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Abstract  

The main concern of this paper is to study the dynamic of a ratio dependent predator-prey system with diffusion and delay. Concretely, we prove that when the system of PDE is persistent the nontrivial equilibrium is globally asymptotically stable.

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Abstract  

A simple method for the determination of uranium and thorium by delayed neutron counting is described. One portion of the sample is irradiated in a reactor and the delayed neutrons are counted. Another portion of the sample is mixed with B4 C powder absorbing the thermal neutrons, and irradiated in the same position. From those data, both uranium and thorium can be calculated when a quantitative calibration has been made beforehand. The detection limits for the pure elements are 0.07 ppm for uranium and 2 ppm for thorium with the minimum analyzing time being 2 min. The accuracy of the method is investigated by comparing results obtained by the method described here with results obtained by epithermal activation analysis.

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A new computerised neutron activation technique permits rapid nondestructive simultaneous elemental and isotopic uranium analysis with various applications, such as nuclear material standardization and safeguards analysis. The technique consists of dual cyclic neutron activation in a differentiated neutron flux spectrum at bare and cadmium covered irradiation positions and subsequent delayed fission neutron counting. Calibration curve fitting by polynomial approximation and data processing by GRAPHER and LOTUS 123 software computer programs gave satisfactory results.

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A paper by Chow [3] contains (i.a.) a strong law for delayed sums, such that the length of the edge of the nth window equals n α for 0 < α < 1. In this paper we consider the kind of intermediate case when edges grow like n=L(n), where L is slowly varying at infinity, thus at a higher rate than any power less than one, but not quite at a linear rate. The typical example one should have in mind is L(n) = log n. The main focus of the present paper is on random field versions of such strong laws.

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Applicability of a small neutron generator and a dual rectangular tube sample transfer system for analyses of U and Th using delayed fission neutron technique has been investigated. A way of optimizing the timing parameters is reported. At a fast neutron flux of 108 n.cm–2s–1, 0.02 w% U can be determined. For thorium determination this method is less sensitive. The Cd difference technique can be used for the simultaneous determination of U and Th but it has lower sensitivity.

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Abstract  

High resolution K X-ray spectrometry preceded by activation with deuterons is an unexplored area in the field of activation analysis. This work describes the capabilities of this technique and evaluates its analytical potential for the specific determination of transition metals in small samples. Detection of the delayed X-rays coupled with their rate of decay provided a unique indication of the target component and its concentration. Analytical conditions are demonstrated for the elements of interest and the potential for application to the general routine determination of the transition metals and some of their isotopes, is discussed.

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Dissolved uranium is selectively removed from 11 of filtered, acidified water using a liquid anion exchange resin (Amberlite LA-1) dissolved in 10 ml of purified kerosene. The organic phase is then analyzed by a standard delayed neutron counting technique. Yields of removed uranium are consistently greater than 90 percent over a measured concentration range of 1.0 to 100 ppb uranium. The absolute detection limit based on 11 of water is 0.06 ppb. Elemental interferences are minimal and the results compare favorably with fluorometric analyses of natural waters.

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Abstract  

We consider the delay differential equation

\documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{upgreek} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} $$\dot x(t) = - \mu x(t) + f(x(t - \tau ))$$ \end{document}
, where µ, τ are positive parameters and f is a strictly monotone, nonlinear C 1-function satisfying f(0) = 0 and some convexity properties. It is well known that for prescribed oscillation frequencies (characterized by the values of a discrete Lyapunov functional) there exists τ* > 0 such that for every τ > τ* there is a unique periodic solution. The period function is the minimal period of the unique periodic solution as a function of τ > τ*. First we show that it is a monotone nondecreasing Lipschitz continuous function of τ with Lipschitz constant 2. As an application of our theorem we give a new proof of some recent results of Yi, Chen and Wu [14] about uniqueness and existence of periodic solutions of a system of delay differential equations.

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At Imperial College uranium is determined at very low levels in environmental samples by delayed neutron counting. High density polyethylene capsules are used for transporting the samples in a pneumatic transfer system, from the reactor, to the neutron detection rig. The detector has a background of 3.6 counts per minute, however the empty transport capsules produce 11.0 counts above the background. We have determined that neither uranium in the polyethylene or from contamination of the capsule nor 17N from the 17O(n,p)17N and 18O(n,d)17N reactions are the cause. This paper reports the results of the investigation into the source of the extra counts.

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Abstract

Introduction  Publication delay, chronological distance between completion of a scientific work and distribution of its achievements as a peer reviewed paper, is a negative phenomenon in scientific information dissemination. It can be further subdivided in successive stages corresponding to the peer review process and the technical preparation of accepted manuscripts. Formal online posting in electronic versions of journals has been considered as a shortening of the process. Objectives  To determine publication delay in a group of leading Food Research journals, as well as factors affecting this lag and also to compute the effect of formal online posting on the distribution of papers in electronic form. Secondary objective is also to study the possible effect of informal posting of papers through some repositories on the publication delay in the field. Methods  14 Food Research journals were selected and 4836 papers published in 2004 were examined. Dates of first submission, submission of revised manuscripts, acceptation, online posting and final publication were recorded for each paper. Analysis  Data collected were analyzed using SPSS and SigmaPlot. Parametric correlation between some variables was determined and ANOVA was performed with BMDP package for significance analysis of differences among journals. Results  average publication delay of papers submitted to the set of selected journals is 348 ± 104 days, with European Food Research and Technology and Journal of Agricultural and Food Chemistry showing the shortest delays. Total delay strongly depends on the peer review process. On average, 85.75% of manuscripts are corrected prior to their acceptance by journals. Online posting of papers prior to their print publication reduces total delay in about 29%. On average, a paper is posted online 260 days after its submission to the set of journals. Conclusions  Publication delay of papers is strongly dependent on the peer review process, which affects most of the manuscripts in the Food Research field. Advanced online publication through formal posting at the editor’s sites only slightly reduces the time between reception and final publication of papers.

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