This file was created by the Typo3 extension sevenpack version 0.7.16 --- Timezone: CET Creation date: 2024-11-22 Creation time: 01-24-46 --- Number of references 50 article Kofahl2016 Magn. Reson. Med. 2016 mrt http://onlinelibrary.wiley.com/doi/10.1002/mrm.26477 10.1002/mrm.26477 A.-L.Kofahl S.Theilenberg J.Bindl D.Ulucay J.Wild S.Napiletzki B.Schu-Schätter A.Vohlen B.Pintea J.Finsterbusch E.Hattingen C.Urbach K.Maier article Raeth2014 Appl. Phys. A 2014 117 3 1041-1044 Refractive index changes &Delta;n in lithium niobate crystals upon irradiation with high-energy protons, deuterons, ³He, and ⁴&alpha; particles (up to 14 MeV/nucleon) are created, and the accompanying, unwanted nuclear activation is investigated. The measurements give answers to the question which ion is the best choice depending on the requirements: largest values of &Delta;n are achieved with ⁴&alpha; particles, low nuclear activation with deuterons, or the best tradeoff between &Delta;n and activation with ³He, respectively. linbo http://link.springer.com/article/10.1007%2Fs00339-014-8524-y 10.1007/s00339-014-8524-y N. L.Raeth J.Goetze K.Peithmann K.Maier article radicke11 Ultrasound in Medicine &amp; Biology 2011 37 253-261 Acoustic radiation contrast in magnetic resonance images is an approach to visualize the changes in ultrasonic loss and viscoelastic changes of the sample with the resolution of a magnetic resonance imaging (MRI) system. By irradiating ultrasound (US) into a tissue-mimicking sample, a displacement along the US beam path caused by the acoustic radiation force is obtained. This displacement varies with the US intensity, the duration of irradiation, the US attenuation and the viscoelastic properties of the sample. US pulses of 2.5 MHz with a duration of 20 ms and an intensity of <17 W/cm(2) are used. An MRI sequence was programmed to produce images in which the magnitude of the displacement is visualized by gray value changes. In addition, a finite element simulation of the measurements was performed to demonstrate the feasibility of the method. Through examination of the measurements and the simulations, information about viscoelastic changes was achieved. In this work, measurements on different breast phantoms are presented. mrt http://www.sciencedirect.com/science/article/pii/S0301562910006186 10.1016/j.ultrasmedbio.2010.11.005 M.Radicke J.Mende A.-L.Kofahl J.Wild D.Ulucay B.Habenstein M.Deimling P.Trautner B.Weber K.Maier article 1742-6596-265-1-012018 J. Phys.: Conf. Ser. 2011 265 1 012018 We present results on the structure of nano-sized particles (Guinier-Preston (GP) and Guinier-Preston-Bagaryatsky (GPB) zones) in Aluminum alloys. Precipitates of alloying elements like Cu, Mg, or Si hinder the motion of dislocations and, thus, are responsible for the strength of AlCuMg- and AlMgSi-alloys - used e.g. as AA2024 (old aircrafts) and AA6013 for the fuselage of the new Airbus A380, respectively. We will discuss the role of quenched-in vacancies for diffusive motion at room temperature (RT) enabling the growth of the precipitates. Using positron annihilation spectroscopy (PAS) – both lifetime and Doppler broadening – gives information on the local atomic environment in the vicinity of vacancies. On the other hand X-ray absorption fine structure (XAFS) spectroscopy is capable of characterizing the local atomic environment around selected elements (Cu, Mg). We will interpret the measured data by comparing them to numerical calculations of PAS and XAFS spectra. However, reliable numerical calculations of spectroscopic quantities are only possible provided that relaxed atomic positions are used as an input. We calculate those employing the ab-initio code SIESTA. Thus, considering decomposition of Al-alloys, we obtain extremely valuable information on the earliest stages, forming immediately after solution heat treatment and quenching, i.e. during the first few minutes of storage at RT. material http://stacks.iop.org/1742-6596/265/i=1/a=012018 10.1088/1742-6596/265/1/012018 T. E. M.Staab B.Klobes I.Kohlbach B.Korff M.Haaks E.Dudzik K.Maier article 1742-6596-262-1-012030 Journal of Physics: Conference Series 2011 262 1 012030 A lot of light aluminium alloys achieve their favourable mechanical properties, especially their high strength, due to precipitation of alloying elements. This class of age hardenable Al alloys includes technologically important systems such as e.g. Al-Mg-Si or Al-Cu. During ageing different precipitates are formed according to a specific precipitation sequence, which is always directed onto the corresponding intermetallic equilibrium phase. Probing the defect state of individual precipitates requires high spatial resolution as well as high chemical sensitivity. Both can be achieved using the finely focused positron beam provided by the Bonn Positron Microprobe (BPM) [1] in combination with the High Momentum Analysis (HMA) [2]. Employing the BPM, structures in the micron range can be probed by means of the spectroscopy of the Doppler broadening of annihilation radiation (DBAR). On the basis of these prerequisites single precipitates of intermetallic phases in Al-Mg-Si and Al-Cu, i.e. Mg 2 Si and Al 2 Cu, were probed. A detailed interpretation of these measurements necessarily relies on theoretical calculations of the DBAR of possible annihilation sites. These were performed employing the DOPPLER program. However, previous to the DBAR calculation the structures, which partly contain vacancies, were relaxed using the ab-initio code SIESTA, i.e. the atomic positions in presence of a vacancy were recalculated. material http://stacks.iop.org/1742-6596/262/i=1/a=012030 B.Klobes B.Korff O.Balarisi P.Eich M.Haaks I.Kohlbach K.Maier R.Sottong