Estimating total cross sections from neutron-nucleus collisions using a simple functional form

Total cross sections for neutron scattering from nuclei, with energies ranging from 10 to 600 MeV and from many nuclei spanning the mass range 6Li to 238U, have been analyzed using a simple, three-parameter, functional form. The calculated cross sections are compared with results obtained by using microscopic (g-folding) optical potentials as well as with experimental data. The functional form reproduces those total cross sections very well. When allowance is made for Ramsauer-like effects in the scattering, the parameters of the functional form required vary smoothly with energy and target mass. They too can be represented by functions of energy and mass.

Elliptic flow of thermal photons in heavy-ion collisions at Relativistic Heavy Ion Collider and Large Hadron Collider

We calculate the thermal photon transverse momentum spectra and elliptic flow in $\sqrt{s_{NN}} = 200$ GeV Au+Au collisions at RHIC and in $\sqrt{s_{NN}} = 2.76$ TeV Pb+Pb collisions at the LHC, using an ideal-hydrodynamical framework which is constrained by the measured hadron spectra at RHIC and LHC. The sensitivity of the results to the QCD-matter equation of state and to the photon emission rates is studied, and the photon $v_2$ is discussed in the light of the photonic $p_T$ spectrum measured by the PHENIX Collaboration. In particular, we make a prediction for the thermal photon $p_T$ spectra and elliptic flow for the current LHC Pb+Pb collisions.

Raman spectroscopic study of the uranyl titanate mineral euxenite (Y,Ca,U,Ce,Th) (Nb,Ta,Ti)2O6

Raman spectra of the uranyl titanate mineral euxenite were analyzed and related to the mineral structure. A comparison is made with the Raman spectra of uranyl oxyhydroxide hydrates. The obsd. bands are attributed to the Ti[n.63743]O and (UO2)2+ stretching and bending vibrations, as well as lattice vibrations of rare-earth ions. The Raman bands of euxenite are in harmony with those of the uranyl oxyhydroxides. The mineral euxenite is metamict as is evidenced by the intensity of the U[n.63743]O stretching and bending modes, which are of lower intensity than expected, and with bands that are significantly broader.

Mid-Holocene sea-level and coral reef demise : U-Th dating of subfossil corals in Moreton Bay, Australia

It is increasingly apparent that sea-level data (e.g. microfossil transfer functions, dated coral microatolls and direct observations from satellite and tidal gauges) vary temporally and spatially at regional to local scales, thus limiting our ability to model future sea-level rise for many regions. Understanding sealevel response at ‘far-field’ locations at regional scales is fundamental for formulating more relevant sea-level rise susceptibility models within these regions under future global change projections. Fossil corals and reefs in particular are valuable tools for reconstructing past sea levels and possible environmental phase shifts beyond the temporal constraints of instrumental records. This study used abundant surface geochronological data based on in situ subfossil corals and precise elevation surveys to determine previous sea level in Moreton Bay, eastern Australia, a far-field site. A total of 64 U-Th dates show that relative sea level was at least 1.1 m above modern lowest astronomical tide (LAT) from at least ˜6600 cal. yr BP. Furthermore, a rapid synchronous demise in coral reef growth occurred in Moreton Bay ˜5800 cal. yr BP, coinciding with reported reef hiatus periods in other areas around the Indo-Pacific region. Evaluating past reef growth patterns and phases allows for a better interpretation of anthropogenic forcing versus natural environmental/climatic cycles that effect reef formation and demise at all scales and may allow better prediction of reef response to future global change.

Anatase-rutile transformation in Fe/TiO2, Th/TiO2 and Cu/TiO2 catalysts and its possible role in metal-support interaction

Based on in-situ Mossbauer and X-ray diffraction studies, it is shown that in the Fe/TiO2 catalyst, the anatase-rutile transformation of the TiO2 support is facilitated by the Fe2+ ions formed during the reduction. The transformation occurs at lower temperatures in Th/TiO2 and Cu/TiO2 compared to pure TiO2. In general, the transformation of anatase to rutile seems to occur at or below the temperature (approximately 770 K) at which strong-metal-support-interaction manifests itself.

