Development of an intense positron beam research facility (POSH) at the HOR

The development is described of an unique intense reactor based positron beam called POSH (POSitrons from the HOR reactor). Positrons are generated by gammas, mainly fission gammas, produced in the reactor core. An assembly of thin tungsten foils, configured as 4 disks containing 10 x 10 mm square channels, serves to convert the gammas by pair-formation into electron positron pairs, to slow down and thermalize the energetic positrons, and, finally to re-emit the positrons as slow positrons (3 eV) from the tungsten surfaces into the vacuum of the vacuum tube containing the source. Electrical fields are applied to the disks and to additional beam focussing electrodes in order to extract the generated slow positrons. Subsequently, the beam is guided away from the reactor core, and through the 4 meters thick concrete shielding by a magnetic solenoid field. In March 1999 a beam intensity of 2 x 108 e+/s was demonstrated in a beam spot of about 10 mm diameter for a reactor power of 2 MW. The main problems in the design have been in coping with the hostile environment and the geometrically difficulties that exist for creating vacuum and electrical fields in a position located closely to the reactor core. In the final design, presented here, gamma heating is employed advantageously to heat the tungsten foils permanently to temperatures of 300 deg. C and higher, so that damage created by fast neutrons is continuously annealed. The intense positron beam will be used to perform high resolution measurements of the electron momentum distribution (2D-ACAR) for depth localized defects, thin layer systems and interfaces. Furthermore, the beam will be used to create a submicrometer size positron beam for defect depth profiling on a lateral scale smaller than a micrometer. It will enable to map 'vacancy type' defects in a 3-D fashion. (author)