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Pesquisa : G01.249.660.250 [Categoria DeCS]
Referências encontradas : 7374 [refinar]
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PMID:28749809
Autor:Piper RK; Mozhayev AV; Murphy MK; Thompson AK
Endereço:*Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA 99352; †National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, MD 20899.
Título:Beyond Californium-A Neutron Generator Alternative for Dosimetry and Instrument Calibration in the U.S.
Fonte:Health Phys; 113(3):183-194, 2017 09.
ISSN:1538-5159
País de publicação:United States
Idioma:eng
Resumo:Evaluations of neutron survey instruments, area monitors, and personal dosimeters rely on reference neutron radiations, which have evolved from the heavy reliance on (α,n) sources to a shared reliance on (α,n) and the spontaneous fission neutrons of californium-252 (Cf). Capable of producing high dose equivalent rates from an almost point source geometry, the characteristics of Cf are generally more favorable when compared to the use of (α,n) and (γ,n) sources or reactor-produced reference neutron radiations. Californium-252 is typically used in two standardized configurations: unmoderated, to yield a fission energy spectrum; or with the capsule placed within a heavy-water moderating sphere to produce a softened spectrum that is generally considered more appropriate for evaluating devices used in nuclear power plant work environments. The U.S. Department of Energy Cf Loan/Lease Program, a longtime origin of affordable Cf sources for research, testing and calibration, was terminated in 2009. Since then, high-activity sources have become increasingly cost-prohibitive for laboratories that formerly benefited from that program. Neutron generators, based on the D-T and D-D fusion reactions, have become economically competitive with Cf and are recognized internationally as important calibration and test standards. Researchers from the National Institute of Standards and Technology and the Pacific Northwest National Laboratory are jointly considering the practicality and technical challenges of implementing neutron generators as calibration standards in the U.S. This article reviews the characteristics of isotope-based neutron sources, possible isotope alternatives to Cf, and the rationale behind the increasing favor of electronically generated neutron options. The evaluation of a D-T system at PNNL has revealed characteristics that must be considered in adapting generators to the task of calibration and testing where accurate determination of a dosimetric quantity is necessary. Finally, concepts are presented for modifying the generated neutron spectra to achieve particular targeted spectra, simulating Cf or workplace environments.
Tipo de publicação: JOURNAL ARTICLE; RESEARCH SUPPORT, U.S. GOV'T, NON-P.H.S.
Nome de substância:975X05H15A (Californium)


  2 / 7374 MEDLINE  
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PMID:29341744
Autor:Liu Z; Huang J; Tyagi M; O'Neill H; Zhang Q; Mamontov E; Jain N; Wang Y; Zhang J; Smith JC; Hong L
Endereço:School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, China.
Título:Dynamical Transition of Collective Motions in Dry Proteins.
Fonte:Phys Rev Lett; 119(4):048101, 2017 Jul 28.
ISSN:1079-7114
País de publicação:United States
Idioma:eng
Resumo:Water is widely assumed to be essential for protein dynamics and function. In particular, the well-documented "dynamical" transition at ∼200 K, at which the protein changes from a rigid, nonfunctional form to a flexible, functional state, as detected in hydrogenated protein by incoherent neutron scattering, requires hydration. Here, we report on coherent neutron scattering experiments on perdeuterated proteins and reveal that a transition occurs in dry proteins at the same temperature resulting primarily from the collective heavy-atom motions. The dynamical transition discovered is intrinsic to the energy landscape of dry proteins.
Tipo de publicação: JOURNAL ARTICLE
Nome de substância:0 (Proteins); 059QF0KO0R (Water)


