High intense positron beam at the new Munich research reactor FRM-II

The Munich Intense POsitron Source MIPOS.facility for producing a high intense positron beam at the new Munich research reactor Forschungs-Reaktor Mu¨nchen II.FRM-II is presented. Positrons are generated by pair production of high 113 .114 energy prompt g-rays from neutron capture in cadmium: Cd n,g Cd. A cadmium cap will be located inside a beamtube at an undisturbed thermal neutron flux of 2=1014 n cmy2 sy1. Model calculation showed that this would lead to a mean capture rate of 1.2=1013 n cmy2 sy1. Thermal load resulting from absorbed g-rays is expected to be less than 4 W cmy2. Inside the cadmium cap a structure of platinum and a stack of tungsten foils is placed for converting the g-rays into positron–electron pairs. Platinum is used as converting material, since the cross section for pair production is even higher q11%.than in tungsten. The maximum of the energy spectrum of the positrons produced is about 800 keV. The tungsten foils also act as moderator. The positrons will be accelerated by electric lenses and guided by magnetic fields. Various arrangements are tested to improve the efficiency of the system. After remoderation of the positron beam an intensity of about 109 slow positrons per second is expected. q1999 Elsevier Science B.V. All rights reserved.