质子辐照不锈钢中氢气泡产生和演化研究

采用质子辐照模拟方法和正电子湮没寿命测量方法研究了质子辐照在国产改进型316L不锈钢中产生的氢气泡及其随辐照质子注量的演化过程。5×10^12,5×10^13,5×10^14／cm^2质子辐照的实验结果表明,辐照在不锈钢中产生氢气泡的尺度随辐照质子注量增加而增大,在质子注量为5×10^14／cm^2时氢气泡尺寸达到0．62nm,气泡的浓度随质子注量增加而减小。The bubble production and evolution in the home-made modified 316L stainless steel have been investigated by the proton irradiation simulation technique and the positron annihilation lifetime measurement. The experimental results of 5 × 10^12, 5 × 10^13 and 5 × 10^14/cm^2 proton irradiations show that the produced bubble size increases, while the bubble concentration decreases, with the increasing of the irradiation proton flueence. At the proton irradiation fluence of 5 × 10^14/cm^2 the bubble size reaches 0.62 nm.     

钨辐射损伤随辐照剂量变化的重离子辐照模拟研究

采用重离子辐照模拟方法和正电子湮没寿命测量技术研究了钨辐射损伤随辐照剂量的变化。20,60和90dpa（每个原子的位移次数）辐照损伤水平的实验结果表明,辐照在钨中产生单空位、双空位、位错和空位团等缺陷;随辐照剂量的增大,单空位、双空位和位错浓度增加,空位团的尺度和浓度都随之增大。Radiation damage in W has been studied as a function of irradiation dose by heavy ion simulation and positron annihilation lifetime measurement. The experimental results of 20, 60 and 90 dpa irradiations illustrate that the mono-and di-vacancies, dislocations and vacancy clusters are produced by the irradiation. The concentrations of the mono-and di-vacancies and dislocations and both the concentration and size of the vacancy clusters or voids all increase with the increasing of the irradiation dose.     

Migration Enthalpy of Thermal Vacancies by Positron Spectroscopy

The trapping of positrons at vacancy site in some materials provide a new and sensitive method for the equilibrium determination of point defect migration enthalpy. Data are presented for commercial Al-Mg a11oys and fitted to a model a11owing presentation in the form of Arrhenius plots, hence the migration enthalpy Hiv^m can be determined by the positron annihilation lifetime technique. The results show that the value of Hiv^m increases as the concentration of Mg increases.     

聚四氟乙烯密封材料的电子束辐射效应研究

采用低能电子束辐射系统地研究了操作氚系统用密封材料（聚四氟乙烯）的辐射效应。利用扫描电镜和X光电子能谱仪分析了材料的表观形貌和表面元素的变化,用X射线粉末衍射仪和热分析仪分析了辐照前后材料的结晶度改变和热稳定性程度,并利用气相色谱仪和质谱仪测定了辐射降解气体的成分和生成量随吸收剂量的变化规律。采用电子自旋共振谱仪研究了材料在辐照过程中产生的中间体（自由基）,利用正电子湮没寿命谱仪分析了材料中的自由体积孔洞及浓度。结果表明,辐照前聚四氟氟乙烯样品的表面是规则的带状晶形结构,经1×10^5Gy辐照后样品的晶状结构遭到了明显的破坏,向非晶态转变显著。辐照后样品的熔点和热分解温度均呈下降趋势,表明样品的热稳定性降低。聚。四氟乙烯材料在氧气气氛中发生的辐射降解最为严重,其次是真空,而在氘气气氛中辐射降解最少。材料的辐射降解主要发生在主链上,即C—C键断裂,而侧基C—F键的断裂相对较少。第三寿命强度I3的变化规律与τ3相反,即随吸收剂量的增大,第三寿命的强度I3降低,表明自由体积孔洞的浓度降低。 The radiation effect of polytetrafluoroethylene used as a kind of sealing material in the tritium system was studied by using electron beam. The superficial configuration and state were analyzed by SEM and XPS respectively, and the crystal degree and thermal stability were determined by XRD and DSC. At the same time the composition of gas products after radiation ,was measured by GC/MS, and the yield of CO2 with the dose increasing was analyzed. The free radicals produced in the process of radiation were studied by ESR, and the content of free volume of PTFE is analyzed by positron annihilation lifetime spectroscopy. The result is that the crystal structure of the sample irradiated at the dose of 1 × 10^5 Gy is of an obvious damage while the structure of the unirradiated sample is crystal band. The transformation of the sample which has high crystal degree amorphous one. The melting point and thermal ecomposition temperature trend sample＇s thermal stability drops. The radiation decomposition of the sample is is remarkable from crystal state to to decline, which indiccates that the mainly the rupture of C--C bond, and less is the rupture of C--F bond. The Intensity I3 of the third lifetime fails with increasing the absorbed dose, which demonstrates the reduction of free volume cave content. Compared with I3, the rule of τ3 is reverse.     

