Positrons as High Sensitive and Non-Destructive Probes for Nanomaterials (instrument NEPOMUC)
In material science and surface physics positron annihilation is used as a non-destructive technique for the investigation of the microstructure and defects on an atomic scale. Since the annihilation parameters depend on the local electronic structure, measurement of the positron lifetime and exact energy distribution of the annihilation radiation reveals material defects with high sensitivity. Therefore, open volume defects such as vacancies, dislocations or grain boundaries on the nanometer scale in metals and semiconductors can be investigated. In addition, information about the size of nanovoids and the free volume in anorganic amorphous materials and polymers is experimentally accessible.
Defect spectroscopy in the near surface region of samples using slow positrons is performed at the positron beam facility NEPOMUC – NEutron induced POsitron source MUniCh. In these experiments, a monoenergetic positron beam is focused onto a sample with a variable material dependent implantation depth which ranges from the surface to about 1500 nm. After thermalisation the positron diffuses through the bulk with a typical diffusion length of 100 nm and annihilates together with an electron of the sample material into two 511 keV γ-quanta.
In a new approach, the chemical information of the positron annihilation site, i.e. the elemental surrounding of open volume defects, could be revealed. Heated brass was chosen as model system in order to show the elemental signature in a first positron. The figure shows the fingerprint of Zn and Cu of the normalized annihilation line for E > 511 keV due to the different electron momentum distribution of these elements. With increasing temperature, i.e. less zinc concentration of the brass sample, the energy distribution approaches the curve recorded for pure copper.
