Use of CR-39 films for nuclear radiation shielding efficacy evaluation of lining materials for combat vehicles

All materials provide, to a lesser or greater extent, shielding against nuclear radiations. Armoured fighting vehicles (AFVs) have steel as the structural material, which appears to be a reasonably good gamma and neutron shield material but a shield of pure iron would not be equally effective against whole range of neutron energies as it has a few resonances in electron volt range, and it reduces energy of fast neutrons to lower energy neutrons. These neutrons will be absorbed through radiative capture and emit gamma radiations. Thus it is essential that an effective shield should contain a large amount of moderating material, hydrogen being preferred with low atomic number materials (B, C, Li) and lead (Pb) to ensure that the neutrons do not diffuse at intermediate energies in the shield as well as gamma attenuation will also take place. In order to have a suitable shield material for armoured vehicles which serves as neutron and gamma radiation attenuator, polyethylene polymer with fillers lining materials are preferred. These materials were evaluated against gamma and fast neutrons using radioactive sources for suitability to fitment into combat vehicle as per the requirement of protection factor values. The detector for gamma radiation was used as Nal(Tl) while for neutron, CR-39 film was used.      

SSRF前端区锯齿墙束流孔洞中子屏蔽的蒙特卡罗估算

主要关于上海同步辐射装置(SSRF)储存环电子引发产生的韧致辐射和中子辐射的研究. 中子和光子经多种组合材 料(厚度在5cm～115cm之间)屏蔽后的剂量特征由蒙特卡罗代码MCNP和EGSnrc估算得到; 蒙特卡罗计算表明, 单一的材料如铅, 铁和聚乙烯对高能中子是无效的生物屏蔽材料, 而组合材料如铅或者铁加聚乙烯和铅或者铁加混凝土被认为是屏蔽高能中子很好的组合材料. 铅铁等高Z材料加点包含有氢的低Z材料如聚乙烯是同时屏蔽高能中子和韧致辐射的一种比较好的组合材料选择.     

On the Electrode Damage and Current Densities of Carbon Arcs

Arc damage on carbon electrodes has been examined as a function of the charge passing through each arc in the range from 10-6 to 10-1 coloumb. Two forms of arcing damage were observed: that due to the so-called vapor arc, which has a very high current density (~6 × 107 A/cm2) and low erosion rate, and that due to the so-called thermionic arc with a lower current density (~2 × 103 A/cm2) and higher erosion rate. Unlike damage observed on metallic electrodes raised rims were not formed around the carbon craters. The vapor mode damage is compared with two known models of cathode erosion. Differences between the vapor and thermionic modes and the transition between them are discussed. A whitish-gray deposit observed on the arced electrodes appears to be a form of graphite.     

微波消解-冷原子荧光法测定煤中痕量汞

微波消解技术具有样品溶解完全、污染少、节约试剂和方便快捷等特点。文章采用微波消解预处理煤样 ,并用冷原子荧光法测定样品中的痕量汞。采用硝酸 盐酸 氢氟酸消解体系 ,进行了微波消解条件的选择 ,并考察了载气流量、屏蔽气流量、负高压和干扰等对测定的影响。在选定的操作条件下 ,汞的检出限为0 0 3ng·mL-1,相对标准偏差 (n =7)小于 5 %。方法准确、快速和方便 ,利用国家标准参考物质煤飞灰GBW 0 84 0 1验证了方法的准确度 ,测定值与参考值基本吻合     

Cavitation erosion by shockwave self-focusing of a single bubble

The ability of cavitation bubbles to effectively focus energy is made responsible for cavitation erosion, traumatic brain injury, and even for catalyse chemical reactions. Yet, the mechanism through which material is eroded remains vague, and the extremely fast and localized dynamics that lead to material damage has not been resolved. Here, we reveal the decisive mechanism that leads to energy focusing during the non-spherical collapse of cavitation bubbles and eventually results to the erosion of hardened metals. We show that a single cavitation bubble at ambient pressure close to a metal surface causes erosion only if a non-axisymmetric energy self-focusing is at play. The bubble during its collapse emits shockwaves that under certain conditions converge to a single point where the remaining gas phase is driven to a shockwave-intensified collapse. We resolve the conditions under which this self-focusing enhances the collapse and damages the solid. High-speed imaging of bubble and shock wave dynamics at sub-picosecond exposure times is correlated to the shockwaves recorded with large bandwidth hydrophones. The material damage from several metallic materials is detected in situ and quantified ex-situ via scanning electron microscopy and confocal profilometry. With this knowledge, approaches to mitigate cavitation erosion or to even enhance the energy focusing are within reach.     