Evolution of native point defects in ZnO bulk probed bypositron annihilation spectroscopy

This paper studies the evolution of native point defects with temperature in ZnO single crystals by positron lifetime and coincidence Doppler broadening (CDB) spectroscopy, combined with the calculated results of positron lifetime and electron momentum distribution. The calculated and experimental results of the positron lifetime in ZnO bulk ensure the presence of zinc monovacancy, and zinc monovacancy concentration begins to decrease above 600℃ annealing treatment. CDB is an effective method to distinguish the elemental species, here we combine this technique with calculated electron momentum distribution to determine the oxygen vacancies, which do not trap positrons due to their positive charge. The CDB spectra show that oxygen vacancies do not appear until 600℃ annealing treatment, and increase with the increase of annealing temperature. This study supports the idea that green luminescence has a close relation with oxygen vacancies.     

Theoretical study on the positron annihilation in Rocksalt structured magnesium oxide

Based on the atomic superposition approximation(ATSUP) and first-principles pseudopotential plane-wave methods,the bulk and Mg mono-vacancy positron lifetime of magnesium oxide were calculated using Arponen-Pajamme and Boron’ski-Nieminen positron-annihilation-rate interpolation formula respectively.The calculated values are in good agreement with experimental values and the first-principles method gives more convincing results.The positron annihilation density spectra analysis reveals that positrons mainly annihilate with valence electrons of oxygen atoms when the magnesium-vacancy appears within magnesium oxide.     

Theoretical study on the Rocksalt structured positron annihilation in magnesium oxide

Based on the atomic superposition approximation （ATSUP） and first-principles pseudopotential plane-wave methods, the bulk and Mg mono-vacancy positron lifetime of magnesium oxide were calculated using Arponen-Pajamme and Borofiski-Nieminen positron-annihilation-rate interpolation formula respectively. The calculated values are in good agreement with experimental values and the first-principles method gives more convincing results. The positron annihilation density spectra analysis reveals that positrons mainly annihilate with valence electrons of oxygen atoms when the magnesium-vacancy appears within magnesium oxide.     

Temperature effect on emission spectra and fluorescence lifetime of the ~4I_(13/2) state of Er~(3+)-doped Gd_2SiO_5 crystal

By measuring the emission spectra and the fluorescence lifetime of the 4 I 13 /2 state of Er 3+ ions in Gd 2 SiO 5 crystal at different temperatures,the effects of temperature on the spectra and the lifetime of the 4 I 13 /2 state are investigated. When the temperature increases,the emission line width for the 4 I 13 /2 → 4 I 15 /2 transition is broadened,and the main emission lines at 1 596,1 609,and 1 644 nm shifte toward shorter wavelengths. The measured lifetime of the 4 I 13 /2 state decreases from 13.2 to 8.4 ms with temperature increase from 13 to 300 K,which is mainly due to the temperature dependence of multiphonon relaxation between the 4 I 13 /2 and 4 I 15 /2 states and the changing population distribution among the Stark levels within the 4 I 13 /2 state. The experimental results imply that low temperature condition is better for the ～1.6-μm laser output.     

Effect of non-condensable gas on a collapsing cavitation bubble near solid wall investigated by multicomponent thermal MRT-LBM

A multicomponent thermal multi-relaxation-time(MRT)lattice Boltzmann method(LBM)is presented to study collapsing cavitation bubble.The simulation results satisfy Laplace law and the adiabatic law,and are consistent with the numerical solution of the Rayleigh-Plesset equation.To study the effects of the non-condensable gas inside bubble on collapsing cavitation bubble,a numerical model of single spherical bubble near a solid wall is established.The temperature and pressure evolution of the two-component two-phase flow are well captured.In addition,the collapse process of the cavitation bubble is discussed elaborately by setting the volume fractions of the gas and vapor to be the only variables.The results show that the non-condensable gas in the bubble significantly affects the pressure field,temperature field evolution,collapse velocity,and profile of the bubble.The distinction of the pressure and temperature on the wall after the second collapse becomes more obvious as the non-condensable gas concentration increases.     

