Berlin 2008 – wissenschaftliches Programm
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VA: Fachverband Vakuumphysik und Vakuumtechnik
VA 4: NEPOMUC positron source and experiments
VA 4.2: Vortrag
Montag, 25. Februar 2008, 15:40–16:00, H 0106
Gas moderation of positrons — •Benjamin Löwe1,2, Klaus Schreckenbach1,2, and Christoph Hugenschmidt1,2 — 1TU München, FRM II, Lichtenbergstr. 1, 85747 Garching — 2TU München, Physik Department E21, James Franck Str., 85748 Garching
A variety of low energy positron experiments need an improved brilliance of the beam by means of a remoderator. Conventionally, a tungsten foil or single crystal is used as a remoderator in transmission or reflection geometry. In this project a novel remoderation unit was developed and tested at the positron beam facility NEPOMUC at the FRM II. This remoderation is based on inelastic positron scattering and the drift of positrons in a suitable gas. The stopping of positrons in nitrogen has already been shown by Surko et al. and is currently used to store positrons in a trap.
Positrons from the NEPOMUC source are decelerated at the entrance of the remoderation chamber to about 50 eV by an electric field and enter into the gas region (about 0.01 mbar). Due to inelastic scattering with gas molecules the positrons lose energy through different processes such as vibrational excitations, electronic excitations and ionisation. Above energies of 8 eV losses due to positronium formation occur. After thermalisation in the gas the positrons drift along the focusing electric field lines. At the exit the moderated positrons have been measured by a retarding potential analyser. The principle of the new gas remoderator and first measurements will be presented.