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 Munchen II) FRM-II is presented. Positrons are generated by pair production of high energy prompt gamma-rays from neutron capture in cadmium: Cd-113(n,gamma)Cd-114. A cadmium cap will be located inside a beamtube at an undisturbed thermal neutron flux of 2 x 10(14) n cm(-2) s(-1). Model calculation showed that this would lead to a mean capture rate of 1.7 x 10(13) n cm(-2) s(-1). Thermal load resulting from absorbed gamma-rays is expected to be less than 4 W cm(-2). Inside the cadmium cap a structure of platinum and a stack of tungsten foils is placed for converting the gamma-rays into positron-electron pairs. Platinum is used as converting material, since the cross section for pair production is even higher (+11%) 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 10(9) slow positrons per second is expected. (C) 1999 Elsevier Science B.V. All rights reserved.