基于Geant4模拟中子输运过程及建立中子监测模型

通过研究Geant4程序的中子输运模型与反应截面库,构建热中子至20 MeV中子的输运过程,为验证Geant4中子输运过程的正确性,通过建立中子监测模型,再根据监测模型设计试验装置,最后将试验装置放入中子参考辐射场中进行测试验证.模拟与实验结果表明:试验装置在241Am-Be中子源和252Cf源中子参考辐射场中测试结果最大误差为2.5％.     

B_4C_P/PI聚酰亚胺复合薄膜耐高温及热中子辐照屏蔽性能研究

以耐高温型聚酰亚胺为基体,微米碳化硼(B_4C)为热中子吸收剂,采用粉体表面改性及超声湿混-热亚胺化成膜工艺成功制备了一系列B_4CP/PI聚酰亚胺复合薄膜,重点探讨了不同B_4C含量条件下复合薄膜的耐热性能和力学性能以及不同B_4C含量、不同复合薄膜厚度条件下复合材料的热中子屏蔽性能。研究表明:采用上述工艺,B_4C功能粒子在聚酰亚胺基体中可均匀分散;B_4CP/PI复合薄膜的耐热性随B_4C含量的增加显著提高,力学性能则呈相反趋势;所制备的B_4CP/PI复合薄膜表现出优异的热中子屏蔽性能,中子透射率I/I0随复合薄膜厚度增加及B_4C含量增加呈指数变化规律。据此,可通过材料结构设计,满足不同领域对该类耐高温中子防护材料的应用需求。     

用于中子屏蔽的碳化硼/超高分子量聚乙烯复合材料研究

采用热压成型工艺,制备用于中子屏蔽的碳化硼/超高分子量聚乙烯(UHMWPE)复合材料,研究了热压温度、硅烷偶联剂添加量、碳化硼含量对材料的冲击强度、弯曲强度等性能的影响,通过FTIR,SEM及能谱分析偶联剂添加效果.同时研究了碳化硼含量对材料中子屏蔽效率的影响.结果表明,当成型温度小于200℃时,温度的提高对材料力学性能有提高作用;KH570硅烷偶联剂对碳化硼有较好的偶联作用,适当添加KH570增强了碳化硼与UHMWPE之间的结合力,当添加量超过3％(质量分数,下同)后,偶联剂在碳化硼表面形成多余吸附层,减弱了基体与颗粒之间的结合力,降低了冲击强度;碳化硼含量提高,颗粒的团聚增加,大颗粒在基体中形成缺陷,导致材料脆性增加,降低了材料冲击强度;中子衰减系数随碳化硼含量增加而提高,随样品厚度的增加而减小;样品厚度较薄时,材料主要吸收热中子,当样品厚度增加时,快中子在材料内部发生多次碰撞,能量降低后也被吸收掉.     

粘结碳化硼材料中子屏蔽性能及制备工艺研究

中子准直器在中国散裂中子源(CSNS)的谱仪上起着限制束流截面与发散度的作用,准直器的结构设计中通过在中子飞行管上间隔设置B4C阻挡块可以起到阻挡、吸收屏蔽杂散中子的作用,为此研制了粘结碳化硼中子阻挡块,并对其成型工艺进行了研究,确定了成型最佳粉体尺寸、粘结剂含量和固化温度.对该中子屏蔽材料的中子衰减系数计算结果表明,波长为0.1×10-10、1×10-10和15×10-10m中子通过该材料的线性衰减系数分别为3.42、30.4和449.9 cm-1.力学冲击和热重实验结果也表明该材料符合在CSNS谱仪中子束线屏蔽使用中的性能要求.     

铝基复合材料摩擦磨损性能研究

为研究碳纤维对Al1O3f/ZL109复合材料摩擦磨损性能的影响,进一步提高金属基体的摩擦磨损性能,利用液态模锻法制备了(Cf,Al2O3f)/ZL109复合材料,并研究了该材料的摩擦磨损性能.结果表明:各种(Cf,Al2O3f)/ZL109复合材料的磨损量均随载荷的增加而增大,但复合材料的磨损量均低于ZL109基体,且在总纤维体积分数为12%的复合材料中,(4?,8%Al2O3f)/ZL109复合材料具有最低的磨损量;各种(Cf,Al2O3f)/ZL109复合材料的摩擦因数均随载荷的增加而减小.(Cf,Al2O3f)/ZL109复合材料的耐磨性由碳纤维与氧化铝纤维性能及基体共同决定.     