Anode erosion during pulsed arcing

An experimental study of the anode erosion rates of Cu, Zr, Ti, Mo, Ta, and W is presented under conditions similar to those used for electrodischarge coating. The arcs are conducted between a small anode and a larger cathode in air with pressures ranging from 10^-4 to 10³ torr. Unipolar arc pulses of 200-400-A peak current and 0.1-ms duration are produced at a 100-Hz pulse repetition rate by an RC circuit. For most materials, the electrode mass loss is primarily from the anode, and the mass loss is independent of pressure for pressures less than 0.1 torr, decreases steeply with increasing pressures in the range 0.1 to 10 torr, and decreases more gradually with increasing pressure above 10 torr. The experimental results are explained by using a limiting case of the integral conservation laws. In the low-pressure region the input energy is expended mainly in the acceleration of the metal vapor, and thus the erosion rate is independent of pressure. In the intermediate-pressure region the metal vapor jet is braked by its interaction with the surrounding gas. In the high-pressure region the vapor jet is completely halted, and vapor transport takes place only by diffusion through the surrounding gas     

DIAMOND MICRO- AND NANOSTRUCTURING BY ACCELERATED CLUSTER EROSION

Reactive accelerated cluster erosion (RACE) has been used for direct micro- and nanostructuring of bulk diamond. Carbondioxide nanoparticles of about 1000 molecules are accelerated to 100keV energy in order to erode a staircase structure or a toothed wheel into diamond by using appropriate movable or fixed masks. Non-reactive erosion by argon clusters is slower but more effectively polishing. Large-scale molecular dynamics simulations show the nanoparticle impacts to lead to transient craters which relax to a very smooth surface via collective elastic recovery. The enhanced or reduced erosion of the respective cluster material is illuminated.     

A Review of Anode Phenomena in Vacuum Arcs

This paper discusses arc modes at the anode, anode temperature measurments, anode ions, transitions of the arc into various modes (principally the anode-spot mode), and theoretical explanations of anode phenomena. A vacuum arc can exhibit five anode discharge modes: 1) a low-current mode in which the anode is basically passive, acting only as a collector of particles emitted from the cathode; 2) a second low-current mode that can occur if the electrode material is readily sputtered (a flux of sputtered atoms will be emitted by the anode); 3) a footpoint mode, characterized by the appearance of one or more luminous spots on the anode (footpoints are much cooler than the true anode spots present in the last two modes); 4) an anode-spot mode in which one large or several small anode spots are present (such spots are very luminous, have a temperature near the atmospheric boiling point of the anode material, and are a copious source of vapor and ions); and 5) an intense-arc mode where an anode spot is present, but accompanied by severe cathode erosion. The arc voltage is relatively low and quiet in the two low-current modes and the intense-arc mode. It is usually high and noisy in the footpoint mode, and it can be either in the anode-spot mode. Anode erosion is low, indeed negative, in the two low-current modes, and it is low to moderate in the footpoint mode. Severe anode erosion occurs in both the anode-spot and intense-arc modes.     

