合肥光源托歇克效应损失电子的探测

 束流寿命是衡量储存环性能的重要参数,直接影响着光源的正常运行。对于合肥光源（HLS）,托歇克（Touschek）寿命是影响束流寿命的重要因素。为了研究Touschek寿命,需要探测由于Touschek效应所损失的电子。介绍了束流寿命的概念,说明了Touschek效应的原理和机制,利用蒙特卡罗软件EGSnrc模拟计算了丢失电子与真空壁的相互作用,通过塑料闪烁体探测器和光电倍增管获得了由于Touschek效应丢失的电子所产生的信号,然后将信号经过放大甄别和符合处理后,用计数器测量了计数率。结果表明：由于Touschek效应而成对丢失的电子的确存在,且电子损失率随流强的降低而减小。这为下一步储存环的能量标定工作做好了前期准备。     

Benchmark measurements and simulations of dose perturbations due to metallic spheres in proton beams

Monte Carlo simulations are increasingly used for dose calculations in proton therapy due to its inherent accuracy. However, dosimetric deviations have been found using Monte Carlo code when high density materials are present in the proton beamline. The purpose of this work was to quantify the magnitude of dose perturbation caused by metal objects. We did this by comparing measurements and Monte Carlo predictions of dose perturbations caused by the presence of small metal spheres in several clinical proton therapy beams as functions of proton beam range and drift space. Monte Carlo codes MCNPX, GEANT4 and Fast Dose Calculator (FDC) were used. Generally good agreement was found between measurements and Monte Carlo predictions, with the average difference within 5% and maximum difference within 17%. The modification of multiple Coulomb scattering model in MCNPX code yielded improvement in accuracy and provided the best overall agreement with measurements. Our results confirmed that Monte Carlo codes are well suited for predicting multiple Coulomb scattering in proton therapy beams when short drift spaces are involved.     

Upgrade of beamline BL08B at Taiwan Light Source from a photon‐BPM to a double‐grating SGM beamline

During the last 20 years, beamline BL08B has been upgraded step by step from a photon beam‐position monitor (BPM) to a testing beamline and a single‐grating beamline that enables experiments to record X‐ray photo‐emission spectra (XPS) and X‐ray absorption spectra (XAS) for research in solar physics, organic semiconductor materials and spinel oxides, with soft X‐ray photon energies in the range 300–1000 eV. Demands for photon energy to extend to the extreme ultraviolet region for applications in nano‐fabrication and topological thin films are increasing. The basic spherical‐grating monochromator beamline was again upgraded by adding a second grating that delivers photons of energy from 80 to 420 eV. Four end‐stations were designed for experiments with XPS, XAS, interstellar photoprocess systems (IPS) and extreme‐ultraviolet lithography (EUVL) in the scheduled beam time. The data from these experiments show a large count rate in core levels probed and excellent statistics on background normalization in the L‐edge adsorption spectrum.     

整体考虑靶和均整器的设计来提高医用加速器剂量率的方法

通常低能医用加速器靶的设计遵从厚靶原则, 即采用厚度等于或大于电子射程的靶, 使靶后无电子污染. 本文提出了新的靶设计概念：设计靶时, 将靶与相关系统(初级准直锥、均整器)作为一个整体考虑, 通过减小靶厚来提高光子剂量率, 利用初级准直锥、均整器来减少电子污染. 采用这种设计概念, 用蒙特卡罗方法对BJ-6—6MV单光子医用电子直线加速器的靶进行优化设计. 模拟计算10cm×10cm野, 30cm×30cm野的深度剂量曲线, 结果表明：较之现有的厚靶, 治疗剂量提高了10%, 同时表面剂量、X射线的品质仍满足国标的要求. 通过薄靶、厚靶的加速器整机对比实验, 已验证了此结论的正确性.     