Formation and preservation of pedogenic carbonates in South India, links with paleo-monsoon and pedological conditions: Clues from Sr isotopes, U-Th series and REEs

The influence of the pedogenic and climatic contexts on the formation and preservation of pedogenic carbonates in a climosequence in the Western Ghats (Karnataka Plateau, South West India) has been studied. Along the climosequence, the current mean annual rainfall (MAR) varies within a 80 km transect from 6000 mm at the edge of the Plateau to 500 mm inland. Pedogenic carbonates occur in the MAR range of 500-1200 mm. In the semi-arid zone (MAR: 500-900 mm), carbonates occur (i) as rhick hardpan calcretes on pediment slopes and (ii) as nodular horizons in polygenic black soils (i.e. vertisols). In the sub-humid zone (MAR: 900-1500 mm), pedogenic carbonates are disseminated in the black soil matrices either as loose, irregular and friable nodules of millimetric size or as indurated botryoidal nodules of centimetric to pluricentimetric size. They also occur at the top layers of the saprolite either as disseminated pluricentimetric indurated nodules or carbonate-cemented lumps of centimetric to decimetric size. Chemical and isotopic (Sr-87/Sr-86) compositions of the carbonate fraction were determined after leaching with 0.25 N HCl. The corresponding residual fractions containing both primary minerals and authigenic clays were digested separately and analyzed. The trend defined by the Sr-87/Sr-86 signatures of both labile carbonate fractions and corresponding residual fractions indicates that a part of the labile carbonate fraction is genetically linked to the local soil composition. Considering the residual fraction of each sample as the most likely lithogenic source of Ca in carbonates, it is estimated that from 24% to 82% (55% on average) of Ca is derived from local bedrock weathering, leading to a consumption of an equivalent proportion of atmospheric CO2. These values indicate that climatic conditions were humid enough to allow silicate weathering: MAR at the time of carbonate formation likely ranged from 400 to 700 mm, which is 2- to 3-fold less than the current MAR at these locations. The Sr, U and Mg contents and the (U-234/U-238) activity ratio in the labile carbonate fraction help to understand the conditions of carbonate formation. The relatively high concentrations of Sr, U and Mg in black soil carbonates may indicate fast growth and accumulation compared to carbonates in saprolite, possibly due to a better confinement of the pore waters which is supported by their high (U-234/U-238) signatures, and/or to higher content of dissolved carbonates in the pore waters. The occurrence of Ce, Mn and Fe oxides in the cracks of carbonate reflects the existence of relatively humid periods after carbonate formation. The carbonate ages determined by the U-Th method range from 1.33 +/- 0.84 kyr to 7.5 +/- 2.7 kyr and to a cluster of five ages around 20 kyr, i.e. the Last Glacial Maximum period. The young occurrences are only located in the black soils, which therefore constitute sensitive environments for trapping and retaining atmospheric CO2 even on short time scales. The maximum age of carbonates depends on their location in the climatic gradient: from about 20 kyr for centimetric nodules at Mule Hole (MAR = 1100 mm/yr) to 200 kyr for the calcrete at Gundlupet (MAR = 700 mm/yr, Durand et al., 2007). The intensity of rainfall during wet periods would indeed control the lifetime of pedogenic carbonates and thus the duration of inorganic carbon storage in soils. (C) 2010 Elsevier Ltd. All rights reserved.

La sacra doctrina y el gnosticismo : (S. Th., I, q. 1, art. 1)

El estudio de la falsa gnosis es algo de lo cual varios pensadores católicos ya han escrito y lo siguen haciendo. Sin embargo, la consideración de este argumento a la luz del pensamiento de Santo Tomás de Aquino no es algo común. El concepto de gnosis (gnw/sij, conocimiento), en sí mismo, no significa algo peyorativo. Por el contrario, Jesucristo hace referencia a los doctores de la ley que han quitado la llave de la ciencia (Lc 11, 52). Por su parte san Pablo confiesa que «no hay sino un solo Dios y Padre, de quien proceden todas las cosas, y nosotros estamos destinados hacia él; y un solo Señor Jesucrito, por quien son todas las cosas, y nosostros también por él. Pero no en todos se halla esa ciencia (h` gnw/sij)». Este conocimiento o gnosis que toma la Revelación en la fe por regla y guía es la gnosis verdadera Nos proponemos en este trabajo realizar una lectura del art. 1º de la Iª Pars de la Suma Teológica y compararlo ante todo con el fundamento mismo de aquello que san Pablo denomina como «la mal llamada ciencia»...

