238.2T-14 Concrete Thixotropy

Fresh concrete can exhibit different rheological behavior when at rest than when flowing. This difference is due to thixotropy, which can have important consequences for formwork pressure, multi-lift casting, slip-form paving, pumping, and segregation resistance. This TechNote defines thixotropy and distinguishes it from other changes in rheological properties; discusses the origins of, test methods for measuring, and factors affecting thixotropy; and concludes with its applications.

HG 238.80-238.90 Thin Section

Dark brown sediment with clasts ranging from small to medium in size and sub-angular to angular. Contains lineations and grain stacking throughout the sample. Rotation and comet structures can also be seen. Rotation structures can be seen around mainly the sub-angular clasts. Patches of clay can also be seen throughout the sample.

Nitrification processes in groundwaters evidenced by modification on the U contents and U-234/U-238 activity ratios

Groundwaters from the Botucatu-Piramboia aquifer system located at the Parana sedimentary basin in Brazil and sampled at Ribeirao Preto city, São Paulo State, were chemically and isotopically analysed, with the aim of evaluating how uranium isotopes U-234 and U-238 can be utilized to monitor the progress of nitrate, the main indicator of agrichemical pollution into an aquifer. Based on the isotope dilution analysis, the U-234/U-238 activity ratio and U content data in the groundwaters were utilized to deduce their proportions in a mixture, suggesting that the water not affected significantly by nitrification still dominates the exploitation system involving the studied wells.

A laboratory study of the transfer of U-234 and U-238 during water-rock interactions in the Carnmenellis granite (Cornwall, England) and implications for the interpretation of field data

Laboratory time-scale experiments were conducted on gravels from the Carnmenellis granite, Cornwall, England, with the purpose of evaluating the release of natural uranium isotopes to the water phase. The implications of these results for the production of enhanced U-234/U-238 activity ratios in Cornish groundwaters are discussed. It is suggested that the U-234/U-238 lab data can be used to interpret activity ratios from Cornwall, even when the observed inverse relationship between dissolved U and U-234/U-238 in leachates/etchates is taken into account. (C) 2001 Elsevier B.V. Ltd. All rights reserved.

Some constraints on the modeling using the uranium isotopes U-238 and U-234 for dating groundwaters from Guarany aquifer, South America

Groundwaters from the Guarany aquifer located at the South American continent and sampled at four wells with described geological sections in São Paulo State, Brazil, were chemically and isotopically analysed with two aims: to evaluate the quality of this important hydrological resource and to investigate the possibility of using the natural uranium isotopes U-234 and U-238 as a chronological tool, since the U-234/U-238 activity ratio and dissolved U content data in groundwater systems have generated models for dating purposes.

Transfer of radon and parent nuclides U-238 and U-234 from soils of the Mendip Hills area, England, to the water phase

Laboratory time scale experiments were conducted on soils from the Mendip Hills area, England, with the purpose of evaluating the release of Rn-222 and their parent nuclides U-238 and U-234 to the water phase and to determine the influence of parameters that can affect the geochemical behaviour of these nuclides in natural systems. The specific surface area of the samples ranged from 43.8 to 52.5 cm(2) g(-1), where the particle size for all soil horizons is lognormally distributed, with modal values of the particle radius undersize ranging from 107 up to 203 mu m. The values for the released radon were between 26 and 194 pCi, which allowed to estimate emanation coefficients for these materials between 0.1 and 0.2, within the context of other values reported elsewhere. Soils derived from Carboniferous limestone and characterized by higher pH, exchangeable calcium, and the presence of U, but with a lower U-231/U-238 activity ratio, yielded the highest values for released Rn; however, this trend was not observed for dissolved U and its respective U-234/U-238 activity ratio, when considering the less aggressive etchant. Uranium is mobilized from rock matrix to A and B horizons in the analysed soil profiles, where its enrichment is about 10 times higher in soils derived from Carboniferous limestone. These data also permitted an evaluation of a theoretical model for the generation of Rn in soils and its transfer to water, in order to interpret the radioactivity due to this gas in groundwaters from the Mendip Hills district, England. (C) 1999 Elsevier B.V. B.V. All lights reserved.

The transfer of uranium isotopes U-234 and U-238 to the waters interacting with carbonates from Mendip Hills area (England)

Laboratory time-scale experiments were conducted on limestone and dolomite gravels from the Mendip Hills area, England, with the purpose of evaluating the release of U-238 and U-234 to different aqueous solutions. The U-234/U-238 activity ratio (AR) lab data were reliable to interpret the field data. The obtained values do not indicate a reduction in the amount of dissolved U and an increase in the AR of the remaining dissolved U as commonly observed for groundwater systems close to redox boundaries. (C) 2000 Elsevier B.V. Ltd. All rights reserved.

