Target chamber for a slow positron beam: optimization of count rate and minimization of backscattering effects

Positrons, which scatter back from the target and annihilate in chamber walls near the detectors, may cause a significant error in annihilation parameters. We have constructed a new UHV target chamber for slow positron beam studies. In our design special care has been taken to reduce the effect of backscattered positrons. Detector wells are designed for two-detector coincidence measurements and they are situated on both sides of the target. The distance of the wells from the target can be adjusted by simple manipulators. This enables optimization regarding the count rate and the rate of backscattered positrons hitting the detector wells. The magnetic field in front of the target is increased by permanent magnets situated behind the target. The increased magnetic field guides the backscattered positrons effectively away from the detectors. The increased magnetic field also focuses the beam spot strongly.