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981.
YongHao Chen XiMeng Chen JiaRong Lei Li An XiaoDong Zhang JianXiong Shao Pu Zheng XinHua Wang 《中国科学:物理学 力学 天文学(英文版)》2014,57(10):1885-1890
Accurate knowledge of the neutron energy spectra is useful in basic research and applications. The overall procedure of measuring and unfolding the fast neutron energy spectra with BC501A liquid scintillation detector is described. The recoil proton spectrum of 241Am-Be neutrons was obtained experimentally. With the NRESP7 code, the response matrix of detector was simulated. Combining the recoil proton spectrum and response matrix, the unfolding of neutron spectra was performed by GRAVEL iterative algorithm. A MatLab program based on the GRAVEL method was developed. The continuous neutron spectrum of 241Am-Be source and monoenergetic neutron spectrum of D-T source have been unfolded successfully and are in good agreement with their standard reference spectra. The unfolded 241Am-Be spectrum are more accurate than the spectra unfolded by artificial neural networks in recent years. 相似文献
982.
Recent progress on polymer dynamics by neutron scattering: From simple polymers to complex materials
The recent (from 2010 onward) contributions of quasielastic neutron scattering techniques (time of flight, backscattering, and neutron spin echo) to the characterization and understanding of dynamical processes in soft materials based on polymers are analyzed. The selectivity provided by the combination of neutron scattering and isotopic—in particular, proton/deuterium—labeling allows the isolated study of chosen molecular groups and/or components in a system. This opportunity, together with the capability of neutrons to provide space/time resolution at the relevant length scales in soft matter, allows unraveling the nature of the large variety of molecular motions taking place in materials of increasing complexity. As a result, recent relevant works can be found dealing with dynamical process which associated characteristic lengths and nature are as diverse as, for example, phenyl motions in a glassy linear homopolymer like polystyrene and the chain dynamics of a polymer adsorbed on dispersed clay platelets. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013 相似文献
983.
Helen Y. Playford Dr. Alex C. Hannon Dr. Emma R. Barney Prof. Richard I. Walton 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(8):2803-2813
A structural investigation is reported of polymorphs of Ga2O3 that, despite much interest in their properties, have hitherto remained uncharacterised due to structural disorder. The most crystalline sample yet reported of γ‐Ga2O3 was prepared by solvothermal oxidation of gallium metal in ethanolamine. Structure refinement using the Rietveld method reveals γ‐Ga2O3 has a defect Fd$\bar 3$ m spinel structure, while pair distribution function analysis shows that the short‐range structure is better modelled with local F$\bar 4$ 3m symmetry. In further solvothermal oxidation reactions a novel gallium oxyhydroxide, Ga5O7(OH), is formed, the thermal decomposition of which reveals a new, transient gallium oxide polymorph, κ‐Ga2O3, before transformation into β‐Ga2O3. In contrast, the thermal decomposition of Ga(NO3)3?9 H2O first forms ε‐Ga2O3 and then β‐Ga2O3. Examination of in situ thermodiffraction data shows that ε‐Ga2O3 is always contaminated with β‐Ga2O3 and with this knowledge a model for its structure was deduced and refined—space group P63mc with a ratio of tetrahedral/octahedral gallium of 2.2:1 in close‐packed oxide layers. Importantly, thermodiffraction provides no evidence for the existence of the speculated bixbyite structured δ‐Ga2O3; at the early stages of thermal decomposition of Ga(NO3)3?9 H2O the first distinct phase formed is merely small particles of ε‐Ga2O3. 相似文献
984.
985.
Dr. Laura J. McCormick Ciaran McDonnell‐Worth Dr. James A. Platts Dr. Alison J. Edwards Dr. David R. Turner 《化学:亚洲杂志》2013,8(11):2642-2651
A series of urea‐derived heterocycles, 5N‐substituted hexahydro‐1,3,5‐triazin‐2‐ones, has been prepared and their structures have been determined for the first time. This family of compounds only differ in their substituent at the 5‐position (which is derived from the corresponding primary amine), that is, methyl ( 1 ), ethyl ( 2 ), isopropyl ( 3 ), tert‐butyl ( 4 ), benzyl ( 5 ), N,N‐(diethyl)ethylamine ( 6 ), and 2‐hydroxyethyl ( 7 ). The common heterocyclic core of these molecules is a cyclic urea, which has the potential to form a hydrogen‐bonding tape motif that consists of self‐associative (8) dimers. The results from X‐ray crystallography and, where possible, Laue neutron crystallography show that the hydrogen‐bonding motifs that are observed and the planarity of the hydrogen bonds appear to depend on the steric hindrance at the α‐carbon atom of the N substituent. With the less‐hindered substituents, methyl and ethyl, the anticipated tape motif is observed. When additional methyl groups are added onto the α‐carbon atom, as in the isopropyl and tert‐butyl derivatives, a different 2D hydrogen‐bonding motif is observed. Despite the bulkiness of the substituents, the benzyl and N,N‐(diethyl)ethylamine derivatives have methylene units at the α‐carbon atom and, therefore, display the tape motif. The introduction of a competing hydrogen‐bond donor/acceptor in the 2‐hydroxyethyl derivative disrupts the tape motif, with a hydroxy group interrupting the N? H???O?C interactions. The geometry around the hydrogen‐bearing nitrogen atoms, whether planar or non‐planar, has been confirmed for compounds 2 and 5 by using Laue neutron diffraction and rationalized by using computational methods, thus demonstrating that distortion of O‐C‐N‐H torsion angles occurs to maintain almost‐linear hydrogen‐bonding interactions. 相似文献
986.
987.
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989.
为提高中子法在线煤质分析仪的测量精度,对其电学系统进行改进。在原设计的基础上重组了部分冗余部件,采用DSP+USB方案优化了多道谱仪,选用11点平滑公式进行数据处理。实验表明,改进后的系统测量精度比原系统显著提高,测量数据与化学检测结果吻合,验证了系统改进措施的可行性。 相似文献
990.
YAN Jie LIU Rong LI Cheng JIANG Li & WANG Mei Institute of Nuclear Physics Chemistry Chinese Academy of Engineering Physics Mianyang China 《中国科学:物理学 力学 天文学(英文版)》2011,(3)
The unfolding of neutron spectra from the pulse height distribution measured by a BC501A scintillation detector is accomplished by the application of artificial neural networks (ANN). A simple linear neural network without biases and hidden layers is adopted. A set of monoenergetic detector response functions in the energy range from 0.25 MeV to 16 MeV with an energy interval of 0.25 MeV are generated by the Monte Carlo code O5S in the training phase of the unfolding process. The capability of ANN was demon... 相似文献