Maik Butterling

Design and Construction of a Slow Positron Beam for Solid and Surface Investigations

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.

Nuclear deformation and neutron excess as competing effects for dipole strength in the pygmy region.

The electromagnetic dipole strength below the neutron-separation energy has been studied for the xenon isotopes with mass numbers A=124, 128, 132, and 134 in nuclear resonance fluorescence experiments using the γELBE bremsstrahlung facility at Helmholtz-Zentrum Dresden-Rossendorf and the HIγS facility at Triangle Universities Nuclear Laboratory Durham. The systematic study gained new information about the influence of the neutron excess as well as of nuclear deformation on the strength in the region of the pygmy dipole resonance. The results are compared with those obtained for the chain of molybdenum isotopes and with predictions of a random-phase approximation in a deformed basis. It turned out that the effect of nuclear deformation plays a minor role compared with the one caused by neutron excess. A global parametrization of the strength in terms of neutron and proton numbers allowed us to derive a formula capable of predicting the summed E1 strengths in the pygmy region for a wide mass range of nuclides.

First Experiments with MePS

MePS (Mono-energetic Positron System) is part of the EPOS system (ELBE Positron Source) in the HZDR (Helmholtz-Zentrum Dresden-Rossendorf). It is one of the installations at ELBE (Electron LINAC for beams with high Brilliance and low Emittance), which supplies a 40-MeV electron beam. MePS makes use of the excellent time structure of the primary electron beam of ELBE (repetition frequency up to 26 MHz; bunch length < 5ps) to produce a pulsed, intense slow positron beam to allow positron lifetime spectroscopy. In order to avoid spurious signals, which, in other systems, are often obtained by positrons reflecting from the sample surface, a bent tube (45°) was added between accelerator and sample chamber. The MePS system has been used to study the pore system of a series of low-k dielectric layers.

Positron spectroscopy of point defects in the skyrmion-lattice compound MnSi

Outstanding crystalline perfection is a key requirement for the formation of new forms of electronic order in a vast number of widely different materials. Whereas excellent sample quality represents a standard claim in the literature, there are, quite generally, no reliable microscopic probes to establish the nature and concentration of lattice defects such as voids, dislocations and different species of point defects on the level relevant to the length and energy scales inherent to these new forms of order. Here we report an experimental study of the archetypical skyrmion-lattice compound MnSi, where we relate the characteristic types of point defects and their concentration to the magnetic properties by combining different types of positron spectroscopy with ab-initio calculations and bulk measurements. We find that Mn antisite disorder broadens the magnetic phase transitions and lowers their critical temperatures, whereas the skyrmion lattice phase forms for all samples studied underlining the robustness of this topologically non-trivial state. Taken together, this demonstrates the unprecedented sensitivity of positron spectroscopy in studies of new forms of electronic order.

Sb-related defects in Sb-doped ZnO thin film grown by pulsed laser deposition

Sb-doped ZnO films were fabricated on c-plane sapphire using the pulsed laser deposition method and characterized by Hall effect measurement, X-ray photoelectron spectroscopy, X-ray diffraction, photoluminescence, and positron annihilation spectroscopy. Systematic studies on the growth conditions with different Sb composition, oxygen pressure, and post-growth annealing were conducted. If the Sb doping concentration is lower than the threshold ∼8 × 1020 cm−3, the as-grown films grown with an appropriate oxygen pressure could be n∼4 × 1020 cm−3. The shallow donor was attributed to the SbZn related defect. Annealing these samples led to the formation of the SbZn-2VZn shallow acceptor which subsequently compensated for the free carrier. For samples with Sb concentration exceeding the threshold, the yielded as-grown samples were highly resistive. X-ray diffraction results showed that the Sb dopant occupied the O site rather than the Zn site as the Sb doping exceeded the threshold, whereas the SbO related deep ...

