I SLAC - PUB - 378 September - 1968 ( MPI ) POSITRON BEAMS

The secondary positron beams produced by linear electron accelerators are used in scattering experiments with stationary targets, in electron-positron colliding beam experiments, and in the production of annihilation photon beams : they now approach primary electron beams in their energy, intensity, and range of application. (To be published in physics Today) Work supported by the U.S. Atomic Energy Commission, The study of elementary subatomic particles is made possible in large measure by the development in recent years of high-energy electron and high-energy proton accelerators. Progress in the field of “accelerator physics” is usually reckoned in terms of the maximum “primary“ beam energies ‘and intensities that the most advanced accelerators are able to produce.. More recently, as higher primary energies and intensities have been achieved, the production of high-energy “secondary” beams of mesons, photons, anti-protons, neutrons and ,antineutrons, positrons (anti-electrons) and even neutrinos and hyperons has assumed considerable importance. Secondary beams are produced when an accelerated beam of primary electrons or protons strikes a suitable target. The photon (x-ray) beams produced in x-ray tubes (Fig. 1) provide a familiar example: electrons emitted from the negative cathodc are accelerated by the electric field toward the positive anode where they emit x-rays with a wide range of energy and direction. Usually, secondary beams are col.limatecl to yield “pencil” beams traveli.ng in a well-defined direction. Unlike photon beams, chargecl particle beams can also be momentum-analyzed by bending the bcnms through a magnetic field. This resuIts in a beam of specific momcniun~ and momcnlum interval. Techniques are also available for distinguishing or fol separating out. pflrticlcs of a particular type. The resulting secondary beams, thou$ still very useful, necessarily have lower energies and much lower intcnsitics than the primary beams that gcncrate them. The positron beams produced internally by linear accelerators (Fig. 2) differ from other secondary beams in that their energies normally exceed the encrf;ics of the primary electrons incident on the positron target or “converter. ” Furthermore, positron intensities can, in principle, exceed primary electron intensities. The high energy is flchieved by usin, 0‘ a major fraction of the linear accelerator to