分子动力学研究MeV质子辐照对生物分子二级结构的影响

利用分子动力学方法对MeV质子辐照多肽分子所引起的分子空间结构变化进行了研究，对比了不同质子能损和模拟条件下分子构象变化的差异。结果表明，在质子轰击下，分子的二级结构在其得到3—5eV能量后出现明显的变化，从α螺旋结构向β折叠逐渐转变，并且这种变化与质子能量的沉积方式有关。与温度效应的对照显示，质子辐照引起的变化与温度所引起的变化有明显的差异：质子辐照产生的分子构象更丰富多样．而温度所引起的变化则比较单一。     

利用粒子辐照技术实现碳纳米管向金刚石的转变

采用低能氢等离子体和中能C^ 离子束辐照技术相结合的方法,实现了碳纳米管向金刚石纳米晶粒的转变,完成了一个从有序(碳纳米管)到无序(无定形碳纳米线)再到有序(金刚石纳米晶)的转变过程。利用透射电子显微镜(TEM)、选区电子衍射(SAD)和拉曼光谱(Raman)等研究了晶粒的微观结构,并对纳米金刚石晶粒的生成机理进行了初步探讨。     

医用回旋加速器生产正电子核素~(18)F条件分析与优化

分析医用回旋加速器正电子核素18F的照射条件和轰击参数对生产的影响,优化生产条件并给出最佳的轰击参数以期获得高效的生产产额。使用Origin 9.0软件绘制核素18F产量随不同质子束流强度和轰击时间的变化趋势曲线,以蒙特卡罗方法建立回旋加速器质子辐照靶室模型,分析不同质子能量、Havar膜和靶水厚度等对核素18F产量的影响,并给出18F生产最佳的束流强度、轰击时间和质子能量等生产参数。回旋加速器运行期间束流应充分聚焦于照射靶室中心位置,最大化的利用束流以引发足够多的核反应;根据质子束流的能量选择合适的Havar膜和靶水厚度,20 Me V质子束流轰击生产正电子核素18F的靶室系统使用Havar膜总计厚度60μm,靶水厚度3 mm,可获得最佳18F产量。总体而言,18F的产量随束流强度而增大,轰击时间越长18F产量越大,但随着轰击时间的延长增长趋势变缓,轰击时间建议60 min左右。正电子核素18F的生产需要选择合适的Havar膜和靶水厚度(当质子能量为20 Me V时,推荐Havar厚度60μm,靶水厚度3 mm),轰击时间建议60 min左右,开机启动稳定一段时间后再开始照射。     

AB-BNCT中子靶物理设计分析

质子加速器适用于为硼中子俘获治疗提供中子源,其中子源强及能谱较反应堆中子源更具可调性。中子靶物理计算分析是加速器中子源设计的基础,为其提供粒子能量、流强等参数需求分析,并为靶体结构尺寸设计、中子慢化和屏蔽分析等提供前端参数。本文利用MCNPX蒙特卡罗程序,通过对质子打靶的中子产额和能谱、靶体能量沉积、打靶后靶材放射性活度和中子出射空间角分布等进行研究,提出能量2.5 MeV质子轰击100～200μm锂靶的设计,并用模拟计算数据论证其合理性。该设计中子源在1 mA流强质子轰击下,源强可达9.74×10~(11) s~(-1);拟设计15 mA、2.5 MeV质子束产生的中子源,在治疗过程中靶材放射性活度累积最大值约为1.44×10~(13) Bq。     

AB-BNCT中子靶物理设计分析

质子加速器适用于为硼中子俘获治疗提供中子源，其中子源强及能谱较反应堆中子源更具可调性。中子靶物理计算分析是加速器中子源设计的基础，为其提供粒子能量、流强等参数需求分析，并为靶体结构尺寸设计、中子慢化和屏蔽分析等提供前端参数。本文利用MCNPX蒙特卡罗程序，通过对质子打靶的中子产额和能谱、靶体能量沉积、打靶后靶材放射性活度和中子出射空间角分布等进行研究，提出能量2.5 MeV质子轰击100～200 μm锂靶的设计，并用模拟计算数据论证其合理性。该设计中子源在1 mA流强质子轰击下，源强可达9.74×10¹¹ s^-1；拟设计15 mA、2.5 MeV质子束产生的中子源，在治疗过程中靶材放射性活度累积最大值约为1.44×10¹³ Bq。     