T. E. M.Staab article ISI:000292331200101 J. Appl. Phys. 2011 109 12 124104 Radiation damage in magnesium-doped lithium niobate crystals, created by low-mass, high-energy ions which have transmitted the entire crystal thickness, leads to an enhanced electrical dark conductivity as well as an enhanced photoconductivity. Experimental results on the electrical properties after ion exposure are given, and an asymmetric dependence of the conductivity as well as refractive index changes on the irradiation geometry with respect to the ferroelectric axis is revealed. linbo http://jap.aip.org/resource/1/japiau/v109/i12/p124104_s1 0021-8979 10.1063/1.3596824 L.Jentjens N. L.Raeth K.Peithmann K.Maier article ISI:000288297300030 Mater. Sci. Eng., A 2011 528 <prt>7-8</prt> 3253-3260 <prt>Although Al alloys based on the Al-Cu-(Mg) system have been investigated for decades, information about the evolution of microstructure on the atomistic level during natural ageing is scarce. Therefore, the early stages of natural ageing in laboratory Al-Cu-(Mg) alloys and in AA2024 were investigated using positron annihilation and X-ray absorption spectroscopy. This complementary approach allows for accessing both essential components of Al alloys, namely vacancies and alloying elements. It is found, that during natural ageing the chemical environment of vacancies is formed both of Cu and Mg atoms and that the rearrangement of vacancy surroundings persists the hardness increase of Al-Cu-Mg alloys. During natural ageing two different regimes of vacancy environment are detected and interpreted in terms of cluster growth and vacancy capture. Features of the near edge structure of the X-ray absorption are interpreted using theoretical calculations obtained by the FEFF8.4 code. Thus the agglomeration of Cu and Mg, which is accompanied by lattice distortions around Cu atoms, can be probed. A minimal size of Cu-Mg co-clusters is proposed for later stages of natural ageing. (C) 2011 Elsevier B.V. All rights reserved.</prt> positron, material http://www.sciencedirect.com/science/article/pii/S0921509311000086 <prt>0921-5093</prt> 10.1016/j.msea.2011.01.002 B.Klobes K.Maier T. E. M.Staab article ISI:000284453300006 Scr. Mater. 2011 64 <prt>1</prt> 21-24 <prt>Positron annihilation spectroscopy was performed to study the microstructure of peak aged Al-Zn-Mg-Cu alloy Applying artificial ageing at different temperatures, the solute content around vacancies is found to be increased compared to as quenched state The similarity of vacancy surroundings of all artificially aged states, except for over-aged ones, suggests that peak strength is caused not only by eta' precipitates but also by GPII zones Moreover, eta' precipitates produced by artificial aging at 403 K are apparently coherent with the Al matrix (C) 2010 Acta Materialia Inc Published by Elsevier Ltd All rights reserved</prt> positron, material http://www.sciencedirect.com/science/article/pii/S1359646210006019 <prt>1359-6462</prt> 10.1016/j.scriptamat.2010.08.054 M.Liu B.Klobes K.Maier article springerlink:10.1007/s00340-011-4681-2 Appl. Phys. B: Lasers Opt. 2011 105 1 113-127 Ferroelectric lithium niobate crystals offer a great potential for applications in modern optics. To provide powerful optical components, tailoring of key material parameters, especially of the refractive index n and the ferroelectric domain landscape, is required. Irradiation of lithium niobate crystals with accelerated ions causes strong structured modifications in the material. The effects induced by low-mass, high-energy ions (such as ³He with 41 MeV, which are not implanted, but transmit through the entire crystal volume) are reviewed. Irradiation yields large changes of the refractive index &Delta;n , improved domain engineering capability within the material along the ion track, and waveguiding structures. The periodic modification of &Delta;n as well as the formation of periodically poled lithium niobate (PPLN) (supported by radiation damage) is described. Two-step knock-on displacement processes, 3 He&rarr;Nb and 3 He&rarr;O causing thermal spikes, are identified as origin for the material modifications. Helmholtz-Institut für Strahlen- und Kernphysik, Universität Bonn, Nussallee 14–16, 53115 Bonn, Germany linbo http://www.springerlink.com/content/c624u15604046572/ Springer Berlin / Heidelberg 0946-2171 10.1007/s00340-011-4681-2 K.Peithmann P.-D.Eversheim J.Goetze M.Haaks H.Hattermann S.Haubrich F.Hinterberger L.Jentjens W.Mader N. L.Raeth H.Schmid M.-R.Zamani-Meymian K.Maier article kohlbach:2010 phys. stat. sol. (b) 2010 247 9 2168--2178 ab initio calculations, crystal defects, positron annihilation spectroscopy, X-ray absorption positron http://onlinelibrary.wiley.com/doi/10.1002/pssb.201046102/abstract WILEY-VCH Verlag 1521-3951 10.1002/pssb.201046102 I.Kohlbach B.Korff T. E. M.Staab article ISI:000283807900017 Acta Mater. 2010 58 <prt>19</prt> 6379-6384 <prt>The influence of microalloying additions of Au to the binary Al-Cu system is investigated utilizing hardness measurements, positron annihilation spectroscopy and X-ray absorption spectroscopy. No room temperature ageing could be observed in the case of the ternary Al-Cu-Au alloy. Instead, quenched-in vacancies were trapped by Au atoms and thus could not catalyse the decomposition of the alloy. However, artificial ageing dissolves the complexes of Au and vacancies. A comparison between experimental and calculated absorption spectra shows that Au atoms did not agglomerate significantly during either natural or artificial ageing. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.