The LLNL Electron-Positron Beams Facility

Introduction

The Lawrence Livermore National Laboratory operates a world-class facility to provide intense beams of positrons for use in materials physics, plasma physics, and particle physics research. Users of the facility include DOE National Security programs, academic researchers, and industry.

The positron facility provides a wide range of capabilities, including high-current beams for investigation of bulk materials properties, low-emittance beams for studies of one-component plasmas, and a positron microprobe.

Experiments underway at the positron facility are probing the mechanisms by which defects arise in materials, of vital importance for science-based stockpile stewardship in this era of a comprehensive ban on nuclear weapons testing.

Positrons

The positron is the anti-particle of the electron. It has the same mass (511 keV/c²) as the electron, but it has a positive charge. When a positron and an electron meet in flight, they annihilate, releasing two 511-keV gamma rays traveling in opposite directions.

Figure 1. Electron-positron annihilation. A positron (e+) and an electron (e-) collide, annihilating each other and producing two back-to-back photons, each of which has an energy of 511 keV.

Two-photon annihilation is the dominant decay mode for positrons in a material; unless a positron collides with an electron, it is stable. Therefore, the lifetime of a positron is directly related to the probability it will encounter an electron.

Positrons are produced at the facility in two ways. Positrons are emitted by some radioactive substances, such as ²²Na (Sodium-22). These particles can be accelerated to precise energies. For experiments requiring a large number of positrons, the high-intensity positron source generates positrons by colliding a high-energy electron beam (about 100 MeV) into a solid target, also known as a "converter."

More information about the positron facility and the experiments being conducted:

The Positron Facility	Experiments
Overview   Positron Production                 Electron Linac                 Pelletron   Positron Microprobe   Positron Trap	Materials Science                 Bulk Defect Analysis                 High-Resolution Defect Analysis   Particle Physics   Plasma Phsyics

This page was created by Dave Knapp