  3 / 7374 MEDLINE  
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PMID:29211773
Autor:Bélanger-Champagne C; Vainionpää H; Peura P; Toivonen H; Eerola P; Dendooven P
Endereço:Helsinki Institute of Physics, Helsinki, Finland.
Título:Design of a novel instrument for active neutron interrogation of artillery shells.
Fonte:PLoS One; 12(12):e0188959, 2017.
ISSN:1932-6203
País de publicação:United States
Idioma:eng
Resumo:The most common explosives can be uniquely identified by measuring the elemental H/N ratio with a precision better than 10%. Monte Carlo simulations were used to design two variants of a new prompt gamma neutron activation instrument that can achieve this precision. The instrument features an intense pulsed neutron generator with precise timing. Measuring the hydrogen peak from the target explosive is especially challenging because the instrument itself contains hydrogen, which is needed for neutron moderation and shielding. By iterative design optimization, the fraction of the hydrogen peak counts coming from the explosive under interrogation increased from [Formula: see text]% to [Formula: see text]% (statistical only) for the benchmark design. In the optimized design variants, the hydrogen signal from a high-explosive shell can be measured to a statistics-only precision better than 1% in less than 30 minutes for an average neutron production yield of 109 n/s.
Tipo de publicação: JOURNAL ARTICLE


  4 / 7374 MEDLINE  
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PMID:28463504
Autor:Stefanopoulos KL; Youngs TGA; Sakurovs R; Ruppert LF; Bahadur J; Melnichenko YB
Endereço:Institute of Nanoscience & Nanotechnology, NCSR "Demokritos" , Aghia Paraskevi, Athens, Greece.
Título:Neutron Scattering Measurements of Carbon Dioxide Adsorption in Pores within the Marcellus Shale: Implications for Sequestration.
Fonte:Environ Sci Technol; 51(11):6515-6521, 2017 Jun 06.
ISSN:1520-5851
País de publicação:United States
Idioma:eng
Resumo:Shale is an increasingly viable source of natural gas and a potential candidate for geologic CO sequestration. Understanding the gas adsorption behavior on shale is necessary for the design of optimal gas recovery and sequestration projects. In the present study neutron diffraction and small-angle neutron scattering measurements of adsorbed CO in Marcellus Shale samples were conducted on the Near and InterMediate Range Order Diffractometer (NIMROD) at the ISIS Pulsed Neutron and Muon Source, STFC Rutherford Appleton Laboratory along an adsorption isotherm of 22 °C and pressures of 25 and 40 bar. Additional measurements were conducted at approximately 22 and 60 °C at the same pressures on the General-Purpose Small-Angle Neutron Scattering (GP-SANS) instrument at Oak Ridge National Laboratory. The structures investigated (pores) for CO adsorption range in size from Å level to ∼50 nm. The results indicate that, using the conditions investigated densification or condensation effects occurred in all accessible pores. The data suggest that at 22 °C the CO has liquid-like properties when confined in pores of around 1 nm radius at pressures as low as 25 bar. Many of the 2.5 nm pores, 70% of 2 nm pores, most of the <1 nm pores, and all pores <0.25 nm, are inaccessible or closed to CO , suggesting that despite the vast numbers of micropores in shale, the micropores will be unavailable for storage for geologic CO sequestration.
Tipo de publicação: JOURNAL ARTICLE
Nome de substância:142M471B3J (Carbon Dioxide)


  5 / 7374 MEDLINE  
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PMID:29045491
Autor:Lee C; Lee S; Lee SJ; Song H; Kim DH; Cho S; Jo K; Han Y; Chung YH; Kim JS
Endereço:Department of Radiological Science, Yonsei University, Wonju, Korea.
Título:Monte Carlo simulation of secondary neutron dose for scanning proton therapy using FLUKA.
Fonte:PLoS One; 12(10):e0186544, 2017.
ISSN:1932-6203
País de publicação:United States
Idioma:eng
Resumo:Proton therapy is a rapidly progressing field for cancer treatment. Globally, many proton therapy facilities are being commissioned or under construction. Secondary neutrons are an important issue during the commissioning process of a proton therapy facility. The purpose of this study is to model and validate scanning nozzles of proton therapy at Samsung Medical Center (SMC) by Monte Carlo simulation for beam commissioning. After the commissioning, a secondary neutron ambient dose from proton scanning nozzle (Gantry 1) was simulated and measured. This simulation was performed to evaluate beam properties such as percent depth dose curve, Bragg peak, and distal fall-off, so that they could be verified with measured data. Using the validated beam nozzle, the secondary neutron ambient dose was simulated and then compared with the measured ambient dose from Gantry 1. We calculated secondary neutron dose at several different points. We demonstrated the validity modeling a proton scanning nozzle system to evaluate various parameters using FLUKA. The measured secondary neutron ambient dose showed a similar tendency with the simulation result. This work will increase the knowledge necessary for the development of radiation safety technology in medical particle accelerators.
Tipo de publicação: JOURNAL ARTICLE
Nome de substância:0 (Protons)