</prt> positron, material http://www.sciencedirect.com/science/article/pii/S1359645410005033 <prt>1359-6454</prt> 10.1016/j.actamat.2010.07.059 B.Klobes O.Balarisi M.Liu T. E. M.Staab K.Maier article paperkalk Medical Physics 2010 37 6347-6356 mrt http://online.medphys.org/resource/1/mphya6/v37/i12/p6347_s1 10.1118/1.3512806 J.Mende J.Wild D.Ulucay M.Radicke A.-L.Kofahl B.Weber R.Krieg K.Maier article Wirtz2010 Phys. Status Solidi A 2010 207 8 1827--1831 The concentration of lattice defects in plastically,deformed metals can be measured by positron annihilation spectroscopy (PAS) with an outstanding sensitivity. The positron acts as a highly mobile atomic probe sensitive to all defects forming an open volume in the lattice. Using a positron microbeam, like the Bonn positron microprobe (BPM), the lateral distribution of these defects in the sub-surface layer can be mapped with a resolution down to one micrometer. In this work the changes in the defect concentration were determined during tension tests on the aluminium alloys AA2024, AA6013 and AA6082. The results show that these changes depend on the configuration and the heat treatment of the alloys. Moreover, alternating load fatigue tests were performed on AA6082. The defect distribution was measured laterally resolved employing the BPM in several early stages of fatigue. Using those results the number of cycles to fatigue failure was extrapolated. The trueness of the prediction was tested by further fatiguing the sample until failure occurs. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim positron, material http://onlinelibrary.wiley.com/doi/10.1002/pssa.200925540/abstract;jsessionid=FD0156421B82E863995F70B5BCB6D028.d01t03 10.1002/pssa.200925540 M.Wirtz P.Eich M.Haaks Th.Loewenhoff K.Maier article Mende2010203 Journal of Magnetic Resonance 2010 203 2 203 - 207 mrt http://www.sciencedirect.com/science/article/pii/S1090780709003747 1090-7807 10.1016/j.jmr.2009.12.019 J.Mende N.Elmiladi C.Höhl K.Maier article Klobes2010 Phys. Rev. B: Condens. Matter Mater. Phys. 2010 82 5 054113 Precipitates forming in decomposable aluminum alloys such as Al-Mg-Si evolve toward the corresponding intermetallic phase, which is beta(Mg(2)Si) in this case, depending on heat-treatment conditions. Individual beta precipitates were produced in an Al-1.11 at. % Mg-0.77 at. % Si alloy and identified using optical as well as electron microscopy. The individual beta precipitates could be investigated with regard to their intrinsic crystal defects using a finely focused positron microbeam provided by the Bonn Positron Microprobe. Comparison with theoretical calculations of the Doppler broadening of annihilation radiation reveals that beta precipitates most likely do not contain vacancies in either sublattice and that 0.16 is the upper bound of the fraction of trapped positrons. The usage of different enhancement factors had only little influence on the calculations whereas the general gradient approximation affected the contribution of Si orbitals, in particular. Additional measurements of the Doppler broadening based on the radioactive source (68)Ge, which emits high-energy positrons probing bulk regions of the sample, were carried out. These measurements show that beta precipitates are sparsely distributed in the Al matrix. positron, material http://prb.aps.org/abstract/PRB/v82/i5/e054113 10.1103/PhysRevB.82.054113 B.Klobes B.Korff O.Balarisi P.Eich M.Haaks K.Maier R.Sottong S.-M.Huehne W.Mader T. E. M.Staab article Zamani-Meymian2010 Appl. Phys. A 2010 98 4 909-912 Irradiation of congruently melting, nominally undoped lithium niobate crystals (LiNbO3) with high-energy, low-mass ³He ions, which are transmitted through the crystal, causes large and stable changes of the refractive index. In the irradiated regions, the extraordinary index is increased whereas the ordinary index is diminished. The decay of these changes upon annealing treatments up to 400°C is investigated, exhibiting a strongly non-monoexponential decay behaviour. Long-term measurements of the refractive index changes yield no pattern erasure on a timescale of four years. linbo http://www.springerlink.com/content/310746l53235q1t4/ <prt>SPRINGER</prt>

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<prt>Article</prt> 0947-8396 10.1007/s00339-009-5475-9 M.-R.Zamani-Meymian L.Jentjens N. L.Raeth K.Peithmann K.Maier article Zamani-Meymian2009 J. Phys. Condensed Matter 2009 21 7 075402 Congruently melting undoped lithium niobate crystals are irradiated with 20 MeV3 He ions which penetrate the entire crystal volume. Radiation damage effects are directly visualized by transmission electron microscopy (TEM) where damage zones with diameters of 4 nm give rise to circular Fresnel fringe contrasts. These regions of modified material, appearing circular in cross-section, are interpreted as damage cascades inflicted by fast Nb and O atoms displaced in knock-on collisions with primary He-3 ions. This two-step displacement process results in local density changes manifested by the contrast behaviour of Fresnel fringes observed in TEM images. linbo http://iopscience.iop.org/0953-8984/21/7/075402/ <prt>IOP PUBLISHING LTD</prt>

