Start-To-End Simulations of a Compact, Linac-Based Positron Source

TL;DR

This paper presents comprehensive simulations of a linac-based positron source that significantly enhances moderation efficiency by decelerating positrons, resulting in a brighter, more usable slow positron beam for material surface studies.

Contribution

It introduces a start-to-end simulation framework for a compact linac-based positron source, demonstrating a 16.3-fold increase in moderation efficiency through positron deceleration.

Findings

15 times more positrons under 500 keV with deceleration

16.3 times improvement in moderation efficiency

Potential for brighter positron sources for material studies

Abstract

Slow positrons are increasingly important to the study of material surfaces. For these kinds of studies, the positrons must have low emittance and relatively high brightness. Unfortunately, fast positron sources like radioactive capsules or linac driven sources have broad energy and angular spread, which make them difficult to capture and use. Moderators are materials that produce slow, mono-energetic positrons from a fast positron beam. Since their efficiencies are typically less than $10^{-3}$ slow $e^+$ per fast $e^+$, research into how to maximize efficiency is of great interest. Previous work has shown that using a linac, one can decelerate the fast positron beam in order to greatly increase moderation efficiency. We present here start-to-end simulations using G4beamline to model a 100 MeV electron beam incident upon a Tungsten target, focused by an adiabatic matching device, and decelerated by a 1.3 GHz, 5-cell pillbox cavity. We show that by decelerating the positrons after their creation we can increase the number of positrons under 500 keV by 15 times, translating to a 16.3 times improvement in moderation efficiency, and therefore leading to a brighter positron source.