High-intensity variable-energy positron beam for surface and near-surface studies

We report on the construction and performance of a magnetically confined slow positron beam. The beam is designed for surface physics and near-surface disorder studies and it operates in ultrahigh vacuum with a base pressure of 3 nPa. The beam uses a Co-58 source with a backscattering W(110) single crystal moderator. We get 4.7 × 10⁶ slow positrons per second striking the sample using a 300 mCi source corresponding to a fast-to-slow positron conversion efficiency 0.28 ± 0.02%. The energy of the incident positrons can be varied from 1 eV to 35 keV. Some applications concerning positron re-emission and diffusion as well as positronium desorption from a surface state are shown.     

Design of a low energy positron beam

A low energy positron beam apparatus has been designed and constructed. Experimental details are presented and the performance characteristics are described.     

Diagnosis and dynamics in a simple low energy medium current electron beam channel

We present a simple experimental setup and an associated method enabling both the non-destructive diagnosis and the calculation of the beam evolution in a low energy medium current electron beam channel, where the space-charge and emittance effects are comparable. The diagnosis makes use of an axially symmetric magnetic lens while a second lens is added to increase the flexibility in the beam processing. The paper emphasizes the three steps involved in the method: the evaluation of the lenses' magnetic field by numerical simulation, the beam diagnosis, and the computation of the beam envelope. The calculation of the magnetic field is based on the finite element method. Subsequently, the beam parameters at the electron source exit – emittance and cross-over radius and position – are found with the modified three gradient method. Finally, the beam dynamics are modeled with the K–V equation adapted for the particular case of axial symmetry. The results obtained in this paper can be used to optimize technological processes, such as welding, hardening, cladding, and surface alloying.     

Xe-implanted zirconium oxycarbide studied by variable energy positron beam

The effect of annealing on defects and the formation of Xe bubbles were investigated in zirconium oxycarbide implanted with 800-keV¹³⁶Xe²⁺ ions at two fluences 1 × 10¹⁵ and 1 × 10¹⁶ Xe/cm². Doppler broadening technique combined with slow positron beam was used. The analysis of the S depth profiles and S-W maps revealed that in the as-implanted samples at both fluences Xe bubbles are not formed. The post-implantation annealing of the samples implanted at 1 × 10¹⁶ Xe/cm² caused formation of Xe bubbles. The response of the lower implantation dose samples to this post implantation annealing was found rather complicated and is discussed.     

Variable energy positron beam study of Xe-implanted uranium oxide

Doppler broadening of annihilation gamma-line combined with a slow positron beam was used to measure the momentum density distribution of annihilating pair in a set of sintered UO₂ samples. The influence of surface polishing, of implantation with 800-keV ¹³⁶Xe²⁺ at fluences of 1 × 10¹⁵ and 1 × 10¹⁶ Xe cm^?2, and of annealing were studied by following the changes of the momentum distribution shape by means of S and W parameters. The program used for this purpose was VEPFIT. At the two fluences in the stoichiometric as-implanted UO₂, formation of Xe bubbles was not detected. The post-implantation annealing and over-stoichiometry in the as-implanted sample caused Xe precipitation and formation of Xe bubbles.     

An ion beam chopping method for low energy time-of-flight measurements

Chopping an ion beam by electrically sweeping it over an aperture produces a degradation of the beam quality. After a short review of the problem, a simple alternative method is presented and experimental results are shown.     

脉冲慢正电子束流的检测

本工作设计了一种通过测量脉冲慢正电子湮没辐射在闪烁探测器的积分效应进行测量和标定脉冲正电子束流强度的方法.通过对探测器的刻度,定量测量了北京慢正电子强束流项目中慢正电子束流的流强和能谱发布,为脉冲正电子束流的直流化设计提供了依据.     

Simulation of a low energy beam transport line

A 2.45 GHz electron cyclotron resonance intense proton source and a low energy beam transport line with dual-Glaser lens were designed and fabricated by Institute of Modern Physics for a compact pulsed hadron source at Tsinghua. The intense proton beams extracted from the ion source are transported through the transport line to match the downstream radio frequency quadrupole accelerator. Particle-in-cell code BEAMPATH was used to carry out the beam transport simulations and optimize the magnetic field structures of the transport line. Emittance growth due to space charge and spherical aberrations of the Glaser lens were studied in both theory and simulation. The results show that narrow beam has smaller aberrations and better beam quality through the transport line. To better match the radio frequency quadrupole accelerator, a shorter transport line is desired with sufficient space charge neutralization.     