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<prt>Article</prt> <prt>0953-8984</prt> <prt>10.1088/0953-8984/21/7/075402</prt> M.-R.Zamani-Meymian K.Peithmann K.Maier H.Schmid W.Mader article springerlink:10.1007/s00340-009-3496-x Appl. Phys. B: Lasers Opt. 2009 95 3 441-445 Irradiation of 5% magnesium-doped lithium niobate crystals (LiNbO 3 :Mg) with high-energy, low-mass 3 He ions, which are transmitted through the crystal, changes the domain reversal properties of the material. This enables easier domain engineering compared to non-irradiated material and assists the formation of small-sized periodically poled domains in LiNbO 3 :Mg. Periodic domain structures exhibiting a width of ≈520 nm are obtained in radiation-damaged sections of the crystals. The ferroelectric poling behavior between irradiated and non-treated material is compared. Universität Bonn Helmholtz-Institut für Strahlen- und Kernphysik Nussallee 14-16 53115 Bonn Germany linbo http://www.springerlink.com/content/c624u15604046572/ Springer Berlin / Heidelberg 0946-2171 10.1007/s00340-009-3496-x L.Jentjens K.Peithmann K.Maier H.Steigerwald T.Jungk article staab:2009 phys. stat. sol. (b) 2009 246 7 1587--1589 61.72.jd, 61.72.Bb, 61.82.Bg positron, material http://onlinelibrary.wiley.com/doi/10.1002/pssb.200844348/abstract WILEY-VCH Verlag 1521-3951 10.1002/pssb.200844348 T. E. M.Staab article ISI:000258650100018 Phys. Status Solidi: RRL 2008 2 <prt>4</prt> 182-184 <prt>We demonstrate the sensitivity of X-ray absorption fine-structure (XAFS) measurements to the earliest stages of decomposition in Al alloys, i.e. just a few minutes after quenching. XAFS is one of the few applicable experimental approaches to this regime. Three different AlCu(Mg) samples were investigated by XAFS at the Cu K edge. Significant changes of the XAFS can be detected in the course of the decomposition in these alloys during the first 15 minutes. Actually, these changes correspond to relaxations of the nearest neighbours towards the absorbing Cu atoms. The Fourier transformation of the XAFS spectra thus leads to a pseudo radial distribution function which reflects this relaxation. In addition, XAFS measurements of the S-phase of AlCuMg are used to decide in favour of the Perlitz and Westgren model for the S-phase. (c) 2008 WILEY-VCH Verlag GmbH &amp; Co. KGaA, Weinheim.</prt> positron, material http://onlinelibrary.wiley.com/doi/10.1002/pssr.200802067/abstract <prt>1862-6254</prt> 10.1002/pssr.200802067 B.Klobes T. E. M.Staab E.Dudzik article Jentjens2008 J. Appl. Phys. 2008 103 3 034104 linbo http://link.aip.org/link/?JAP/103/034104/1 AIP 10.1063/1.2838207 L.Jentjens H.Hattermann K.Peithmann M.Haaks K.Maier M.Kösters article Staab2008 Appl. Surf. Sci. 2008 255 1 132-135 We present preliminary results on the structure of nano-sized particles (clusters of solute atoms or Guinier-Preston-Bagaryatsky (GPB) zones) in aluminum alloys. Hindering the motion of dislocations, these GPB zones (precipitates) are responsible for the strength of AlCuMg-alloys - used e.g. as AA2024 - for the outer shell plates in aircraft industry. We will discuss the role of quenched-in vacancies for the formation and growth of the precipitates. Using positron annihilation (Doppler broadening together with the recently developed tool of high-momentum analysis (HMA)) and X-ray absorption spectroscopy measurements, we are able to characterize the local atomic environment in the vicinity of vacancies and selected elements (Cu) forming the precipitates. The interpretation will be based on a comparison to numerical calculations of positron lifetimes and momentum distributions as well as simulated X-ray absorption spectra. Often reliable numerical calculations of experimental quantities are possible only if relaxed atomic positions - calculated employing an ab-initio code like SIESTA - are used as input. ((C) 2008 Elsevier B.V. All rights reserved. material http://www.sciencedirect.com/science/article/pii/S0169433208012427 10.1016/j.apsusc.2008.05.201 T. E. M.Staab M.Haaks H.Modrow article Haaks2008a Appl. Surf. Sci. 2008 255 1 84-88 Unique information about the chemical vicinity of positron annihilation sites is provided by the contribution of high electron momenta to the Doppler spectrum, since this momentum range is characteristic for the annihilation with core electrons and hence element specific. However, the corresponding energy region in the spectrum is overlaid by a huge background caused by the annihilation radiation itself and the Compton spectrum of other gamma lines having an energy above 511 keV. Usually these backgrounds are reduced by measuring both annihilation quanta in coincidence. By mathematically analyzing the background contributions, we open another possibility to obtain the high-momentum region employing one single germanium detector. A necessary precondition is employing either background-free positron beams or a low-background positron source, e.g. Ge-68, instead of the widely used positron emitter Na-22. The Ge-68-source emits positrons with an endpoint energy of about 1.9 MeV, where as the contribution of gamma quanta having higher energies than the annihilation radiation at 511 keV is negligible low. When analyzing spectra from metals and semiconductors according to the described background subtraction, the same information contained in the momentum range up to 35 x 10(3)m(0)c or beyond can be extracted, as if the spectra were measured employing a coincidence setup with two Ge-detectors. (C) 2008 Elsevier B.V. All rights reserved. positron http://www.sciencedirect.com/science/article/pii/S0169433208011859 10.1016/j.apsusc.2008.05.313 M.Haaks T. E. M.Staab article springerlink:10.1007/978-3-540-74325-5_23 Advances in Solid State Physics 2008 47 289-300 We describe an alternative approach for a reliable lifetime prediction employing the local concentration of lattice defects as a precursor for fatigue failure. We present positron annihilation spectroscopy (PAS) as a non-destructive technique sensitive for defect concentrations in the range relevant to plasticity in metals. The Bonn Positron Microprobe (BPM), a currently unique device, provides a fine focused positron beam with a selectable beam diameter from 5 to 200 µm assisted by an inbuilt fully functional scanning electron microscope (SEM). Using the BPM, plasticity and