2.45 MeV小型D-D中子管准直屏蔽结构设计

为获取小角度出射的单能中子源,采用蒙特卡罗软件对小型D-D中子管产生的能量为2.45 MeV的4立体角中子源进行了准直屏蔽结构设计。准直屏蔽结构分为准直器和捕获穴,准直器采用铁+含硼聚乙烯+铅的三层过滤结构,用于屏蔽照射野外杂散中子,捕获穴主要功能是增加反方向中子的弹性散射次数,从而降低出射束低能散射中子的比例。通过MCNP模拟得到了准直器各层材料的最佳厚度和出射孔尺寸以及捕获穴最佳结构。经验证,中子照射野处2.45 MeV的中子通量比照射野外大三个量级,中子照射野处低能中子通量比2.45 MeV的中子通量低一个量级,墙壁外总剂量率（中子+）在2.5 Gy/h以下。该研究对于小角度单能中子源的快速获取具有一定的实用价值,获取的中子源可用于中子剂量仪器有效性检验、中子监测仪性能测试等方面的研究。     

细线径螺旋线长脉冲产生技术

 提出了双路输出的螺旋脉冲形成线(PFL)结构,该结构内置用于充电的高耦合变压器,在螺旋PFL的两端各自输出一个脉冲,副路匹配输出时主路输出脉冲波形具有良好的平顶品质,主路输出脉冲能量占储能的大部分,解决了Korovin提出的螺旋PFL充电问题。对比分析双路输出的螺旋 PFL与SINUS-700/130两种技术路线,结果发现双路输出的螺旋 PFL改善了输出脉冲的平顶质量,输出功率提高29%,但是副路输出占用11%的储能,不易充分利用。     

Increasing energy relaxation time of superconducting qubits with nonmagnetic infrared filter and shield

One of the primary origins of the energy relaxation in superconducting qubits is the quasiparticle loss. The quasiparticles can be excited remarkably by infrared radiation. In order to minimize the density of quasiparticle and increase the qubit relaxation time, we design and fabricate the infrared filter and shield for superconducting qubits. In comparison with previous filters and shields, a nonmagnetic dielectric is used as the infrared absorbing material, greatly suppressing the background magnetic fluctuations. The filters can be made to impedance-match with other microwave devices. Using the as-fabricated infrared filter and shield, we increased the relaxation time of a transmon qubit from 519 ns to 1125 ns.     

Physico-chemical behavior of carbon materials under high temperature and ion irradiation

Carbon/carbon (C/C) composites have been chosen for the conception of the thermal shield of the “Solar Probe” space mission. To understand their behavior under solar aggressions (high temperature and ion irradiation) and know the possible interactions with the shipped-in instruments, these materials are tested in a facility that allows to partially simulate the solar environment and to carry out in situ measurements. The mass loss rate of the materials is recorded and the composition and distribution of the ejected species are followed using an opened source mass spectrometer and a post-treatment XPS analysis of the condensed species on a gold-plated copper target is also performed. Besides this, the sputtering processes of graphite at high temperature and ion irradiation have been theoretically investigated in order to understand the main parameters that can influence the erosion of the material and predict accurately the physico-chemical behavior of the C/C composites.     

A Review of Anode Phenomena in Vacuum Arcs

This paper discusses are modes at the anode, experimental results pertinent to anode phenomena, and theoretical explanations of anode phenomena. A vacuum are can exhibit five anode discharge modes: (1) a low current mode in which the anode is basically passive, acting only as a collector of particles emitted from the cathode; (2) a second low current mode that can occur if the electrode material is readily sputtered (a flux of sputtered atoms will be emitted by the anode); (3) a footpoint mode, characterized by the appearance of one or more small luminous spots on the anode (footpoints are generally much cooler than the true anode spots present in the last two modes); (4) an anode spot mode in which one large or several small anode spots are present (such spots are very luminous, have a temperature near the atmospheric boiling point of the anode material, and are a copious source of vapor and ions); and (5) an intense are mode where an anode spot is present, but accompanied by severe cathode erosion. The are voltage is relatively low and quiet in the two low current modes and the intense are mode. It is usually high and noisy in the footpoint mode, and it can be either in the anode spot mode. Anode erosion is low, indeed negative, in the two low current modes, and it is low to moderate in the footpoint mode. Severe anode erosion occurs in both the anode spot and intense are modes. The dominant mechanism controlling the formation of an anode spot appears to depend upon the electrode geometry, the electrode material, and the current waveform of the particular vacuum are being considered. In specific experimental conditions, either magnetic constriction in the gap plasma, or gross anode melting, or local anode evaporation can trigger the transition. However, the most probable explanation of anode spot formation is a combination theory, which considers magnetic constriction in the plasma together with the fluxes of material from the anode and cathode as well as the thermal, electrical, and geometric effects of the anode in analyzing the behavior of the anode and the nearby plasma.     

