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The creation of an electron space charge in a dipole magnetic trap and the subsequent injection of positrons have been experimentally demonstrated. Positrons (5 eV) were magnetically guided from their source and injected into the trapping field generated by a permanent magnet (0.6 T at the poles) using a cross field E × B drift, requiring tailored electrostatic and magnetic fields. The electron cloud is created by thermionic emission from a tungsten filament. The maximum space charge potential of the electron cloud reaches −42 V, which is consistent with an average electron density of (4 ± 2) × 10 12 m − 3 and a Debye length of (2 ± 1) cm. We demonstrate that the presence of this space potential does not hamper efficient positron injection. Understanding the effects of the negative space charge on the injection and confinement of positrons represents an important intermediate step toward the production of a confined electron–positron pair plasma. [ABSTRACT FROM AUTHOR]

Lossless injection of positrons into a magnetic dipole trap and their subsequent confinement have been demonstrated. Here, we investigate by numerical single-particle simulations how the radial distribution of positrons in the trap is affected by the measurement itself, the choice of injection parameters, the asymmetry of the electric potential, and by elastic collisions. The results are compared to experimental data. A comprehensive understanding of these effects is a milestone on the road to creating an electron–positron plasma in a trap with a levitating superconducting coil. [ABSTRACT FROM AUTHOR]

Single crystalline YBa2Cu3O7-δ (YBCO) thin films were grown by pulsed laser deposition in order to probe the oxygen deficiency δ using a mono-energetic positron beam. The sample set covered a large range of δ (0.191<δ<0.791) yielding a variation of the critical temperature T c between 25 and 90K. We found a linear correlation between the Doppler broadening of the positron electron annihilation line and δ determined by X-ray diffraction. Ab-initio calculations have been performed in order to exclude the presence of Y vacancies and to ensure the negligible influence of potentially present Ba or Cu vacancies to the found correlation. Moreover, scanning with the positron beam allowed us to analyze the spatial variation of δ, which was found to fluctuate with a standard deviation of up to 0.079(5) within a single YBCO film. [ABSTRACT FROM AUTHOR]

We report the instrumentation of a new positron diffractometer that is connected to the high-intensity positron beam at the neutron induced positron source Munich. Crucial elements for the adaption of the positron beam are presented, which include the magnetic field termination, the optional transmission-type remoderator for brightness enhancement, and the electrostatic system for acceleration and beam optics. The positron trajectories of the remoderated and the twofold remoderated beam have been simulated to optimize the system, i.e., to obtain a coherent beam of small diameter. Within a first beamtime, we tuned the system and characterized the direct beam. For the twofold remoderated beam of 10 keV energy, we experimentally observe a beam diameter of d < 1.3 mm, which agrees well with the simulation.

We report on a novel device for particle acceleration based on elevation of the potential energy of beam pulses. This so-called energy elevator is particularly beneficial if both the particle source and the sample have to be near ground potential due to experimental constraints. We applied this new technique to enable depth dependent measurements of re-emitted positrons using the surface spectrometer at the NEPOMUC positron beam facility. First, a two-stage bunching system is used to generate positron pulses with a repetition rate of 5 MHz and a duration of 1.663(5) ns before their energy is raised to several keV. The whole system was shown to work with an exceptional efficiency of 88%. We demonstrated the usability of our setup by investigating the positron re-emission spectra of Ni and Pd as function of positron implantation energy. For Ni the positron work function could be determined to be ΦNi (+)=-1.4(2)eV. In addition, as predicted by theory, our experimental findings imply a positive positron work function for Pd.

Angular correlation of annihilation radiation (ACAR) is a well established technique for the investigation of the electronic structure. A major limitation of ACAR studies is the available positron flux at a small spot on the sample. For this reason, the focus of this work is put on the discussion of a newly developed source-sample stage of the new 2D-ACAR spectrometer at Technische Universität München which uses an optimized static magnetic field configuration to guide the positrons onto the sample. The achieved spot diameter is d(FWHM) = 5.4 mm, with a high efficiency over the whole energy spectrum of the (22)Na positron source. The implications of the performance of the source-sample stage are discussed with regard to 2D-ACAR measurements of single crystalline α-quartz, which serves as a model system for the determination of the total resolution. A value of (1.53 × 1.64) mrad(2) FWHM was achieved at room temperature.