Use of superconducting LINACs for positron generation

Recently, superconducting LINACs became available to obtain high-energy electron beams [1]. The advantage of these rather compact accelerators is the high electron bunch charge at very high repetition rates up to > 20 MHz under cw operating conditions. Moreover, the electron bunch length is extremely short (< 5 ps). A single accelerator stage containing several RF cavities may supply beam energies as large as 20 MeV. Thus, such electron beams are ideal hosts for the generation of bunched, intense sources of either slow or fast positrons. The Radiation Source ELBE [2] at the Research Centre Dresden-Rossendorf (FZD) has two accelerator stages providing an electron beam of 40 MeV with 26 MHz maximum repetition rate, and an average electron current of 1 mA. In close collaboration of the University Halle and the FZD the positron source EPOS (ELBE Positron Source) is under construction [3]. The recent progress of the EPOS project will be demonstrated. EPOS consists of a digital, pulsed monoenergetic positron beam (MePS), a conventional lifetime/Doppler spectrometer (CoPS), and a setup for gamma-induced positron annihilation spectroscopy (GiPS). While the CoPS and GiPS systems are available for user operation, the MePS system is still under construction. First positrons were moderated and fed into the positron lab. In the moment, the chopper/buncher/accelerator system is under assembly. The GiPS system has now been used in several regular user sessions. Positrons are generated inside the sample by an intense, pulsed photon beam (up to 20 MeV). Thus, the whole sample volume will be investigated even for rather thick samples up to a sample volume of 20x20x20 mm³. This is very useful for liquids, coarse powders, biological tissue, NDT experiments, and other bulky samples. GiPS as well as CoPS are operated in the moment with analogue detector systems. However, the development of digital systems is close to be completed.
[1] http://elan.desy.de/
[2] http://www.fzd.de/db/Cms?pNid=144
[3] http://positron.physik.uni-halle.de/EPOS/