  6 / 7374 MEDLINE  
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PMID:28961587
Autor:Tian C; Guo H; Lv N; Xu P; Lv W
Endereço:*Xi'an Research Institute of Hi-Tech, Xi'an 710025, China.
Título:Analysis on Enrichment and Mass of Uranium Materials With Time-Correlated Measurement.
Fonte:Health Phys; 113(5):387-391, 2017 Nov.
ISSN:1538-5159
País de publicação:United States
Idioma:eng
Resumo:Effective determination for enrichment of the uranium materials in heavy shielding is technically difficult to solve. Various technical methods so far have been proposed to deal with this problem. With the time-correlated coincidence measurement method, 14 MeV neutrons and fission spectrum neutrons are chosen to interrogate the uranium samples with different masses and enrichments. The fission spectrum neutrons are obtained by slowing down the 14 MeV neutrons. Results have shown that a good linear relationship exists between masses of the uranium materials (≤10% enriched U) that possess the same enrichment. A linear relationship also exists between enrichments of the uranium materials that possess the same mass.
Tipo de publicação: JOURNAL ARTICLE
Nome de substância:4OC371KSTK (Uranium); AR09D82C7G (Deuterium)


  7 / 7374 MEDLINE  
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PMID:28961582
Autor:Schauer DA
Endereço:*ICRU, 7910 Woodmont Ave., Suite 400 Bethesda, MD 20814.
Título:Neutron Metrology in the United States-Where We've Been, Where We Are Now and What We Need to Do Moving Forward.
Fonte:Health Phys; 113(5):347-352, 2017 Nov.
ISSN:1538-5159
País de publicação:United States
Idioma:eng
Resumo:Neutron metrology in the United States must be based on traceability to standards maintained by the National Institute of Standards and Technology (NIST). This article reviews the history of NIST's neutron-metrology efforts, the loss of those capabilities, and attempts to restore them. Recommendations are made to ensure that neutron dosimetry performed in the United States meets the requirements set forth by the International Standards Organization and other international and national authorities.
Tipo de publicação: HISTORICAL ARTICLE; JOURNAL ARTICLE


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PMID:28749952
Autor:Merk B; Litskevich D; Bankhead M; Taylor RJ
Endereço:University of Liverpool, School of Engineering, Liverpool, United Kingdom.
Título:An innovative way of thinking nuclear waste management - Neutron physics of a reactor directly operating on SNF.
Fonte:PLoS One; 12(7):e0180703, 2017.
ISSN:1932-6203
País de publicação:United States
Idioma:eng
Resumo:A solution for the nuclear waste problem is the key challenge for an extensive use of nuclear reactors as a major carbon free, sustainable, and applied highly reliable energy source. Partitioning and Transmutation (P&T) promises a solution for improved waste management. Current strategies rely on systems designed in the 60's for the massive production of plutonium. We propose an innovative strategic development plan based on invention and innovation described with the concept of developments in s-curves identifying the current boundary conditions, and the evolvable objectives. This leads to the ultimate, universal vision for energy production characterized by minimal use of resources and production of waste, while being economically affordable and safe, secure and reliable in operation. This vision is transformed into a mission for a disruptive development of the future nuclear energy system operated by burning of existing spent nuclear fuel (SNF) without prior reprocessing. This highly innovative approach fulfils the sustainability goals and creates new options for P&T. A proof on the feasibility from neutronic point of view is given demonstrating sufficient breeding of fissile material from the inserted SNF. The system does neither require new resources nor produce additional waste, thus it provides a highly sustainable option for a future nuclear system fulfilling the requests of P&T as side effect. In addition, this nuclear system provides enhanced resistance against misuse of Pu and a significantly reduced fuel cycle. However, the new system requires a demand driven rethinking of the separation process to be efficient.
Tipo de publicação: JOURNAL ARTICLE
Nome de substância:0 (Radioactive Waste); 0 (Radioisotopes)