Design of a neutron polarizer using polarizing super mirrors for the TOF-SANS instrument at the J-PARC

Small-angle neutron scattering technique using polarized neutrons is powerful for studying structures in the range between nm and μm of magnetic materials. In addition, they have been used for the incident beam of focusing-geometry SANS instruments using a magnetic neutron lens, where a high polarization degree of about 99.9% is necessary because the imperfectness of the neutron polarization increases the background level. We are going to install such a magnetic focusing system on the new time-of-flight SANS (TOF-SANS) instrument at the J-PARC so as to make q_min smaller than 10⁻³ Å⁻¹ and improve the resolution of the conventional TOF-SANS at low q. As a polarizing device of the instrument, two V-shaped polarizing super mirrors arranged in crossed geometry to enhance the polarization degree has been considered. In this paper, we present the concept and the detailed design of this device and its performance estimated by Monte Carlo simulations.     

Simultaneous estimation of the temperature and heat rate distributions within the combustion region by a new inverse radiation analysis

A new inverse radiation analysis is presented for estimating the heat rate and temperature distributions in the combustion region from the information of the temperature and heat flux profiles of wall elements in the system. The Monte Carlo method is employed to solve the radiative heat transfer equation. The inverse radiation problem is posed as a minimization problem of the least squares criterion, which is solved by the conjugate gradient method. The performance of the present technique of inverse analysis is evaluated and the effects of the errors of the absorption coefficient, emissivity and convective heat transfer coefficient on the inverse analysis are investigated. The results show that the present technique is robust and yields accurate estimation even with noisy measurement.     

Multiple temperature model for the information preservation method and its application to nonequilibrium gas flows

The information preservation (IP) method has been successfully applied to various nonequilibrium gas flows. Comparing with the direct simulation Monte Carlo (DSMC) method, the IP method dramatically reduces the statistical scatter by preserving collective information of simulation molecules. In this paper, a multiple temperature model is proposed to extend the IP method to strongly translational nonequilibrium gas flows. The governing equations for the IP quantities have been derived from the Boltzmann equation based on an assumption that each simulation molecule represents a Gaussian distribution function with a second-order temperature tensor. According to the governing equations, the implementation of IP method is divided into three steps: molecular movement, molecular collision, and update step. With a reasonable multiple temperature collision model and the flux splitting method in the update step, the transport of IP quantities can be accurately modeled. We apply the IP method with the multiple temperature model to shear-driven Couette flow, external force-driven Poiseuille flow and thermal creep flow, respectively. In the former two cases, the separation of different temperature components is clearly observed in the transition regime, and the velocity, temperature and pressure distributions are also well captured. The thermal creep flow, resulting from the presence of temperature gradients along boundary walls, is properly simulated. All of the IP results compare well with the corresponding DSMC results, whereas the IP method uses much smaller sampling sizes than the DSMC method. This paper shows that the IP method with the multiple temperature model is an accurate and efficient tool to simulate strongly translational nonequilibrium gas flows.     

随机介质背散射二维Mueller矩阵的数值模拟与分析

推导了随机介质背散射Mueller矩阵的直接计算公式,并运用矢量Monte Carlo方法进行了数值模拟.结果表明随机介质背散射二维Mueller矩阵方位关系随散射系数的减小而增强,而与微粒大小关系不大;Mueller矩阵元素绝对值的空间分布随径向呈近似指数规律衰减,矩阵元素的方位变化具有周期性.对称系统的二维Mueller矩阵的花样图中仅有7幅独立,其余9幅可通过对称、旋转变换得到.     

