Hou Qing

Simulation of Helium Behaviour in Titanium Crystals Using Molecular Dynamics

The behaviour of helium in Ti crystals at 300 K has been investigated by means of the molecular dynamics. The study is focused on the influences of He–Ti interaction on the aggregation of helium atoms in the substrate. When a Born–Mayer potential is used to describe the He–Ti interaction, the He atoms are unable to cluster with each other due to the weak bridge barrier that cannot trap the helium atoms, Whereas using a He–Ti potential that is constructed by fitting the ab initio pairwise He–Ti potential, the clustering of He atoms can be observed. The results indicate that suitable He–Ti potential plays an important role in the formation of He clusters in metals. Moreover, it is noted that the shape of the formed He cluster is irregular, and the produced defect prefers to congregating on one side of the He cluster rather than spreading symmetrically around it.

Helium-Related Defect Evolution in Titanium Films by Slow Positron Beam Analysis

Various helium-containing titanium films were deposited on Si substrates by magnetron sputtering under different helium/argon (He/Ar) ambiances. Helium concentrations and corresponding depth profiles in the Ti films are obtained by elastic recoil detection analysis (ERDA). X-ray diffraction (XRD) measurements are carried out to evaluate the crystallization of the titanium films. Vacancy-type defects and their depth profiles were revealed by slow positron beam analysis (SPBA). It is found that the defect-characteristic parameter S rises with the increment of the He/Ar flow ratios. The variation of.. indicates the formation and evolution of various He-related defects, with uniform distribution into the depth around 400 nm.

Justification of a Monte Carlo Algorithm for the Diffusion-Growth Simulation of Helium Clusters in Materials

A theoretical analysis of a Monte Carlo (MC) method for the simulation of the diffusion-growth of helium clusters in materials is presented. This analysis is based on an assumption that the diffusion-growth process consists of first stage, during which the clusters diffuse freely, and second stage in which the coalescence occurs with certain probability. Since the accuracy of MC simulation results is sensitive to the coalescence probability, the MC calculations in the second stage is studied in detail. Firstly, the coalescence probability is analytically formulated for the one-dimensional diffusion-growth case. Thereafter, the one-dimensional results are employed to justify the MC simulation. The choice of time step and the random number generator used in the MC simulation are discussed.

A sputtering calculation: Angular dependence of sputtering yields

Abstract The material erosion induced by physical sputtering resulted from bombardments of hydrogen, helium and their isotopes is a critical problem for fusion technique. An important feature for such ion-induced sputtering is isotropic incidence condition. Therefore it is necessary to develop a sputtering theory which can be applied to the case of obliquely incident ion beams. The sputtering theory based on the bipartition model of ion transport has recently extended to treat the problem of sputtering calculation for the case of obliquely incident ion beam. In the present paper some calculation results have been given out. Comparison shows that the angular dependence of sputtering yields given by the bipartition model is in reasonable agreement with that given by TRIM-SP Monte Carlo code and experimental measurements.

Monte Carlo dosimetry of a new (90)Y brachytherapy source.

Purpose In this study, we attempted to obtain full dosimetric data for a new 90Y brachytherapy source developed by the College of Chemistry (Sichuan University) for use in high-dose-rate after-loading systems. Material and methods The dosimetric data for this new source were used as required by the dose calculation formalisms proposed by the AAPM Task Group 60 and Task Group 149. The active core length of the new 90Y source was increased to 4.7 mm compared to the value of 2.5 mm for the old 90Sr/90Y source. The Monte Carlo simulation toolkit Geant4 was used to calculate these parameters. The source was located in a 30-cm-radius theoretical sphere water phantom. Results The dosimetric data included the reference absorbed dose rate, the radial dose function in the range of 1.0 to 8.0 mm in the longitudinal axis, and the anisotropy function with a θ in the range of 0° to 90° at 5° intervals and an r in the range of 1.0 to 8.0 mm in 0.2-mm intervals. The reference absorbed dose rate for the new 90Y source was determined to be equal to 1.6608 ± 0.0008 cGy s–1 mCi–1, compared to the values of 0.9063 ± 0.0005 cGy s–1 mCi–1 that were calculated for the old 90Sr/90Y source. A polynomial function was also obtained for the radial dose function by curve fitting. Conclusions Dosimetric data are provided for the new 90Y brachytherapy source. These data are meant to be used commercially in after-loading system.

