Resolution of the frozen-charge paradox in stopping of channeled heavy ions

The long-standing problem of the lacking signature of a Barkas effect in the stopping of swift heavy ions under channeling conditions has been analyzed theoretically. The stopping model provides explicit dependences on impact parameter and allows for projectile screening and higher-order Z(1) corrections. The analysis differentiates between principal target shells. A distinct Barkas correction is found in accordance with standard theory. It is less pronounced for channeled than for random stopping because of the dominance of outer target shells. Varying contributions from different target shells to the stopping force may give rise to an inversion of the commonly observed variation with ion energy and charge state of the Barkas correction.     

Giant Barkas effect observed for light ions channeling in Si

Measurements of the electronic energy loss are presented for (4)He and (7)Li ions channeling along the Si main axial directions at intermediate to high projectile energies. The Barkas effect, an energy-loss enhancement proportional to the third power of the projectile charge at high energies, is clearly separated from other processes. It reaches about 50% for Li ions channeling along the Si [110] direction. The observed Barkas contribution from the valence-electron gas is in fair agreement with the Lindhard model.     

Energy loss straggling of fast charged particles

Energy loss straggling of fast charged particles colliding with atoms have been considered in the eikonal approximation. The result is represented in the form of the Fano formula with a nonperturbative correction. The known nonperturbative Titeica formula (which is transformed to the Fano formula when perturbation theory is applicable) is obtained only under certain approximations in eikonal calculations. It has been shown that straggling calculated with allowance for nonperturbative effects at large charges of the projectile can be significantly different (by an order of magnitude) from the results obtained by Titeica, Fano, and Bohr. Energy loss straggling of fast highly charged ions on hydrogen and copper atoms have been calculated. The latter results are compared to experimental data.     

Binary theory of antiproton stopping

Binary theory of electronic stopping, developed recently with the aim of quantifying stopping forces on swift heavy ions, has been applied to antiproton stopping. Essential ingredients in the theory are inverse-Bloch and shell corrections. The numerical input consists of the excitation spectrum of the stopping material, characterized by bundled oscillator strengths extracted from tabulated optical properties. Predicted stopping forces for eight solid materials agree well with experimental data, in particular for Si where measurements span over two decades of projectile energy. Large discrepancies were found with stopping data for helium extracted from annihilation time measurements. Received 22 February 2001     

Interplay of charge exchange and projectile excitation in the stopping of swift heavy ions

The electronic energy loss of a dressed ion penetrating through matter is commonly considered as being synonymous with the sum of the excitation energies of the target and the projectile in atomic collisions undergone during the passage. We show that this is not justified in projectile-ionizing collisions and discuss some consequences. Received 23 October 2002 / Received in final form 1st December 2002 Published online 18 February 2003     

Electron captures by positrons from hydrogenic ions in nonideal classical plasmas

In nonideal classical plasmas, the electron captures by positrons from hydrogenic ions are investigated. An effective pseudopotential model taking into account the plasma screening effects and collective effects is applied to describe the interaction potential in nonideal plasmas. The classical Bohr-Lindhard model has been applied to obtain the electron capture radius and electron capture probability. The modified hyperbolic trajectory method is applied to the motion of the projectile positron in order to visualize the electron capture probability as a function of the impact parameter, nonideal plasma parameter, projectile velocity, and plasma parameters. The results show that the electron capture probability in nonideal plasmas is always greater than that in ideal plasmas descried by the Debye-Hückel potential, i.e., the collective effect increases the electron capture probability. It is also found that the collective effect is decreased with increasing the projectile velocity. Received 21 January 2000 and Received in final form 27 April 2000     

Electronic structure of three-dimensional quantum dots

We study the electronic structure of three-dimensional quantum dots using the Hartree-Fock approximation. The confining potential of the electrons in the quantum dot is assumed to be spatially isotropic and harmonic. For up to 40 interacting electrons the ground-state energies and ground-state wavefunctions are calculated at various interaction strengths. The quadrupole moments and electron densities in the quantum dot are computed. Hund's rule is confirmed and a shell structure is identified via the addition energies and the quadrupole moments. While most of the shell structure can be understood on the basis of the unperturbed non-interacting problem, the interplay of an avoided crossing and the Coulomb interaction results in an unexpected closed shell for 19 electrons. Received 5 November 2001 / Received in final form 12 November 2002 Published online 1st April 2003 RID="a" ID="a"e-mail: vorrath@physnet.uni-hamburg.de     

