isopropylphenol, 5b, and 3-azido-4-isopropylphenol, 11b. Although 5b is the expected product of N3- trapping of the oxenium ion, kinetic analysis shows that it is produced by a kinetically bimolecular reaction of N3- with 3b. No oxenium ion is involved. The sulfonamide 4c predominantly undergoes a rearrangement reaction under acidic and neutral conditions, but a minor component of the reaction yields 4-tert-butylcresol, 17, and 2-azido-4-tert-butylphenol, 5c, in the presence of N3-. These products may indicate that 4c generates the oxenium ion 1c, but they are generated in very low yields (ca. 10%) so it is not possible to definitively conclude that 1c has been produced. If 1c has been generated, the N3--trapping data indicate that it is a very short-lived and reactive species in H2O. Comparisons with similarly reactive nitrenium ions indicate that the lifetime of 1c is ca. 20-200 ps if it is generated, so it must react by a preassociation process. Density functional theory calculations at the B3LYP/6-31G*//HF/6-31G* level coupled with kinetic correlations also indicate that the aqueous solution lifetimes of 1a-c are in the picosecond range. 相似文献
Increasing convex-along-rays functions are defined within an abstract convexity framework. The basic properties of these functions including support sets and subdifferentials are outlined. Applications are provided to unconstrained global optimization using the concept of excess function. 相似文献
Large enhancements have been observed in the sub-barrier fusion cross sections for Ti+Ni systems in our previous studies.
Coupled channel calculations incorporating couplings to 2+ and 3− states failed to explain these enhancements completely. A possibilty of transfer channels contributing to the residual enhancements
had been suggested. In order to investigate the role of relevant transfer channels, measurements of one- and two-nucleon transfer
were carried out for 46,48Ti+61Ni systems. The present paper gives the results of these studies. 相似文献
Ion mobility/mass spectrometry techniques are employed to investigate the binding of Zn2+ to the nine-residue peptide hormone oxytocin (OT, Cys1-Tyr2-Ile3-Gln4-Asn5-Cys6-Pro7-Leu8-Gly9-NH2, having a disulfide bond between Cys1 and Cys6 residues). Zn2+ binding to OT is known to increase the affinity of OT for its receptor [Pearlmutter, A. F., Soloff, M. S.: Characterization of the metal ion requirement for oxytocin-receptor interaction in rat mammary gland membranes. J. Biol. Chem. 254, 3899–3906 (1979)]. In the absence of Zn2+, we find evidence for two primary OT conformations, which arise because the Cys6–Pro7 peptide bond exists in both the trans- and cis-configurations. Upon addition of Zn2+, we determine binding constants in water of KA = 1.43 ± 0.24 and 0.42 ± 0.12 μM?1, for the trans- and cis-configured populations, respectively. The Zn2+ bound form of OT, having a cross section of Ω = 235 Å2, has Pro7 in the trans-configuration, which agrees with a prior report [Wyttenbach, T., Liu, D., Bowers, M. T.: Interactions of the hormone oxytocin with divalent metal ions. J. Am. Chem. Soc. 130, 5993–6000 (2008)], in which it was proposed that Zn2+ binds to the peptide ring and is further coordinated by interaction of the C-terminal, Pro7-Leu8-Gly9-NH2, tail. The present work shows that the cis-configuration of OT isomerizes to the trans-configuration upon binding Zn2+. In this way, the proline residue regulates Zn2+ binding to OT and, hence, is important in receptor binding.
T2?-weighted blood oxygen level-dependent functional magnetic resonance imaging is adversely affected by susceptibility-induced field gradients in brain regions adjoining air interfaces that cause image distortion and signal dropout. Reducing slice thickness diminishes signal dropout but is accompanied by reduced signal-to-noise ratio (SNR). This study proposes simultaneous excitation of subslices with total width equal to the desired slice thickness, employing alternating Hadamard-encoded radiofrequency pulses coupled with incoherent addition of the subslices to achieve reduction of through-plane dephasing with minimal SNR loss but at the expense of a reduction in temporal resolution. Using a sensory task and hypercapnic challenge with breathholding (BH), results with two subslices per slice and a twofold reduction in temporal resolution show improved activation relative to a conventional acquisition. Average (eight subjects) T-scores in the BH task increased by 16% (P<.0003), and activation extent increased by 12% (not significant). In frontal brain regions, significant improvements in BH activation extent (11.4%, P<.05) and T-scores (18%, P<.0002) were demonstrated. Higher temporal resolution can be achieved by tradeoff of SNR. 相似文献