Effects of packing materials on the sensitivity of RadFET with HfO2 gate dielectric for electron and photon sources

The radiation sensing field effect transistor (RadFET) with SiO₂ gate oxide has been commonly used as a device component or dosimetry system in the radiation applications such as space research, radiotherapy, and high-energy physics experiments. However, alternative gate oxides and more suitable packaging materials are still demanded for these dosimeters. HfO₂ is one of the most attractive gate oxide materials that are currently under investigation by many researchers. In this study, Monte Carlo simulations of the average deposited energy in RadFET dosimetry systems with different package lid materials for point electron and photon sources were performed with the aim of evaluating the effects of package lids on the sensitivity of the RadFET by using HfO₂ as a gate dielectric material. The RadFET geometry was defined in a PENGEOM package and electron–photon transport was simulated by a PENELOPE code. The relatively higher average deposited energies in the sensitive region (HfO₂ layer) for electron energies of 250?keV–20?MeV were obtained from the RadFET with the Al₂O₃ package lid despite of some deviations from the general tendency. For the photon energies of 20–100?keV, the average amount of energy deposited in RadFET with Al₂O₃ package was higher compared with the other capped devices. The average deposited energy in the sensitive region was quite close to each other at 200?keV for both capped and uncapped devices. The difference in the average deposited energy of the RadFET with different package lid materials was not high for photon energies of 200–1200?keV. The increase in the average deposited energy in the HfO₂ layer of the RadFET with Ta package lid was higher compared with the other device configurations above 3?MeV.     

Step wandering due to the structural difference of the upper and the lower terraces

We consider the wandering instability of steps due to a gap in the lifetime of adatoms for evaporation on the upper and the lower terraces. Our study is meant to explain the step wandering observed in the growth of Si(1 1 1) surface near its structural transition temperature. With a linear stability analysis and Monte Carlo simulations, we show that the instability of an isolated step occurs in growth if adatoms on the upper terrace evaporate more easily than those on the lower terrace. For the instability of a vicinal face, additional features are considered as the motion of the phase boundary and the mass flow across it during the phase transformation. It is found that steps and phase boundaries wander in-phase with a rather well-defined periodicity when evaporation is weak. We compare the result with that for a system with a gap in the diffusion coefficient. The simulation results show that the first mechanism is more effective to make the wandering steps in-phase and that the second mechanism induces step wandering in a wider range of parameters.     

The emerging role of 4D synchrotron X‐ray micro‐tomography for climate and fossil energy studies: five experiments showing the present capabilities at beamline 8.3.2 at the Advanced Light Source

Continuous improvements at X‐ray imaging beamlines at synchrotron light sources have made dynamic synchrotron X‐ray micro‐computed tomography (SXR‐µCT) experiments more routinely available to users, with a rapid increase in demand given its tremendous potential in very diverse areas. In this work a survey of five different four‐dimensional SXR‐µCT experiments is presented, examining five different parameters linked to the evolution of the investigated system, and tackling problems in different areas in earth sciences. SXR‐µCT is used to monitor the microstructural evolution of the investigated sample with the following variables: (i) high temperature, observing in situ oil shale pyrolysis; (ii) low temperature, replicating the generation of permafrost; (iii) high pressure, to study the invasion of supercritical CO₂ in deep aquifers; (iv) uniaxial stress, to monitor the closure of a fracture filled with proppant, in shale; (v) reactive flow, to observe the evolution of the hydraulic properties in a porous rock subject to dissolution. For each of these examples, it is shown how dynamic SXR‐µCT was able to provide new answers to questions related to climate and energy studies, highlighting the significant opportunities opened recently by the technique.     

FLUKA-LIVE—an embedded framework, for enabling a computer to execute FLUKA under the control of a Linux OS

This paper describes a Linux-based OS framework for integrating the FLUKA Monte Carlo software (currently distributed only for Linux) into a CD-ROM, resulting in a complete environment for a scientist to edit, link and run FLUKA routines—without the need to install a UNIX/Linux operating system. The building process includes generating from scratch a complete operating system distribution which will, when operative, build all necessary components for successful operation of FLUKA software and libraries.Various source packages, as well as the latest kernel sources, are freely available from the Internet. These sources are used to create a functioning Linux system that integrates several core utilities in line with the main idea—enabling FLUKA to act as if it was running under a popular Linux distribution or even a proprietary UNIX workstation.On boot-up a file system will be created and the contents from the CD will be uncompressed and completely loaded into RAM—after which the presence of the CD is no longer necessary, and could be removed for use on a second computer. The system can operate on any i386 PC as long as it can boot from a CD.     

