Electrostatic Positron Beam Research Articles

High-Resolution Measurements of e^{+}+H_{2}O Total Cross Section.

Using a purely electrostatic positron beam, the total cross section of positrons scattering from H_{2}O has been measured for the first time with a high angular discrimination (≃1°) against forward scattered projectiles. Results are presented in the energy range (10-300)eV. Significant deviations from previous measurements are found which are, if ascribed entirely to the angular acceptances of various experimental systems, in quantitative accord with abinitio theoretical predictions of the differential elastic scattering cross section.

An electrostatic positron beam for atomic and molecular collision experiments

SynopsisA new electrostatic positron beam at UCL is described. The beam incorporates a re-moderator to achieve brightness enhancement, time of flight facility for beam energy determination and improved signal to background ratio and a position sensitive detector for 2-D imaging of the beam spot. The apparatus is being employed for measuring the total cross section of positron scattering from krypton and helium in the energy range of (10 — 200) eV. Results will be presented at the conference.

An electrostatic brightness-enhanced timed positron beam for atomic collision experiments

An electrostatic positron beam, incorporating a remoderation stage to yield brightness enhancement with time-of-flight capabilities, has been developed and characterized. The beam is tunable in an energy range (5–500) eV with an angular divergence of ≃ 1° and an energy resolution of ≲ 1%.

A Full Electrostatic High-Brightness Slow Positron Beam Apparatus Using a Remoderator in Conjunction with a Hemispherical Energy Analyzer

The development of a full electrostatic slow positron beam apparatus at the Tokyo University of Science is described. The performance of different types of material as the primary moderator and the energy resolution of the beam are detailed.

An electrostatic slow positron beam apparatus for positron scattering experiments

An electrostatic slow positron beam system has been developed for positron scattering studies. The apparatus uses a remoderator in reflection geometry in conjunction with an electrostatic hemispherical energy analyzer for the brightness enhancement. It is possible to determine absolute total scattering cross section values under magnetic field-free conditions down to a few mG. Measurement of the energy distribution of the positrons that are reemitted from the remoderator and total cross sections for positron-He scattering are presented.

Slow positron moderator studies at Brandeis – Collaborative work with Karl F. Canter

AbstractThe collaborative research work carried out with Karl F Canter on slow positron moderators at Brandeis University is presented. A compact electrostatic positron beam has been designed and constructed for the determination of slow positron yield of single crystalline W(100) moderator foil with provision for in‐situ heating and surface characterization. Moderation yield is found to gradually increase with successive heating cycles, which is also correlated with the removal of surface and bulk carbon‐vacancy complexes. The work on inert Ar gas moderator is also briefly discussed. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Spin-polarization of an electro-static positron beam

We constructed an electro-static positron beam apparatus. We fabricated a simple spin-polarimeter composed of a permanent magnet with a surface magnetic field of 0.65 T and an iron pole piece. The longitudinal spin-polarization of the positron beam was determined to be 0.3 by analyzing the magnetic field dependence of the Doppler broadening of annihilation radiation from a fused silica specimen. The effect of spin rotation was examined using an iron poly-crystal and a simple E × B filter.

Improvement of the electrostatic positron beam apparatus and observation system for RHEPD experiment

We have observed reflection high energy positron diffraction (RHEPD) using the electrostatic positron beam apparatus, where positrons were accelerated at 20 keV and converged using three einzel lenses and a collimator. To reduce beam energy dispersion and gamma ray background, a 45° electrostatic prism and einzel lenses were newly added to the original apparatus. The positron beam is further collimated to 1 mm diameter. Final beam energy dispersion and angular divergence are less than 0.1 keV and 0.1°, respectively. Any noises arising from the gamma rays are not observed. We also fabricated the positron beam observation system with a dynamic range of approximately 4.3×10 9. The above improvements enabled to detect the fine structures of diffraction patterns and rocking curves.

Reflection high-energy positron diffraction at solid surfaces by improved electrostatic positron beam

We report observation of reflection high-energy positron diffraction (RHEPD) at the Si(1 1 1) surface using an electrostatic positron beam. The improvement of the beam (i.e., reduction of beam energy spread) was critical to obtaining the higher-order diffraction spots. We could observe the first Laue zone, which was not seen in our previous work. This allows us to study the surface Debye temperature. The obtained rocking curve represents the enhancement of the fourth Bragg peak, as expected from diffraction theory, in addition to previously confirmed total reflection and the first Bragg peak.