fatigue can be measured with a lateral resolution from some microns up to the range of millimeters. Employing laterally resolved PAS and the empirical supposition of a linear relation between the defect concentration and the logarithm of the number of fatigue cycles, the point of failure was successfully predicted on the common carbon steel AISI 1045. For a generalization of the precursor method, a minimal model of fatigue based on a cellular automaton was developed. First results from a one-dimensional implementation are presented. Helmholtz Institut für Strahlen- und Kernphysik, Universität Bonn, Nußallee 14-16, 53115 Bonn, Germany positron http://www.springerlink.com/content/k4lx118586wl3227/ Haug, Rolf Springer Berlin / Heidelberg Advances in Solid State Physics 978-3-540-74324-8 10.1007/978-3-540-74325-5_23 M.Haaks P.Eich J.Fingerhuth I.Müller article springerlink:10.1007/s10751-008-9628-6 Hyperfine Interactions 2008 181 21-26 When applied to a sample, ultrasound (US) gives rise to a displacement of tissue and a flow in a liquid due to the acoustic radiation pressure. These movements depend on the viscoelastic properties of the sample and can be visualized precisely with an MRI scanner using diffusion- sensitive pulse sequences. In this paper, measurements will be presented, which show the visualization of the US under variation of its parameters in different liquids and in tissue. University of Bonn Helmholtz-Institut für Strahlen-und Kernphysik Nussallee 14–16 53115 Bonn Germany mrt http://www.springerlink.com/content/0926650r6tw30w11/ Springer Netherlands 0304-3843 10.1007/s10751-008-9628-6 M.Radicke A.Engelbertz B.Habenstein M.Lewerenz O.Oehms P.Trautner B.Weber S.Wrede K.Maier article Haaks2008 Cirp Annals-Manufacturing Technology 2008 57 1 537--540 Positron annihilation spectroscopy (PAS) provides extreme sensitivity for the detection of lattice defects from a concentration of 10(-6) defects per atom. PAS is a versatile and non-destructive tool for the study of plasticity and fatigue in solid-state materials. Scanning positron microscopy (SPM) expands the capabilities of PAS into the micron range. Recent results of defect imaging by SPM on plastically deformed and fatigued metals and semiconductors will be presented in this paper. A new method estimating the remaining useful life of fatigued components by employing the S-parameter as a precursor for failure will be introduced. (C) 2008 CIRP. positron, material http://www.sciencedirect.com/science/article/pii/S0007850608001121 10.1016/j.cirp.2008.03.109 M.Haaks article klobes:2008 phys. stat. sol. (RRL) 2008 2 5 224--226 The very early stages of decomposition during room temperature storage, i.e. just a few minutes after quenching, are investigated by positron annihilation lifetime spectroscopy for both an AlMgSi alloy and an AlCuMg alloy. It turns out that by freezing the decomposition kinetics during measurements we can detect vacancy–solute atom pairs. The formation of larger solute clusters with structural vacancies is seen by an increase of the mean positron lifetime in the course of storage at room temperature (RT). Earlier findings concerning aging at RT were unable to discover this effect. The detected changes are interpreted in terms of cluster formation. Thus we show that positron annihilation spectroscopy (PAS) is one of the very few methods to access early stages of decomposition in metallic alloys. Moreover, the lower limit of the concentration of quenched-in vacancy-like defects is calculated to be at least 2 × 10^–5 per atom. 61.43.Dq, 61.72.jd, 64.75.Op, 78.70.Bj positron, material http://onlinelibrary.wiley.com/doi/10.1002/pssr.200802123/abstract WILEY-VCH Verlag 1862-6270 10.1002/pssr.200802123 B.Klobes T. E. M.Staab M.Haaks K.Maier I.Wieler article Staab2007 Phys. Status Solidi: RRL 2007 1 5 172--174 We propose X-ray absorption (XAS) measurements as a novel approach to determine the atomic structure of pre-Guinier-Preston zones. These nano-clusters are formed during very early stages of aging AlCu alloys, immediately after solution heat treatment and quenching. X-ray absorption near-edge structure (XANES) spectra were taken from technical aluminum alloys at the copper K edge, revealing the local atomic environment of copper. The spectra of - after solution heat treatment - freshly quenched and of artificially aged alloys differ significantly from each other. We compare the measured XANES spectra with those calculated by the FEFF-8 code. We show the importance of employing realistic, i.e. relaxed, atomic coordinates around the scattering atom type from ab-initio calculations with SIESTA. Atomic structure of a pre-GPZ from SIESTA: a small disk of five Cu atoms (red) on a <prt>100</prt> plane in fcc Al (blue). Note that Al atoms in direct neighborhood of copper relax towards the small Cu disk. (online colour at: www.pss-rapid.com) (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. material http://onlinelibrary.wiley.com/doi/10.1002/pssr.200701090/abstract 10.1002/pssr.200701090 T. E. M.Staab M.Haaks C.Zamponi H.Modrow K.Maier article PSSC:PSSC200675869 Phys. Status Solidi C 2007 4 10 4036--4039 Lutetium oxyorthosilicate (LSO) was employed as scintillator material in a positron lifetime (PALS) fast-slow spectrometer. Compared to the widely used BaF2 scintillator material, LSO has a single exponential decay with 40 ns, a high density of 7.49 g/cm3, an effective nuclear charge of 65.8 and a superb light output. With this features photon rates above 5 × 106 should be possible, which affords to obtain lifetime spectra within a period of minutes. The physical and mechanical properties make LSO attractive as scintillator material for PALS measurements. The capability of LSO for PALS was studied and compared to BaF2. As a first test of the spectrometer, a lifetime spectrum of well annealed nickel was measured and analyzed. