Positron-Annihilation Lifetime Spectroscopy for Materials Science

Early experiments on the interaction of positrons (the anti-particles of electrons) with materials revealed a significant sensitivity on the electronic structure. Especially, open-volume defects, such as vacancies, vacancy agglomerates, and dislocations cause attractive electric potentials due to the lack of the repulsive positive potential of the nuclei. With diffusion lengths in the order of 100 nm positrons probe large volumes before getting trapped at positively charged defects which in turn results in a sensitivity of defect concentrations of about 1 in 107 atoms (in metals) . While annihilation lifetimes increase with increasing defect sizes due to reduced local electron densities, one can also infer the momentum-distributions of the annihilation electrons which in turn tell about the chemical compositions in the vicinity of defects. Doppler-broadening spectroscopy and annihilation lifetime spectroscopy have therefore found widespread applications in defect studies in pure metals, alloys, and semiconductors. With increasing defect sizes the formation of the electron-positron bound state – called Positronium (Ps) – becomes possible. While the spin-parallel triplet state has a vacuum annihilation lifetime of 142 ns, this annihilation lifetime gets reduced when the Ps bounces off the walls of porous materials and flipping to the spin-singlet state with 125 ps. In contrast to standard intrusion techniques, porosimetry studies with positrons can be applied for closed porosity as well.
The Helmholtz-Center at Dresden-Rossendorf operates several user beamlines for materials research employing positron annihilation. SPONSOR (Slow POsitroN System Of Rossendorf) uses moderated positrons from 22Na decay which are post-accelerated to energies from 27 eV to 37 keV which are guided magnetically towards the samples under study [1]. The energy dependent range allows performing depth-dependent (coincidence) Doppler-broadening spectroscopy of thin films with thicknesses up to about 1 µm. SPONSOR has been extended by a new installation called AIDA (Apparatus for In-Situ Defect Analysis) which additionally allows temperature-dependent positron annihilation spectroscopy (PAS) from 50 to 1200 K, in-situ ion irradiation and sputtering with noble and reactive gases (up to 5keV ion energy), thin film deposition (Molecular Beam Epitaxy), and four-point probe resistometry. First experiments with this facility on open volume defects in Fe60Al40 alloys have been performed and the results will be presented [2]. Two other user facilities dedicated to positron annihilation lifetime and Doppler-broadening studies in materials research are being operated at a superconducting electron linear accelerator. Hard X-rays from electron-bremsstrahlung generate positrons from pair production. Both installations employ bunched continuous-wave (CW) electron beams with energies between 15 MeV and 30 MeV. The CW-operation results in significantly reduced pile-up effects in the detectors in comparison to normal conducting accelerators. Electron bunch lengths below 10 ps FWHM allows positron annihilation lifetime spectroscopy measurements with high timing resolutions. The bunch repetition rate is adjustable to 26 MHz / 2n, n=0, 1, 2 ... 16 matching wide spans in positron or positronium lifetimes. The GiPS (Gamma-induced Positron Source) generates energetic electron-positron pairs inside the sample under investigation from hard x-rays impinging onto the sample [3]. Therefore, the source is especially suited for materials which are not qualified for vacuum conditions or because they are imposing hazardous conditions or intrinsic radioactivity. Exemplary defect studies on the skyrmoin-lattice compound MnSi [4] will be presented. MePS (the Monoenergetic Positron Source) utilizes positrons with fixed energies ranging from 500 eV to 16 keV[3]. A magnetic beam transport system guides positrons to the samples under investigation. A dedicated chopper/buncher system is used to maintain a high timing resolution for depth-dependent annihilation lifetime studies in thin films. The signal-to-noise ratio is beyond 104 while lifetime resolutions of around 280 ps FWHM have been obtained. Applications of porosimetric studies in low-k dielectrics will be presented. [6].
The MePS facility has partly been funded by the Federal Ministry of Education and Research (BMBF) with the grant PosiAnalyse (05K2013). The initial AIDA system was funded by the Impulse- und Networking fund of the Helmholtz-Association (FKZ VH-VI-442 Memriox). The AIDA facility was funded through the Helmholtz Energy Materials Characterization Platform.

[1] W. Anwand, et al., Defect and Diffusion Forum Vl. 331 25 (2012).
[2] M. O. Liedke, et al., Journal of Applied Physics 117 163908 (2015).
[3] M. Butterling, et al., Nuclear Instruments and Methods in Physics Research B 269, 2623 (2011).
[4] M. Reiner, et al., Scientific Reports 6, 29109 (2016).
[5] M. Jungmann, et al., Journal of Physics: Conference Series 443, 012088 (2013)
[6] A. Uedono, et al., Applied Surface Science 368, 272 (2016).