Molecular Dynamics Simulations of Helium Behaviour in Titanium Crystals

Molecular dynamics simulations are performed to investigate the behaviour of helium atoms in titanium at a temperature of 300K. The nucleation and growth of helium bubble has been simulated up to 50 helium atoms. The approach to simulate the bubble growth is to add helium atoms one by one to the bubble and let the system evolve. The titanium cohesion is based on the tight binding scheme derived from the embedded atom method, and the helium–titanium interaction is characterized by fitted potential in the form of a Lennard-Jones function. The pressure in small helium bubbles is approximately calculated. The simulation results show that the pressure will decrease with the increasing bubble size, while increase with the increasing helium atoms. An analytic function about the quantitative relationship of the pressure with the bubble size and number of helium atoms is also fitted.

Phoenix 2000, A 3D conformal Treatment Planning System

Phoenix 2000 is a 3D conformal treatment planning system developed by the center for radiation physics of Sichuan University cooperating with the Sichuan Tumor Hospital. This paper gives a brief account of the system, which meets the needs of modern radiation therapy. In the aspect of medical image technology, the system can reconstruct a stereoscopic display of the anatomy structure and the target volumes directly by means of CT data. And it can conveniently provide users with automatic, semi-automatic or manual-operated way to sketch contour of the target volumes and the anatomy, and with a flexible tool to reconstruct the 3D images of them. The dose algorithms in this system include photon beam algorithm and electron beam algorithm. For the former we use the improved Convolution/ Superposition method, in which the concept of equivalent radiation source is applied to carefully treat the radiation field perturbation induced by bone. For the latter the bipartition model for electron transport, which has been extended from 1D inhomogeneous media to 3D inhomogeneous media, is used.

Single-parameter expression for ions reflection from surfaces

Based on a detailed analysis of the Boltzmann equation for ion transport in solids, it has been shown that for low-energy ions incident on a heavy element target the distribution function of the ions can be determined by one single parameter, called the scaled transport cross-section, which was defined earlier [1]. This means that the transport quantities of different ion-target-energy combinations should be similar only when their scaled transport cross-section is the same. To test this significant conclusion, we undertook a set of systematic and extensive calculations of reflection coefficients using the improved bipartition model of ion transport. The systematic calculations include 3410 ion-target-energy combinations, namely H, D, T, He, Li, B, C, N, O, Ne ions with energy ranges from 10 eV to 1 MeV normally incident to C, Al, Cu, Mo, Ag, W, Au, Pb, U targets. The only restrictions isM1/M2<1/6. The calculations verify that particle and energy reflection coefficients present an excellent one-to-one correspondence to the scaled transport cross-section. Furthermore, based on the calculations, universal expressions for both particle reflection coefficients and energy reflection coefficients for normal ion incidence have been obtained by fitting the numerical data. By comparing the results calculated by the universal expressions with experimental and Monte Carlo data, it is shown that the expression can describe reflection coefficients well.

Theoretical Calculation of Differential Cross Section for e+-Ar and e--Ar Elastic Scattering

The differential cross sections for e--Ar and e+-Ar elastic scattering are calculated by relativistic partial wave expansion method. In the processes an incident particle (electron or positron) is treated as moving relativistically in an effective central potential. For electron-atom scattering, the effective potential is calculated by relativistic self-consistent-field (SCF) method with the Slater local exchange approximation. For the positron scattering, the effective potential is the electrostatic potential determined by the ground-state electron charge density calculated by the SCF method. For atomic target polarization due to the incident charge particle(electron or positron), the effect is treated approximately by adding a static polarization potential to the SCF potentials. The calculated results show that the polarization effect is very important for the low energy positron-atom scattering.