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) 29.40.Mc, 78.70.Bj positron http://dx.doi.org/10.1002/pssc.200675869 WILEY-VCH Verlag 1610-1642 10.1002/pssc.200675869 M.Haaks R.Valentini R.Vianden article PSSC:PSSC200675814 Phys. Status Solidi C 2007 4 10 3465--3468 Several samples of the common ferritic tool steel AISI 1045 were fatigued in cyclic load tests. The local distribution of the Von-Mieses stress σVM was simulated using the finite elements method (FEM). In the regions of interest, where σVM reaches maximum values, the defect distribution was measured spatially resolved by Doppler-spectroscopy (DBAR) employing the Bonn Positron Microprobe (BPM). The lateral distribution of the S-parameter, which could be described by a simple model derived from linear fracture mechanics, corresponds well with the simulated Von-Mieses stress. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) 07.78.+s, 47.11.Fg, 62.20.Fe, 62.20.Mk, 78.70.Bj positron, material http://dx.doi.org/10.1002/pssc.200675814 WILEY-VCH Verlag 1610-1642 10.1002/pssc.200675814 P.Eich M.Haaks R.Sindelar K.Maier article haaks:2006 Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 2006 569 3 829 - 836 Positron annihilation positron http://www.sciencedirect.com/science/article/B6TJM-4KY8785-G/2/b876b6af4babf991b360ef15a1cf65f1 0168-9002 10.1016/j.nima.2006.08.117 M.Haaks T. E. M.Staab K.Maier article Peithmann2006b Appl. Phys. B 2006 82 3 419-422 Irradiation of lithium-niobate crystals (LiNbO3) with fast, high-energy 3He ions changes the refractive index in the interaction region where the ions speed through the material. Thus an inhomogeneous flux density profile can be used for a tailored modification of the optical properties of LiNbO3 crystals, without employing ion implantation. A new method to fabricate embedded, polarization sensitive channel waveguides in LiNbO3 utilizing accelerated 3He ions with an energy of 40 MeV is demonstrated. linbo http://www.springerlink.com/content/ymmmw7g171131779/ 10.1007/s00340-005-2002-3 K.Peithmann M.-R.Zamani-Meymian M.Haaks K.Maier B.Andreas K.Buse H.Modrow article Peithmann2006a J. Opt. Soc. Am. B 2006 23 10 2107-2112 Irradiation of lithium niobate crystals with 41 MeV ³He ions causes strong changes of the ordinary and extraordinary refractive indexes. We present a detailed study of this effect. Small fluence of irradiation already yields refractive index changes about 5×10⁻⁴; the highest values reach 3×10⁻³. These index modulations are stable up to 100°C and can be erased thermally, for which temperatures up to 500°C are required. A direct correlation between the refractive index changes and the produced lattice vacancies is found. linbo http://www.opticsinfobase.org/abstract.cfm?URI=josab-23-10-2107 10.1364/JOSAB.23.002107 K.Peithmann M.-R.Zamani-Meymian M.Haaks K.Maier B.Andreas I.Breunig article Haaks2006 Phys. Status Solidi A 2006 203 4 R31--R33 Spatially resolved studies on plasticity in a polycrystalline sample of the ferritic carbon steel C45E (ATSI 1045) were performed after deformation in a three-point bending test. The local effects of deformation were investigated by scanning the sample with two different methods: Positron Annihilation Spectroscopy (S-parameter) and Debye-Scherrer diffraction (reflex broadening). A simple relation between the results of both experiments and the true strain over the cross-section of the bent sample is presented in this letter. Comparing the methods a linear correlation between the lattice distortion of a-iron and the defect sensitive positron annihilation parameter is found. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. positron, material http://onlinelibrary.wiley.com/doi/10.1002/pssa.200622063/abstract 10.1002/pssa.200622063 M.Haaks I.Müller A.Schoeps H.Franz article staab:2006 J. Mater. Sci. 2006 41 1059-1066 Martin-Luther Universität Halle-Wittenberg Department of Physics Friedemann-Bach-Platz 6 D-06108 Halle/Saale Germany Friedemann-Bach-Platz 6 D-06108 Halle/Saale Germany 10.1007/s10853-005-3640-x positron, material http://www.springerlink.com/content/g227068l7m16v451/ Springer Netherlands 0022-2461 10.1007/s10853-005-3640-x T.Staab R.Krause-Rehberg U.Hornauer E.Zschech article springerlink:10.1007/s10853-005-3640-x Journal of Materials Science 2006 41 1059-1066 We investigate the artificial aging process of the weldable Al-alloys AA-6013 (AlMgSiCu) and AA-6061 (AlMgSi) by positron annihilation spectroscopy and Vickers micro-hardness measurements. We find a correlation between a minimum of the measured positron lifetime and the maximum hardness. To explain these findings AlZn will be discussed as a well known example of positron interaction with different types of precipitations in aluminum. Additionally, we calculate positron annihilation parameters for pure Al, Si, Mg, and \mbox<prt>Mg</prt>_2\mbox<prt>Si</prt> finding support for our interpretation. For AA-6013 and AA-6061 the increase of the average positron lifetime after reaching the maximum hardness is attributed to the formation of semi-coherent precipitates. Hence, using positron annihilation it is possible to follow very sensitively the precipitation sequence even in technical aluminum alloys. Finally, we discuss which implications the aging may have on microstructural changes. Martin-Luther Universität Halle-Wittenberg Department of Physics Friedemann-Bach-Platz 6 D-06108 Halle/Saale Germany Friedemann-Bach-Platz 6 D-06108 Halle/Saale Germany positron, material http://www.springerlink.com/content/g227068l7m16v451 Springer Netherlands 0022-2461 