Positron facilities in Europe

Positron annihilation is a well established method in solid state physics and material science. The positron being a very sensitive probe, can give very precise information on the momentum distribution of electrons in metals and alloys as well as on lattice defects in crystals. Starting with the energy distribution of positrons from a radioactive decay, the current development is directed more to monoenergetic positrons of variable energy and of high intensity. Pulsed and continuous beams of moderated positrons have been designed and developed to study very efficiently surface and near surface regions in metals and semiconductors. Especially a pulsed positron beam with narrow beam pulses (∼ 150 ps) enables positron lifetime experiments as a function of the positron energy. The impact of intense positron beams is straightforward: a decrease of the counting time. There are various possibilities and approaches to realize intense positron beams. Developments at several institutes in Europe are under way. The aim is to obtain a beam intensity in the order of ∼ 1010 positrons/sec. Parallel to the installment of intense positron beams, the development of positron microscopes is pursued. Two types of positron microscopes are being set up: a scanning positron microscope with a pulsed beam of 100 ps and a beam diameter of 1 μm and a positron reemission microscope with about the same beam diameter. (orig.)