Determination of the U-238 spontaneous fission decay constant without neutron irradiation

We have developed a methodology for measuring the decay constant of the spontaneous fission of U-238, lambda(f), using nuclear particle track detectors where thermal neutron irradiation is unnecessary. This methodology is based on the fact that the radiation damage caused by spontaneous fission of trans-uranium elements bearing a mass number close to 238 are similar to U-238 spontaneous-fission ones. Loading a thick source of uranium (thickness greater than the fission fragment range) with a small amount of a suitable trans-uranium element (for instance, Pu-242, which presents a spontaneous fission half-life of 6.75(.)10(10) y), it is possible to determine the observation efficiency of a particle-track detector for fission fragments. Procedures concerning our thick source manufacture and uniformity tests of the trans-uranium distribution are also presented. These results make it possible for the exposure of thick uranium sources (without trans-uranium element) to lead to a lambda(f) value.

NUCLEAR ALIGNMENT - CLASSICAL DYNAMIC-MODEL FOR THE U-238 U-238 SYSTEM

Dynamical properties of the U-238-U-238 system at the classical turning point, specifically the distance of closest approach, the relative orientations of the nuclei, and deformations have been studied at the sub-Coulomb energy of E(lab) = 6.07 MeV/nucleon using a classical dynamical model with a variable moment of inertia. Probability of favorable alignment for anomalous positron-electron pair emission through vacuum decay is calculated. The calculated small favorable alignment probability value of 0.116 is found to be enhanced by about 16% in comparison with the results of a similar study using a fixed moment of inertia as well as the results from a semiquantal calculation reported earlier.

Ellen Evans Cathcart Papers - Accession 238 - M111 (143)

The Ellen Evans Cathcart Papers consist of biographical sketches of Mrs. Ellen Evans Cathcart (1869-1952), first woman member of the Democratic National Committee who was also instrumental in the founding of the Children’s Bureau. Also included in the collection are newspaper clippings, magazine articles, correspondence, reports, certificates, citations, and photographs relating to her work as supervisor of the Children’s Bureau of South Carolina and her involvement in the women’s suffrage movement. Of particular note is an annual report of the South Carolina Council of Defense which outlines the achievements of women of South Carolina in war work and a program book of the 47th annual convention of the National American Woman Suffrage Association.

Cattura neutronica su u-238: Studio della regione delle risonanze risolte.

Le attuali sezioni d’urto di reazioni indotte da neutroni non sono sufficientemente accurate per essere utilizzate in progetti di ricerca di frontiera. Per soddisfare la richiesta di nuovi dati nucleari è stata recentemente costruita al CERN di Ginevra la facility n_TOF (neutron Time Of Flight). Le caratteristiche che contraddistinguono questa facility sono l’elevato flusso istantaneo di neutroni, l’ampio spettro energetico e l’eccellente risoluzione energetica. In questa tesi viene analizzata la sezione d’urto di cattura neutronica su 238U, fondamentale per la realizzazione dei reattori nucleari di nuova generazione che prevedono la sostituzione del combustibile 235U con 238U, molto più abbondante in natura e che permette di ridurre drasticamente l’accumulo di scorie nucleari. Nonostante le numerose misure già effettuate su questo elemento, ancora non si è raggiunta la precisione necessaria per le tecnologie nucleari emergenti quali i reattori di quarta generazione e gli ADS (Accelerator Drive System). Inoltre la parametrizzazione della sezione d’urto in termini di matrice R, non è così accurata quanto richiesto. In questo lavoro si è dapprima studiata l’assegnazione dello spin delle risonanze riportata in letteratura, elemento fondamentale per la successiva analisi delle risonanze e la parametrizzazione della sezione d’urto della reazione 238U(n, ). Parallelamente a questa attività si è studiata la funzione di risposta dello spettrometro n_TOF, che descrive la risoluzione energetica dei neutroni incidenti e va quindi a modificare la forma delle risonanze. I risultati ottenuti sono quindi stati confrontati con quelli in letteratura per poter essere validati e per poter verificare eventuali migliorie.

Acta andreae et bartholomaei (bhg 2056, cant 238): Edizione critica e commento della versione greca

Gli Atti di Andrea e Bartolomeo sono un testo cristiano apocrifo, probabilmente ascrivibile al V secolo d.C. e collocabile in àmbito egizio. Tale testo è tramandato in greco, copto, arabo ed etiopico e il presente lavoro consiste nell’edizione critica della versione greca, sinora inedita. La storia narra l’evangelizzazione della Partia per opera degli apostoli Andrea e Bartolomeo e di un antropofago dalle sembianze mostruose (Cristomeo/ Cristiano), convertito al cristianesimo e divenuto instrumentum fidei.