碳化硼粒度对无压浸渗高体分铝基复合材料微观组织和力学性能的影响

选用平均粒度为2μm和38μm的碳化硼颗粒,分别制备100%(质量分数) 38μm、20%(质量分数) 2. 0μm+80%(质量分数) 38μm和100%(质量分数) 2. 0μm的碳化硼预制体,以无压浸渗法制备三种高体分B_4C/Al复合材料,研究碳化硼颗粒对复合材料的物相组成、微观组织和力学性能的影响。结果表明,三种复合材料均由Al、B_4C、Al_3BC、AlB_2和富Fe-Mn相组成。当增强相完全为大颗粒碳化硼时,复合材料内部碳化硼均匀分布于铝基体,此时界面反应程度较弱,界面产物AlB_2和Al_3BC呈随机分布的特征,且复合材料的硬度和抗弯强度分别为23. 2HRC和406 MPa。由于小颗粒碳化硼具有较高的比表面积,其与熔融状态的铝合金(以下简称"熔铝")实际接触面积较大,使得两者之间发生剧烈的界面反应。因此,当增强相中引入20%(质量分数)小颗粒碳化硼时,复合材料内铝基体消耗量增加,大颗粒碳化硼仍近乎均匀分布,颗粒间组织表现为剩余的细颗粒B_4C和铝均匀分布于界面产物内。由于初始增强相体积分数和陶瓷相界面产物含量均增加,复合材料的硬度提升至40. 02HRC,抗弯强度略有提升(425 MPa),但应变量有所降低。当增强相完全为小颗粒碳化硼时,剧烈的界面反应大量消耗铝合金基体,使得Al_3BC和AlB_2成为B_4C/Al复合材料的主要物相,微观组织呈现为剩余的小颗粒B_4C和铝均匀分布于陶瓷相基体内,复合材料硬度提升至56. 8HRC。然而,由于小颗粒碳化硼在高温烧结过程中存在封闭微孔缺陷且这些缺陷将保留于复合材料内,使得复合材料的弯曲强度降低至248 MPa。     

用空间电荷限制电流法测量非晶硅基薄膜的隙态密度

利用常规的层状结构的空间电荷限制电流法,测得了具有一定厚度 d 的 GD-a-Si_(1-x)C_x∶H 和 GD-a-Si_(1-x)N_x∶H 膜不同含量 x 时的隙态分布 N(E):对 GD-a-Si_(1-x)C_x∶H 膜(d(?)1μm),当 x 为0、0.1、0.8时,平衡费米能级附近处的隙态密度 N(E_(?)~o)分别为2×10~(15)、4×10~(15)、6.2×10~(16)/cm~3·eV,对 GD-a-Si_(1-x)N_x∶H 膜(d(?)1μm),当 x 为0、0.05、0.2时,N(E_F~o)分别为2×10~(15)、3×10~(15)、4.5×10~(16)/cm~3·eV;得到了 GD-a-Si∶H 膜的隙态分布与膜厚度的关系,发现随着膜厚度的增加 N(E_F~o)在减小,当 d<1μm 时,N(E_F~o)约为10~(16)/cm~3·eV 的数量级,当 d>1μm 时,N(E_F~o)约为10~(15)/cm~3·eV 的数量级。对共面电极结构的样品,用温度调制空间电荷限制电流法(TM-SCLC),测得了 GD-a-Si∶H 膜的隙态分布,并对光处理前后的样品进行比较,发现强光照后存在有光诱导效应。我们对所得结果作了初步说明。     

a-Si_(1-x)Nx:H 光电导的改善及其原由

氮的掺入能够改善 a-Si∶H 的光电导性。用 ESR 方法难以分析其原因。吸收系数的弱吸收段却能显示出氮造成 a-Si∶H 缺陷态密度的变小。当掺氮量 x(?)0.01时,a-Si_(1-x)N_x 之ημT 值呈最大,在 E_(?)下1.2eV 处缺陷态密度最低,约3.3×10~(15)/cm~3。过量地掺氮则导致缺陷态密度增加,光电导降低。     

TiB2掺杂对碳纤维增强锂铝硅复合材料抗氧化性的影响

目的 研究含有不同质量分数的TiB2的Cf/LAS复合材料的抗弯强度变化规律。方法 以碳纤维、TiB2、硝酸锂、硝酸铝、硅溶胶和氨水为主要原料,采用溶胶凝胶法结合浆料浸渍-热压烧结工艺,制备了加入TiB2的碳纤维增强锂铝硅微晶玻璃（LAS）复合材料Cf/LAS(TiB2),利用XRD、FTIR、万能材料试验机、SEM等对Cf/LAS(TiB2)复合材料的性能和微观结构进行研究。结果 TiB2的质量分数为1%, 3%, 5%, 9%的Cf/LAS(TiB2)复合材料,在1000 ℃热处理30 min后的强度保留率分别为2.9%, 29.1%, 44.0%, 50.0%。结论 材料的强度保留率随TiB2含量的增多而上升,因此Cf/LAS(TiB2)复合材料的抗氧化性随着TiB2含量的增加而增强。     