强流脉冲质子束轰击19F靶产生6-7MeV准单能脉冲γ射线初步实验研究

给出了利用PIN半导体探测器和ST401塑料闪烁体配合光电倍增管测量的强流脉冲质子束轰击含^19F核素的靶产生的6—7MeV准单能脉冲γ射线的初步实验结果。理论上计算了质子束轰击C2F4靶产生的准单能脉冲γ射线产额和核反应截面随入射质子能量的变化曲线，提出了采用质子束传输法分离和降低轫致辐射干扰的方法，利用ST401配合光电倍增管和PIN半导体探测器测量了质子传输不同距离后轰击C2F4靶产生的6—7MeV准单能脉冲γ射线谱以及相对于轫致辐射的时间延迟数据，并根据飞行时间确定了束流峰值时刻的质子能量，还通过实验验证了Cu、Cr和不锈钢靶及其中所含的杂质不能产生明显的其它能量的γ射线干扰。     

MeV能量Si~+引起二次离子发射的研究

利用北京大学2×1.7 MV串列加速器终端的飞行时间(TOF)谱仪,分别用1.5、2.0和3.0MeV三种能量的初级束Si+轰击样品来研究其二次离子发射现象,使用的样品包括石墨、碳纳米管等。结果表明,采用合理的降噪方法后得到了高信噪比和高时间分辨率的二次离子质谱,实现了全质量范围无遗漏记谱。利用H-C12峰刻度之后,计算得出二次离子各成分的最可能构成及产额,碳纳米管样品表面氢质量含量为8.15%。同时,发现MeV能区二次离子产额与Si+阻止本领之间的关系并非简单的正相关。     

PE/CNT复合材料对空间质子的屏蔽效能仿真分析

随着遥感卫星对观测精度需求的增长，国内外积极探索新型高效质子防护材料。为研究聚乙烯掺杂碳纳米管屏蔽质子效能，本文利用Geant4软件开展质子屏蔽仿真，探究碳纳米管掺杂浓度、管壁直径、排布方式和管壁层数对复合材料质子屏蔽效能的影响规律，并与纯聚乙烯进行对比分析。结果表明，质子屏蔽效能对掺杂浓度、管壁直径和排布方式较敏感，受管壁层数影响相对较小；在高掺杂浓度、大管壁直径和规则排布的情况下，复合材料表现出更优异的质子屏蔽效能；相同质量厚度下，碳纳米管掺杂浓度为10%和20%的复合材料质子屏蔽后的电离剂量分别比纯聚乙烯最大降低了7.40%和12.83%。本文研究结果为辐射防护材料设计提供了数据支撑。     

荷电离子轰击靶材料损伤潜径迹的原子水平观测研究

叙述了用扫描隧道显微镜和原子力显微镜观测研究荷电离子轰击靶材料的损伤潜径迹的状况和进展，观测研究了Ａｕ离子和Ｈ＾＋轰击高定向石墨的ＳＴＭ。给出了损伤形貌、损伤范围、表面损伤数密度和离子注入剂量的关系，并对损伤过程进行了分析和讨论。     

一种用于SPM分析的大气气溶胶单颗粒样品的制备方法

介绍了一种用于扫描质子微探针分析的大气气溶胶单颗粒样品的制备方法。通过实验摸索出一种适合于支撑颗粒物的材料聚乙烯醇缩丁醛(PVB)。大气颗粒物被直接采集在PVB薄膜上从而制成样品,保持了颗粒物的原始信息。用显微镜观察了采集的颗粒物,颗粒物之间完全分开,颗粒物间距合适。样品的扫描质子微探针试验结果表明,所制成的单颗粒样品能经受住质子束的长久轰击。对PVB薄膜的本底测试表明,这种膜的本底很小,不会对被测颗粒物的元素产生干扰。     

Structural change of carbon nanotubes produced by Si ion beam irradiation

Multi-walled carbon nanotubes (MWCNTs) were irradiated by 40 keV Si ion beam with different doses. The structural change of the MWCNTs was revealed by transmission electron microscopy, high-resolution transmission electron microscopy and Raman spectroscopy. The structural characterization after irradiation shows that the formation of amorphous carbon nanowires proceeds through two periods, carbon nanotube – semi-solid amorphous carbon nanowire with hollow structure – solid amorphous carbon nanowire. Based on the interaction between energetic particles and carbon nanotubes, the structural transformation process and corresponding mechanisms are discussed. A model is presented to illustrate the structural change of carbon nanotubes with increased irradiation dose.     

