First experiments with the MePS

The MePS system (Mono-energetic Positron Spectroscopy) is located in the Helmholtz Center Dresden-Rossendorf (HZDR) [1]. It is one of the installations at ELBE (Electron Linac for beams with high Brilliance and low Emittance) which supplies a 40-MeV electron beam [2]. MePS makes use of the excellent time structure of the primary electron beam of ELBE (repetition frequency up to 26 MHz; bunch length < 5ps) to produce a pulsed, intense slow-positron positron beam to allow positron lifetime spectroscopy.
In autumn 2011 the system was completed and tested. A new moderator was successfully installed (tungsten meshes on a tungsten foil). Moreover, the accelerator stage together with a sample chamber was added. At a rather low primary electron beam current of 48 μA (maximum current: 1 mA) a count rate of 9400 cps was obtained in a BaF2/PMT detector close to the sample. In order to avoid spurious signals which are in other systems often obtained by positrons being reflected from the sample surface, a bended tube (45°) was added between accelerator and sample chamber. Although up to now no chopper stage is in use, the signal to background ratio is 104. However, a chopper will be used in the future to improve this ratio and the time resolution which is now only about 500 ps.
The MePS system has been used to study the pore system of a series of low-k dielectric layers
References
[1] http://positron.physik.uni-halle.de/EPOS/
[2] www.HZDR.de/db/Cms?pNid=145.