Positron lifetime calculations of defects in fusion irradiated beryllium

Numerical quantum-mechanical positron lifetime calculations were performed for mono-vacancies, di-vacancies, tri-vacancies and small nano-voids containing helium and hydrogen in neutron irradiated beryllium. Helium and hydrogen atoms in the sample after the irradiation are considered as atoms forming interstitial O-type loops. Spherical clusters of vacancies are included in the calculations as a reference. It was found that the presence of He and H atoms significantly changes the positron lifetime in irradiated beryllium. A correlation between the positron lifetime and mutual position of vacancies in nano-voids and interstitial loops was established.     

Application of the model of two prompt-neutron groups for analysis ofβ eff rossi-α measurements

Main Science Center of the Russian Federation — Physics and Power Engineering Institute. Translated from Atomnaya énergiya, Vol. 80, No. 1, pp. 61–66, January, 1996.     

A model for analysis of fuel behaviour in transients

For many applications, analysis of fuel element behaviour must take non-linear thermal, and elasto-plastic effects into account. This is particularly true if the fuel undergoes large deformations and rapid temperature transients. To meet this need a multi-dimensional fuel model based on finite element stress and thermal analysis has been developed. The model is solved for the transient temperature distribution by a step-by-step time incremental procedure. The temperature is then introduced into the elasto-plastic analysis as a thermal load and stresses and deformations are calculated. A model for treatment of creep and a special element for the gap between fuel pellet and cladding is incorporated together with semi-empirical procedures for calculating fission gas release, fuel pellet to cladding heat transfer coefficients, etc.The fuel model has been compared with both analytical solutions and in-reactor experimental results. The observed and predicted results are in good agreement.     

A new UHV system for channeling/blocking analysis of solid surfaces and interfaces

This paper describes principles, design and tests of a new UHV system for structural analysis of solid surfaces and interfaces with medium energy ion scattering (50–500 keV H⁺ or He⁺), combined with channeling and blocking. The chamber contains a high precision, three-axis manipulator for accurate alignment of samples under investigation and a toroidal electrostatic analyser for simultaneous energy selection and angular detection of reflected ions over a range of scattering angles of 30°. With a sample loading chamber containing up to seven samples, samples can be transferred to the manipulator without breaking the vacuum of the main system.     

A point-hydraulics model for flow stability analysis

The strong variation of the thermal–physical properties of supercritical fluids in the vicinity of the pseudo-critical line results in challenging tasks in thermal–hydraulic design of a supercritical water-cooled reactor (SCWR). One of the challenging tasks is to understand and to predict the dynamic behavior and flow stability of supercritical water-cooled systems. The present study introduces a new method, the so-called point-hydraulics model (PHM), to establish a criterion for predicting the onset of a self-sustaining flow oscillation in a closed cooling system. Four dimensionless numbers are clarified which affect the onset of flow instability. Based on this model, stability maps are derived which can be applied to any kind of fluids and operating conditions. The PHM model is applied to a simplified system cooled by supercritical water. Parameters affecting the flow stability are discussed. A good agreement is achieved between the PHM model and the numerical results obtained using the SASC code which was developed specifically for analyzing the dynamic behavior of systems cooled by supercritical fluids.     

A pulse shape discrimination of CsI(T1) crystal with 6He beam

The performance test of a CsI(T1) crystal (70×27×23 mm~3) was performed by applying the pulse shape discrimination technique for identification oflight charged pafflcles , The crystal is coupled to a photomultiplier tube during an experiment with ~6He beam. The pulse waveform is fully recorded by employing a high precision digital oscilloscope. The fast and slow gates are used for the pulse shape discrimination and the best values for the gate widths were determined to be 0.5 μs and 1.67 μs, respectively. The ~6He, ~4He and ~3He are successfully discnminated with this technique.     

A pulse shape discrimination of CsI(Tl) crystal with ~6He beam

The performance test of a CsI(Tl) crystal (70×27×23 mm3) was performed by applying the pulse shape discrimination technique for identification of light charged particles .The crystal is coupled to a photomultiplier tube during an experiment with 6He beam.The pulse waveform is fully recorded by employing a high precision digital oscilloscope.The fast and slow gates are used for the pulse shape discrimination and the best values for the gate widths were determined to be 0.5 μs and 1.67 μs,respectively.The 6He,4He and 3He are successfully discriminated with this technique.     

用正电子湮没实验研究PbWO_4新型闪烁晶体绿光发光机理

通过正电子湮没寿命谱研究了PbWO_4晶体退火处理前后缺陷的变化;发现氧退火后,晶体正电子寿命值τ_2变小,正电子捕获率κ增大,真空退火反之。并且PbWO_4晶体氧退火后发光主峰位从440nm移到485nm的绿光处,而真空退火晶体发射谱谱形并未变化。从不同退火处理对晶体的影响,提出了PbWO_4晶体中铅空位形成WO_3＋O~-发绿光的发光模型。     

A constitutive model for structural analysis of fusion magnets

A constitutive model to simulate the elastic-plastic and slip actions of fusion magnets under operating or abnormal loads is outlined. To represent the elastic-plastic responses a unified material homogenization procedure based on the existing composite technology was applied to obtain an effective incremental stress-strain relationship for the heterogeneous, laminated magnets. The inter-layer slip behavior of the magnets was represented by a friction-type model from which slip deformation can be calculated for the situation where inter-layer shear stresses exceed the bonding strength of the adhesives. Numerical results of three example problems are presented to demonstrate the utility of the proposed model.     