Antiproton and proton energy loss straggling at keV energies

The slowing-down process of point-like charged particles in matter has been investigated by measuring the energy straggling for antiprotons and protons in Al, Ni and Au. A comparison with binary theory shows good agreement for Al and Au. For Ni, experimental data are not as convincing. In particular for the aluminum target, the Barkas-like effect of reduced energy straggling for antiprotons compared to protons is visible in the experimental data and a nearly velocity-proportional straggling is found, in good agreement with binary theory.     

Measurement of the Barkas effect in hydrogen

The stopping power of gaseous hydrogen for positive and negative muons at energies ranging from 3 to 100 keV has been measured by time-of-flight. A pronounced Barkas effect was observed: the energy loss in hydrogen for negative muons was found to be significantly smaller than that for positive muons. Received: 23 April 1998 / Accepted: 7 May 1998     

多次聚束的空间电荷波理论

本文采用空间电荷波的观念,讨论了包括电子相互作用的多次聚束问题。所得结果,在电流接近以零为极限时,简化成费恩柏格(E.Feenberg)由运动学观点出发的结果。但是空间电荷效应的包括,显示了多次聚束系统一些不能由运动学方法得到的特徵。本文的分析,可以应用在一系列的微波电子器件上。文中对这方面,也提供了一些具体的方向。     

Coherent transport in disordered metals: zero dimensional limit

We consider non-equilibrium transport in disordered conductors. We calculate the interaction correction to the current for a short wire connected to electron reservoirs by resistive interfaces. In the absence of charging effects we find a universal current-voltage-characteristics. The relevance of our calculation for existing experiments is discussed as well as the connection with alternative theoretical approaches. Received 2 September 2002 Published online 29 October 2002     

Intense radioactive-ion beams produced with the ISOL method

For fifty years the isotope separation on-line (ISOL) technique has been used for the production of radioactive-ion beams (RIBs). Thick-target ISOL facilities can provide very intense RIBs for a wide range of applications. The important design parameters for an ISOL facility are efficiency, rapidity and selectivity of all steps of the separation process. To achieve the anticipated beam intensities with the next-generation RIB facilities, the production rate in the ISOL target has to be increased by orders of magnitude. This is only possible by adapting the projectile beam for optimum production cross-sections and simultaneously minimizing the target heating due to the electronic stopping power of charged-particle projectiles. ISOL beams of 75 different elements have been produced up to now and further beam development is under way to produce a still greater variety of isotopes and to improve existing beams in intensity and purity. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: Ulli.Koster@cern.ch     

Intensity and detuning dependence of fine structure changing collisions in a 85 Rb magneto-optical trap

In an experimental study, the multi-ionisation of metallic clusters (Na_n) has been analysed in collisions with light ions in low charge states (H⁺, He⁺, He²⁺, O³⁺) at collision velocities below 1 a.u. Cluster ions are produced in charge states up to 5+. The average charge of the nano-particles is found to increase linearly with the variation of projectile velocity and the square of the effective projectile charge, well in agreement with the electronic stopping power of the bulk material. A fraction of 50% to 30% of the total projectile energy loss (decreasing with velocity) is transferred into vibrational modes in good agreement with recent theoretical predictions. Received 8 November 2000 and Received in final form 26 January 2001     

First results from laser spectroscopy on bunched radioactive beams from the JYFL ion-beam cooler

A new RFQ ion-beam cooler and buncher, installed after the mass-separating magnet of the ion guide isotope separator, IGISOL, JYFL, has dramatically increased the scope of on-line laser spectroscopy at this facility. The device, operated in a bunching mode, has permitted new measurements on short-lived radionuclei in the Ti, Zr and Hf chains at a sensitivity two orders of magnitude greater than that previously achieved. The device has also opened new prospects for laser-based nuclear spectroscopy at the facility, particularly collinear resonance ionisation spectroscopy. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: pc@mags.ph.man.ac.uk     