POSITRON LIFETIME STUDIES OF Y1-xCaxBa2Cu3O7-δ AND Y1-xCaxBa2Cu3-xFexO7-δ

In this paper it is reported that the measurement of the bulk positron lifetime as a function of substitution content x in the temperature range from 70 to 220K was performed in two high-T_c superconducting systems, Y_1-xCa_xBa₂Cu_3-xFe_xO_7-δ and Y_1-xCa_xBa₂Cu₃O_7-δ. It was found that τ_B of both systems decreases significantly with x and the temperature dependence of τ_B is very weak in normal state. In lower temperature region (Tc), a dramatic x-dependent temperature variation of τ_B was observed in the Ca-substituted system: from a decrease of τ_B with decreasing T to an increase of τ_B. With increasing x, the temperature dependence of τ_B remains weak in the Ca- and Fe- substituted systems. Compared with the experimental data of positron lifetime in other substituted systems and the calculation of the positron density distribution, the authors suggest that positron bulk lifetime spectra behaviour can be interpreted by the physical model based on the transfer of electron density between the CuO₂ planes and Cu-O chains. Therefore, the study of positron lifetime spectra provides a useful means to detect the local charge density and to study the correlation between the electronic structure and the high-T_c superconductivity.     

Positron annihilation studies of defects in helium-irradiated silicon

Positron annihilation studies have been carried out on Si-n irradiated with He⁺ ions at the V.U.B. cyclotron, to a dose of 4×10¹⁷ He/cm². No temperature dependence on the S-parameter and lifetimes is seen below the irradiation temperature. The positron lifetime associated to the created defects is 290 ps. During the isochronal annealing, this lifetime stays constant up to 700 K. It is attributed to the annihilation of positrons from large vacancy-clusters filled with He atoms. From the isochronal annealing results, only one annealing stage is seen. This annealing stage which extends over a long range of temperature 700–1000 K, is ascribed to the degassing of helium atoms from defects and the growth of vacancy-clusters. The lifetime of positrons in those defects reaches a value of about 530±30 ps at 1000 K, indicating that the vacancy-clusters formed have a mean size of more than 8 vacancies.     

Positron annihilation study of helium clustering in alpha irradiated copper

Abstract

Helium clustering in alpha irradiated copper has been investigated by positron annihilation spectroscopy. Pure copper samples have been homogeneously helium implanted using a cyclotron, yielding helium concentrations of 100 appm and 400 appm. Post-implantation positron lifetime and Doppler broadened annihilation lineshape measurements have been carried out on these Cu samples as a function of isochronal annealing temperature. An annealing stage observed in the isochronal annealing curve viz., a marked reduction in the resolved lifetime τ₂ and an increase of its intensity I ₂, is explained as due to the formation of helium bubble embryos. At higher annealing temperatures, τ₂ corresponding to helium bubbles increases and saturates while its intensity I ₂ decreases, indicating an increase in the size of the bubble with a concomitant decrease in the bubble concentration. This stage is interpreted to be the bubble growth stage. From an analysis of positron lifetime parameters in the growth stage, helium stom density, bubble size and bubble concentration have been deduced at various annealing temperatures. The bubble characteristics are found to be affected by the helium dose. The present results on direct helium implanted Cu are compared with those of our earlier study on n-irradiated Cu-B, where helium was introduced using ¹⁰B(n, α)⁷ Li reaction.     

Helium bubbles in neutron-irradiated copper-boron studied by positron annihilation

The influence of helium, introduced by the ¹⁰B(n, α)⁷Li reaction, on the evolution of defect structure in copper containing a few hundred ppm boron has been studied by detailed positron lifetime and two-photon angular correlation measurements, supplemented by TEM studies. In the as-irradiated state of Cu-B, two lifetime components have been resolved. The shorter lifetime, τ₁, = 167 ps of 97% intensity, has been understood as due to positron trapping at small helium-vacancy complexes, while the longer lifetime τ₂ = 450 ps of 3% intensity is explained as due to helium-free voids. Marked changes in the annihilation characteristics observed at 670 K are interpreted in terms of the nucleation of microbubbles, controlled by thermally activated helium migration to vacancy traps. Corroborative evidence for the onset of helium clustering is obtained from the change in the average size of positron traps as deduced from the smearing of the measured angular correlation spectra. Helium bubbles and helium-free voids coexisting in the system have been distinguished by a three-component analysis of the lifetime spectra. Bubbles are found to be stable beyond the temperature of dissociation of voids. The size and concentration of bubbles, determined independently by TEM measurements, are in accordance with the positron annihilation results in the growth stage. The observed positron lifetime at higher annealing temperatures has been analysed by relating the annihilation rate to helium atom density and helium pressures in bubbles evaluated. These pressures are in satisfactory agreement with the estimates of equilibrium pressures, leading to the conclusion that bubble relaxation occurs by the mechanism of thermal vacancy condensation.     