Origin of Th/U variations in chondritic meteorites

Isotope dilution thorium and uranium analyses of the Harleton chondrite show a larger scatter than previously observed in equilibrated ordinary chondrites (EOC). The linear correlation of Th/U with 1/U in Harleton (and all EOC data) is produced by variation in the chlorapatite to merrillite mixing ratio. Apatite variations control the U concentrations. Phosphorus variations are compensated by inverse variations in U to preserve the Th/U vs. 1/U correlation. Because the Th/U variations reflect phosphate ampling, a weighted Th/U average should converge to an improved solar system Th/U. We obtain Th/U=3.53 (1_-mean=0.10), significantly lower and more precise than previous estimates.

To test whether apatite also produces Th/U variation in CI and CM chondrites, we performed P analyses on the solutions from leaching experiments of Orgueil and Murchison meteorites.

A linear Th/U vs. 1/U correlation in CI can be explained by redistribution of hexavalent U by aqueous fluids into carbonates and sulfates.

Unlike CI and EOC, whole rock Th/U variations in CMs are mostly due to Th variations. A Th/U vs. 1/U linear correlation suggested by previous data for CMs is not real. We distinguish 4 components responsible for the whole rock Th/U variations: (1) P and actinide-depleted matrix containing small amounts of U-rich carbonate/sulfate phases (similar to CIs); (2) CAIs and (3) chondrules are major reservoirs for actinides, (4) an easily leachable phase of high Th/U. likely carbonate produced by CAI alteration. Phosphates play a minor role as actinide and P carrier phases in CM chondrites.

Using our Th/U and minimum galactic ages from halo globular clusters, we calculate relative supernovae production rates for ²³²Th/²³⁸U and ²³⁵U/²³⁸U for different models of r-process nucleosynthesis. For uniform galactic production, the beginning of the r-process nucleosynthesis must be less than 13 Gyr. Exponentially decreasing production is also consistent with a 13 Gyr age, but very slow decay times are required (less than 35 Gyr), approaching the uniform production. The 15 Gyr Galaxy requires either a fast initial production growth (infall time constant less than 0.5 Gyr) followed by very low decrease (decay time constant greater than 100 Gyr), or the fastest possible decrease (≈8 Gyr) preceded by slow in fall (≈7.5 Gyr).

Termocronologia (U-Th)He e reativação da margem continental do sudeste do Brasil: seção Serra da Mantiqueira - Gráben da Guanabara

