Test facility for a neutron induced positron source

At the research reactor FRM of the Technical University of Munich, an in-pi le intensive slow positron source was developed. Neutron capture gamma rays created by Cd-113(n,gamma)Cd-114 convert into e(+)-e(-) pairs by gamma abs orption in tungsten foils. Moderated positrons are emitted from the foil su rface 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 sole noid. Zn 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 use d to measure the annihilation signal of the incoming positrons. This facili ty allows to optimize acceleration potentials and guide fields, as well as settings of earth magnetic compensation. With a continuous flux of 4 X 10(7 ) neutrons/cm(2)s, the intensity of the primary positron beam was about 10( 2) slow positrons per second. Recently, the profile of the positron beam, a s 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. (C) 1999 Elsevier Science B.V. All rights reserved .