基于LINAC慢正电子源的EGS4模拟

介绍基于ＢＥＰＣ ＬＩＮＡＣ和ＢＦＥＬ ＬＩＮＡＣＥ慢正电子束设备中正电子源部分的Ｍｏｎｔｅ－Ｃａｒｌｏ模拟过程。结果表明,从Ｔａ靶出射的快正电子产额在ＢＥＰＣ ＬＩＮＡＣ和ＢＦＥＬ ＬＩＮＡＣ运行兼容模式时分别为１．８４＊１０＾１０／ｓ和３．９＊１０＾１１／Ｓ。     

Development of an electrostatic high-brightness slow positron beam apparatus for positron scattering experiments

A new apparatus for the study of positron scattering on gaseous target is presented. The apparatus uses electrostatic lens elements and a remoderator in a reflection geometry for the brightness enhancement. The system will provide a high quality positron beam with a small diameter and a small angular divergence, even at low energies.     

A pulsing system for low energy positrons

A pulsing system for a low energy positron beam of variable energy was developed. The system consists of a beam chopper and a beam buncher. The performance of the system was tested in various positron lifetime measurements. A time resolution of 135 ps was achieved for the pulsing system at positron energies in the range between 0.5 keV and 28 keV.     

Irradiation effect on PET surface using low energy argon ion beam

A study of various physical properties of a PET film irradiated with an Ar beam is reported. SEM images and UV–VIS and FTIR spectra were obtained for a number of ionic fluxes and three irradiation times. Small changes in the energy gap of degradated samples were found, and the SEM images indicate that the optimum homogeneity and roughness are reached after 30 min of irradiation. These results may well be of practical interest. A modified saddle field ion source was used as a preparation tool of the surface of polyethylene terephthalate PET polymer substrate to be ready for coating or thin film deposition. Argon ion beam was used for this purpose, where the scanning electron microscope (SEM) shows that, the best sample is the one which was irradiated to 30 min, where this sample is more homogenous and roughness than other irradiated samples. Also the (UV–VIS) spectrum tells us that, there is small change on energy gap and this is meaning that, the change on electric properties is small also. In this case the sample is more homogenous and of higher roughness than other irradiated samples.     

Dynamic response of target electrons upon low and medium energy ion impact

We found that differential scattering cross sections for medium and low energy He⁺ and Ne⁺ impact on high Z-atoms were significantly enhanced compared with those calculated from the inter-atomic potential based on the Hartree–Fock–Slater atomic model coupled with the bare nuclear charge of a projectile. The enhanced scattering cross sections determined experimentally are reproduced well by a simple model that the center of gravity of target electrons is shifted toward the projectile during a large-angle collision. The shift from the target nucleus is expressed as a function of inter-nuclear distance in terms of a dipole moment (Z₁ and Z₂: atomic numbers of projectile and target, : polarizability, e: electron charge). The effective polarizability β (≡Z₁/Z₂) is expressed as a function of ion velocity v [10⁷ cm/s], in the form β = 0.079 exp[−0.46v].     

Lifetime and dimensions of electron (or positron) beam in a storage ring at low current

This article gives the results of experiments on the lifetime and dimensions of the beam for currents at which these quantities are independent of the number of particles. They are studied in relation to energy, voltage on the resonator, etc. Good agreement is obtained between experiment and theory.Translated from Atomnaya Énergiya, Vol. 22, No. 3, pp. 200–203, March, 1967.     

一种测量辐射加工用加速器电子束能量的方法

根据射程法原理,在上海二级标准剂量学实验室对辐射加工用加速器电子束能量的测量方法进行了实验研究,并在3个辐射工厂对3台3MeV、20mA高频高压型加速器的电子束进行了能量测定.结果表明,在1-3MeV能量范围内,能量测定准确度优于±10%.     

低能电子束循环辐射硫化天然橡胶乳液

应用250keV低能电子束循环辐射硫化二丙烯酸壬酯（1,9-nonanedilo diacrylate，NDDA）敏化的天然橡胶乳液，探讨了影响低能电子束循环辐射硫化天然橡胶乳液的因素。结果表明，提高电子束束流，可缩短辐射硫化时间；40％的天然橡胶乳液更适合低能电子束的辐射硫化；适当提高循环乳液的重量有利于辐射硫化乳液的质量控制。     

注砷硅片低能电子束退火

本文报道了用低能大面积电子束处理注砷硅片的实验结果。由四探针和背散射、沟道效应测量结果表明，用本方法退火的样品具有电激活率高和砷原子再分布小的优点。