Positron production using a 9 MeV electron linac for the GBAR experiment

TL;DR

This paper describes a low-energy electron linac that efficiently produces slow positrons for the GBAR experiment, offering a safer and more effective alternative to radioactive sources.

Contribution

It introduces a novel 9 MeV electron linac setup for positron production, demonstrating high flux and safety advantages over traditional radioactive sources.

Findings

Produces 5×10^7 slow positrons per second

Ensures no radioactive activation beyond 10 MeV

Achieves safe radiation levels outside shielding

Abstract

For the GBAR (Gravitational Behaviour of Antihydrogen at Rest) experiment at CERN's Antiproton Decelerator (AD) facility we have constructed a source of slow positrons, which uses a low-energy electron linear accelerator (linac). The driver linac produces electrons of 9 MeV kinetic energy that create positrons from bremsstrahlung-induced pair production. Staying below 10 MeV ensures no persistent radioactive activation in the target zone and that the radiation level outside the biological shield is safe for public access. An annealed tungsten-mesh assembly placed directly behind the target acts as a positron moderator. The system produces $5\times10^7$ slow positrons per second, a performance demonstrating that a low-energy electron linac is a superior choice over positron-emitting radioactive sources for high positron flux.