CVI-GSI工艺制备C/C-SiC复合材料的组成结构与力学性能

采用密度为1.0g/cm~3的C/C素坯,联合化学气相渗透(CVI)和气相渗硅(GSI)2种工艺制备C/C-SiC复合材料,研究CVI C/C-SiC复合材料中间体的密度对CVI-GSI C/C-SiC复合材料物相组成、微观结构及力学性能的影响。结果表明:随着CVI C/C-SiC复合材料中间体密度的增大,CVI-GSI C/C-SiC复合材料C含量增多,残余Si含量减少,SiC含量先增多后减少,CVI-GSI C/C-SiC复合材料的密度先增大后减小;随着CVI C/C-SiC复合材料中间体的密度由1.27g/cm~3增加到1.63g/cm~3时,得到的CVI-GSI C/C-SiC复合材料的力学性能先升高后降低。当CVI C/C-SiC复合材料密度为1.42g/cm~3时,制得的CVI-GSI C/C-SiC复合材料力学性能最好,其弯曲强度为247.50MPa,弯曲模量为25.63GPa,断裂韧度为10.08MPa·m~(1/2)。     

Observation of the 3He(n,tp) Reaction by Detection of Far-Ultraviolet Radiation

We have detected Lyman alpha radiation, 121.6 nm light produced from the n = 2 to n = 1 transition in atomic hydrogen, as a product of the ³He(n, tp) nuclear reaction occurring in a cell of ³He gas. The predominant source of this radiation appears to be decay of the 2p state of tritium produced by charge transfer and excitation collisions with the background ³He gas. Under the experimental conditions reported here we find yields of tens of Lyman alpha photons for every neutron reaction. These results suggest a method of cold neutron detection that is complementary to existing technologies that use proportional counters. In particular, this approach may provide single neutron sensitivity with wide dynamic range capability, and a class of neutron detectors that are compact and operate at relatively low voltages.     

Quantum molecular tunneling of NH4 + ions in mixed ammonium-alkali halide systems

The rotational excitations of NH₄ ⁺ ions in dilute solution in metal alkali halide lattices have been studied at 4K by inelastic neutron scattering technique. It is found that the ions exist in matrix isolation at very low ammonium ion concentration,c (c → 0), and it transforms to orientational glass phase on increase ofc. It is shown that the onset of the orientational glass phase is moderated through the strain field generated by the substituted impurity in the lattice. The variation of the intensity profiles with increase ofc have been successfully explained by a phenomenological model based on gaussian distributed heights.     

A New Method for Precision Cold Neutron Polarimetry Using a 3He Spin Filter

We present a new method for precision measurement of the capture flux polarization of a polychromatic (white), continuous cold neutron beam, polarized by a ³He spin filter. This method allows an in situ measurement and does not require knowledge of the neutron beam wavelength distribution. We show that a polarimetry precision of 0.1 % is possible.     

Electron-Impact Total Ionization Cross Sections of Hydrocarbon Ions

The Binary-Encounter-Bethe (BEB) model for electron-impact total ionization cross sections has been applied to CH2+, CH3+, CH4+, C2H2+, C2H4+, C2H6+ and H₃O⁺. The cross sections for the hydrocarbon ions are needed for modeling cool plasmas in fusion devices. No experimental data are available for direct comparison. Molecular constants to generate total ionization cross sections at arbitrary incident electron energies using the BEB formula are presented. A recent experimental result on the ionization of H₃O⁺ is found to be almost 1/20 of the present theory at the cross section peak.     

Polarized 3He Spin Filters for Slow Neutron Physics

Polarized ³He spin filters are needed for a variety of experiments with slow neutrons. Their demonstrated utility for highly accurate determination of neutron polarization are critical to the next generation of betadecay correlation coefficient measurements. In addition, they are broadband devices that can polarize large area and high divergence neutron beams with little gamma-ray background, and allow for an additional spin-flip for systematic tests. These attributes are relevant to all neutron sources, but are particularly well-matched to time of flight analysis at spallation sources. There are several issues in the practical use of ³He spin filters for slow neutron physics. Besides the essential goal of maximizing the ³He polarization, we also seek to decrease the constraints on cell lifetimes and magnetic field homogeneity. In addition, cells with highly uniform gas thickness are required to produce the spatially uniform neutron polarization needed for beta-decay correlation coefficient experiments. We are currently employing spin-exchange (SE) and metastability-exchange (ME) optical pumping to polarize ³He, but will focus on SE. We will discuss the recent demonstration of 75 % ³He polarization, temperature-dependent relaxation mechanism of unknown origin, cell development, spectrally narrowed lasers, and hybrid spin-exchange optical pumping.     