Carbon nanostructures produced through ion irradiation

Several nanostructures we produced by ion irradiation have been reviewed in this paper. By using ions to irradiate two ultrahigh molecular weight polyethylene targets respectively, it was found that small fullerenes C20 and C26 were grown, adding two members to the fullerene family. Meanwhile, crystalline diamonds also have been produced by Ar+ ions irradiation of graphite. In the experiment of double ions Ni+ and Ar+ irradiation, nanoscale ar-gon bubbles formed. On the other side, when multi-wall carbon nanotubes were irradiated by C+, many MWCNTs evolved to amorphous carbon nanowires and amorphous carbon nanotubes. And there are possible welding in the crossed nanotubes.     

Formation of Pellet-Cladding Bonding Layer in High Burnup BWR Fuels

Formation process of the pellet-cladding bonding layer was studied by EPMA, XRD, and SEM/TEM for the oxide layer on a cladding inner surface and the bonding layer in irradiated fuel rods. Specimens were prepared from fuels which had been irradiated to the pellet average burnups of 15, 27 and 42 GWd/t in BWRs. In the lower burnup specimens of 15 and 27GWd/t, no bonding layer was found, while the higher burnup specimens of 42 and previously reported 49 GWd/t had a typical bonding layer. A bonding layer which consisted of two regions was found in the latter fuels. One region of the inner surface of the cladding was made up mainly of ZrO₂. The structure of this ZrO₂ consisted of cubic phase, while no monoclinic crystals were found. The other region, near the pellet surface, had both a cubic solid solution of (U, Zr)O₂ and amorphous phase. Even in the lower burnup specimens having no bonding layer, cubic ZrO₂ phase was identified in the cladding inner oxide layer. The formation process of the bonding layer were discussed in connection with phase transformation by irradiation damage of fission products and conditions for contact of pellet and cladding.     

Magnetic force microscopy studies on the magnetic ordering in organic materials induced by high-energy proton irradiation

In this study, ferromagnetic microstructures in highly oriented pyrolytic graphite and superparamagnetic spots in polyimide foils were created by 2.25 MeV proton microbeam irradiation and characterized using atomic and magnetic force microscopy. For this purpose, graphite samples were irradiated with cross-like patterns of 15 μm × 15 μm size using ion fluences in the range of (0.003–2.5) × 10¹⁸ cm⁻². The irradiated crosses showed strong magnetic signals and a complex domain structure in the magnetic images depending on the geometrical dimensions of the crosses. Furthermore, polyimide foils were irradiated with microspots and fluences in the range of (0.016–3.1) × 10¹⁹ cm⁻². Magnetic force microscopy shows very strong phase shifts in these irradiated areas.     

Atomic-scale structure of irradiated GaN compared to amorphised GaP and GaAs

We have compared the atomic-scale structure of ion irradiated GaN to that of amorphised GaP and GaAs. While continuous and homogenous amorphised layers were easily achieved in GaP and GaAs, ion irradiation of GaN yielded both structural and chemical inhomogeneities. Transmission electron microscopy revealed GaN crystallites and N₂ bubbles were interspersed within an amorphous GaN matrix. The crystallite orientation was random relative to the unirradiated epitaxial structure, suggesting their formation was irradiation-induced, while the crystallite fraction was approximately constant for all ion fluences beyond the amorphisation threshold, consistent with a balance between amorphisation and recrystallisation processes. Extended X-ray absorption fine structure measurements at the Ga K-edge showed short-range order was retained in the amorphous phase for all three binary compounds. For ion irradiated GaN, the stoichiometric imbalance due to N₂ bubble formation was not accommodated by Ga–Ga bonding in the amorphous phase or precipitation of metallic Ga but instead by a greater reduction in Ga coordination number.     

