Requirements for samples to be used in Positron Annihilation Experiments

1. Positron Lifetime Measurement and Doppler-Coincidence Spectroscopy:

We need for our spectrometers here in Halle two identical samples for the measurement. The samples will be measured in "Sandwich Geometry", i.e. the two sample halfs surround the 22Na-source which is packed into thin (about 2 mm) Al foil.

The samples should be larger than the source, so that the minimum size is about 8 x 8 mm. When temperature-dependent measurements are desired, the samples should fit into the cryostats. Maximum size is therefore about 10 x 10 mm. For room temperature measurements, the sample size is almost unlimited.

In order to ensure that 99.9% of the positrons will stop and annihilate in the sample, a certain sample thickness is required. It is called "information depth" in the following figure. When your samples are thinner, one can make a stack of several samples, or one can put a reference sample with known positron lifetime spectrum behind the sample under investigation. When your sample material is not indicated in the figure, please interpolate according to the mass density. The surface quality is not very important, but the surface should not be damaged (grinding paper, diamond saw, etc.).

Here is a photo of our 4 lifetime spectrometers and the Doppler-Coincidence spectrometer (at the left) which is combined with a lifetime spectrometer:


1. VEPAS (Variable-energy positron annihilation spectroscopy):

When it is necessary to study thin layers (epitaxy, ion implantation, surface damage), the use of our monoenergetic positron beam (photo below) is necessary. One sample of a size of 1 x 1 cm is required.

You need further information? Please contact R. Krause-Rehberg.