Conceptual design of an intense positron source based on an LIA

Accelerator based positron sources are widely used due to their high intensity. Most of these accelerators are RF accelerators. An LIA (linear induction accelerator) is a kind of high current pulsed accelerator used for radiography. A conceptual design of an intense pulsed positron source based on an LIA is presented in the paper. One advantage of an LIA is its pulsed power being higher than conventional accelerators, which means a higher amount of primary electrons for positron generations per pulse. Another advantage of an LIA is that it is very suitable to decelerate the positron bunch generated by bremsstrahlung pair process due to its ability to adjustably shape the voltage pulse. By implementing LIA cavities to decelerate the positron bunch before it is moderated, the positron yield could be greatly increased. These features may make the LIA based positron source become a high intensity pulsed positron source.     

Stopping power of fast channeled protons in Hartree-Fock approximation

The electronic energy loss of MeV-protons in crystals is investigated in the Hartree-Fock approximation for the core electrons of the target. It turns out that each core electron can be regarded as stopping the proton independent of all other electrons without restriction by the Pauli principle. For the channeling stopping power the impact parameter dependent energy loss of a proton moving rectilinearly past a crystal ion is calculated in first Born approximation. Low excitations of the core electrons lead to a long range impact parameter dependence, whereas high excitations contribute to the energy loss proportional to the electron density sampled along the proton trajectory. The results are applied to 4 MeV proton channeling along the main channels of Si using Clementi wave functions for the core electrons and a free electron approximation for the valence electrons. The comparison with the experimental results of Clarket al. yields good agreement. In the high velocity limit a reduction of the channeling stopping power to 0.64(0.83) of the random value is predicted for Si(C).     

Technique of symmetric type quasi-linear electron pulse duration modulation

A novel technique of symmetric type quasi-linear electron pulse duration modulation is proposed. The salient feature from the conventional photoelectron gun is the introduction of the alternating electric field resonator. The electric field that results is synchronously controlled to generate the desired quasi-linear differential energy modulation on the electron pulse passing through. The effect resulted directly is that the leading electrons undergo negative energy modulation and decelerate, while the rear ones positive energy modulation and accelerate, which eventually leads to electron-pulse-duration modulation. The technical details are demonstrated, together with an illustration example.     

Contribution of valence s+p type electrons to the high momentum components of the electron-positron momentum density in gold

It is generally believed that the high momentum components (HMCs) of the angular correlation of annihilation radiation or Doppler broadening spectra map the positron annihilation with predominately core electrons, containing only a small fraction coming from localised d+f-type valence electrons. In present work, we study how far the contribution of valence electrons to the HMCs of the electron-positron (e-p) momentum density can be neglected. Calculations are performed for gold within both the independent particles model (IPM) and including the e-p correlation effects non-locally, using the weighted density approximation (WDA). In particular, the HMCs due to valence s+p-type electrons are compared with their core and d+f-type valence counterparts.     

Gamma spectra resulting from the annihilation of positrons with electrons in a single core level

The first gamma spectra associated with the annihilation of positrons with individual core levels (Cu 3p and Ag 4p) are presented. The spectra were obtained by measuring the energy of gamma rays time coincident with Auger electrons emitted as a result of positrons annihilating with a selected core level. Relativistic calculations show good agreement with experiment over a limited range of momenta. However, statistically significant differences indicate that the measurements can provide an impetus to new calculations of many body effects in positron-core electrons annihilation.     

辐照电子在光纤芯处能量沉积的计算

为了用尽可能低的辐照电子能量在光纤芯处形成大的性质改变,模拟了0.1～1 MeV能量的电子辐照二氧化硅.发现每一特定能量的电子辐照二氧化硅时,在其中有个能量沉积最快的位置.计算得到0.447 4 MeV能量的电子在单模光纤中心能量沉积最快,分析发现对于这个能量的电子多数可以穿透到光纤芯处,电子能量在光纤芯处的沉积主要是由于电子能量的减小造成的.这些结果可为用电子辐照光纤制作光器件时的初始电子能量选择提供参考.     

Theoretical Aspects of Dynamic Nuclear Polarization in the Solid State: The Influence of High Radical Concentrations on the Solid Effect and Cross Effect Mechanisms

