Papers by Keyword: Positron Beam

Abstract: The use of Coincidence Doppler-Broadened (CDB) positron annihilation spectra for surface analysis requires ensuring that a significant fraction of the annihilation signal originates from positrons annihilating at the surface. We present measurements of CDB spectra obtained using a high-purity germanium (HPGe) detector in coincidence with a sodium iodide (NaI) detector for a series of incident positron beam energies ranging from 2 eV to 20 keV on multilayer graphene (6–8 layers) on copper. The CDB data were analyzed by fitting the Doppler spectra using a linear combination of spectra derived from measured data obtained at the incident beam energies mentioned together with a Gaussian function representing the spectrum associated with para-positronium decay. These fits were used to determine the fraction of incident positrons that annihilate at the surface and in the bulk. We compare the results obtained from fitting the full Doppler-broadened spectra with those derived from a VEPFIT analysis of the S parameters as a function of beam energy, using the same set of Doppler spectra. Both approaches indicate that most annihilation events occur at the surface for beam energies below 5 keV. However, the results from full spectrum fitting suggest a significantly larger bulk annihilation fraction than VEPFIT in the 100 eV to 5 keV range, highlighting the importance of accounting for bulk contributions when using CDB for surface analysis at beam energies above 100 eV.

Abstract: Brightness enhancement by re-moderation of highly intense slow positron sources at nuclear reactors or large electron accelerators is an inevitable requirement for most positron experiments. We demonstrate the feasibility of a new layout for re-moderation combining for the first time the high reliability of magnetic beam guiding, the high efficiency of reflection re-moderation, correction of spherical aberrations in the primary focussing lens and loss-free extraction of the re-moderated beam by an E x B-filter. In the case of the intense positron source NEPOMUC at the research reactor FRM II in Munich, up to 10-fold intensity and 100-fold brightness are predicted for the re-moderated positron beam, compared to the current situation.

Abstract: A new bunched positron (e⁺) beam is currently under final development at the AntiMatter Laboratory (AML) of the University of Trento. The positron beam has been designed to deliver bunches containing up to a few 10⁵ positrons compressed to durations of less than 2 ns and with a tunable energy range between 1 and 21 keV. Thanks to the specifications of this new apparatus, different experiments based on the production of positronium (Ps) in nanostructured e⁺/Ps converters have been planned. A silicon target with nanochannels connected to laser-written buried micro-cavities is under development. The intent is to produce Ps atoms in the nanochannels and collect them in the micro-cavities to explore the mechanisms of Ps/cavity walls interaction and Ps/Ps interaction. Moreover, Ps clouds emitted into vacuum after implantation of e⁺ bunches in nanochanneled e⁺/Ps converters will be employed to perform measurements of the entanglement of the three annihilation gamma-rays of Ps and the inertial sensing on Ps. In this work, the characteristics of the new positron beam at the AML of Trento will be presented and the scheduled experiments will be illustrated.

Abstract: Commercial composite membranes for water purification were studied by means of the variable-energy positron annihilation γ-ray technique. The positron incident energy E dependence of R parameter (3γ/2γ ratio) analyzed from positron annihilation γ-ray spectra revealed the layer structure consisting of a dense top layer and a porous substrate for the membranes. Thickness of the dense layer for each membrane was evaluated from the R(E) based on a positron implantation stopping profile.

Abstract: After a general introduction to the field of resistive switching and spin electronics and the role of defects therein, recent investigations on the above mentioned topics including positron beams are reviewed. An ongoing project at the Helmholtz Centre Dresden-Rossendorf to further extend such investigations is briefly outlined and expected benefits are mentioned.

Abstract: Depth resolved positron beam results of Si and metal silicides are highlighted in terms of identifying the nature of vacancy defects in amorphous and disordered Si and phase transformation and defect evolution as a consequence of silicide formation, respectively.

Abstract: Positron transmission experiments were performed on free-standing poly(methyl-methacrylate) (PMMA) and polystyrene (PS) films of nanometric thicknesses made by spin coating. The power_law equation z1/2(E) )=(α/ρ)En was determined from the measurements of the S-parameter as a function of the positron implantation energy. These transmission experiments indicate that n = 1.90(±0.08) and α = 1.33(±0.10) μg cm-2 which deviates from the values found by Algers et al. (n = 1.71(±0.05) and α = 2.8(±0.2) μg cm-2) and the commonly used parameters (n = 1.6 and α = 4.0 μg cm-2).

Abstract: Well-ordered 2-dimensional (2D) hexagonal and 3-dimensional (3D) cubic mesoporous silicon oxide thin films prepared using triblock Poly(ethylene oxide)-Poly(Propylene oxide)-Poly(ethylene oxide) copolymer species (P123, F127) as the structure-directing agents, are studied by positron beam analysis in parallel with X-ray reflection measurements. It is observed that in the two films with equivalent porosity and pore size (normal to the film surface direction), the shape of mesopores considerably affects positron annihilation behavior. The narrowed positron annihilation Doppler broadening in the 2D hexagonal mesoporous film may suggest a higher positronium formation probability there, owing to a larger effective open volume area originated from the extension of pore channels parallel to the film substrate.