Test facility for a neutron induced positron source

At the research reactor FRM of the Technical University of Munich, an in-pile intensive slow positron source was developed. Neutron capture gamma rays created by 113Cd n,g.114Cd convert into eq–ey pairs by gamma absorption in tungsten foils. Moderated positrons are emitted from the foil surface and accelerated by electric lenses to a few keV. The positron beam is guided to a remoderation stage outside the reactor pool by a magnetic solenoid. In order to test various improvements of the experimental set-up the positron source was positioned at the accessible end of an external neutron guide outside the reactor building. To minimize background, a curved beam tube was installed at the end of the beamline. A germanium detector was used to measure the annihilation signal of the incoming positrons. This facility allows to optimize acceleration potentials and guide fields, as well as settings of earth magnetic compensation. With a continuous flux of 4=107 neutronsrcm2s, the intensity of the primary positron beam was about 102 slow positrons per second. Recently, the profile of the positron beam, as well as photo electrons emitted from the tungsten foils, were studied by a space-resolving micro channel plate detector combined with a fluorescent screen and a CCD-camera. q1999 Elsevier Science B.V. All rights reserved