Compressing Spin-Polarized 3He With a Modified Diaphragm Pump

Nuclear spin-polarized ³He gas at pressures on the order of 100 kPa (1 bar) are required for several applications, such as neutron spin filters and magnetic resonance imaging. The metastability-exchange optical pumping (MEOP) method for polarizing ³He gas can rapidly produce highly polarized gas, but the best results are obtained at much lower pressure (~0.1 kPa). We describe a compact compression apparatus for polarized gas that is based on a modified commercial diaphragm pump. The gas is polarized by MEOP at a typical pressure of 0.25 kPa (2.5 mbar), and compressed into a storage cell at a typical pressure of 100 kPa. In the storage cell, we have obtained 20 % to 35 % ³He polarization using pure ³He gas and 35 % to 50 % ³He polarization using ³He-⁴He mixtures. By maintaining the storage cell at liquid nitrogen temperature during compression, the density has been increased by a factor of four.     

可见光下利用ZrO2(Er3+)/TiO2光催化降解海水中柴油污染

通过共沉淀法自制上转换材料与TiO_2复合的纳米光催化剂;利用SEM、XRD等方法对光催化剂进行表征;通过改变催化剂掺杂比、pH值、催化剂投入量、光照时间和柴油初始浓度研究了可见光下光催化降解海水中柴油污染的影响因素以及复合光催化剂ZrO_2(Er3+)/TiO_2的利用效率,通过正交试验优化海水中柴油污染的降解;进行动力学分析,计算总反应速率表达式。结果表明,当柴油初始浓度为0.20g/L,催化剂投加量为0.8g/L,催化剂掺杂比为40%,pH为7,光照时间为2.5h时,复合光催化剂的利用效率最高,柴油的去除率达到87.74%。ZrO_2(Er3+)/TiO_2在可见光下能够有效地降解海水中的柴油污染。     

Synthesis of monodisperse spherical core-shell SiO2-SrAl2Si2O8: Eu2+ phosphors by hydrothermal homogeneous precipitation method

Nanocrystalline SrAl₂Si₂O₈ :Eu²⁺ phosphor layers were coated on nonaggregated, monodisperse and spherical SiO₂ particles using a hydrothermal homogeneous precipitation. After annealing at 1100 °C, core-shell SiO₂@SrAl₂Si₂O₈ :Eu²⁺ particles were obtained. They were characterized with x-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy and photoluminescence techniques. XRD analysis confirmed the formation of SiO₂ @SrAl₂Si₂O₈ :Eu²⁺ particles; it indicated that the SrAl₂Si₂O₈ :Eu²⁺ shells on SiO₂ particles consisted of hexagonal crystallites. The core-shell phosphors obtained are well-dispersed submicron spherical particles with a narrow size distribution. The thickness of the coated layer is approximately 20–40 nm. Under ultraviolet excitation (361 nm), the particles emit blue light at about 440 nm due to the Eu²⁺ ions in their shells.     

Zn2(OH)3VO3∶Eu3+纳米晶的水热合成及发光性能

以NaVO3、Zn(NO3)2·6H2O为原料,采用水热法合成了铕离子掺杂Zn2(OH)3VO3纳米晶体。运用X射线粉末衍射、扫描电子显微镜、荧光测试仪对纳米晶进行了表征,研究了水乙醇体系不同pH值对产物晶相、微观形貌及荧光性能的影响。在溶剂体系pH值为7~10时,得到片状的Zn2(OH)3VO3,而在pH值为4~6和11~12时得到Zn3(OH)2V2O7·2H2O,表明强酸性和强碱性的溶剂体系都不太适合Zn2(OH)3VO3的合成,而最佳的合成体系为中性偏碱性;荧光测试表明在pH=10的水乙醇体系下合成的Zn2(OH)3VO3∶8%Eu3+纳米片的荧光性能最好,其中发射主峰位于612nm处。     

The Pb–Pb and O–Pb Nuclear Spin Coupling in Ba(Pb,Bi)O3 Oxides

We report the ¹⁷O, ²⁰⁷Pb NMR and ¹⁷O–²⁰⁷Pb SEDOR study of superconducting BaPb_1-x Bi _x O₃ (x 0.35) oxides. Inhomogeneous The results points to the formation of microscopically inhomogeneous distribution of the carrier density in metallic phase. The local spin susceptibility is increased around Bi cation within an area, which is delimited by its two first cation shells.