The development of a stress analysis code for nuclear graphite components in gas-cooled reactors

Most of the UK nuclear power reactors are gas-cooled and graphite moderated. As well as acting as a moderator the graphite also acts as a structural component providing channels for the coolant gas and control rods. For this reason the structural integrity assessments of nuclear graphite components is an essential element of reactor design. In order to perform graphite component stress analysis, the definition of the constitutive equation relating stress and strain for irradiated graphite is required. Apart from the usual elastic and thermal strains, irradiated graphite components are subject to additional strains due to fast neutron irradiation and radiolytic oxidation. In this paper a material model for nuclear graphite is presented along with an example of a stress analysis of a nuclear graphite moderator brick subject to both fast neutron irradiation and radiolytic oxidation.     

Energy loss of protons in carbon nanotubes: Experiments and calculations

We have studied the energy loss of protons in multi-walled carbon nanotube (MWCNT) samples, both experimentally and theoretically. The experiments were done in transmission geometry, using 6 and 10 keV proton beams, with the MWCNT targets dispersed on top of a ～20 nm-thick holey carbon coated TEM grid (amorphous carbon film, a-C). The energy loss of protons interacting with the MWCNTs and the amorphous carbon film is obtained after analyzing the signals coming from both types of carbon allotropes. The electronic energy loss of protons is calculated using the dielectric formalism, with the target energy loss function built from optical data. Comparison of experimental and theoretical data indicates that model calculations appropriate for three-dimensional (bulk) targets substantially overestimate the energy loss to MWCNTs. In contrast, a recent parameterization of the dielectric function of MWCNTs predicts significantly lower stopping power values compared to the bulk models, which is more in line with the present experimental data when considering the additional stopping mechanisms that are effective in the keV range.     

The evolution of hydrogen from proton irradiated glassy carbon

Glassy carbon was irradiated with 15 keV H⁺ ion beam. It was observed that the implanted hydrogen is unstable in material and evolves as H, H₂ and H₂O. Post-irradiation evolution of H, H₂ and H₂O from proton irradiated glassy carbon was monitored by temperature programmed desorption (TPD) in the period of 30 days. In between irradiation and TPD measurements the irradiated samples were stored in air. The evolution of the molecular hydrogen, although the protons are implanted deeply below the surface of the disordered glassy carbon, proceeds over the same mechanism as in the case of low-energy H-atoms chemisorbed on the very surface of an ideal graphite structure.     

AFM and STM investigation of carbon nanotubes produced by high energy ion irradiation of graphite

Carbon nanotubes (CNT) were produced by high energy, heavy ion irradiation (215 MeV Ne, 246 MeV Kr, 156 MeV Xe) of graphite. On samples irradiated with Kr and Xe ions large craters were found by atomic force microscopy, these are attributed to sputtering. Frequently one or several CNTs emerge from the craters. Some of the observed CNTs showed a regular vibration pattern. No other carbon based materials, like amorphous carbon or fullerenes were evidenced. Focused ion beam cuts were used to compare CNTs with surface folds on graphite.     

Phase transformation during silica cluster impact on crystal silicon substrate studied by molecular dynamics simulation

The process of a silica cluster impact on a crystal silicon substrate is studied by molecular dynamics simulation. At the impact loading stage, crystal silicon of the impact zone transforms to a locally ordered molten with increasing the local temperature and pressure of the impact zone. And then the transient molten forms amorphous silicon directly as the local temperature and pressure decrease at the impact unloading stage. Moreover, the phase behavior between the locally ordered molten and amorphous silicon exhibits the reversible structural transition. The transient molten contains not only lots of four-fold atom but also many three- and five-fold atoms. And the five-fold atom is similar to the mixture structure of semi-Si-II and semi-bct5-Si. The structure transformation between five- and four-fold atoms is affected by both pressure and temperature. The structure transformation between three- and four-fold atoms is affected mostly by temperature. The direct structure transformation between five- and three-fold atoms is not observed. Finally, these five- and three-fold atoms are also different from the usual five- and three-fold deficient atoms of amorphous silicon. In addition, according to the change of coordination number of atoms the impact process is divided into six stages: elastic, plastic, hysteresis, phase regressive, adhesion and cooling stages.