Papers by Keyword: Slow Positron Beam

Abstract: A slow positron beamline has been developed using a 30-MeV L-band electron linear accelerator at Kyoto University. Based on particle-transport simulations to evaluate positron generation, the optimum thickness of the tungsten converter was determined. Through positron trajectory simulations in a Penning trap (linear storage section), the electrode types were compared to realize the best storage performance; the importance of the uniformity and precision of magnetic fields using supplementary coils was highlighted. During the commissioning experiments, positron transport to the beamline end was confirmed, along with the operation of the linear storage section.

Abstract: Low energy positron beams have become a valuable tool for depth dependent characterization of defects and micro- and mesoscale porosities, as well as for fundamental positron and positronium interactions. Some of the recent developments in the generation of secondary positron beams using particle accelerators are presented and the underlying physics and technologies are discussed.

Abstract: Tantalum nitride (TaN) thin films were deposited using magnetron sputtering method under different N₂/Ar ratio condition. Slow positron beam was used to analyze the microstructure of those films. The results show that the films which deposited at low N₂/Ar flow ratio contain more vacancy-like defects, and the corresponding S parameter is relatively large. The sheet resistance measurement displays that ohms-per-square greatly increase with increased N₂/Ar ratio. And the reasons could be related to nonstoichiometry-induced vacancies and lattice distortions.

Abstract: We describe the status of a project for obtaining an intense beam of polarized slow positrons at the Extreme Light Infrastructure - Nuclear Physics (ELI-NP) at Magurele (near Bucharest, Romania) [1]. Positrons will be created via pair production and moderated at a tungsten target using the pulsed brilliant gamma beam which will be produced by Compton back-scattering of circularly polarized laser photons on electrons from a warm linac beam [2]. Simulations of the interaction of circularly polarized γ‑rays of energies up to 3.5 MeV and an intensity of 2.4×10¹⁰ γ/s with the target, moderation of created positrons and beam formation are discussed. The optimization of the target design showed that the primary slow positron beam can be obtained with intensity of 1‑2×10⁶ e⁺/s. The primary beam will be transversally polarized with a degree of polarization of ~30%. We discuss the necessity of changing the e⁺ beam polarization from transversal to longitudinal by an electrostatic 90˚ bender which is proposed to work in combination with a remoderator. Simulations show that neither the remoderator nor the electrostatic bender will change the degree of e⁺ beam polarization. The longitudinally polarized e⁺ can be successfully transported to the sample chambers without depolarization, but with reduced intensity (by approximately one order of magnitude) due to the remoderation. We present a convertor-moderator assembly with a hole which will allow creating positrons in parasitic mode, i.e., simultaneously with the nuclear physics experiments at ELI-NP. The positron spectroscopy laboratory at ELI-NP will be user dedicated and the beam will have the highest intensity of polarized slow positrons for material science in the world and therefore it could become a unique tool for investigation of magnetic samples.

Abstract: A simple pulsed slow positron beam based on a Penning-trap has been designed and is being constructed in Wuhan University. The cooled positrons from the trap with very low energy dispersion are dumped and chopped to pulses of a few tens of ns in width. Positron pulses are bunched by adjusting the potential of drift tube using an arbitrary wave generator, then accelerated to the target. Influences of the pulse width, the energy dispersion of positrons etc. on the time resolution of bunched positron pulse are simulated. The result shows that lower energy dispersion of positrons leads to much narrower positron pulses, indicating that a trap-based slow positron beam has a great advantage in developing a pulsed slow positron beam.

Abstract: A study of irradiation-induced damage in HAVAR^® foils was initiated in order to extract the highest proton dose the foils can sustain. The lattice structure of HAVAR^® foils in different metallurgic conditions is presented, as well as visible internal structure, measured by Transmission Electron Microscopy (TEM). Positron Annihilation Spectroscopy (PAS) techniques were used to investigate these foils, and another foil that had been irradiated to the maximal proton dose limit, set by the manufacturer to a total charge of 1 mAh (= 3.6 C). PAS techniques included Doppler broadening (DB) measurement in the SPONSOR beam and lifetime (LT) measurements, both carried at Helmholtz-Zentrum Dresden-Rossendorf (HZDR). Both positron spectroscopy methods show clear differences between the investigated foils, with distinguished characteristics for annealed, cold-rolled and irradiated foils. The advantages of using a slow positron beam to study thin foils and defect profiles, over a table-top LT spectrometer, are discussed and demonstrated by the HAVAR^® measurements.

Abstract: On the basis of the design and construction of the slow positron beam SPONSOR at the Helmholtz-Centre Dresden-Rossendorf an example is given how to build-up a simple slow positron beam for solid surface investigations within a short time and without high financial costs. The system uses a ²²Na source and consists of three main parts: (1) the source chamber with a thin film tungsten moderator used in transmission, and a pre-accelerator stage, (2) the vacuum system with magnetic transport, a bent tube for energy selection and an accelerator, (3) the sample chamber with a sample holder, Ge detectors and (4) facilities for remote control and data acquisition. These parts are described in detail. The paper is preferentially addressed to beginners in the field of slow positron beam techniques and other readers being generally interested in positron annihilation spectroscopy.

Abstract: The degradation of ZnO/Silicone coating system under 90keV proton irradiation varying from 5×1014cm-2 to 1×1016cm-2, was studied by slow positron annihilation spectrum. Effective diffusion length calculated by Variable energy positron fit (VEPFIT) shows a dramatically break in a dose of 1×1015cm-2. It is ascribed to the increase of crosslink density and decrease of free volume and hole during the proton irradiation. Furthermore, positron has shown a satisfying sensitivity in detecting the nano-scale defect on ZnO/Silicone system.

Abstract: Slow positron beam are used to study defect structures in Co doped and undoped ZnO films prepared by Pulsed Laser Deposition (PLD) at 400°C, 600°C, 700°C on c-plane sapphire. Comparing with ZnO samples, Co doped ZnO samples have larger positron effective diffusion length (Leff), which change in different tendencies depending on the growth temperature. Crystal structures of the samples are investigated by X-ray diffraction (XRD) and wurtzite ZnO could be observed in Co doped samples.