Positron-Annihilation Lifetime Spectroscopy using Electron Bremsstrahlung

A new type of an intense source of positrons for materials research has been set up at the superconducting electron linear. The source employs hard X-rays from electron- bremsstrahlung production generating energetic electron-positron pairs inside the sample under investigation. CW-operation allows performing experiments with significantly reduced pile-up artefacts in the detectors compared to pulsed mode operation in conventional accelerators. The high-resolution timing of the accelerator with bunch lengths below 10 ps full width at half maximum (FWHM) allows positron annihilation lifetime spectroscopy (PALS) measurements with high time resolution. A single-component annihilation lifetime of Kaptonhas been measured as (381.3 ± 0.3) ps. Employing segmented detectors for the detection of both annihilation photons allows for the first time to perform a 4D tomographic reconstruction of the annihilation sites including the annihilation lifetime.

Study of Neutron Induced Defects in Ceramics using the GiPS Facility

Preliminary results are presented from a study of neutron irradiation damage in Sapphire and B4C, produced with a fluence of 6?1018 n/cm2 and ~1015 n/cm2, respectively. Measurements were performed at the GiPS facility and the SPONSOR beam at HZDR, and in the PAL spectrometer at NRCN. Bulk and vacancies lifetimes were identified in the Sapphire, ~150ps and ~188ps, respectively, with complete trapping in the irradiation induced vacancies. Irradiation damage in B4C found to be limited to the surface. A single lifetime of ~166ps was measured in both irradiated and non-irradiated samples, and was associated with the bulk.

Flash lamp annealing of tungsten surfaces marks a new way to optimized slow positron yields

Tungsten in the form of a mono-crystalline foil with an optimum thickness of about 2 μm is often used as a positron moderator in mono-energetic positron beams with 22Na positron sources. The efficiency of such a moderator strongly depends on its prior heat treatment, i.e. an annealing procedure with considerable difficulty at temperatures of about 2000 °C under vacuum conditions. Flash lamp annealing (FLA) has been tested as new method to quickly anneal W foils in order to produce easy manageable, low-cost moderators with a high efficiency. With FLA, just the surface of a W foil is heated above the melting point (3422°C) within 1 to 3 ms, i.e. without melting the whole foil volume. In this way, a surface cleaning is reached connected with a considerable increase in the positron diffusion length. Conventional polycrystalline W foils of 9 μm ± 25% thickness, heat treated by FLA, were characterized and tested as positron moderators. First promising tests result in a moderator efficiency of ~3*10−4 and clearly demonstrate that FLA is also applicable to tungsten meshes.

Annihilation Lifetime Spectroscopy Using Positrons from Bremsstrahlung Production

A new type of a positron annihilation lifetime spectroscopy (PALS) system has been set up at the superconducting electron accelerator ELBE [ at Helmholtz-Zentrum Dresden-Rossendorf. In contrast to existing source-based PALS systems, the approach described here makes use of an intense photon beam from electron bremsstrahlung which converts through pair production into positrons inside the sample under study. The article focusses on the production of intense bremsstrahlung using a superconducting electron linear accelerator, the production of positrons inside the sample under study, the efficient detector setup which allows for annihilation lifetime and Doppler-broadening spectroscopy simultaneously. Selected examples of positron annihilation spectroscopy are presented.

Low Background Digital Coincidence Spectrometer – A Tool for Investigation of Positron Annihilation in Flight

A digital spectrometer for low background gamma ray spectroscopy equipped with two high purity Ge detectors and a 12-bit two channel digitizer was employed for the investigation of positron annihilation-in-flight. Measurements were performed for positrons emitted by 68Ge/68Ga and 22Na radioisotopes and annihilated in Cu and Mg targets. The contribution of the two-quantum positron annihilation-in-flight was clearly resolved in coincidence two-dimensional gamma ray energy spectra. The contribution of positrons annihilated in flight has a hyperbolic shape described well by the relativistic theory.