不锈钢网/铝板多冲击防护屏高速撞击防护性能实验研究

 通过对铝Whipple防护结构进行扩展变形,设计出不锈钢网/铝板组合多冲击防护屏,并利用二级轻气炮对其进行高速撞击实验,撞击速度为3.93~4.25 km/s,弹丸直径为6.35 mm。分析了不同规格不锈钢网、不同间距组合以及网格间结膜对不锈钢网/铝板多冲击防护屏高速撞击防护性能的影响。结果表明：不锈钢网位于防护屏的最后层有利于碎片云的扩散;不锈钢网位于防护屏最前层不利于撞击粒子的初次破碎;丝网几何参数、防护层间距组合是提高不锈钢网/铝板多冲击防护屏高速撞击防护性能的重要参数;网格间结膜有助于弹丸撞击动能的吸收。     

Numerical study on the splitting of a vapor bubble in the ultrasonic assisted EDM process with the curved tool and workpiece

Electrical discharge machining (EDM) is a powerful and modern method of machining. In the EDM process, a vapor bubble is generated between the tool and the workpiece in the dielectric liquid due to an electrical discharge. In this process dynamic behavior of the vapor bubble affects machining process. Vibration of the tool surface affects bubble behavior and consequently affects material removal rate (MRR). In this paper, dynamic behavior of the vapor bubble in an ultrasonic assisted EDM process after the appearance of the necking phenomenon is investigated. It is noteworthy that necking phenomenon occurs when the bubble takes the shape of an hour-glass. After the appearance of the necking phenomenon, the vapor bubble splits into two parts and two liquid jets are developed on the boundaries of the upper and lower parts of the vapor bubble. The liquid jet developed on the upper part of the bubble impinges to the tool and the liquid jet developed on the lower part of the bubble impinges to the workpiece. These liquid jets cause evacuation of debris from the gap between the tool and the workpiece and also cause erosion of the workpiece and the tool. Curved tool and workpiece affect the shape and the velocity of the liquid jets during splitting of the vapor bubble. In this paper dynamics of the vapor bubble after its splitting near the curved tool and workpiece is investigated in three cases. In the first case surfaces of the tool and the workpiece are flat, in the second case surfaces of the tool and the workpiece are convex and in the third case surfaces of the tool and workpiece are concave. Numerical results show that in the third case, the velocity of liquid jets which are developed on the boundaries of the upper and lower parts of the vapor bubble after its splitting have the highest magnitude and their shape are broader than the other cases.     

Erosion products of plasma facing materials formed under ITER-like transient load and deuterium retention in them

Erosion of the plasma-facing materials in particular evaporation of the materials in a fusion reactor under intense transient events is one of the problems of the ITER. The current experimental data are insufficient to predict the properties of the erosion products, a significant part of which will be formed during transient events (edge-localized modes (ELMs) and disruptions). The paper concerns the experimental investigation of the graphite and tungsten erosion products deposited under pulsed plasma load at the QSPA-T: heat load on the target was 2.6 MJ/m² with 0.5 ms pulse duration. The designed diagnostics for measuring the deposition rate made it possible to determine that the deposition of eroded material occurs during discharge, and the deposition rate is in the range (0.1–100) × 10¹⁹ at/(cm² s), which is much higher than that for stationary processes. It is found that the relative atomic concentrations D/C and D/(W + C) in the erosion products deposited during the pulse process are on the same level as for the stationary processes. An exposure of erosion products to photonic energy densities typical of those expected at mitigated disruptions in the ITER (pulse duration of 0.5–1 ms, integral energy density of radiation of 0.1–0.5 MJ/m2) significantly decreases the concentration of trapped deuterium.     

