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.     

A scanning positron microscope for defect analysis in materials science

The realisation of a scanning positron microscope will be presented and discussed. A positron beam with a variable energy from 0.5 to 30 keV, with a spot diameter of 1 μm or below, can be scanned over an area of 0.6 × 0.6 mm². This beam is formed after a double stage stochastic cooling (moderation) of positrons emitted from a radioactive isotope. In addition the positron beam will be pulsed in order to have a well-defined time base for positron lifetime measurements. In the system included is a conventional scanning electron microprobe for surface analysis. The design of the scanning positron microscope is dominated by the special demands of positron physics.     

Low-Energy Positron Injector

The LEPTA low-energy positron accumulator, which is to be used for producing directed fluxes of positronium and antihydrogen atoms, is under development at the Joint Institute of Nuclear Research. The monochromatic positron injector, operating in the pulsed mode, in the accumulator must generate a positron beam with intensity 10⁸–10⁹ particles in a pulse with duration less than 300 nsec, the positron energy is 10 keV, the relative energy spread in the beam is less than 2·10^–3, and the beam radius is 0.5 cm.Radioactive ²²Na serves as a positron source. The positrons at the exit from the source are decelerated in a solid target and enter the magnetic trap. There they are once again decelerated in a gas to thermal velocity and accumulate in ~100 sec. For injection into the accumulator, the positrons are pulled out of the trap by a pulsed electric field and acclerated up to the required energy.     

用成像板探测正电子

用成像板对常用正电子源＾３２Ｎａ进行了一系列感光实验。结果显示,激发辐射的光子强度值ｆ（ｐ）与成像板记记载的正电子流强ｆ（ｅ＾＋）成正比关系。在北京慢正电子束设备上进行了单慢正电子的感光实验,结果表明,成像板是慢正电子的一种优良的二维位置灵敏探测器。     

The Stuttgart positron beam, its performance and recent experiments

A monoenergetic MeV positron (e⁺) beam, with a flux at present of 6 × 10⁴ e⁺/s in the energy range of 0.5 to 6.5 MeV, has been installed at the Stuttgart Pelletron accelerator. The stabilization and the absolute calibration of the energy E is monitored by a Ge detector with real-time feedback; a relative energy stability of ΔE/E 10⁻⁴ is obtained. So far, e⁺e⁻ scattering and annihilation-in-flight experiments for investigating the low-energy e⁺e⁻ interaction as well as β⁺ γ positron lifetime measurements in condensed matter have been performed. The advantages of the β⁺ γ method compared to the conventional γγ coincidence technique have been demonstrated. Recently, triple-coincidence positron “age-momentum correlation” measurements have been carried out on fused quartz. A brief account is given on the development of a “positron clock” aiming at a substantial improvement of the time resolution of the β⁺ γ positron lifetime measurements.     

Positron implantation profiles in layered samples

The paper presents theoretical and experimental studies of the positron implantation profiles in layered samples. The Monte Carlo simulations performed using the GEANT4 toolkit reveal accumulation of positrons in the denser layer embedded between two less dense layers. That effect is significant not only for low energy positrons at slow monoenergetic positron beams but also for high energy positrons which are emitted from radioactive nuclei in conventional experiments. Measurements of the positron implantation profile into the samples which consist of silver and aluminium foils of different thickness show profile features which correspond well with those simulated by the GEANT4 toolkit for examined cases. We propose to call this phenomenon the accumulation effect of energetic positrons.     

正电子致电离截面实验中束流强度的在线测量

本文采用HPGe探测器实时收集了正电子碰撞厚Ti靶伴随产生的湮灭光子,并结合HPGe探测器对放置在碰撞点处²²Na标准源产生的511 keV湮灭光子的探测效率刻度值,得到了8~9.5 keV正电子引起Ti原子内壳层电离截面实验中正电子束流强度的实时测量结果。结果表明,在实验测量的38 h内,基于²²Na标准源慢正电子束流装置产生的正电子束流强度不稳定,随时间的变化有着不同程度的衰减,且存在小幅度波动现象。因此,低能正电子致原子内壳层电离截面实验中应采用在线法获取慢正电子束流强度。     

Calculations of positron absorption in targets

Using the multiple-step method for calculation of transmission of charged particles through matter, the density distributions of thermalized positrons in a number of materials (6 ? Z ? 42) have been obtained under irradiation by monoenergetic positrons and positrons from ²²Na and ⁵⁸Ga sources. Fluctuations of energy losses, absorber boundaries, annihilation of positrons have been taken into account. Elastic scattering of the positrons was calculated according to Mott's theory. One can use the obtained data when constructing similar theoretical distributions and when determining the positron annihilation probabilities for a wide range of materials involving compounds.     

