The generation and acceleration of positrons

Abstract

Positrons will be produced in the University of Saskatchewan Linear Electron Accelerator by allowing bursts of electrons accelerated by the first linac section to fall on a thick target. The resulting positrons can be accelerated by the three succeeding linac sections. The longitudinal equations of motion for positrons in a linear accelerator have been solved and results are presented for the field configuration of the Saskatchewan linac sections. These results indicate that the minimum energy spread at output occurs if the positron burst is injected with an initial phase of 120° and the magnetic confining system is designed for positrons of 2 MeV total initial energy. For reasonable phase spread and energy spread at injection these injection parameters will yield a positron beam of energy 39.4 ± 0.5 MeV at the end of the first positron accelerating section and of energy 114.5 ± 1.0 MeV at the end of the third positron accelerating section. Two tentative designs are presented for confining the positron beam within the accelerator aperture. The better system should give a net conversion efficiency of 4 x 10-4 e+/e- and a time average positron current of 0.07 µA. The inferior, but less costly, system should give a net conversion efficiency of 0.7 x 10-4 e+/e- and a time average positron current of 0.01 µA.