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

Eye catcher

SPONSOR - The Slow-Positron System of Rossendorf

 

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 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 and it is sensitive to size and concentration of vacancy-like 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.

 

SPONSOR - SW 8
Implantation profiles for positrons in silicon for various incident energies (left). Definition of line shape parameters S and W for the 511 keV two-photon annihilation quanta (right).
Illustration: Maik Butterling (Download)

 

SPONSOR - L+ 8
Example of an experimentally determined S parameter for varying positron implantation energies (left). Taking into account the depth distribution one derives a depth-dependent defect type variation (calculated with VEPFIT).
Illustrations: Maik Butterling (Download)

 

Characteristics of the mono-energetic positron beam SPONSOR

o    magnetically guided positron beam from an intensive  22Na source

o    beam diameter: d ~ 4 mm

o    accelerating potential for positrons: 30 eV to 36 keV

o    annihilation energy resolution: (1.09 + 0.01) keV at 511 keV

o    two Germanium detectors for coincident Doppler Broadening Spectroscopy

 

 

SPONSOR - Setup 8
Schematics of the monoenergetic positron beam system SPONSOR for depth-resolved defect characterization.
Illustrations: Maik Butterling (Download)

COMPASS – Compact multi detector system for Positron Annihilation Spectroscopy Studies

 

The multi detector system COMPASS primarily serves for Positron Annihilation Lifetime Spectroscopy (PALS) and consists of four barium fluoride detectors, a He cryostate for sample cooling and a conventional  22Na positron source. When using a cryostate or measuring larger samples the distance between detector and sample has to be increased. This also increases the measurement time due to a smaller effective solid angle that can be covered. To overcome this four detectors are used at COMPASS.

In contrast to standard lifetime setups using two detectors COMPASS is flexible regarding the detectors responsibility: each tube can serve as start or stop detector. If one detector registers the start signal (1.27 MeV for 22Na) one of the remaining three detectors can deliver the stop signal. So a total number of 12 possible detector combinations results in 12 lifetime spectra which can be analyzed separately or averaged.

The amount of necessary analog electronic for signal processing would be rather complex and expensive. Therefore the system is fully digital using just a fast digitizer and an external coincidence for triggering. The signal processing and data evaluation is realized by software provided by the positron group of Prof. Reinhard Krause-Rehberg in Halle.

COMPASS can be extended using an additional germanium detector in order to realize Doppler Broadening Spectroscopy as well as the Age-Momentum Correlation.

 


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