Radiation damage in neutron transmutation doped-InP

InP crystals irradiated at different thermal neutron fluences have been investigated by optical absorption and positron lifetime measurements. Irradiation produces a shift of the absorption edge towards long wavelengths and the appearance of an absorption band centered at 1.24 eV. The absorption edge is fully recovered after annealing but the absorption band remains. Positron lifetime spectra are analysed with two components. The longer lifetime, τ₂ = 300 ps, has an intensity that increases with the fluence and falls down after annealing. This component has been associated to positrons trapped at In-monovacancy related defects. The absorption tail has been proposed to be due to optical transitions involving vacancies that act as acceptors while the 1.24 eV band has been tentatively ascribed to anion antisites.     

Positron affinity for precipitates in reactor pressure vessel steels

The sensitivity of positron annihilation spectroscopy to irradiation-induced precipitates in reactor pressure vessel steels is discussed in the light of recent positron affinity and lifetime calculations. Carbide and nitride precipitates are found to trap positrons only if they contain metal vacancies. Copper precipitates are also attractive to positrons but they are probably detected through annihilation at the precipitate-matrix interface. These findings are related to available experimental data.     

Dense positrons and γ-rays generation by lasers interacting with convex target

We use quantum electrodynamics particle-in-cell simulation to study the generation of dense electron–positron plasma and strong γ-ray bursts in counter-propagating laser beam interactions with two different solid targets, i.e. planar(type I) and convex(type II). We find that type II limits fast electron flow most effectively. while the photon density is increased by about an order of magnitude and energy by approx. 10%–20% compared with those in type I target. γ-photon source with an ultrahigh peak brilliance of 2?×?1025 photons/s/mm2/mrad2/0.1% BW is generated by nonlinear Compton scattering process. Furthermore, use of type II target increases the positron density and energy by 3 times and 32% respectively, compared with those in type I target. In addition, the conversion efficiencies of total laser energy to γ-rays and positrons of type II are improved by 13.2% and 9.86% compared with type I. Such improvements in conversion efficiency and positron density are envisaged to have practical applications in experimental field.     

Comparative study of elastic scattering of low-energy electrons, positrons and protons in solids

The differential cross section (DCS) and total cross section (TCS) for elastic scattering of 0.1 eV–1 keV electrons, positrons and protons by atoms bound in solids (Z = 3–82) are calculated using the partial wave expansion technique. The DCS for low-energy positrons decreases in the forward direction due to the cancellation of the electrostatic interaction by the polarization of the atom. This cancellation effect for positrons results in a much smaller TCS than that for electrons at low energy, whereas, at high energy, they are close to each other due to dominant electrostatic interaction. The DCS for protons shows a strong forward scattering. This results in a TCS much greater than that for positrons, whereas, for large angles, the DCS is equal to that of positrons (except for very low energy).     

4.5 tesla magnetic field reduces range of high-energypositrons-potential implications for positron emission tomography

We have theoretically and experimentally investigated the extent to which homogeneous magnetic fields up to 7 Tesla reduce the spatial distance positrons travel before annihilation (positron range). Computer simulations of a noncoincident detector design using a Monte Carlo algorithm calculated the positron range as a function of positron energy and magnetic field strength. The simulation predicted improvements in resolution, defined as full-width at half-maximum (FWBM) of the line-spread function (LSP) for a magnetic field strength up to 7 Tesla: negligible for F-18, from 3.35 mm to 2.73 mm for Ga-68 and from 3.66 mm to 2.68 mm for Rb-82. Also a substantial noise suppression was observed, described by the full-width at tenth-maximum (FWTM) for higher positron energies. The experimental approach confirmed an improvement in resolution for Ga-68 from 3.54 mm at 0 Tesla to 2.99 mm FWHM at 4.5 Tesla and practically no improvement for F-18 (2.97 mm at 0 Tesla and 2.95 mm at 4.5 Tesla). It is concluded that the simulation model is appropriate and that a homogeneous static magnetic field of 4.5 Tesla reduces the range of high-energy positrons to an extent that may improve spatial resolution in positron emission tomography     

Status of Work on the VÉPP-5 Injection Complex

The VÉPP-5 injection complex under construction at the Institute of Nuclear Physics of the Siberian Branch of the Russian Academy of Sciences is a powerful source of intense electron and positron bunches at 510 MeV, which covers all needs of the electron–positron colliding beam setups currently operating and under construction at the Institute of Nuclear Physics. The complex includes a 285 MeV linear electron accelerator, a 510 MeV linear positron accelerator, and an accumulator–cooler with beam injection and ejection channels. Intense work on the design, assembly, and tuning of the linear electron accelerator has been conducted in the last 2 yr. As a result, by August 2002 the linear electron accelerator was put into operation with all standard subsystems. By this time, the isochronous achromatic turning of the electron beam, a system for converting electrons into positrons, and the first accelerating structure of the linear positron accelerator were assembled and put into operation. All this made it possible to accelerate the positron beam up to 75 MeV. Preliminary results of tests of the linear accelerators are presented.     

