Positron annihilation spectroscopy (PAS)

Positron annihilation spectroscopy (PAS)

Head:
Dr. sc.nat. Gerhard Brauer

  • Co-worker:
    Dipl.-Phys. Wolfgang Anwand


 Goal:

  • Investigation of vacancy-type defects in solids and surfaces

Research topics:

 

  • investigations of vacancy-type defects caused by ion implantation
  • investigations of precipitates.
  • investigations of radiation-induced damage.
  • investigations of thin films and interfaces.
  • methodical developments.

 

Results


Equipment:

  • conventional positron lifetime and Doppler broadening spectroscopy
    system 1 :combined conventional positron lifetime and Doppler broadening measuring system,sample temperatures ranging from 10 K to room temperature.system 2 :single lifetime system, sample at room temperature.

  • mono-energetic positron beam for Slow Positron Implantation Spectroscopy (SPIS)
    magnetically guided mono-energetic positron beam,beam spot diameter 4 mm, high vacuum conditions,
    accelerator voltage from 30 V to 36 kV, measurement of the Doppler broadening of the positron
    annihilation line in dependence on the incident positron energy using a Ge detector,
    resolution of the Ge detector (1.09 + 0.01) keV at 511 keV.

mono-energetic positron beam apparatus of the PAS laboratory at Rossendorf


Method

Slow Positron Implantation Spectroscopy (SPIS):

Positrons of predetermined energies E (30 eV – 36 keV) are implanted at depths of up to a few micrometer in the sample. 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 parameter S. S is higher for positrons trapped at and annihilated in open-volume defects. 
The W parameter (“wing” or core annihilation parameter) is taken in high-momentum region far from the center. The parameters S an W are calculated as the normalized area of the curve in a fixed energy interval - see sketch below.

 

Sketch of the annihilation line changes


Definition of lineshape parameters S and W

 

Experimental setup
 

1 positron source      2 Helmholtz coils     3 solenoids    4 accelerator    5 iris aperture 
6 beam movement coils    7 valve     8 Ge-detector     9 sample chamber 

Data

The figure shows an S parameter vs. positron energy E plot of a 6H-SiC sample implanted with Ge+ ions. Based on the S(E)-data, a numerical solution of the positron diffusion equation is possible using the software package VEPFIT [1]. Thereby a Makhovian profile for the distribution of the thermalized positrons in the sample is assumed.


Evaluation

The fit is based on the assumption of a certain number of box-shaped depth profiles of vacancy-type defects created by the Ge+ implantation. The width of a box represents the depth and the height of a box is connected with size and concentration of a given defect.

[1] A.van Veen et al., in Positron Beams for Solids and Surfaces, edited by P.J.Schulz et al.,
     AIP Conf. Proc. No. 218 (AIP New York 1990), p. 171
 


URL of this article
https://www.hzdr.de/db/Cms?pOid=10956


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