Nepomuc - The New Positron Beam Facility At Firm Ii

An in-pile positron source is installed in the vicinity of the reactor core of the new research reactor FRM-II. This neutron induced positron source at Munich NEPOMUC is based on absorption of high-energy prompt gamma-rays from thermal neutron capture in Cd-113. For this purpose, a cadmium cap is placed inside the tip of the inclined beam tube SR 11 in the heavy water moderator tank. Inside the cadmium cap a structure of platinum foils is placed in order to convert the high-energy gamma-radiation into positron-electron pairs. Due to the negative positron work function, moderation in annealed platinum leads to emission of monoenergetic positrons. After acceleration to several keV by electrical lenses the positron beam is magnetically guided in a solenoid field of 7.5 mT. The beam profile is determined with a micro-channel plate (MCP) detector at the end of the beam line. In addition, a NaI-scintillator detects the 511 keV gamma-radiation of the positrons which annihilate at the front side of the MCP in order to perform intensity measurements as a function of acceleration energy and magnetic guide field.