Monte Carlo simulation of many-arm star polymers in two-dimensional good solvents in the bulk and at a surface

A Monte Carlo technique is proposed for the simulation of statistical properties of many-arm star polymers on lattices. In this vectorizing algorithm, the length of each arml is increased by one, step by step, from a starting configuration withl=1 orl=2 which is generated directly. This procedure is carried out for a large sample (e.g., 100,000 configurations). As an application, we have studied self-avoiding stars on the square lattice with arm lengths up tol _max=125 and up tof=20 arms, both in the bulk and in the geometry where the center of the star is adsorbed on a repulsive surface. The total number of configurations, which behaves asNl ^{G–1 fl} , where=2.6386 is the usual effective coordination number for self-avoiding walks on the square lattice, is analyzed, and the resulting exponents G=(f) and _s (f) for the bulk and surface geometries are found to be compatible with predictions of Duplantier and Saleur based on conformai invariance methods. We also obtain distribution functions for the monomer density and the distance of the end of an arm from its center. The results are consistent with a scaling theory developed by us.     

On the sampling distribution of Allan factor estimator for a homogeneous Poisson process and its use to test inhomogeneities at multiple scales

The Allan factor (AF) is a statistic widely used to assess if the rate of occurrences of an event tends to cluster and show persistence in a range of space and/or time scales. For a homogeneous Poisson process, the relationship between AF and the space/time is expected to be constant, thus denoting the lack of clustering and persistence in the occurrence process. However, in time series analysis, conclusions about the persistence of the underlying process have been usually drawn by visual inspection of the diagrams of AF estimates versus scale, without applying any formal statistical test. This study investigates the sampling distribution function of the AF estimator when the underlying process is homogeneous Poissonian. Monte Carlo simulations show that the distribution of the AF estimator is described by a gamma distribution whose mean and variance can be deduced by the delta method. Therefore, the derived analytical distribution of the AF estimator can be used to build a formal statistical test to evaluate the significance of the AF fluctuation against the Poissonian hypothesis across a range of space/time scales. As an example, we apply the AF-based test to analyse the time series of the number of rainfall observations exceeding fixed high thresholds in order to study the properties of the rate of occurrence of the extreme values over a wide range of time scales.     

Development of a system for measuring the complex impedance of borosilicate glasses at high pressures and temperatures: Application to the study of Li- and Na-doped borosilicate glasses

An apparatus for measuring the complex impedance of samples with high impedances is described. Complex impedance spectra were collected from a range of borosilicate glasses of composition (B₂O₃)₄(Li₂O)(LiBr)_x(NaBr)_1−x at pressures and temperatures ranging from 1 to 5 GPa and 350 to 450 °C, respectively. These data were used to determine AC conductivities and activation energies in order to test the Modified Random Network model of glass structure. Our results are in line with the predictions of this theory.     

Contribution of the hadronic component of a virtual photon to the structure function for charm leptoproduction at high <Emphasis Type="Italic">x</Emphasis> and <Emphasis Type="Italic">Q</Emphasis><Superscript>2</Superscript>

The contribution of the hadronic component of a virtual photon to the structure function for charm leptoproduction is calculated. This contribution comes from the scattering of c quarks of a virtual photon on proton quarks and gluons. A comparison of the results of our calculations for this structure function with relevant data obtained by the European Muon Collaboration (EMC) for μ⁺p scattering gives reasons to believe that the contribution of the resolved photon may be responsible for the excess in these EMC data over the predictions of the model of photon-gluon fusion at high momentum transfers. Therefore, it may become possible to describe the EMC data without resort to the hypothesis of a nonperturbative charmed-quark admixture in the proton wave function (intrinsic-charm hypothesis).