Electro-produced slow positrons

Abstract

During the last 6 years it has been demonstrated that electro-produced intense beams of slow positrons are possible. High energy electrons from an accelerator generate bremsstrahlung in a thick conversion target of high element number Z. The photons produce electronpositron pairs and a small fraction of the positrons may be moderated to thermal energies. A review is given of the existing slow positron beam lines using this technique. At accelerator energies of 100 MeV total conversion efficiencies of several slow positrons per 10⁶ primary electrons have been obtained, resulting in average intensities of several 10⁹ slow positrons per second or more than 10⁵ slow positrons in pulses having a duration of a few ns. A further increase in intensity by at least one order of magnitude seems possible at higher accelerator energies.