Radioactive sources for laboratory experiments

The first reactor-based slow positron beam was developed by Lynn at Brookhaven [13]. In this system, housed in the reactor building, a copper ball was irradiated in the reactor core (63Cu(n,y)64Cu) and transferred automatically into the source chamber. It was dropped into a cmcible and evaporated on to a tungsten backing. The strong MCu source therefore acted as a self-moderator for the production of slow positrons. At the Munich reactor positrons are produced by pair production by gamma rays emitted in the reaction 3Cd(n,Y) 4Cd [14]. 

It is of great interest to note that after Cherry s observation of positron emission from a solid surface the first beam system—developed in the late 60 s—was based at a LINAC facility [15]. Bremsstrahlung gamma radiation from the energetic (50MeV) LINAC electrons create electron-positron pairs in a Ta target the fast positrons thus created are then moderated (see section 8) to form the slow positron beam. The efficiency of this process clearly depends on the LINAC electron energy and the thickness of the converter. 

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