Contact

Dr. Andreas Wagner
Head Nuclear Physics
a.wagnerAthzdr.de
Phone: +49 351 260 - 3261
Fax: +49 351 260 - 13261

Wolfgang Anwand
Nuclear Physics
w.anwandAthzdr.de

Prof. Reinhard Krause-Rehberg
Nuclear Physics
reinhard.krause-rehbergAtphysik.uni-halle.de

Dr. Maciej Oskar Liedke
Postdoc
Nuclear Physics
m.liedkeAthzdr.de
Phone: +49 351 260 - 2117

Dr. Kay Potzger
Head Interface Magnetism
Project-group head
k.potzgerAthzdr.de
Phone: +49 351 260 - 3244, 2411
Fax: +49 351 260 - 13244

orangener PfeilPositron Studies of Defects (PSD-17), HZDR 3. - 9. September, 2017

The aims of this workshop are to bring together positron scientists who interested in studying defects in materials for various applications and to provide an international platform to present and discuss recent results and achievements as well as on new experimental and theoretical methods in this field.

orangener PfeilWorkshop on Methods of Porosimetry and Applications, HZDR, 21.-23.10.2015

The workshop focussed on various methods of porosimetry, such as N2 absotrption, Hg-intrusion, SAXS, SANS, and positron annihilation.

Eye catcher

Positron Annihilation Spectroscopy at the HZDR

Positron annihilation spectroscopy allows studying a variety of phenomena and material properties on an atomic scale. Being the anti-particle of electrons, positrons are used to probe material defects at low concentrations and  with high sensitivity. With the advantage of being a non-destructive materials research method, positron annihilation has been developed as a well established  tool for investigations of metals, semiconductors, polymers and porous materials.

When a positron hits an electron, both particles annihilate into electromagnetic radiation which is emitted as two or three photons depending on the relative spin orientation of positron and electron. In the two-photon case both photons are emitted back-to-back with an energy of about 511 keV. In positron-annihilation spectroscopy we detect the annihilation photons and derive informations about defects in crystalline materials.

Different material defects, like dislocations, grain boundaries, single or cluster vacancies, voids, alter the energy spectrum of the emitted photons and the lifetime of positrons in characteristic ways due to varying momentum distributions of the annihilating electron and their concentration. The lack positively charged atom cores at defects generates a local repulsive potential for positrons which leads to trapping at neutral or negatively charged defects. This specific process allows studying very low defect concentrations and defect sizes on the nm-scale.

Two positron-annihilation techniques are being employed at HZDR 

  • Positron-annihilations lifetime-spectroscopy (PALS) measures the elapsed time between the implantation of the positron into the material and the emission of annihilation radiation. Positrons are trapped preferentially in atomic defects which in turn have a locally smaller electron density leading to an extended positron lifetime. The PALS technique therefore is a sensitive method to derive sizes and concentration of vacancy-type defects like nano-cavities. The positron annihilation lifetime has a characteristic value for all elemental materials and defects. As an example: defect-free iron shows a positron lifetime of 108 ps while the single-atom vacancy shows 175 ps.
  • Doppler-broadening spectroscopy (DBS) employs the energy-momentum conservation during positron annihilation. The momentum of the electron-positron pair prior to annihilation is being transferred to the anniliation quanta. In the case of two-photon annihilation the 511 keV photons are slightly but significantly shifted in energy in the laboratory frame resembling a Doppler-effect. Since the main contribution of the electron-positron momentum stems from the orbital momentum of the electron, DBS is a sensitive probe for the local chemical surrounding of defects. Both, decorations of defects with impurity atoms and precipitations of materials in alloys can be investigated.

Depending on the initial energy positrons are implanted into the material with a certain range distribution (Makhov-distribution). After slowing down to thermal energies in a few picoseconds, positrons diffuse inside the material on a typical scale of 10 - 100 nm until they are being trapped in defects. Using monoenergetic positron beams, depth-dependent defect characterization of thin films can be performed.

Smallest atomic defects in crystals, metals, semiconductors and polymers can be investigated, like voids on nm-scales, but also chemical structures in fluids and biological systems.

In close collaboration with the Center for Material Science at Martin-Luther-University Halle-Wittenberg various setups are employed in positron annihilation spectroscopy aiming both at the fundamental understanding of condensed matter and defect formation and applied research for material durability, intended defect-engineering, electronic components, .

Positrons - theory 8
Sensitivity of positron-annihilation spectroskopy in comparison to other standard techniques used in materials research (left). Fate of a positron after implantation into host material (right)
Illustration: Maik Butterling (Download)

Neues ELBE-Logo

Positrons at the ELBE-center

Several setups with complementary specifications are used in in-house research or by external users. The ELBE Positron Source (EPOS) is a unique combination of five different setups available at the ELBE-center for high-power radiation sources. Two setups use the high-intensity electron beam from a superconducting electron LINAC as a driver for secondary positron production.