The design of a spin-polarized slow positron beam

Spin-polarized low-energy positrons were considered as useful probes for studying electron spin states of both surface and bulk materials. Due to the spin-dependent interactions between electrons and positrons, the formation of positronium (Ps), an electron-positron bound system, can be distinguished from different electron spin states. Recently, a positron source of 18F has been developed for a spin polarized slow positron beam at the institute of physical and chemical research (RIKEN). The design of an electrostatic positron beam will be discussed in conjunction with a spin rotator.

An electrostatically-focussed positron beam for surface studies

Construction of an electrostatic positron beam mainly for surface studies is under way. In order to obtain information related to surface states with moderate intensity beams, scanning a target with positron beams is planned. Positron trajectories are calculated for various configurations of lens elements and potentials given to them. Several moderators are evaluated for the slow positron generation. A single crystal tungsten foil gives the best performance among the materials tested. In case of the single crystal tungsten, heat treatment is crucial for obtaining good results.

A new electrostatic positron beam for surface studies

A new electrostatic positron beam has been constructed and tested. The energy range spans more than three decades, from less than 50 eV to more than 50 keV. The machine can operate in a low-energy mode (50 eV to 2 keV) with a beam diameter (FWHM) from 2.9 to 1.7 mm or in a high-energy mode (1 keV to 50 keV) with a FWHM from 1.4 to 1.0 mm. The transverse spot position on the target is very stable, with wandering being much smaller than the beam diameter. The apparatus transmission function is very flat, with an intensity change of only 30% when going from 10 eV to 50 keV. The product of moderation and optical transmission efficiency is , corresponding to 800-1000 with a 1 mCi source. The apparatus shows low noise in S-parameter measurements. The background of the high-energy wing of the annihilation line is also one of the best reported. Optical and constructional details are given.

The electrostatic slow positron beam for the positron-induced ion-desorption spectroscopy

We have constructed an electrostatic positron beam for positron-induced ion-desorption spectroscopy. The design and construction of the instrument is described.

A high performance electrostatic positron beam

A new electrostatic positron beam for Doppler broadening measurements in the near surface layers has been constructed and tested in the Trento laboratory. The beam energy is tuneable from 50 eV to 50 keV. The diameter of the beam spot at the sample is two mm FWHM, when a commercial 22Na source of 3 mm in diameter is used. The beam could be easily upgraded to become a microbeam. The apparatus is described and the preliminary tests are presented.

Development of a reflection geometry positron reemission microscope

The design, construction, and development of an electrostatic positron beam system for reemitted positron microscopy is described. Positrons from a 1.5 GBq 22Na source are moderated and a primary 10 mm diameter beam formed. The brightness of the beam is subsequently increased by two further stages of focusing and remoderation. The resultant, smaller diameter beam is then focused and impinges on the sample of interest at an angle of 45° to the surface. The incident positron beam optics and reemitted positron imaging optics, integrated to minimise interference between the two, are in reflection geometry to permit the imaging of thick samples. The reemitted positron ensemble is accelerated, magnified, and focused to form an image at the face of a position sensitive detector; contrast in the image is determined by the variation in the probability of reemission across the surface of the sample. The results of tests on the performance of the beam delivery optics are presented together with proposed improvements and possible applications for the prototype system.

Design and simulation of the PSI electrostatic positron beam

For the design of the brightness enhancement stages a detailed phase space analysis based on unconventional focusing configurations and on-ray tracing simulations has resulted in a reduction of the field-extracted beam of 10 cm diameter and E T of 20 eV down to a spot size of 8 mm. After a second remoderation stage reducing the beam size by a further factor of 25, a straightforward electrostatic transport of the positrons to the target at ground potential allows the beam to be tuned between a few eV and 60 keV. The main result is summarized in a beam phase-space at the target of less than 300 μm 40 mrad for energies above a few 100 eV increasing to 500 μ m 120 m at a few eV.

Antihydrogen by positronium-antiproton collisons

A method for producing antihydrogen,\(\bar H\), by positronium-antiproton collisions is described. Included are the calculated capture cross sections, and short descriptions of the electrostatic slow positron beam, positronium-formation, and the antiproton ion trap to be used for antihydrogen production. With present available technology, collimated monoenergetic\(\bar H\) beams with an energy of a few keV and an intensity in the order of one per second can be produced by this method. Possible enhancements of this rate are discussed.

Antihydrogen Production by Positronium-Antiproton Collisions in an Ion Trap

A method for producing antihydrogen by the p̄ + Ps → H̄ + e- reaction is described. Included are the calculated capture cross sections, and short descriptions of the electrostatic slow positron beam, positronium-formation, and the antiproton ion trap to be used for antihydrogen production. With present available technology, collimated monoenergetic H̄ beams with an energy of a few keV and an intensity in the order of one per second can be produced by this method. Possible enhancements of this rate are discussed.