Fine-structure quenching and multiplet distortion in a screened Coulomb atom

Theoretical results presented in this paper reflect that the relativistic fine-structure due to the mass-velocity, spin-orbit and Darwin terms is sensitive to the screening strength parameter in an exponential screened Coulomb hydrogen atom, that is sometimes used to model a plasma-embedded atom. With stronger screening the fine-structure correction undergoes a gradual suppression in magnitude, but contributes to the total binding energy in an increasing proportion, indicating that the relativistic contribution to binding may become quite significant in the ultra-low binding regime under large screening strength. In the presence of screening the l-independence of the fine-structure correction as predicted by the Dirac theory progressively disappears, and a departure from the Z⁴-scaling law of the correction occurs along the H-isoelectronic sequence of ions - both the effects become accentuated with growing screening strength. In conjunction with screening-induced removal of the Coulomb degeneracy of non-relativistic levels, these result in a deformed multiplet structure for the screened Coulomb atom. Received 31 May 1999 and Received in final form 20 September 1999     

The role of sample rotation and oblique ion incidence on quantum-dot formation by ion sputtering

Starting from a theory recently proposed by Kahng et al. that explains the formation of ordered dots by ion sputtering under normal ion incidence, it was demonstrated that extending this theory to ion sputtering under oblique ion incidence with simultaneous sample rotation offers the self-organized formation of dots by ion sputtering for a large variety of different ion/material combinations. While for sputtering under normal incidence the shape of the collision cascade must be anisotropic, where the lateral straggling exceeds the longitudinal straggling, this constraint is not necessary under oblique incidence. Received: 16 May 2001 / Accepted: 10 September 2001 / Published online: 17 October 2001     

Impulse-scaling in a laser-driven in-tube accelerator

The laser-driven in-tube accelerator (LITA) is a unique device for laser propulsion. It is characterized by the acceleration of a projectile in a tube. The thrust performance can be improved by exploiting a confinement effect. In the experiment, a 3.0-g projectile is vertically launched, and the momentum coupling coefficient is measured for various monoatomic gases. The measured coupling coefficient is almost proportional to the reciprocal of the speed of sound. The same impulse generation characteristics are obtained in simplified situations that are analyzed based on conservation relations. Received: 26 August 2002 / Accepted: 8 February 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +81-22/217-5284, E-Mail: sasoh@ifs.tohoku.ac.jp RID="**" ID="**"Present address: also at Institute of Advanced Aerospace Technology, Seoul National University, Seoul, 151-742, Korea     

多电荷离子与C,Ne和Ar原子碰撞中K壳层电子俘获截面的计算

本文提出了一种计算多电荷离子与原子碰撞中K壳层电子俘获截面的新方法,称为模型势Oppenheimer-Brinkman-Krammer近似(MPOBK)。在该方法中我们用模型势来描述K电子所处的有效场,同时在波函数中考虑屏蔽效应。我们得到的K壳层总俘获截面是一个表解析达式。对H~ 、He~(2 )-Ne,H~ 、He~(2 )-C及O~(5 ,6 )-Ar体系的计算表明本文结果较好地与实验数据一致。     

Density functional calculations for shell closures in Mg clusters

Motivated by recent measurements on Mg clusters we discuss the electronic structure and shell closures of these type of systems in the framework of self-consistent mean fields derived from density-functional theory. The ionic background is treated at different levels of refinement: spherical jellium model, and the spherically-averaged-pseudo-potential scheme (SAPS) with local as well as non-local pseudo-potentials. The ionic positions in SAPS are optimized using a Metropolis simulated annealing. It is shown that the details of ionic background influence sensitively the electronic levels sequence near the Fermi energy. In particular, the non-local effects from the pseudo-potential change the relations between states with high and with low angular momentum. Some of these effects go into the right direction towards experiment. Received 3 July 2001 and Received in final form 29 October 2001