  • Mono-energetic Positron Spectroscopy – MePS: From the primary ELBE electron beam a monoenergetic positron beam is created by pair production at a thungsten target. The unique time structure of the ELBE beam is thereby transfered on the positron beam which results in a pulsed positron source with high repetition rate, high intensity and selectable implantation energies. With this beam measurements at surfaces and thin layers can be done performed with high depth resolution.
    Parameters of the positron beam  
    kinetic energy 0.5 - 15 keV
    pulse length 250 ps FWHM
    repetition rate 1.625 - 13 MHz
    positron flux 106 / s

    Schematics MePS positron beam

    Schematics of the positron beam facility MePS.
  • Gamma-induced Positron Spectroscopy – GiPS: At the bremsstrahlung facility a beam of photons from bremsstrahlung production is created. When hitting the sample positrons from pair production are generrated throughout the entire sample volume. Pairs of annihlation-quanta are detected by four sets of Germanium detectors with high energy resolution and Barium-Fluoride detectors with high timing resolution. The setup is suitable for thick samples (>= 1cm³) of solids, liquids, bilogical samples, and even gases. Due to the efficient background-suppression, samples with intrinsic radioactivity (like for example reactor pressure vessel steels) can ebe investigated as well.
    Parameters of the GiPS facility  
    Photon energies max. 16 MeV
    pulse length ~ 10 ps
    repetition rate 26 MHz / 2n n=0... 6
     

    Schematics GiPS facility

    Schematics of the positron annihilation lifetime spectroscopy system GiPS.
  • Source-Based Positron Spectroscopy: Souce-based positron-sources (like ²²Na) are available for complementary and off-beam lifetime measurements (LT) and for depth-dependent Doppler-broadening spectroscopy using a monoenergetic positron beam (SPONSOR).

    Schematics SPONSOR positron beam

    Schematics of the positron beam facility SPONSOR.
  • In-situ Characterisation of Defects: AIDA
    An new facility called "Apparatus for in-situ Defect Analysis" has been set-up which allows positron annihilation spectroscopy to be applied on thin functional films which are created using ion implantation or vapor deposition. Key point is the possibility to perform these investigations near to the material surface on the atomic scale and in very early stages of defect formation which shall lead to a deeper understanding on the dynamics of defect generation.

All three systems have different thematic priorities and cover as part of the EPOS system standard PAS techniques. Because of the unique time structure of the positron beam both MEPS and GiPS are not limited to (Coincidence) Doppler Broadening Spectroscopy (DBS and CDBS), but also for lifetime spectroscopy and the age-momentum correlation called AMOC. The high intensity of the beam accounts for short measurement time which make it possible to study temperature depending behaviour and maybe even dynamic transitions.The EPOS system has been realized by the Interdisciplinary Center of Materials Science (CMAT) of the Martin-Lither-University in cooperation with the Helmholtz-Zentrum Dresden-Rossendorf. It is devoted to fundamental research, materials research and it's facilities are explicitely open to external users.

Recent publications

Induced conductivity in sol-gel ZnO films by passivation or elimination of Zn vacancies
Winarski, D. J.; Anwand, W.; Wagner, A.; Saadatkia, P.; Selim, F. A.; Allen, M.; Wenner, B.; Leedy, K.; Allen, J.; Tetlak, S.; Look, D. C.

ZnO Luminescence and scintillation studied via photoexcitation, X-ray excitation, and gamma-induced positron spectroscopy
Ji, J.; Colosimo, A. M.; Anwand, W.; Boatner, L. A.; Wagner, A.; Stepanov, P. S.; Trinh, T. T.; Liedke, M. O.; Krause-Rehberg, R.; Cowan, T. E.; Selim, F. A.

Defect studies of Mg films deposited on various substrates
Hruška, P.; Čížek, J.; Anwand, W.; Bulíř, J.; Drahokoupil, J.; Stráská, J.; Melikhova, O.; Procházka, I.; Lančok, J.

Surface sealing using self-assembled monolayers and its effect on metal diffusion in porous low-k dielectrics studied using monoenergetic positron beams
Uedono, A.; Armini, S.; Zhang, Y.; Kakizaki, T.; Krause-Rehberg, R.; Anwand, W.; Wagner, A.

Defects in zinc oxide grown by pulsed laser deposition
Ling, F. C. C.; Wang, Z.; Ho, L. P.; Younas, M.; Anwand, W.; Wagner, A.; Su, S. C.; Shan, C. X.

Positron spectroscopy of point defects in the skyrmion-lattice compound MnSi
Reiner, M.; Bauer, A.; Leitner, M.; Gigl, T.; Anwand, W.; Butterling, M.; Wagner, A.; Kudejova, P.; Pfleiderer, C.; Hugenschmidt, C.

Characterisation of irradiation-induced defects in ZnO single crystals
Prochazka, I.; Cizek, J.; Lukac, F.; Melikhova, O.; Valenta, J.; Havranek, V.; Anwand, W.; Skuratov, V. A.; Strukova, T. S.

Contact

Dr. Andreas Wagner
Head Nuclear Physics
a.wagnerAthzdr.de
Phone: +49 351 260 - 3261
Fax: +49 351 260 - 13261

Wolfgang Anwand
Nuclear Physics
w.anwandAthzdr.de

Prof. Reinhard Krause-Rehberg
Nuclear Physics
reinhard.krause-rehbergAtphysik.uni-halle.de

Dr. Maciej Oskar Liedke
Postdoc
Nuclear Physics
m.liedkeAthzdr.de
Phone: +49 351 260 - 2117

Dr. Kay Potzger
Head Interface Magnetism
Project-group head
k.potzgerAthzdr.de
Phone: +49 351 260 - 3244, 2411
Fax: +49 351 260 - 13244