SPONSOR - The Slow-Positron System of Rossendorf
SPONSOR is a device for depth-resolved materials investigation with atomic resolution using Slow-Positron Doppler Broadening Spectroscopy. Positrons are generated through ß+ decays of the radioisotope 22Na and they are slowed down using a tungsten moderator foil. The obtained mono-energetic positrons are then accelerated towards the sample with typical energies in the range of 30 eV to ~ 36 keV.
Positron implantation spectroscopy
Magnetically guided positrons from a 22Na source of predetermined energies E (30 eV - 36 keV) are implanted at depths of up to a few micrometer into the sample. Then, the positrons reduce their energy within a few pico-seconds via inelastic scattering (thermalization) and diffuse through the lattice (10 ... 100 nm depending on trapping defects) until they annihilate with an electron. The resulting implantation profile can be described using an energy- and depth-depending Makhovian distribution .
The motion of positron-electron pairs prior to annihilation causes a Doppler broadening of the photopeak in the measured energy spectrum of the annihilation photons characterized by the lineshape parameters S and W. The value of S depends on the size and concentration of vacancy-like defects and is higher for an increasing amount of positrons being trapped at and annihilating in open-volume defects. The W parameter (“wing” or core annihilation parameter) is taken at the high-momentum region away from the center. Since the positron annihilation takes place with core electrons, the W parameter probes the chemical surrounding of the annihilation site. The parameters S and W are calculated as the normalized area of the curve in a fixed energy interval. The correlation between both parameters varies for different defect types.
Left: Implantation profiles for positrons in silicon for various incident energies.
Right: Definition of line shape parameters S and W for the 511 keV two-photon annihilation quanta.
Fitting routines like VEPFIT  can be used to obtain the positron diffusion length from the measured S-parameter curve. This parameter allows for further characterization of defects and can be used to determine the quality of a material. A combination of the diffusion length with the energy-dependent positron implantation profiles helps to obtain and visualize a depth-resolved defect profile.
Left: Example of an experimentally determined S parameter for varying positron implantation energies.
Right: Taking into account the depth distribution one derives a depth-dependent defect type variation (calculated with VEPFIT ) which treats the positron diffusion and trapping numerically.
Characteristics of the mono-energetic positron beam SPONSOR
- magnetically guided positron beam from an intensive 22Na source
- beam diameter: d ~ 4 mm
- accelerating potential for positrons: 30 eV to 36 keV, selectable
- annihilation energy resolution: (1.09 + 0.01) keV at 511 keV
- two Germanium detectors for coincident Doppler Broadening Spectroscopy
Schematics of the monoenergetic positron beam system SPONSOR for depth-resolved defect characterization.
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- A.F. Makhov, The Penetration of Electrons into Solids. 1. The Intensitiy of an Electron Beam, Transverse Paths of Electrons. Fiz. Tverd. Tela+, 2(9):1934–1941,1961.
- A. van Veen A, H. Schut, J. de Vries, R.A. Haakvoort, M.R Ijpma, Analysis of positron profiling data by means of VEPFIT, In Positron Beams for Solids and Surfaces, vol. 218. Edited by: Schultz PJ, Massoumi GR, Simpson PJ. New York: AIP; 1991:171–218.