10.1007/s10853-005-3640-x T.Staab R.Krause-Rehberg U.Hornauer E.Zschech article PhysRevLett.95.125502 Phys. Rev. Lett. 2005 95 125502 material http://link.aps.org/doi/10.1103/PhysRevLett.95.125502 American Physical Society 10.1103/PhysRevLett.95.125502 T. E. M.Staab R. M.Nieminen M.Luysberg Th.Frauenheim article Haaks2005 Phys. Status Solidi A 2005 202 4 R38--R40 We present a novel method for analyzing the data from momentum distribution measurements employing positron annihilation, using just one single Ge-detector, but employing a mathematically sufficiently well suited model for background subtraction. For a variety of high purity metals the resulting momentum distributions have exactly the same shapes up to 35 x 10(-3) m(0)c, as those obtained with coincidence Doppler broadening of the annihilation radiation using a two-detector system. This data analysis applies to any positron source without high energy gammas or a slow positron beam system with just a single Ge-detector and enables the possibility of analysing the chemical surrounding at the positron trapping site without any additional hardware. In particular, in combination with a slow positron microbeam, it allows a spatially resolved measurement of the electron's momentum distribution. (c) 2005 WILEY-VCH Verlag GmbH W Co. positron http://onlinelibrary.wiley.com/doi/10.1002/pssa.200510004/abstract 10.1002/pssa.200510004 M.Haaks T. E. M.Staab K.Saarinen K.Maier article Zamponi2004 J. Mater. Sci. 2004 39 23 6951--6956 Fatigue cracks have been generated in the commercial aluminium alloys AA2024 and AA6013 and analysed with a positron microprobe. This instrument provides laterally resolved positron annihilation measurements, which are sensitive to lattice defects like vacancies and dislocations. These commercial alloys have undergone a solution heat treatment and quenching prior to fatigue testing. Subsequently, they have been aged at room temperature and 190degreesC for AA2024 and AA6013, respectively. We performed the fatigue crack generation both in air and under the influence of a chemically aggressive environment (artificial seawater). Due to the corrosive environment hydrogen is probably produced at the fresh fractured surface in the vicinity of the crack tip. We discuss the possible implications of in-diffused hydrogen on the produced lattice defects, especially when there is a delayed migration of vacancies in the lattice, due to a reduced mobility. (C) 2004 Kluwer Academic Publishers. positron, material http://www.springerlink.com/content/ru611m96v58t4024/ 10.1023/B:JMSC.0000047537.16498.1d C.Zamponi S.Sonneberger M.Haaks I.Müller T.Staab article Andreas2004 Appl. Phys. Lett. 2004 84 19 3813 We report reductions of the refractive index of congruently melting lithium niobate crystals of up to 6×10⁻³ by exposure of z-cut samples with high-energy ⁴He²⁺ and D⁺ particles which are transmitted through the crystals. linbo http://apl.aip.org/resource/1/applab/v84/i19/p3813_s1 10.1063/1.1741031 B.Andreas K.Peithmann K.Buse K.Maier article Muller2004 Mater. Sci. Forum 2004 445-6 498-500 For the construction of heavy duty mechanical components it is essential to know the lifetime of the sample under dynamical load conditions. Material fatigue tests in rail vehicle axles were first made in the year 1858 by August Wohler [1]. Up to now the measurement of a Wohler diagram remains extraordinarily time and money consuming. Since iron based materials fail at a critical defect density independent of cycle number and load, we are able to extrapolate the remaining lifetime of the sample from a few cycles. With positron annihilation measurements employing the Bonn Positron Microprobe different stress states in standard samples can be observed as a function of position [2,3,4]. This leads to a novel non-destructive way to determine a complete Wohler diagram (SN-diagram) with just one sample. positron, material http://www.scientific.net/MSF.445-446.498 10.4028/www.scientific.net/MSF.445-446.498 I.Müller K.Bennewitz M.Haaks T. E. M.Staab S.Eisenberg T.Lampe K.Maier article Haaks2004 Mater. Sci. Forum 2004 445-6 96-98 We have performed a study of the temperature dependence of the S-parameter on slightly deformed (7%) copper over a temperature range from room temperature (300 K) down to 7 mK, using a (3)He/(4)He dilution refrigerator. The sample used is expected to contain a spectrum of lattice defects with both deep and shallow traps. The data show a decrease of the S-parameter with temperature down to about 1K and a slight increase below 800 mK. positron http://www.scientific.net/MSF.445-446.96 10.4028/www.scientific.net/MSF.445-446.96 M.Haaks C.Tramm I.Müller T.Staab C.Zamponi P.Herzog K.Maier article Staab2004 Mater. Sci. Forum 2004 445-6 510-512 Spatially resolved positron annihilation spectroscopy has been used for materials science investigations. In undoped semi-insulating GaAs we investigated the sub-surface damage due to scratches produced with a single-diamond grain. We. clearly identify different regions of damage, which have been characterized both by scanning electron microscopy and the positron microbeam. The. latter reveals signs of plastic deformation in front of the scratch indicated by the defects detected. The observed ductile behavior of GaAs at room temperature is discussed, considering how this usually very brittle material can undergo a brittle-to-ductile transition when exposed to high pressure. positron, material www.scientific.net/MSF.445-446.510 10.4028/www.scientific.net/MSF.445-446.510 T. E. M.Staab C.Zamponi M.Haaks I.Müller S.Eichler K.Maier article Bennewitz20020 Zeitschrift für Metallkunde 2002 93 8 778--783 The fatigue behavior of iron-based materials has been