Dynamic nuclear polarization (DNP) is used to enhance signals in NMR and MRI experiments. During these experiments microwave (MW) irradiation mediates transfer of spin polarization from unpaired electrons to their neighboring nuclei. Solid state DNP is typically applied to samples containing high concentrations (i.e. 10–40?mM) of stable radicals that are dissolved in glass forming solvents together with molecules of interest. Three DNP mechanisms can be responsible for enhancing the NMR signals: the solid effect (SE), the cross effect (CE), and thermal mixing (TM). Recently, numerical simulations were performed to describe the SE and CE mechanisms in model systems composed of several nuclei and one or two electrons. It was shown that the presence of core nuclei, close to DNP active electrons, can result in a decrease of the nuclear polarization, due to broadening of the double quantum (DQ) and zero quantum (ZQ) spectra. In this publication we consider samples with high radical concentrations, exhibiting broad inhomogeneous EPR line-shapes and slow electron cross-relaxation rates, where the TM mechanism is not the main source for the signal enhancements. In this case most of the electrons in the sample are not affected by the MW field applied at a discrete frequency. Numerical simulations are performed on spin systems composed of several electrons and nuclei in an effort to examine the role of the DNP inactive electrons. Here we show that these electrons also broaden the DQ and ZQ spectra, but that they hardly cause any loss to the DNP enhanced nuclear polarization due to their spin-lattice relaxation mechanism. Their presence can also prevent some of the polarization losses due to the core nuclei.     

激光等离子体中高能电子各向异性压强的粒子模拟

直接驱动惯性约束聚变(ICF)的实现需要对靶丸进行严格的对称压缩,以达到自持热核反应(点火)所需的条件.快点火方案的应用降低了对靶丸压缩对称性以及驱动能量的要求,但压缩及核反应过程中良好的靶丸对称性无疑有助于核反应增益的提高.本文研究了快点火方案中高能电子注入高密等离子体后导致的各向异性电子的压强张量.这一现象存在于ICF快点火方案中的高能电子束"点火"及核反应阶段.鉴于高能电子加热离子过程以及靶丸核反应自持燃烧过程的时间较长,高密靶核会由于超高的各向异性压强的作用破坏高密靶丸的对称性,降低核燃料密度,进而降低了核燃料燃烧效率以及核反应增益.     

Doppler broadening of positron annihilation photons in metals

The Doppler broadening of positron annihilation photons was measured in 17 metals. A model which considers the positron lifetime spectra in metals to be composed of terms for annihilation with conduction and core electrons and surface centers of low electron momentum is used to correlate calculated core annihilation rates with the Doppler lineshape. Ta metal was doped with defects with high energy implantations of¹⁴N⁺⁴ ions at variable doses. Differences in the Doppler linewidths were ascribed as being principally a reflection of the probabilty of annihilation with core electrons relative to annihilation with conduction electrons.     

Calculation of the electric field gradient tensor from energy band structures

The electric field gradient tensor (EFG) can be measured accurately by various experimental techniques. The theoretical understanding, however, was restricted to point charge models, Sternheimer antishielding factors and model calculations for a restricted number of compounds. We have developed a method which obtains the EFG from a full potential linearized augmented plane wave (LAPW) energy band structure calculation. Starting from the total crystal charge density (including the core electrons) the EFG is obtained numerically without further approximations. We have applied our method successfully to all hep metals up to Cd, to semiconductors, and to insulators such as lithiumnitride or cuprite. Good agreement with experiment is found and we predict interesting changes in the sign of the EFG in the 3d and 4d transition metal series. The aspherical distribution of the valence electrons determines 80 or 90% of the total EFG and the influence of the core electrons is small. Even for the 3d and 4d metals the asphericity of the valence p electrons dominates over the d contribution to the EFG due to the different radial behavior of p and d wave functions.     

Sxaps spectra of pyrolytic graphite as a function of the angle of incidence of the incoming electrons

Soft X-ray appearance potential (SXAPS) spectra of pyrolytic graphite as a function of the angle of incidence of the incoming electrons have been measured to obtain information about surface plasmon excitation. Plasmons can be coupled with a core hole and two slow electrons above the Fermi level or with a fast incoming electron. In the last case angular dependent plasmon excitation can be observed. Coupling with the fast incoming electron is observed, but band structure effects cannot be neglected to explain the observed structure. Measurements were performed on clean and hydrogen-contaminated pyrolytic graphite. A signal increase over the whole spectrum was observed after hydrogen contamination, probably due to a rearrangement of the surface lattice structure. Bombardment of the surface with electrons of 2 keV gave a decrease in spectral intensity probably due to surface damage.     

A simple approximate sum rule for correlation-state structure in XPS

We adduce a simple approximate relation for the second moment of the correlation-state structure associated with core ionization by high-energy photons and observed in XPS. The relation follows easily on assumption that the removal of a core electron produces an effect on the remaining electrons which can be described by a perturbation potential. A few simple applications are discussed.     

Geometry optimization in the presence of external forces: a theoretical model for enforced structural changes in molecules

Recently, we proposed a very simple quantum chemical model to simulate the effect of external forces acting on a single molecule [Mol. Phys. 107, 2403 (2009)]. It is based on optimizing the geometry of a molecule with an external force applied to selected pairs of nuclei. In this study we extend this model by considering interactions of external forces not only with the nuclei but also with their electrons, in particular their core electrons, which can be viewed as ‘rigidly’ connected to a nucleus. In the proposed revised model an external force acts on an object which consists of the nucleus of an atom and its 1s core electrons. It is shown in this study that such a model predicts the same conformational (structural) changes in a molecule as our simpler model where the external forces interact with the nuclei only. However, the magnitude of the forces required to cause these changes is now lower and within the range of forces used in real AFM experiments.     