Computer calculation of positron diffusion and annihilation in atomic hydrogen

We have performed a computer-aided analysis of position behaviour in atomic hydrogen. Effect of electric, magnetic and temperature fields on the diffusion and annihilation of positrons has been studied. Electric field is varied over a wide range of 0–200 V cm⁻¹ amagat⁻¹, magnetic field over 0–30 kG while the temperature range considered is 300–10,000°K. The position decay rate decreases with electric and temperature fields but increases with magnetic fields. However, the effect of these fields is reversed on the diffusion coefficient.     

Experimental investigation of positron self-trapping near the vapour-liquid critical point of hellium-4

Positron self-trapping in helium near the vapor-liquid critical point is investigated using the positron annihilation lifetime technique. A 13% decrease in the slowing-down time for positrons to reach the self-trapped state is observed at the critical temperature 5.190 K relative to the slowing-down time at 5.200 K.     

利用正电子湮没技术研究钾掺杂钨合金中的缺陷

钨合金中钾的掺杂会引入大量的缺陷,如尺寸几十纳米的钾泡、高密度的位错以及微米量级的晶粒带来的晶界等,这些缺陷的浓度和分布直接影响合金的服役性能.本文运用正电子湮没谱学方法研究钾掺杂钨合金中的缺陷信息,首先模拟计算了合金中各种缺陷的正电子湮没寿命,发现钾的嵌入对空位团、位错、晶界等缺陷的寿命影响很小;然后测量了不同钾含量掺杂钨合金样品的正电子湮没寿命谱,建立三态捕获模型,发现样品中有高的位错密度和低的空位团簇浓度,验证了钾对位错的钉扎作用,阐述了在钾泡形成初期是钾元素与空位团簇结合并逐渐长大的过程;最后使用慢正电子多普勒展宽谱技术表征了样品中缺陷随深度的均匀分布和大量存在,通过扩散长度的比较肯定了钾泡、晶界等缺陷的存在.     

Positron annihilation studies of vacancy-type defects

The annihilation radiation of low energy positrons gives information on the electronic and defect structure of solids. There are three conventionally measurable quantities: the positron lifetime, the angular correlation of 2 annihilation radiation and the Doppler-broadened annihilation line shape. In the presence of lattice defects the annihilation characteristics show considerable changes. This is due to positron trapping at defects like vacancies and their agglomerates, voids, dislocations and grain boundaries. The concentration of defects can be deduced from the ratio of trapped and free positrons.The annihilation characteristics are different for different defect configurations. Positrons reveal vacancy agglomeration and the lifetime of trapped positrons gives estimates on the size of microvoids in the range of 2–10 Å. Various examples on the study of equilibrium and non-equilibrium defects, radiation damage and defect annealing are presented. Special emphasis is given to vacancy recovery and vacancy-impurity interactions in electron and neutron irradiated bcc transition metals like Fe, Mo, Nb, Ta.     

退火处理Fe43Co43Hf7B6Cu1非晶合金的正电子湮没研究

对熔体急冷法制备的Fe₄₃Co₄₃Hf₇B₆Cu₁非晶合金进行了200,300,400和500 ℃保温30 min的退火处理,用正电子湮没寿命谱、X射线衍射、穆斯堡尔谱等方法研究了退火后试样的结构及结构缺陷变化.结果表明,在非晶合金的制备态,正电子主要在非晶基体相空位尺寸的自由体积中湮没,湮没寿命τ₁为158.4 ps,强度I₁关键词： 43Co₄₃Hf₇B₆Cu₁非晶')" href="#">Fe₄₃Co₄₃Hf₇B₆Cu₁非晶 退火处理 正电子湮没寿命 结构与结构缺陷     

Al-4%Ag合金中Ag原子偏析的低温正电子湮没参数研究

分别研究了823 K淬火处理和20%形变量的Al-4%Ag低温下Ag析出物对正电子的捕获行为的变化。采用正电子湮没寿命谱(PALS)技术和符合多普勒展宽能谱(CDBS)在温度范围10~293 K内对其进行表征。多普勒展宽能谱结果表明2种样品中均存在Ag析出物。正电子寿命谱的解谱结果中的各组分给出了Ag析出物随测量温度的变化规律。在170 ~273 K之间,正电子湮没行为具有较强的温度依赖性。但对于两个具有不同类型缺陷的样品,在低于170 K时观察到样品中Ag析出物捕获正电子能力出现了差异。随着测量温度的降低,淬火样品中的Ag析出物的正电子寿命和强度基本不变。在低于170 K的测量中,形变样品中的Ag析出物对正电子的捕获能力仍旧存在着较强的温度依赖性,但是变化幅度在逐渐减弱。当测量温度提升到室温(273~293 K),越来越多的正电子从Ag析出物中逃逸,逐渐回到自由状态或被其他深陷阱所捕获,失去了对温度的依赖性。