Construction and performance of a slow positron beam guided by an increasing magnetic field

A positron beam apparatus designed for defect-profile studies has been constructed. The system uses a 30 mCi 22Na source with a W foil+mesh transmission moderator. Most parts of the source chamber, beam line and target chamber are made of aluminum in order to avoid possible distortion of the positron beam by residual magnetization of stainless steel. Positron energy can be varied from 20 eV to 35 keV. An increasing magnetic field is adopted, aiming to reduce the beam diameter at all energies. The fast positron background is eliminated by a magnetic deflection system, where the positron beam is first deflected about 37° upwards and then downwards by the same angle to return the drifted beam to the axis.