  9 / 7374 MEDLINE  
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PMID:28707339
Autor:Hiromoto T; Meilleur F; Shimizu R; Shibazaki C; Adachi M; Tamada T; Kuroki R
Endereço:Quantum Beam Science Center, Japan Atomic Energy Agency, Tokai, Ibaraki, 319-1195, Japan.
Título:Neutron structure of the T26H mutant of T4 phage lysozyme provides insight into the catalytic activity of the mutant enzyme and how it differs from that of wild type.
Fonte:Protein Sci; 26(10):1953-1963, 2017 Oct.
ISSN:1469-896X
País de publicação:United States
Idioma:eng
Resumo:T4 phage lysozyme is an inverting glycoside hydrolase that degrades the murein of bacterial cell walls by cleaving the ß-1,4-glycosidic bond. The substitution of the catalytic Thr26 residue to a histidine converts the wild type from an inverting to a retaining enzyme, which implies that the original general acid Glu11 can also act as an acid/base catalyst in the hydrolysis. Here, we have determined the neutron structure of the perdeuterated T26H mutant to clarify the protonation states of Glu11 and the substituted His26, which are key in the retaining reaction. The 2.09-Å resolution structure shows that the imidazole group of His26 is in its singly protonated form in the active site, suggesting that the deprotonated NÉ›2 atom of His26 can attack the anomeric carbon of bound substrate as a nucleophile. The carboxyl group of Glu11 is partially protonated and interacts with the unusual neutral state of the guanidine moiety of Arg145, as well as two heavy water molecules. Considering that one of the water-binding sites has the potential to be occupied by a hydronium ion, the bulk solvent could be the source for the protonation of Glu11. The respective protonation states of Glu11 and His26 are consistent with the bond lengths determined by an unrestrained refinement of the high-resolution X-ray structure of T26H at 1.04-Å resolution. The detail structural information, including the coordinates of the deuterium atoms in the active site, provides insight into the distinctively different catalytic activities of the mutant and wild type enzymes.
Tipo de publicação: JOURNAL ARTICLE
Nome de substância:EC 3.2.1.17 (Muramidase)


  10 / 7374 MEDLINE  
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PMID:28704557
Autor:Shahmohammadi Beni M; Hau TC; Krstic D; Nikezic D; Yu KN
Endereço:Department of Physics and Materials Science, City University of Hong Kong, Kowloon Tong, Hong Kong, China.
Título:Monte Carlo studies on neutron interactions in radiobiological experiments.
Fonte:PLoS One; 12(7):e0181281, 2017.
ISSN:1932-6203
País de publicação:United States
Idioma:eng
Resumo:Monte Carlo method was used to study the characteristics of neutron interactions with cells underneath a water medium layer with varying thickness. The following results were obtained. (1) The fractions of neutron interaction with 1H, 12C, 14N and 16O nuclei in the cell layer were studied. The fraction with 1H increased with increasing medium thickness, while decreased for 12C, 14N and 16O nuclei. The bulges in the interaction fractions with 12C, 14N and 16O nuclei were explained by the resonance spikes in the interaction cross-section data. The interaction fraction decreased in the order: 1H > 16O > 12C > 14N. (2) In general, as the medium thickness increased, the number of "interacting neutrons" which exited the medium and then further interacted with the cell layer increased. (3) The area under the angular distributions for "interacting neutrons" decreased with increasing incident neutron energy. Such results would be useful for deciphering the reasons behind discrepancies among existing results in the literature.
Tipo de publicação: JOURNAL ARTICLE
Nome de substância:059QF0KO0R (Water)



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