A non linear model for the PWR fuel assembly seismic analysis

In seismic PWR core analysis, the non linear behavior of fuel assemblies has to be studied taking into account variations of stiffness and damping due to the slippage of rods in spacer grids. Based on the linear CEA’s assembly model, principally composed of two beams representing sets of rods and thimbles, a linear model is described. In the second part of this paper the slippage and loss of contact of rods on a grid relic is analyzed. Based on the Coulomb friction model and plasticity analogy, a grid model constituted of elastoplastic hinge and non linear springs is proposed to simulate the progressive slippage of rods inside the cells. In the third part, a non linear assembly model is designed and validated with experimental data. This model represented with success the increase of damping and decay of frequency when the magnitude of the dynamic loading increases.     

Precision storage lifetime measurements of highly charged heavy ions in the CSRe storage ring using a Schottky resonator

Schottky mass spectrometry utilizing heavy-ion storage rings is a powerful technique for the precise mass and decay half-life measurements of highly charged ions. Owing to the nondestructive ion detection features of Schottky noise detectors, the number of stored ions in the ring is determined by the peak area in the measured revolution frequency spectrum. Because of their intrinsic amplitude-frequency characteristic（AFC）, Schottky detector systems exhibit varying sensitivities at different frequencies. Using low-energy electron-cooled stored ions, a new method is developed to calibrate the AFC curve of the Schottky detector system of the Experimental Cooler Storage Ring（CSRe） storage ring located in Lanzhou, China. Using the amplitude-calibrated frequency spectrum, a notable refinement was observed in the precision of both the peak position and peak area. As a result, the storage lifetimes of the electron-cooled fully ionized ⁵⁶Fe²⁶⁺ ions were determined with high precision at beam energies of 13.7 and 116.4 MeV/u, despite of frequency drifts during the experiment. When electron cooling was turned off, the effective vacuum condition experienced by the 116.4 MeV/u ⁵⁶Fe²⁶⁺ ions was determined using amplitude-calibrated spectra, revealing a value of 2 × 10^-10 mbar, which is consistent with vacuum gauge readings along the CSRe ring. The method reported herein will be adapted for the next-generation storage ring of the HIAF facility under construction in Huizhou, China. It can also be adapted to other storage ring facilities worldwide to improve precision and enhance lifetime measurements using many ions in the ring.     

胶体模型与金属中点缺陷的研究

在用胶体模型的密度泛函理论方法系统地计算金属Ａｌ中的点缺陷和类点缺陷电子结构的基础上，参照有关的实验资料进行比较，探讨了胶体模型方法的可靠性、局限性及对其局限性的修正方法。分析比较了胶体模型、Ｐｕｓｋａ的无相互作用原子模型、ＦＬＡＰＷ等常见的密度泛函理论计算方案，探讨了各自的利弊。得出了胶体模型方法对于研究金属中的小微空洞（平均半径小于４ｎｍ）是一种有效、简捷的近似计算方法的结论。     

A Time of Flight-Energy spectrometer for stopping power measurements in Heavy Ion-ERD analysis at iThemba LABS

The quantitative analysis of thin layers using Heavy Ion-Elastic Recoil Detection (HI-ERD) can be reliably performed if the stopping powers of the probing ions and recoils in a given target matrix are known accurately. Unfortunately for many projectile/target combinations experimental data is limited and where available, deviations of up to 50% between experiment and theory have been reported. This presentation describes the assembly of a Time of Flight-Energy (ToF-E) detector system developed for HI-ERD analysis and adapted for stopping power measurements at iThemba LABS. First results from energy loss measurements of 0.1-0.5 MeV/nucleon ²⁸Si and ⁸⁴Kr ions in ZrO₂ are presented and compared with predictions of the widely used SRIM2003 (Stopping Range of Ions in Matter).     

MB形容器体积对BGO晶体探测效率的蒙特卡罗优化设计

本研究采用蒙特卡罗方法MCNP对BGO(Bi4 Ge3 O12)晶体探测器Marinelli Breaker(简称MB)容器的优化设计;气体放射源MB容器内的气体发生β 衰变后,接着正电子与负电子湮没后发射γ射线.设计中,首先通过计算γ射线的数量来确定BGO晶体探测器金属包壳厚度与峰值效率关系;计算结果显示当BGO金属包壳厚度为0.7mm～1.0mm时,峰值效率达到最大.根据这一金属尺寸,我们进一步设计了MB容器的最佳尺寸,这种尺寸使探测器的探测效率较高而费用较少.从而达到满足生产这种探测器的要求.