Fe—B超细微晶合金的正电子湮没研究

用正电子湮没技术对急地制备了Ｆｅ－Ｂ合金结晶态超细微粒（ＣＵＦＰ）的微结构进行了分析，寿命谱研究结果与正电子三态湮没模型相吻合，研究了Ｆｅ－Ｂ－ＣＵＦＰ的结晶与非晶两种结构的微缺陷，并推测Ｆｅ－Ｂ－ＣＵＦＰ聚集体（纳米晶膜）材料具有类原子气态微结构的可能性。     

掺杂BaTiO3系PTC材料的正电子湮没研究

采用正电子湮没寿命谱学的方法研究了ＢＳＴｉＯ３系ＰＴＣ材料中掺Ｎｂ５＋、Ｙ３＋及不同掺杂量所引起的缺陷变化，并讨论了不同掺杂对ＰＴＣ效应的影响。     

High density γ-ray emission and dense positron production via multi-laser driven circular target

A diamond-like carbon circular target is proposed to improve γ-ray emission and pair production with a laser intensity of 8×10²² Wcm⁻² by using 2D particle-in-cell simulations with quantum electrodynamics. It is found that the circular target can enhance the density of γ-photons significantly more than a plane target, when two colliding circularly polarized lasers irradiate the target. By multi-laser irradiating the circular target, the optical trap of lasers can prevent the high energy electrons accelerated by laser radiation pressure from escaping. Hence, γ-photons with a high density of beyond 5000n_c are obtained through nonlinear Compton backscattering. Meanwhile, 2.7×10¹¹ positrons with an average energy of 230 MeV are achieved via the multiphoton Breit–Wheeler process. Such an ultrabright γ-ray source and dense positron source can be useful in many applications. The optimal target radius and laser mismatching deviation parameters are also discussed in detail.     

A dual parameter (S versus τ) positron annihilation spectrometer utilizing the β+-γ-γ coincidence applied to the study of defects in AL

The construction of a dual parameter spectrometer utilizing the detection of the β⁺ -γ cascade is presented. The lifetimes of the positrons were measured with start pulses from a thin transmission β⁺ scintillator and with stop pulses from a γ scintillator. The pair momentum distribution was measured with a GeLi spectrometer using Doppler broadening of the annihilation γ line. The data accumulation rate of the dual parameter spectrometer was ～ 8 s^?1 with a 10 μCi⁶⁸Ge positron source. The resolution of the lifetime measurement was ～ 330 ps (fwhm) and the efficiency of the GeLi detector ～10%. The dual parameter spectrometer has been applied to study aluminium samples as a function of their deformation and annealing. The dual parameter spectra were analyzed by calculating the S parameters from the Doppler broadenings as a function of the positron lifetime. The results indicate a strong increase of the S parameter values in the longer lifetimes measured with the samples containing defects.     

Low-Energy Positron Accumulator for Generating Directed Positron Fluxes (LEPTA Project)

The LEPTA (Low Energy Particle Toroidal Accumulator) under construction at the Joint Institute of Nuclear Research is described. The goal of this project is to produce a small accumulator for 10-keV positrons with electronic cooling of the circulating positron beam. The main purpose of the accumulator is to generate an intense flux of the electron–positron ground state, so-called positronium. The parameters of the first experiments on a positronium flux and the current status of the work on this project are discussed.     

Gamma-induced Positron Spectroscopy (GiPS) at a superconducting electron linear accelerator

A new and unique setup for Positron Annihilation Spectroscopy has been established and optimized at the superconducting linear electron accelerator ELBE at Helmholtz-Zentrum Dresden-Rossendorf (Germany). The intense, pulsed (26 MHz) photon source (bremsstrahlung with energies up to 16 MeV) is used to generate positrons by means of pair production throughout the entire sample volume. Due to the very short gamma bunches (< 5 ps temporal length), the facility for Gamma-induced Positron Spectroscopy (GiPS) is suitable for positron lifetime spectroscopy using the accelerator’s radiofrequency as time reference. Positron lifetime and Doppler broadening Spectroscopy are employed by a coincident measurement (Age-Momentum Correlation) of the time-of-arrival and energy of the annihilation photons which in turn significantly reduces the background of scattered photons resulting in spectra with high signal to background ratios. Simulations of the setup using the GEANT4 framework have been performed to yield optimum positron generation rates for various sample materials and improved background conditions.