Calculation of the Knight shift in palladium

The direct and the exchange core polarization (ECP) contributions of the conduction electrons to the Knight shift of palladium are evaluated. To obtain the wave functions for the conduction electrons and the partial densities of states at the Fermi surface a KKR energy band calculation was performed. The contributions of the core electrons to the Knight shift were determined by using the moment perturbation method (MP). Electron-electron interactions are taken into account by individual enhancement factors for thed ands electrons. The agreement between the theoretical results and the available experimental data is quite satisfactory.     

Calculation of the Knight shift in palladium

The direct and the exchange core polarization (ECP) contributions of the conduction electrons to the Knight shift of palladium are evaluated. To obtain the wave functions for the conduction electrons and the partial densities of states at the Fermi surface a KKR energy band calculation was performed. The contributions of the core electrons to the Knight shift were determined by using the moment perturbation method (MP). Electron-electron interactions are taken into account by individual enhancement factors for thed ands electrons. The agreement between the theoretical results and the available experimental data is quite satisfactory.     

Band-offset effect on localization of carriers and p-type doping of InAs/GaAs core–shell nanowires

The electronic properties and p-type doping mechanism of InAs/GaAs core–shell nanowires are studied by using the first-principles calculations within density-functional theory. The core–shell structure of nanowires creates one-dimensional band offset at the InAs/GaAs interface. The magnitude of band offset depends on the sizes of core and shell. We find that a highly efficient p-type doping in InAs/GaAs core–shell nanowires can be achieved by introducing the Cd-impurity into the GaAs shell, utilizing the band-offset effect. It is because the valence-band electrons can spontaneously transfer to the Cd-impurity level, resulting in one-dimensional hole gas in the InAs core of nanowires.     

Remark on the conductivity sum rule for the optical absorptions of alkali metals

In reference to the optical absorption data for K, Rb, and Cs, it is shown that, when an optical coupling between conduction and core electrons is strong, the Pauli-forbidden transitions of conduction electrons to occupied core states play an important role in the sum rule applied restrictively to the conduction electron system.     

Looking at magnetism with photoemission

With photoelectron spectroscopy one can directly observe the electrons in incomplete shells and study their interaction with the core electrons. The greatest relevance to magnetism lies in the ability to map the dispersion of the electronic bands, resolving even the spin-splitting in ferromagnetic transition metals. Additional information is obtained by measuring the spinpolarization of the photoelectrons. In rare earth systems the occupancy of the 4f shell is unambiguously determined by the characteristic final-state multiplet structure. This feature has found application in the study of mixed-valence compounds. Core level spectra yield information about the electronic configuration of the atom rather than about the collective aspects of magnetism. In insulators the core s-electron spectra of paramagnetic ions exhibit a splitting due to the coupling of the spin of the core hole to that of the incomplete outer shell. The interpretation of such data, especially from the deeper core levels, is significantly complicated by many-electron processes caused by the creation of the core hole. They may involve not only the familiar conduction electron screening, but also charge transfer to the photoexcited atom in the final state.     

Runaway Electron Measurements in the EAST Tokamak

An experimental study of runaway electrons in the EAST tokamak has been performed by a recently developed multi‐channel hard x‐ray diagnostics based on NaI(TL) scintillator detectors. It is found that in the current quench phase, the inductive loop voltage plays an important role in the generation of runaway electrons. And the avalanche mechanism was the main mechanism for runaway electrons after the disruptions. The distribution and transportation of runaway electrons were also investigated by multi‐channel hard x‐ray diagnostics. It is also found that the intensity of runaway electrons emission in the core plasma was much higher than those in the downside of the cross‐section, while the emission intensity of runaway electrons in the core plasma was almost the same. Calculated shrinking coefficient of runaway electrons emission after the plasma disruption was about 26 m/s according to the experimental data (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Plasma-induced neutrino helicity flip in a supernova core and a constraint on the Dirac neutrino magnetic moment

We investigate the neutrino helicity flip under supernova core conditions, where the left-handed neutrinos being produced can be converted into right-handed neutrinos sterile with respect to weak interactions owing to the interaction of the magnetic moments with plasma electrons and protons. In calculating the probability for the conversion neutrino scattering by plasma components, we take into account the polarization effects attributable to both electrons and protons in the photon propagator. Based on realistic models with radial distributions and time evolution of physical parameters in a supernova core, we have obtained upper limits on the Dirac neutrino magnetic moment averaged over flavors and time from the condition that the influence of the right-handed neutrino emission on the total cooling time scale should be limited.