A evolução da margem continental do sudeste do Brasil tem sido discutida por diversos autores desde meados do século passado até os dias atuais, especialmente no contexto da origem e evolução dos escarpamentos e das bacias tafrogênicas. Buscou-se contribuir com novos dados sobre a evolução da área a partir da aplicação da termocronologia de baixa temperatura (U-Th)/He em apatita, que oferece uma sensibilidade significativa para registrar movimentações tectônicas na crosta superior. Foi possível obter idades em 107 cristais de apatita de 18 amostras do embasamento coletadas no perfil com orientação NW-SE, numa seção entre a Serra da Mantiqueira e o Gráben da Guanabara. As idades corrigidas variam entre 250,1 8,7 Ma e 43,5 1,9 Ma (2 σ) e as não corrigidas entre 174,13 3,03 Ma e 27,07 0,60 Ma (1 σ). O Neocretáceo, o Eocretáceo e o Paleoceno são os principais registros no conjunto de dados, em ordem de importância. No Neocretáceo, o intervalo entre 83,6 e 72,1 Ma (Campaniano) representa o maior destaque nos registros termocronológicos, embora os outros registros (Maastrichtiano e Santoniano) também estejam presentes e sejam importantes. As idades do Neocretáceo destacam a importância dos eventos tectonomagmáticos e soerguimento regional na história térmica dessa área, inclusive com idades (~86 Ma) atribuídas ao contexto de soerguimento da Serra do Mar. As idades do Eocretáceo indicam o registro de eventos térmicos mais antigos, vinculados à evolução pré-rifte. Já os dados do Paleoceno estariam associados ao evento de reativação responsável pela implantação do sistema de riftes continentais (~65 Ma) e as idades do Eoceno, restritas à borda de falha da bacia de Resende (49,7 Ma e 43,5 Ma), à reativação do sistema de riftes nessa área. A dispersão de idades foi interpretada como efeito dos danos de radiação já que muitos grãos apresentam correlação entre idade e concentração de urânio (eU). Os padrões de tempo-temperatura (t-T), definidos a partir dos modelos HeFTy calibrados para o modelo de difusão que considera os efeitos de danos de radiação nos cristais, registraram eventos de resfriamento rápido, os quais mostram correlação direta com episódios de reativação e soerguimento na margem continental e com registros nas bacias continentais e marginais. O padrão de aumento das idades com a elevação, assim como da costa em direção ao interior é observado, mas mostra-se alterado pela ocorrência de idades mais jovens associadas à complexa evolução dessa margem continental com desnivelamentos de blocos vinculados à tectônica pós-rift, numa situação que ressalta a influência dos episódios de reativação. As estimativas de denudação total variam entre 1,2 e 2,8 km. As taxas de erosão variam entre 15,2 e 35,3 m/Ma. A evolução da área indica não apenas a influência de um evento específico mas, possivelmente, uma combinação de episódios que se alternaram e/ou atuaram em conjunto em determinados períodos. Os eventos de reativação mais antigos, combinados com os mais recentes, exibem os seus remanescentes na paisagem (serras da Mantiqueira e do Mar e os grábens e bacias sedimentares) e assumem papel fundamental na evolução da área. Os registros de tais episódios podem ser observados nas histórias térmicas das rochas e nos depósitos correlativos nas bacias sedimentares marginais e intracontinentais.

Core design options for high conversion BWRs operating in Th- 233U fuel cycle

Several options of fuel assembly design are investigated for a BWR core operating in a closed self-sustainable Th-233U fuel cycle. The designs rely on an axially heterogeneous fuel assembly structure consisting of a single axial fissile zone "sandwiched" between two fertile blanket zones, in order to improve fertile to fissile conversion ratio. The main objective of the study was to identify the most promising assembly design parameters, dimensions of fissile and fertile zones, for achieving net breeding of 233U. The design challenge, in this respect, is that the fuel breeding potential is at odds with axial power peaking and the core minimum critical power ratio (CPR), hence limiting the maximum achievable core power rating. Calculations were performed with the BGCore system, which consists of the MCNP code coupled with fuel depletion and thermo-hydraulic feedback modules. A single 3-dimensional fuel assembly having reflective radial boundaries was modeled applying simplified restrictions on the maximum centerline fuel temperature and the CPR. It was found that axially heterogeneous fuel assembly design with a single fissile zone can potentially achieve net breeding, while matching conventional BWR core power rating under certain restrictions to the core loading pattern design. © 2013 Elsevier B.V. All rights reserved.

BWR fuel design options for self-sustainable Th-233U fuel cycle

In this work, we investigate a number of fuel assembly design options for a BWR core operating in a closed self-sustainable Th-233U fuel cycle. The designs rely on axially heterogeneous fuel assembly structure in order to improve fertile to fissile conversion ratio. One of the main assumptions of the current study was to restrict the fuel assembly geometry to a single axial fissile zone "sandwiched" between two fertile blanket zones. The main objective was to study the effect of the most important design parameters, such as dimensions of fissile and fertile zones and average void fraction, on the net breeding of 233U. The main design challenge in this respect is that the fuel breeding potential is at odds with axial power peaking and therefore limits the maximum achievable core power rating. The calculations were performed with BGCore system, which consists of MCNP code coupled with fuel depletion and thermo-hydraulic feedback modules. A single 3-dimensional fuel assembly with reflective radial boundaries was modeled applying simplified restrictions on maximum central line fuel temperature and Critical Power Ratio. It was found that axially heterogeneous fuel assembly design with single fissile zone can potentially achieve net breeding. In this case however, the achievable core power density is roughly one third of the reference BWR core.