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

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 × 10 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.

Efficient positron trapping and extraction with a center-hole SiC remoderator

“…Note that a SiC-based positron trap is only suitable for trapping pulsed beams with pulse widths less than a few microseconds, since the positrons re-emitted from the SiC must be caught in a pulsed manner. Thus, the SiC-based trap is suitable for the microsecond pulsed positron beams supplied by LINAC-based facilities such as CERN AD (GBAR Collaboration) [14], KEK IMSS SPF [15], and AIST [16]. In application, the trap system can serve as an efficient converter of a LINAC-based positron beam to a controllable, high-quality burst beam, and can store a large number of positrons (10 9 -10 10 e + ) when used in combination with another accumulator.…”

“…Positron beams supplied by nuclear reactors and electron linear accelerators (LINACs) have intensities a few orders of magnitude greater than that of typical radioisotope-based beams. For example, LINAC-based positron beams with intensities of 10 7 -10 8 e + s −1 are available at the European Organization for Nuclear Research (CERN)'s antiproton decelerator (AD) by the gravitational behaviour of antihydrogen at rest (GBAR) Collaboration [14], the slow positron facility (SPF), Institute of Materials Structure Science (IMSS), High Energy Accelerator Research Organization (KEK) [15], and the National Institute of Advanced Industrial Science and Technology (AIST) [16]. Feeding such beams into a buffer-gas trap should improve the trapping rate and thereby increase the number of accumulated positrons.…”

Positronium Physics and Biomedical Applications

“…With e − e + pair production, photons with energy larger than around 1.2 MeV are implanted in materials with high atomic number Z such as tungsten and platinum, and their energy is converted to the mass of e − e + pairs. The high-energy photons can be generated via bremsstrahlung from decelerating electrons previously accelerated to relativistic energy by employing an electron linear accelerator (LINACs), see for example Charlton et al (2021); Howell et al (1982); and Wada et al (2012). As an alternative, γ rays can be released from nuclear processes (see for example Hawari et al (2011); Hugenschmidt et al (2008); Sato et al (2015);and Schut et al (2004)).…”

A large-momentum-transfer matter-wave interferometer to measure the effect of gravity on positronium

“…In [71][72][73], a superconductive LINAC operates at 92 MHz with an average electron current of 2.5 mA and an acceleration energy of 10 MeV, producing at least 10 16 fast electrons per second with a beam spot lower than 1 mm and a divergence of 50 µrad. Taking into account the e + /e − conversion efficiency [74], about 10 10 e + s −1 are expected. Considering the Ps − /e + [63] and the simulated Ps/Ps − conversion efficiencies, about 0.5 × 10 8 Ps s −1 are expected to enter the interferometer.…”