investigated by rotating bending testing, employing positron annihilation spectroscopy to probe defects on the atomic level. Positron annihilation spectra have been recorded at various stages of material fatigue. The defect density has been determined by analysing the line shape of the Doppler broadening of the annihilation radiation in the photo peak. The line shape parameter (S parameter), a measure of the defect density, showed a linear relation to the logarithm of the number of loadings, thus from only a small number of loadings it is possible to determine the remaining useful life of the sample. Furthermore, along the longitudinal sample axis spatially resolved line-scans are taken using the Bonn Positron Microprobe. Due to the special sample geometry, the stress gradient allows to obtain the S parameter for different values of the applied load using the very same sample. This leads to a way to determine a complete Wohler diagram using a non-destructive method for just one sample. positron K.Bennewitz M.Haaks T.Staab S.Eisenberg T.Lampe K.Maier article Holzwarth2001 Appl. Phys. A: Mater. Sci. Process. 2001 73 4 467--475 Positron lifetime measurements were carried out at room temperature before and after isochronous annealing of cylindrical, machined fatigue specimens and of round slabs of austenitic stainless steel AISI 316 L deformed in compression. Annealing experiments are evaluated in terms of vacancy migration and sinking to grain boundaries and dislocations. The model assumes spherical grains with a homogeneous initial distribution of vacancies. A vacancy migration enthalpy of H-V(M) = (0.9 +/-0.15) eV was found. It is concluded that positron trapping at dislocation lines does not significantly contribute to positron lifetime measurements at room temperature and that single vacancies are the dominating positron traps. Positron annihilation depth profiling on cross-sectional areas prepared from machined specimens using a positron microprobe with 10 mum spatial resolution shows that machining of cylindrical specimens creates vacancies up to 5 mm below the surface. positron http://www.springerlink.com/content/ccl9fjychy51w7k3/ 10.1007/s003390100781 U.Holzwarth A.Barbieri S.Hansen-Ilzhöfer P.Schaaff M.Haaks article Mannig1999 Appl. Surf. Sci. 1999 149 1-4 217--220 In cooperation with the ZEISS/LEO GmbH a positron microprobe has been constructed. Additionally, a conventional scanning electron microscope (SEM) is integrated in the setup. Measurements on radiation defects in Mo and Cu samples, made by 1 MeV electron irradiation in a transmission electron microscope (TEM), are presented. The results coincide with the expected damage profiles. Because of the small beam diameter-the great advantage of the positron microprobe-only very small areas (about 20 to 30 mu m in diameter) have to be damaged. Therefore, the irradiation times required to produce samples for investigations of radiation defects caused by electrons are reduced enormously. This means that much higher defect concentrations can be produced in an acceptable time and the positron measurements can cover a wider range of defect concentrations. (C) 1999 Elsevier Science B.V. All rights reserved. positron http://www.sciencedirect.com/science/article/pii/S0169433299002044 10.1016/S0169-4332(99)00204-4 U.Männig K.Bennewitz H.Bihr M.Haaks W.Sigle C.Zamponi K.Maier article haaks:1999 Applied Surface Science 1999 149 1-4 207 - 210 Positron positron, material http://www.sciencedirect.com/science/article/B6THY-3Y9RHVF-1F/2/cbeb57b4666f7bccedb3610ba1e9e2e5 0169-4332 10.1016/S0169-4332(99)00202-0 M.Haaks K.Bennewitz H.Bihr U.Männig C.Zamponi K.Maier article Greif1997 Mater. Sci. Forum 1997 255-2 641-643 In cooperation with the ZEISS/LEO GmbH a positron microprobe has been developed which allows positron annihilation measurements with a spatial resolution in the micron range. Therefore, a commercially available scanning electron microscope (SEM) has been modified and equipped with an additional facility for positron-annihilation measurements. A Na-22 source and a cylindrical tungsten moderator have been integrated in the electron optical system of the SEM which now may be used in the conventional SEM-mode to image a specimen and in another mode to perform local positron-annihilation measurements. As a first test the fatigue damage in a copper plate after a three-point bending test was investigated with the positron microbeam. The profile of the S-parameter along the cross section of the copper plate coincides well with the expected feature of the damage distribution. positron http://www.scientific.net/MSF.255-257.641 10.4028/www.scientific.net/MSF.255-257.641 H.Greif M.Haaks U.Holzwarth U.Männig M.Tongbhoyai K.Maier article Greif1997a Appl. Phys. Lett. 1997 71 15 2115--2117 In cooperation with Zeiss/LEO GmbH, a monoenergetic positron source has been integrated in the electron optical system of a scanning electron microscope by help of a magnetic prism, The electron optics serves both to image the specimen with electrons and to form a positron microbeam that allows local positron-annihilation measurements with a resolution in the micron range. The fatigue damage profile along the cross section of a copper plate after a three-point bending test has been investigated. The obtained S-parameter profile coincides well with the expected fatigue damage distribution. (C) 1997 American Institute of Physics. positron http://apl.aip.org/resource/1/applab/v71/i15/p2115_s1 10.1063/1.120451 H.Greif M.Haaks U.Holzwarth U.Männig M.Tongbhoyai T.Wider K.Maier J.Bihr B.Huber article Bihr1997 Eur. J. Cell Biol. 1997 74 69--69 positron J.Bihr H.Greif M.Haaks U.Holzwarth B.Huber U.Männig M.Tongbhoyai K.Maier