The Time-Resolved Characterization of Erosion Products from High-Current, Copper Vacuum Arcs

A vacuum arc at high enough current can produce gross melting on electrode surfaces as a consequence of anode spot formation and other high-current electrode phenomena. Erosion from the electrodes under this condition is much more rapid than at low-current (where material loss occurs principally from the cathode) and is a process that is presently poorly understood. The present work is aimed at characterizing the erosion products from cathode and anode surfaces during high-current arcs on copper electrodes for single half cycles (60 Hz) arcs having peak currents of 30 kA. Fully open gap lengths were approximately 18 mm. Among the findings were the following. a) Erosion rate determined by electrode weight loss was approximately 8 mg/C of arcing. b) Droplets ejected from the electrodes had masses varying from a few tenths to a few tens of micrograms and velocities typically up to 40 m/s, although higher velocities are seen. c) The greatest number of droplets are produced at, or just after the current peak, and higher droplet velocities are seen in this same time interval. d) Erosion in vapor form detected in the plane of the cathode surface and moving radially is a maximum just after the peak of current and is relatively abundant. Such vapor is essentially absent in the anode plane.     

蒸气压对激光辐照靶材烧蚀速率的影响

研究了强激光辐照碳/碳复合材料靶材引起的烧蚀现象及蒸气压对烧蚀速率的影响。基于傅里叶定律,建立了强激光辐照靶材的热传导模型,模拟了忽略蒸气压影响时烧蚀速率随功率的变化;通过Mott-smith近似方法描述了Knudsen层间断区域,分析了间断两侧表面粒子状态参数;结合质量连续方程和蒸气压与温度关系方程,并由气体状态方程描述蒸气流状态,对蒸气压条件下激光烧蚀碳/碳复合材料靶材的速率随功率变化的关系进行了数值模拟。结果表明,在高能激光对靶材的烧蚀过程中,蒸气压力变化会导致靶材的饱和蒸气温度发生变化,进而影响烧蚀速率且使其随功率呈非线性变化,与忽略蒸气压作用时的线性变化规律相差较大,从理论上解释了忽略蒸气压导致的实验数据与理论结果的差异。     

First principles investigation on Li or Sn codoped hexagonal tungsten bronzes as the near-infrared shielding material

The 52% energy of the solar radiation is contributed by near-infrared radiation (NIR, 780-2500 nm). Therefore, the material design for the energy-saving smart window, which can effectively shield NIR and has acceptable visible transmittance, is vital to save the energy consumed on the temperature control system. It is important to find a non-toxic stable material with excellent NIR-shielding ability and acceptable visible transmittance. The systematic first-principles study on Li_xSn_yWO₃ (x=0, 0.33, 0.66, and y=0, 0.33) exhibits that the chemical stability is a positive correlation with the doping concentration. After doping, the Fermi-energy upshifts into the conduction band, and the material shows metal-like characteristics. Therefore, these structures Li_xSn_yWO₃ (except the structure with x=0.33 and y=0) show pronounced improvement of NIR shielding ability. Our results indicate that when x=0 and y=0.33, the material exhibits the strongest NIR-shielding ability, satisfying chemical stability, wide NIR-shielding range (780-2500 nm), and acceptable visible transmittance. This work provides a good choice for experimental study on NIR shielding material for the energy-saving window.     

单壁碳纳米管膜及其三聚氰胺甲醛树脂复合材料的光电特性

单壁碳纳米管能够强烈吸收光线,尤其是在近红外区域,并能将光能转换成热能.同时,单壁碳纳米管还具有相当大的将热能转换成电能的能力.通过真空过滤方法,将由化学气相沉积生成的单壁碳纳米管阵列制备成单壁碳纳米管膜.根据研究的需要设计了一个简单的单壁碳纳米管膜光伏性质测试实验装置,并在其两端成功地实现了由红外光转换为电压输出.通过功能化步骤,制备了单壁碳纳米管/三聚氰胺甲醛树脂复合材料膜,实验结果表明该复合材料能产生符号相反的输出电压.这预示着单壁碳纳米管及其三聚氰胺甲醛树脂复合材料在光电领域具有良好的应用前景.