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, .
|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)|
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 tungsten 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 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 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 of the positron beam facility SPONSOR.
- Conventional Positron Annihikation Lifetime Spectroscopy - LT
In order to perform pre-characterizations of the samples and to allow for beam-independent measurements we use a conventional positron-annihilatoin lifetime setup which is again fed by the radioactive decay of Na-22.
- 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 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-Luther-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.
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